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 the lowest eutectic point composition proportion of a system is solved through an iteration method at specific temperature and pressure by utilizing a phase diagram thermodynamic calculation method and following a Gibbs free energy minimization principle. The mixed molten salt is mainly composed of KNO3, NaNO2 and a small amount of metal chlorate (KCl, CaCl2 or NaCl), and a ternary mixed molten salt system with the characteristics of low melting point and high decomposition temperature is obtained. According to the combination mode, the cost is reduced, the stability is improved, and the working temperature range is widened. The design and preparation method disclosed by the invention is simple, safe and convenient to operate.

Description

Technical Field

[0001] The present invention belongs to the technical field of molten salt heat storage and transfer, and particularly relates to a ternary mixed molten salt heat storage and transfer medium and a preparation method thereof. Background Art

[0002] As an important energy storage means, molten salt heat storage technology is widely used in fields such as solar thermal power generation, power peak shaving, new energy consumption, and industrial waste heat utilization. The core of it is the research and application of molten salt heat storage materials. In order to achieve high-quality controllable utilization of clean energy, it is urgent to develop molten salt heat storage technology to realize the energy concentration and quality improvement of molten salt heat storage. Currently, the development of molten salt heat storage technology is mainly restricted by problems such as high melting point of molten salt, narrow working temperature range, and poor thermal stability. Therefore, it is urgent to design and develop multi-component molten salts with low melting points and wide working temperature ranges to improve the energy conversion efficiency of molten salts in actual applications.

[0003] When studying the eutectic composition of multi-component molten salt systems, the traditional trial-and-error approach has been difficult to meet the needs of modern research due to its inherent limitations. This method explores the eutectic composition of the system phase diagram through repeated experiments, resulting in a significant extension of the experimental period. Especially when dealing with multi-component systems, the exponentially increasing number of experimental groups and the need for repetitive tests will directly drive up research costs and limit research efficiency. More critically, the complex multiphase reaction kinetics and the formation of intermediate phases in multi-component molten salt systems make it face significant challenges in accurately observing phase change behaviors and analyzing data during the experimental process, further exacerbating the uncertainty of experimental results. In addition, the exploration ability of the traditional trial-and-error approach for high-dimensional component spaces is limited by the blindness of experimental design, and it is difficult to effectively capture non-linear phase equilibrium relationships and structure-property relationships. Therefore, in the face of the research needs for low melting points of complex multi-component molten salt systems, it is urgent to develop an efficient research strategy based on the combination of theoretical simulation and experimental verification to break through the technical bottleneck of the traditional trial-and-error approach in constructing high-dimensional phase diagrams. Summary of the Invention

[0004] An object of an embodiment of the present invention is to provide a ternary mixed molten salt heat storage and transfer medium to reduce the melting point of the molten salt, broaden the working temperature range of the molten salt, and improve the thermal stability;

[0005] A second object of an embodiment of the present invention is to provide a preparation method of a ternary mixed molten salt heat storage and transfer medium.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is a ternary hybrid molten salt heat transfer and storage medium, which comprises the following components: KNO3, NaNO2 and metal chloride; wherein, the metal chloride comprises one of KCl, CaCl2, and NaCl.

[0007] Further, when the metal chloride is KCl, the molten salt heat transfer and storage medium comprises the following components by weight percentage: 59.06 wt%-61.00 wt% of KNO3, 36.00 wt%-37.76 wt% of NaNO2, and 3.00 wt%-3.43 wt% of KCl.

[0008] Further, when the metal chloride is CaCl2, the molten salt heat transfer and storage medium comprises the following components by weight percentage: 57.50 wt%-57.75 wt% of KNO3, 36.50 wt%-37.05 wt% of NaNO2, and 5.20 wt%-6.00 wt% of CaCl2.

[0009] Further, when the metal chloride is NaCl, the molten salt heat transfer and storage medium comprises the following components by weight percentage: 59.71 wt%-61.26 wt% of KNO3, 36.83 wt%-38.49 wt% of NaNO2, and 1.80 wt%-3.00 wt% of NaCl.

[0010] Further, the molten salt heat transfer and storage medium comprises the following components by weight percentage:

[0011] 59.06 wt% of KNO3, 37.76 wt% of NaNO2, and 3.18 wt% of KCl.

[0012] Further, the molten salt heat transfer and storage medium comprises the following components by weight percentage:

[0013] 57.68 wt% of KNO3, 36.97 wt% of NaNO2, and 5.35 wt% of CaCl2.

[0014] Further, the molten salt heat transfer and storage medium comprises the following components by weight percentage:

[0015] 59.71 wt% of KNO3, 38.49 wt% of NaNO2, and 1.80 wt% of NaCl.

[0016] A preparation method of a ternary mixed molten salt heat storage and transfer medium as described above, wherein potassium nitrate, sodium nitrite, and one of three metal chlorides, namely calcium chloride, potassium chloride, or sodium chloride, are respectively ground into powder and kept at 100-120°C for 24-30 hours; based on the eutectic point of the molten salt heat storage and transfer medium, the powders are proportioned, and the three proportioned powders are mixed evenly, and heated to 100-120°C at a heating rate of 3-7°C·min -1 ; keep warm for 1-1.5 hours at this temperature; then heat at a heating rate of 8-15°C·min -1 to 300-400°C until all solids are melted, then keep warm for 1-3 hours, and finally cool to room temperature.

[0017] Further, the specific process of proportioning the powders based on the eutectic point of the molten salt heat storage and transfer medium is as follows:

[0018] S1. Obtain the entropy value, heat capacity, and Gibbs energy data of pure molten salt at different temperatures through the table lookup method;

[0019] S2. Determine the Gibbs free energy thermodynamic model:

[0020] G(T) = A + BT + CTlnT + DT 2

[0021] where G(T) is the Gibbs free energy function of pure substance, T is the Kelvin temperature, and A, B, C, and D are fitting parameters to be determined;

[0022] S3. After measuring the Gibbs energy data of the molten salt at different temperatures, use the least squares method to fit the Gibbs free energy expression; determine the fitting parameters to obtain the thermodynamic model expression of the Gibbs free energy of pure molten salt;

[0023] S4. Compile thermodynamic data files for three ternary system molten salts of KNO3-NaNO2-CaCl2, KNO3-NaNO2-KCl, and KNO3-NaNO2-NaCl. Among them, the substitution melt model is used to describe the Gibbs free energy of the binary system molten salt melt phase, and the intermediate phase is ignored;

[0024] S5. Respectively optimize the thermodynamic data of the three ternary system molten salts of KNO3-NaNO2-CaCl2, KNO3-NaNO2-KCl, and KNO3-NaNO2-NaCl through the optimization module of the phase diagram calculation software; obtain a relatively accurate thermodynamic database through optimization;

[0025] S6. Import the obtained thermodynamic database into the phase diagram calculation software, and calculate and draw the phase diagram of the ternary system molten salt through the phase diagram calculation software;

[0026] S7. For the phase diagram obtained by calculation, find the intersection points of the liquidus surface in the phase diagram. The temperature corresponding to the intersection point is the calculated lowest melting point. The proportions of each component at this temperature are the preliminary determined proportions of the lowest melting point of the system. Then, conduct experiments using these proportions and measure the melting point of these proportions. The measured melting point is the lowest melting point obtained from the experiment.

[0027] S8. Compare the lowest melting point obtained from the calculation results with the lowest melting point value measured in the experiment. If the error is less than 30%, the proportions of each component corresponding to the lowest melting point obtained from the phase diagram calculation are the proportions of the lowest eutectic point of the system. If the error is greater than 30%, the thermodynamic database of the ternary system molten salt needs to be optimized again.

[0028] Further, the binary system molten salts in S4 include seven binary system molten salts: KNO3 - NaNO2, KNO3 - CaCl2, KNO3 - KCl, KNO3 - NaCl, NaNO2 - CaCl2, NaNO2 - KCl, and NaNO2 - NaCl.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention abandons the trial - and - error method for obtaining the eutectic point composition ratio of the molten salt system, and adopts a thermodynamic method of phase diagram for calculating the lowest eutectic point of the ternary system by following the principle of minimum Gibbs free energy. The design of the entire ternary molten salt system is more convenient, and the low - eutectic - point multi - component molten salts can be quickly screened out. The present invention creatively uses KNO3 and NaNO2 as the main components, and adds an appropriate amount of CaCl2 or KCl, NaCl, to obtain a ternary mixed molten salt system with a lowest eutectic point of 95.1 °C and a melting enthalpy of 150.6 J·g -1 , and a decomposition temperature as high as 705.52 °C. The melting points of the molten salts in this system are 128.1 °C, 52.1 °C, and 31.7 °C lower than those of Solar salt, Hietc salt, and HietcXL salt respectively, and the decomposition temperatures are 129.62 °C, 171.32 °C, and 191.22 °C higher than those of Solar salt, Hietc salt, and HitecXL salt respectively. It has the advantages of low cost, high stability, and a wide working temperature range. At the same time, according to the preparation method of the ternary mixed molten salt of the present invention, it is simple, safe, and easy to operate. It is applicable to fields such as molten salt energy storage and solar thermal power generation. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0031] Figure 1 is the flowchart of Embodiment 1 of the present invention;

[0032] Figure 2 is the DSC curve of Embodiment 2 of the present invention;

[0033] Figure 3 is the DSC curve of Embodiment 3 of the present invention;

[0034] Figure 4 is the DSC curve of Embodiment 4 of the present invention;

[0035] Figure 5 is the DSC curve of Embodiment 5 of the present invention;

[0036] Figure 6 is the DSC curve of Embodiment 6 of the present invention;

[0037] Figure 7 is the DSC curve of Embodiment 7 of the present invention;

[0038] Figure 8 is the DSC curve of Embodiment 8 of the present invention;

[0039] Figure 9 is the DSC curve of Embodiment 9 of the present invention;

[0040] Figure 10 is the DSC curve of Embodiment 10 of the present invention;

[0041] Figure 11 is the DSC curve of Embodiment 11 of the present invention;

[0042] Figure 12 is the DSC curve of Embodiment 12 of the present invention;

[0043] Figure 13 is the DSC curve of Embodiment 13 of the present invention;

[0044] Figure 14 is the curve of the change of mass with temperature of Embodiment 2 of the present invention;

[0045] Figure 15 is the curve of the change of mass with temperature of Embodiment 6 of the present invention;

[0046] Figure 16 is the curve of the change of mass with temperature of Embodiment 10 of the present invention.

[0047] Figure 17 is the DSC curve of Comparative Example 1;

[0048] Figure 18 is the DSC curve of Comparative Example 2;

[0049] Figure 19 is the DSC curve of Comparative Example 3;

[0050] Figure 20 is the DSC curve of Comparative Example 4;

[0051] Figure 21 is the DSC curve of Comparative Example 5;

[0052] Figure 22 is the DSC curve of Comparative Example 6;

[0053] Figure 23 is the DSC curve of Comparative Example 7;

[0054] Figure 24 is the curve of the mass change of Comparative Example 7 with temperature;

[0055] Figure 25 is the DSC curve of Comparative Example 8;

[0056] Figure 26 is the curve of the mass change of Comparative Example 8 with temperature. Detailed implementation manners

[0057] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0058] The existing multi-component molten salts (more than four components) have more components. As the number of components increases, the thermal decomposition reactions of the components in the multi-component molten salts may be superimposed on each other, resulting in a decrease in the overall decomposition temperature or the appearance of uncontrollable intermediate phases. Moreover, the more components are introduced, the side reactions between the components will be correspondingly aggravated, reducing the thermal stability of the system, especially more likely to decompose or coke at high temperatures. In the existing ternary system, NaNO2 will slowly decompose above 450°C, seriously affecting the working temperature range of the molten salt system.

[0059] This embodiment proposes a ternary hybrid molten salt heat transfer and storage medium with a low melting point and a wide temperature range. Using potassium nitrate (KNO3) and sodium nitrite (NaNO2) as the main components, and doping with optimized proportions of calcium chloride (CaCl2) or potassium chloride (KCl), sodium chloride (NaCl), an improved ternary molten salt system is constructed. Based on the principle of minimum Gibbs free energy, thermodynamic calculations are carried out to optimize the ternary systems of KNO3-NaNO2-CaCl2, KNO3-NaNO2-KCl, and KNO3-NaNO2-NaCl. The thermodynamic models of pure substances are used to describe the Gibbs free energy of KNO3, NaNO2, CaCl2, KCl, and NaCl, and the substitution melt model is used to describe the Gibbs free energy of the six binary system melt phases of KNO3-NaNO2, KNO3-KCl, KNO3-CaCl2, KNO3-NaCl, NaNO2-KCl, NaNO2-CaCl2, and NaNO2-NaCl. By iteratively adjusting the model parameters until the model fits the experimental data. Through the thermodynamic calculation of the phase diagram, a ternary hybrid molten salt system with a eutectic point as low as 95.1 °C, a melting enthalpy of 150.6 J·g -1 , and a decomposition temperature as high as 705.52 °C is obtained.

[0060] In some specific embodiments, the specific components of the ternary hybrid molten salt heat transfer and storage medium include KNO3, NaNO2, and metal chlorides. The specific components and types of metal chlorides are as follows:

[0061] A. 59.06 wt% - 61.00 wt% of KNO3, 36.00 wt% - 37.76 wt% of NaNO2, 3.00 wt% - 3.43 wt% of KCl;

[0062] B. 57.50 wt% - 57.75 wt% of KNO3, 36.50 wt% - 37.05 wt% of NaNO2, 5.20 wt% - 6.00 wt% of CaCl2;

[0063] C. 59.71 wt% - 61.26 wt% of KNO3, 36.83 wt% - 38.49 wt% of NaNO2, 1.80 wt% - 3.00 wt% of NaCl.

[0064] In some specific embodiments, the preparation method of the ternary hybrid molten salt heat transfer and storage medium is as follows: Grind potassium nitrate, sodium nitrite, and calcium chloride (or one of potassium chloride and sodium chloride) into powder form and keep them at 100 - 120 °C for 24 h - 30 h; Mix the corresponding potassium nitrate, sodium nitrite, and calcium chloride (potassium chloride, sodium chloride) evenly according to the eutectic point ratio of the required molten salt heat transfer and storage medium; At a rate of 3 - 7 °C·min -1Heat it to 100 - 120 °C at a heating rate of, and hold for 1 - 1.5 h; then heat it to 300 - 400 °C at a heating rate of 8 - 15 °C·min -1 until all the solids are melted, and then hold for 1 - 3 h to make the molten salt fully and evenly mixed; finally, cool it to room temperature.

[0065] Example 1

[0066] This example provides a method for determining the composition ratio of the lowest eutectic point of a ternary molten salt heat storage medium. As Figure 1 shown, it is specifically carried out according to the following steps:

[0067] S1. Obtain the entropy value, heat capacity, and Gibbs energy data of pure molten salt at different temperatures through the table lookup method;

[0068] S2. To facilitate the determination of the thermodynamic model of the Gibbs free energy of pure molten salt in the follow-up, determine the expression of the Gibbs free energy thermodynamic model:

[0069] G(T) = A + BT + CTlnT + DT 2

[0070] where G(T) is the Gibbs free energy function of the pure substance, T is the Kelvin temperature, and A, B, C, and D are the parameters to be fitted.

[0071] S3. After measuring the Gibbs energy data of the molten salt at different temperatures, use the least squares method to fit the Gibbs free energy expression; determine the parameters to be fitted to obtain the thermodynamic model expression of the Gibbs free energy of pure molten salt.

[0072] S4. Compile the thermodynamic data files of three ternary system molten salts, namely KNO3 - NaNO2 - CaCl2, KNO3 - NaNO2 - KCl, and KNO3 - NaNO2 - NaCl. Among them, the substitution melt model is used to describe the Gibbs free energy of the binary system molten salt melt phase, and the intermediate phase is ignored;

[0073] including seven binary system molten salts, namely KNO3 - NaNO2, KNO3 - CaCl2, KNO3 - KCl, KNO3 - NaCl, NaNO2 - CaCl2, NaNO2 - KCl, and NaNO2 - NaCl;

[0074] S5. Respectively optimize the thermodynamic data of the three ternary system molten salts of KNO3 - NaNO2 - CaCl2, KNO3 - NaNO2 - KCl, and KNO3 - NaNO2 - NaCl through the optimization module of the phase diagram calculation software; obtain a relatively accurate thermodynamic database through optimization;

[0075] S6. Import the obtained thermodynamic database into the phase diagram calculation software, and calculate and plot the phase diagram of the ternary system molten salt through the phase diagram calculation software;

[0076] S7. Through the phase diagram obtained by calculation, find the intersection points of the liquidus surface in the phase diagram. The temperature corresponding to the intersection point is the calculated lowest melting point. The proportion of each component at this temperature is the preliminary determined ratio of the lowest melting point of the system. Conduct experiments with this ratio and test the melting point of this ratio. The measured melting point is the lowest melting point obtained from the experiment;

[0077] S8. Compare the lowest melting point obtained from the calculation results with the lowest melting point value measured in the experiment. If the error is less than 30%, the proportion of each component corresponding to the lowest melting point obtained from the phase diagram calculation is the ratio of the lowest eutectic point of the system. If the error is greater than 30%, it is necessary to optimize the thermodynamic data of the ternary system molten salt again to obtain a more accurate thermodynamic database.

[0078] Example 2

[0079] This example provides a method for preparing a low-melting-point wide-temperature-range ternary mixed molten salt, which is specifically carried out according to the following steps:

[0080] S1. After calibrating the electronic balance, weigh 57.68 wt% of KNO3, 36.97 wt% of NaNO2, and 5.35 wt% of CaCl2; grind them into powder and keep them at 100 °C for 24 h.

[0081] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 5 °C·min -1 , and keep it at a constant temperature of 100 °C for 1 h to remove the moisture remaining in the salt; then, heat it at a heating rate of 10 °C·min -1 to 400 °C, and keep it at this temperature for 2 h. Pour it out in the molten state, cool it naturally to room temperature, and grind it into powder to obtain the ternary mixed molten salt.

[0082] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , and the test and analysis of the experimental results are as Figure 2 shown. The lowest melting temperature of the molten salt in this example is 95.10 °C, and the latent heat is 150.60 J·g -1 .

[0083] S4. Use a thermal analyzer (STA) to test the prepared ternary mixed molten salt sample. The test range is RT - 650.0 °C, and the heating rate is 10.0 °C·min -1, the test and analysis of the experimental results are as Figure 14 shown. The decomposition temperature of the ternary mixed molten salt in this embodiment is 705.52 °C.

[0084] Example 3

[0085] A preparation method of a low-melting-point and wide-temperature-range ternary mixed molten salt includes the following steps.

[0086] S1. After calibrating the electronic balance, weigh 57.50 wt% of KNO3, 36.50 wt% of NaNO2, and 6.00 wt% of CaCl2; grind them into powder and keep them at 110 °C for 26 h.

[0087] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 5 °C·min -1 , and keep it at 100 °C for 1 h to remove the residual moisture in the salt; then, heat it to 400 °C at a heating rate of 10 °C·min -1 , keep it at this temperature for 2 h, pour it out in the molten state, naturally cool it to room temperature, and grind it into powder to obtain the ternary mixed molten salt.

[0088] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , the test and analysis of the experimental results are as Figure 3 shown. The lowest melting temperature of the molten salt in this embodiment is 95.94 °C, and the latent heat is 131.20 J·g -1 .

[0089] Example 4

[0090] A preparation method of a low-melting-point and wide-temperature-range ternary mixed molten salt includes the following steps.

[0091] S1. After calibrating the electronic balance, weigh 57.60 wt% of KNO3, 37.00 wt% of NaNO2, and 5.40 wt% of CaCl2; grind them into powder and keep them at 120 °C for 28 h.

[0092] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 5 °C·min -1 , and keep it at 100 °C for 1 h to remove the residual moisture in the salt; then, heat it at a heating rate of 10 °C·min -1Heat it at a heating rate to 400 °C, hold it at this temperature for 2 h, pour it out in a molten state, naturally cool it to room temperature, and grind it into a powder to obtain the ternary mixed molten salt.

[0093] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , and the test and analysis of the experimental results are as Figure 4 shown. The lowest melting temperature of the molten salt in this example is 107.16 °C, and the latent heat is 104.00 J·g -1 .

[0094] Example 5

[0095] A preparation method of a low-melting-point wide-temperature-range ternary mixed molten salt includes the following steps.

[0096] S1. After calibrating the electronic balance, weigh 57.75 wt% of KNO3, 37.05 wt% of NaNO2, and 5.20 wt% of CaCl2; grind it into a powder and hold it at 110 °C for 30 h.

[0097] S2. Place the weighed three molten salts into a clean Al2O3 crucible, mix them evenly, cover the Al2O3 crucible lid, and then place it in a muffle furnace. Heat it at a heating rate of 5 °C·min -1 , and keep it at a constant temperature of 100 °C for 1 h to remove the water remaining in the salt; then, heat it at a heating rate of 10 °C·min -1 to 400 °C, hold it at this temperature for 2 h, pour it out in a molten state, naturally cool it to room temperature, and grind it into a powder to obtain the ternary mixed molten salt.

[0098] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , and the test and analysis of the experimental results are as Figure 5 shown. The lowest melting temperature of the molten salt in this example is 108.25 °C, and the latent heat is 125.60 J·g -1 .

[0099] Example 6

[0100] A preparation method of a low-melting-point wide-temperature-range ternary mixed molten salt includes the following steps.

[0101] S1. After calibrating the electronic balance, weigh 59.06 wt% of KNO3, 37.76 wt% of NaNO2, and 3.18 wt% of KCl; grind it into a powder and hold it at 100 °C for 28 h.

[0102] S2. Weigh out the three molten salts and place them in a clean Al2O3 crucible, mix them evenly, cover the Al2O3 crucible lid, then place them in a muffle furnace, and heat at a heating rate of 5 °C·min -1 to raise the temperature, and keep it at 100 °C for 1 h to remove the moisture remaining in the salts; then, heat at a heating rate of 15 °C·min -1 to raise the temperature to 400 °C, hold at this temperature for 2 h, pour out in the molten state, naturally cool to room temperature, and grind into powder form to obtain the ternary mixed molten salt.

[0103] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample, with the test temperature range of 50 °C - 350 °C and a heating rate of 10 °C·min -1 , and the test and analysis of the experimental results are as Figure 6 shown. The lowest melting temperature of the molten salt in this example is 136.90 °C, and the latent heat is 124.70 J·g -1 .

[0104] S4. Use a thermal analyzer (STA) to test the prepared ternary mixed molten salt sample, with the test range of RT - 650.0 °C and a heating rate of 10.0 °C·min -1 , and the test and analysis of the experimental results are as Figure 15 shown. The decomposition temperature of the ternary mixed molten salt in this example is 728.13 °C.

[0105] Example 7

[0106] A method for preparing a ternary mixed molten salt, comprising the following steps.

[0107] S1. After calibrating the electronic balance, weigh out 60.52 wt% of KNO3, 36.15 wt% of NaNO2, and 3.33 wt% of KCl; grind to powder form and keep at 110 °C for 28 h.

[0108] S2. Weigh out the three molten salts and place them in a clean Al2O3 crucible, mix them evenly, cover the Al2O3 crucible lid, then place them in a muffle furnace, and heat at a heating rate of 5 °C·min -1 to raise the temperature, and keep it at 100 °C for 1 h to remove the moisture remaining in the salts; then, heat at a heating rate of 14 °C·min -1 to raise the temperature to 400 °C, hold at this temperature for 2 h, pour out in the molten state, naturally cool to room temperature, and grind into powder form to obtain the ternary mixed molten salt.

[0109] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample, with the test temperature range of 50 °C - 350 °C and a heating rate of 10 °C·min -1, the test and analysis of the experimental results are as Figure 7 shown, its lowest melting temperature is 138.90 °C, and the latent heat is 120.30 J·g -1 .

[0110] Example 8

[0111] A method for preparing a ternary mixed molten salt includes the following steps.

[0112] S1. After calibrating the electronic balance, weigh 61.00 wt% of KNO3, 36.00 wt% of NaNO2, and 3.00 wt% of KCl; grind them into powder and keep them at 120 °C for 26 h.

[0113] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 5 °C·min -1 and keep it at 100 °C for 1 h for water removal to remove the residual water in the salt; then, heat it at a heating rate of 13 °C·min -1 to 400 °C and keep it at this temperature for 2 h. Pour it out in a molten state, cool it naturally to room temperature, and grind it into powder to obtain the ternary mixed molten salt.

[0114] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , the test and analysis of the experimental results are as Figure 8 shown, its lowest melting temperature is 139.40 °C, and the latent heat is 130.60 J·g -1 .

[0115] Example 9

[0116] A method for preparing a ternary mixed molten salt includes the following steps.

[0117] S1. After calibrating the electronic balance, weigh 59.57 wt% of KNO3, 37.00 wt% of NaNO2, and 3.43 wt% of KCl; grind them into powder and keep them at 120 °C for 24 h.

[0118] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 5 °C·min -1 and keep it at 100 °C for 1 h for water removal to remove the residual water in the salt; then, heat it at a heating rate of 12 °C·min -1Heat it to 380 °C at a heating rate, hold it at this temperature for 2 h, pour it out in a molten state, cool it naturally to room temperature, and grind it into powder to obtain the ternary mixed molten salt.

[0119] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , and the test and analysis of the experimental results are as Figure 9 shown. Its lowest melting temperature is 137.60 °C, and the latent heat is 125.30 J·g -1 .

[0120] Example 10

[0121] A preparation method of a low-melting-point and wide-temperature-range ternary mixed molten salt includes the following steps.

[0122] S1. After calibrating the electronic balance, weigh 59.71 wt% of KNO3, 38.49 wt% of NaNO2, and 1.80 wt% of NaCl; grind them into powder and hold them at 110 °C for 30 h.

[0123] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 7 °C·min -1 , and keep it at a constant temperature of 120 °C for 1.5 h to remove the moisture remaining in the salt; then, heat it at a heating rate of 11 °C·min -1 to 360 °C, hold it at this temperature for 3 h, pour it out in a molten state, cool it naturally to room temperature, and grind it into powder to obtain the ternary mixed molten salt.

[0124] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , and the test and analysis of the experimental results are as Figure 10 shown. The lowest melting temperature of the molten salt in this example is 137.53 °C, and the latent heat is 92.70 J·g -1 .

[0125] S4. Use a thermal analyzer (STA) to test the prepared ternary mixed molten salt sample. The test range is RT - 650.0 °C, and the heating rate is 10.0 °C·min -1 , and the test and analysis of the experimental results are as Figure 16 shown. The decomposition temperature of the ternary mixed molten salt in this example is 727.41 °C.

[0126] Example 11

[0127] A preparation method of a ternary mixed molten salt with a low melting point and a wide temperature range, comprising the following steps.

[0128] S1. After calibrating the electronic balance, weigh 61.26 wt% of KNO3, 36.83 wt% of NaNO2, and 1.91 wt% of NaCl; grind them into powder and keep them at 100 °C for 28 h.

[0129] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 6 °C·min -1 and keep it at 120 °C for 1 h for water removal to remove the residual water in the salt; then, heat it at a heating rate of 10 °C·min -1 to 340 °C and keep it at this temperature for 2 h. Pour it out in the molten state, naturally cool it to room temperature, and grind it into powder to obtain the ternary mixed molten salt.

[0130] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , and the test and analysis results are as Figure 11 shown. The lowest melting temperature of the molten salt in this example is 138.07 °C, and the latent heat is 111.06 J·g -1 .

[0131] Example 12

[0132] A preparation method of a ternary mixed molten salt with a low melting point and a wide temperature range, comprising the following steps.

[0133] S1. After calibrating the electronic balance, weigh 60.49 wt% of KNO3, 37.66 wt% of NaNO2, and 1.85 wt% of NaCl; grind them into powder and keep them at 100 °C for 30 h.

[0134] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 4 °C·min -1 and keep it at 110 °C for 1.5 h for water removal to remove the residual water in the salt; then, heat it at a heating rate of 9 °C·min -1 to 320 °C and keep it at this temperature for 3 h. Pour it out in the molten state, naturally cool it to room temperature, and grind it into powder to obtain the ternary mixed molten salt.

[0135] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1, the test and analysis of the experimental results are as Figure 12 shown. The lowest melting temperature of the molten salt in this example is 140.45 °C, and the latent heat is 79.44 J·g -1 .

[0136] Example 13

[0137] A preparation method of a ternary mixed molten salt with a low melting point and a wide temperature range includes the following steps.

[0138] S1. After calibrating the electronic balance, weigh 60.00 wt% of KNO3, 37.00 wt% of NaNO2, and 3.00 wt% of NaCl; grind them into powder and keep them at 110 °C for 24 h.

[0139] S2. Place the weighed three molten salts into a clean Al2O3 crucible and mix them evenly. Cover the Al2O3 crucible lid and place it in a muffle furnace. Heat it at a heating rate of 3 °C·min -1 , and keep it at 100 °C for 1 h to remove the water remaining in the salt; then, heat it at a heating rate of 8 °C·min -1 to 300 °C, and keep it at this temperature for 1 h. Pour it out in the molten state, cool it naturally to room temperature, and grind it into powder to obtain the ternary mixed molten salt.

[0140] S3. Use a differential scanning calorimeter (DSC) to test the prepared ternary mixed molten salt sample. The test temperature range is 50 °C - 350 °C, and the heating rate is 10 °C·min -1 , the test and analysis of the experimental results are as Figure 13 shown. The lowest melting temperature of the molten salt in this example is 137.62 °C, and the latent heat is 82.54 J·g -1 .

[0141] In this embodiment, taking KNO3 and NaNO2 as the main components and adding an appropriate amount of metal chloride, a ternary mixed molten salt system with a eutectic point as low as 95.10 °C and a melting enthalpy of 150.60 J·g -1 is obtained. The melting point of the molten salt in this system is 128.10 °C, 52.10 °C, and 31.70 °C lower than that of Solar salt, Hietc salt, and HitecXL salt respectively, and the decomposition temperature is 129.62 °C, 171.32 °C, and 191.22 °C higher than that of Solar salt, Hietc salt, and HitecXL salt respectively. The melting enthalpy is 43.20 J·g -1 , 69.46 J·g -1 , and 95.17 J·g -1. It has the advantages of low cost, high stability, and a wide working temperature range. At the same time, according to the design and preparation method of the ternary mixed molten salt of this embodiment, it is simple, safe, and convenient to operate.

[0142] Comparative Example 1

[0143] Different from Example 2, the content of KNO3 is 63 wt%, the content of NaNO2 is 32 wt%, and the content of CaCl2 is 5 wt%; the remaining steps are the same as those in Example 2. The test and analysis results of the experimental results are as Figure 17 shown. Its melting point is 208.55 °C, which is significantly higher than 100 °C, and its melting enthalpy is 67 J·g -1 , which is significantly lower than the system of Example 2.

[0144] Comparative Example 2

[0145] Different from Example 2, the content of KNO3 is 58 wt%, the content of NaNO2 is 40 wt%, and the content of CaCl2 is 2 wt%; the remaining steps are the same as those in Example 2. The test and analysis results of the experimental results are as Figure 18 shown. Its melting point is 207.77 °C, which is significantly higher than 100 °C, and its melting enthalpy is 64.89 J·g -1 , which is significantly lower than the system of Example 2.

[0146] Comparative Example 3

[0147] Different from Example 6, the content of KNO3 is 60 wt%, the content of NaNO2 is 35 wt%, and the content of CaCl2 is 15 wt%; the remaining steps are the same as those in Example 6. The test and analysis results of the experimental results are as Figure 19 shown. Its melting point is 212.20 °C, which is significantly higher than 100 °C, and its melting enthalpy is 70.12 J·g -1 , which is significantly lower than the system of Example 6.

[0148] Comparative Example 4

[0149] Different from Example 6, the content of KNO3 is 55 wt%, the content of NaNO2 is 40 wt%, and the content of CaCl2 is 5 wt%; the remaining steps are the same as those in Example 6. The test and analysis results of the experimental results are as Figure 20 shown. Its melting point is 182.42 °C, which is significantly higher than 100 °C, and its melting enthalpy is 62.79 J·g -1 , which is significantly lower than the system of Example 6.

[0150] Comparative Example 5

[0151] Different from Example 10, the content of KNO3 is 65 wt%, the content of NaNO2 is 30 wt%, and the content of CaCl2 is 5 wt%; the remaining steps are the same as those in Example 10. The test and analysis results of the experimental results are as Figure 21As shown, its melting point is 320.82 °C, significantly higher than 100 °C, and its melting enthalpy is 105.70 J·g -1 , significantly lower than the system of Example 10.

[0152] Comparative Example 6

[0153] Different from Example 10, it contains 58 wt% KNO3, 41 wt% NaNO2, and 1 wt% CaCl2; the remaining steps are the same as those in Example 10. The test and analysis results of the experimental results are as Figure 22 shown, its melting point is 391.24 °C, significantly higher than 100 °C, and its melting enthalpy is 111.89 J·g -1 , significantly lower than the system of Example 10.

[0154] Comparative Example 7

[0155] Different from Example 2, it is heated at a rate of 10 °C·min -1 and kept at 150 °C for 3 h to remove the residual moisture in the salt; then, it is heated to 500 °C at a rate of 20 °C·min -1 and held at this temperature for 4 h, poured out in the molten state, naturally cooled to room temperature, and ground into powder to obtain the ternary mixed molten salt; the remaining steps are the same as those in Example 2. The test and analysis results of the experimental results are as Figure 23 and Figure 24 shown, its melting point is 141.52 °C. Although it is close to 100 °C, its decomposition temperature is 658.28 °C, not reaching 700 °C, and its melting enthalpy is 115.36 J·g -1 , significantly lower than the system of Example 2.

[0156] Comparative Example 8

[0157] Different from Example 2, it is heated at a rate of 1 °C·min -1 and kept at 70 °C for 45 min to remove the residual moisture in the salt; then, it is heated to 200 °C at a rate of 5 °C·min -1 and held at this temperature for 45 min, poured out, naturally cooled to room temperature, and ground into powder to obtain the ternary mixed molten salt; the remaining steps are the same as those in Example 2. The test and analysis results of the experimental results are as Figure 25 and Figure 26 shown, its melting point is 129.43 °C. Although it is close to 100 °C, its decomposition temperature is 671.59 °C, not reaching 700 °C, and its melting enthalpy is 93.75 J·g -1 , significantly lower than the system of Example 2.

[0158] Each embodiment in this specification is described in a related manner. For the same or similar parts among the embodiments, reference can be made to each other. Each embodiment focuses on the differences from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and for the related parts, reference can be made to the corresponding description in the method embodiment.

[0159] The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A ternary hybrid molten salt heat transfer and energy storage medium, characterized in that, It includes the following components: KNO3, NaNO2 and metal chlorides; wherein, the metal chloride includes one of KCl, CaCl2, and NaCl.

2. The ternary hybrid molten salt heat transfer and energy storage medium according to claim 1, characterized in that When the metal chloride is KCl, the molten salt heat transfer and storage medium includes the following components by weight percentage: 59.06wt%-61.00wt% of KNO3, 36.00wt%-37.76wt% of NaNO2, 3.00wt%-3.43wt% of KCl.

3. A ternary mixed molten salt heat transfer and energy storage medium according to claim 1, characterized in that, When the metal chloride is CaCl2, the molten salt heat transfer and storage medium includes the following components by weight percentage: 57.50wt%-57.75wt% of KNO3, 36.50wt%-37.05wt% of NaNO2, 5.20wt%-6.00wt% of CaCl2.

4. A ternary hybrid molten salt heat transfer and storage medium according to claim 1, characterized in that, When the metal chloride is NaCl, the molten salt heat transfer and storage medium includes the following components by weight percentage: 59.71wt%-61.26wt% of KNO3, 36.83wt%-38.49wt% of NaNO2, 1.80wt%-3.00wt% of NaCl.

5. A ternary hybrid molten salt heat storage and transfer medium according to claim 2, characterized in that, The molten salt heat transfer and storage medium includes the following components by weight percentage: 59.06wt% of KNO3, 37.76wt% of NaNO2, 3.18wt% of KCl.

6. The ternary hybrid molten salt heat transfer and energy storage medium according to claim 3, wherein The molten salt heat transfer and storage medium includes the following components by weight percentage: 57.68wt% of KNO3, 36.97wt% of NaNO2, 5.35wt% of CaCl2.

7. A ternary hybrid molten salt heat transfer and storage medium according to claim 3, characterized in that, The molten salt heat transfer and storage medium includes the following components by weight percentage: 59.71wt% of KNO3, 38.49wt% of NaNO2, 1.80wt% of NaCl.

8. The preparation method of a ternary hybrid molten salt heat transfer and energy storage medium according to claim 1, characterized in that, Potassium nitrate, sodium nitrite, and one of the three metal chlorides, namely calcium chloride, potassium chloride, or sodium chloride, are each ground into powder form and kept at 100 - 120 °C for 24 - 30 h; based on the eutectic point of the molten salt heat transfer and storage medium, the powders are proportioned, and the three proportioned powders are mixed evenly and heated at a heating rate of 3 - 7 °C·min -1 to 100 - 120 °C and kept warm for 1 - 1.5 h; then heated at a heating rate of 8 - 15 °C·min -1 to 300 - 400 °C until all the solids are melted, then kept warm for 1 - 3 h, and finally cooled to room temperature.

9. The preparation method of a ternary hybrid molten salt heat transfer and storage medium according to claim 8, characterized in that, The specific process of proportioning the powder based on the eutectic point of the molten salt heat transfer and storage medium is as follows: S1. Obtain the entropy value, heat capacity, and Gibbs energy data of pure molten salts at different temperatures through the table lookup method; S2. Determine the Gibbs free energy thermodynamic model: G(T) = A + BT + CTlnT + DT 2 Wherein, G(T) is the Gibbs free energy function of pure substances, T is the Kelvin temperature, and A, B, C, and D are parameters to be fitted; S3. After measuring the Gibbs energy data of the molten salt at different temperatures, use the least squares method to fit the Gibbs free energy expression; determine the parameters to be fitted to obtain the thermodynamic model expression of the Gibbs free energy of pure molten salt; S4. Compile thermodynamic data files for three ternary system molten salts of KNO3-NaNO2-CaCl2, KNO3-NaNO2-KCl, and KNO3-NaNO2-NaCl. Among them, the Gibbs free energy of the molten phase of the binary system molten salt is described using the substitution melt model, ignoring the intermediate phase; S5. Thermodynamically optimize the thermodynamic data of the three ternary system molten salts of KNO3-NaNO2-CaCl2, KNO3-NaNO2-KCl, and KNO3-NaNO2-NaCl written respectively through the optimization module of the phase diagram calculation software; obtain a relatively accurate thermodynamic database through optimization; S6. Import the obtained thermodynamic database into the phase diagram calculation software, and calculate and plot the phase diagram of the ternary system molten salt through the phase diagram calculation software; S7. Through the phase diagram obtained by calculation, find the intersection points of the liquidus surface in the phase diagram. The temperature corresponding to the intersection point is the calculated lowest melting point. The proportion of each component at this temperature is the preliminary determined ratio of the lowest melting point of the system. Conduct experiments using this ratio and measure the melting point of this ratio. The measured melting point is the lowest melting point obtained from the experiment; S8. Compare the lowest melting point obtained from the calculation result with the lowest melting point value measured in the experiment. If the error is less than 30%, the proportion of each component corresponding to the lowest melting point obtained from the phase diagram calculation is the ratio of the lowest eutectic point of the system. If the error is greater than 30%, the thermodynamic database of the ternary system molten salt needs to be optimized again.

10. The preparation method of a ternary hybrid molten salt heat transfer and storage medium according to claim 9, characterized in that, The binary system molten salts in the S4 include seven binary system molten salts of KNO3 - NaNO2, KNO3 - CaCl2, KNO3 - KCl, KNO3 - NaCl, NaNO2 - CaCl2, NaNO2 - KCl, and NaNO2 - NaCl.

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