A heat transfer and heat storage material of a low-melting-point and wide-temperature-range hybrid molten salt and a preparation method thereof

Abstract

The present invention discloses a low-melting-point and wide-temperature-range hybrid molten salt heat transfer and energy storage material and a preparation method thereof. The molten salt material is composed of KNO3, NaNO2, and KNO2. The specific proportions are as follows: the mass percentage of KNO3 is 38-55%, the mass percentage of NaNO2 is 35-48%, and the mass percentage of KNO2 is 2.5-18%. The measured melting point of the low-melting-point and wide-temperature-range hybrid molten salt heat transfer and energy storage material provided by the present invention is about 140 °C, the decomposition temperature is about 650 °C, the average specific heat in the liquid state is about 1.48 J / (g·K), and the average density is about 1.95 g / cm 3 , the average thermal conductivity is about 0.4 W / (m·K), and the heat storage density is greater than 680 kJ / kg. This hybrid salt has low cost, a wide working temperature range, low corrosivity, good fluidity, and is very suitable for heat transfer and energy storage in fields such as solar thermal power generation, compressed air energy storage, flexibility transformation of thermal power plants, and heat storage heating with off-peak electricity.

Description

Technical Field

[0001] The present invention belongs to the technical field of physical heat transfer and energy storage in high and new technologies, and particularly relates to a hybrid molten salt heat transfer and heat storage material with a low melting point, a high decomposition temperature, a low cost, and low corrosiveness. Background Art

[0002] Solar thermal power generation technology has the same quality as conventional thermal power generation technology and is a key technology for achieving the "dual carbon goal of 3060". Molten salt, as an excellent heat transfer and energy storage medium, has now been successfully applied in various solar power plants. Currently, in solar thermal power generation systems, the main heat transfer and heat storage media are Solar salt (40 wt% KNO3 - 60 wt% NaNO3, operating temperature range of 290 - 565 °C) and Hitec salt (53 wt% KNO3 - 7 wt% NaNO3 - 40 wt% NaNO2, operating temperature range of 200 - 535 °C). However, the inherent disadvantages of these commercial hybrid molten salts are high melting points, low upper operating temperatures, and narrow normal liquid temperature ranges, making it difficult to use them above 600 °C. In solar thermal power generation systems, too high a melting point of molten salt is likely to cause pipeline blockage due to freezing; the upper operating temperature of molten salt is relatively low, and molten salt is prone to decomposition at high temperatures, resulting in low steam parameters at the turbine inlet and low power generation efficiency.

[0003] In order to improve the efficiency of solar thermal power generation and reduce costs, the third-generation solar thermal power generation technology uses an sCO2 Brayton cycle to achieve the power generation process. When the operating temperature of a solar thermal power plant rises above 600 °C, the power generation efficiency of the sCO2 Brayton cycle can be increased to 50%, which can reduce the cost per kilowatt-hour of solar thermal power generation and has significant economic benefits. Most of the hybrid molten salts reported in the open literature with a use temperature greater than 600 °C are mixed carbonates and chlorides. However, when mixed carbonates and chlorides are used at high temperatures, they may increase the corrosion effect on metals, posing a threat to the safety of the system. On the other hand, the melting points of mixed carbonates and chlorides are relatively high, increasing the risk of pipeline blockage due to freezing. The currently disclosed hybrid molten salts are difficult to meet this condition.

[0004] In order to scale up the accommodation of wind and solar power generation, technologies such as compressed air energy storage, heat pump heat storage, and flexible regulation of thermal power plants are important ways for large-scale accommodation of wind and solar power. Large-scale energy storage methods urgently need molten salt heat storage materials with a wide temperature range. Therefore, developing hybrid molten salt materials with a low melting point and a high upper operating temperature is an urgent need for solar thermal power generation and large-scale energy storage technologies.

[0005] Most of the traditional mixed molten salts are developed by trial and error. The number of mixtures with different proportions in multi-component molten salts increases exponentially with the increase of the number of components, resulting in low screening efficiency of the trial-and-error method. In order to reduce the blindness of screening mixed molten salts, using the material thermodynamics theory and phase diagram calculation to guide the development of new mixed molten salts is one of the most effective methods for eutectic salt screening at present. It can quickly and accurately predict the eutectic composition and eutectic temperature of the multi-component mixed salt system. The thermodynamic calculation of the phase diagram based on molten salts can theoretically guide the preparation of mixed molten salts and reduce the workload of molten salt screening. The present invention precisely selects a mixed molten salt heat storage material with a low melting point, a high decomposition temperature, and a large heat storage density based on theoretical analysis, phase diagram optimization, and experimental verification. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to optimize a mixed molten salt with a low melting point, a low cost, and a high decomposition temperature, whose melting point is lower than 150 °C, the decomposition temperature is about 650 °C, and it does not contain precious metal salts.

[0007] To solve the above problems, the present invention discloses a formula of a mixed molten salt heat transfer and heat storage material with a low melting point and a wide temperature range. The screening method of this material is based on the phase diagram thermodynamics theory (the minimum total Gibbs free energy of the system), and the sub-regular solution model is used to solve the phase diagram of the KNO3-NaNO2-KNO2 ternary system, so as to determine the component composition of the lowest eutectic point of the system, and the calculation results are verified by experiments.

[0008] A mixed molten salt heat transfer and heat storage material with a low melting point and a wide temperature range described in the present invention is composed of KNO3, NaNO2, and KNO2. In the mixed molten salt material, the mass percentage of KNO3 is 38-55%, the mass percentage of NaNO2 is 35-48%, and the mass percentage of KNO2 is 2.5-18%.

[0009] To achieve the above invention purpose, a preparation method of a mixed molten salt heat transfer and heat storage material with a low melting point and a wide temperature range described in the present invention includes the following steps:

[0010] S1. According to the component composition of the lowest eutectic point of the ternary mixed molten salt material calculated based on the phase diagram thermodynamics theory, use a high-precision balance to weigh three elemental salts, place them in a crucible and mix them evenly to form a solid mixed molten salt;

[0011] S2. Place the solid mixed molten salt in a drying oven for constant temperature drying treatment;

[0012] S3. Transfer the dried solid mixed molten salt to a muffle furnace, set the heating program, heat it to the set temperature at an appropriate heating rate, and maintain constant temperature heating for a certain time to completely melt the mixed molten salt to obtain a molten salt;

[0013] S4. Take out the molten salt from the muffle furnace and place it in the drying oven until the molten salt is completely cooled.

[0014] S5. Use an ultrafine pulverizer to pulverize the cooled mixed molten salt into mixed molten salt powder.

[0015] S6. Place the mixed molten salt powder in an evaporating dish and conduct constant-temperature drying treatment in the drying oven for subsequent experimental use.

[0016] Further, the temperature of the drying oven is set at 100 - 110 °C, and the duration of constant-temperature drying is more than 48 h.

[0017] Further, the heating rate of the muffle furnace is set at 5 - 10 °C / min.

[0018] Further, the heating temperature of the muffle furnace is set at 250 - 300 °C above the melting point of the mixed molten salt, and the duration of constant-temperature heating is more than 12 h.

[0019] Further, the fineness of the pulverized mixed molten salt powder is 20 - 250 mesh.

[0020] The beneficial effects of the present invention are as follows:

[0021] (1) The novel mixed molten salt heat transfer and energy storage material with low melting point and wide temperature range described in the present invention has a melting point of about 140 °C, a decomposition temperature of about 650 °C, an average specific heat in the liquid state of about 1.48 J / (g·K), an average density of about 1.95 g / cm 3 , and an average thermal conductivity coefficient of about 0.4 W / (m·K), and a heat storage density greater than 680 kJ / kg. This material has a wide liquid temperature range and a large heat storage density.

[0022] (2) The novel mixed molten salt heat transfer and energy storage material with low melting point and wide temperature range described in the present invention does not contain precious metal salts, greatly reducing the cost of the heat storage system.

[0023] (3) The screening method of the novel mixed molten salt heat transfer and energy storage material with low melting point and wide temperature range described in the present invention is based on the phase diagram thermodynamics theory, uses the sub-regular solution model to solve the phase diagram of the KNO3-NaNO2-KNO2 ternary system, determines the component composition of the system's eutectic point, and has high screening efficiency.

[0024] (4) The novel mixed molten salt heat transfer and energy storage material with low melting point and wide temperature range described in the present invention is a multi-component eutectic salt formed by mixing nitrates and nitrites, with low corrosion, low viscosity, and good fluidity.

[0025] (5) The hybrid molten salt heat transfer and energy storage material with low melting point and wide temperature range described in the present invention can be applied in the fields of solar thermal power generation, compressed air energy storage, industrial waste heat recovery, molten salt heat pump energy storage, off-peak electricity energy storage for heating, and flexibility transformation of thermal power plants, etc., which can increase the stability of the safe operation of the entire system. Description of the Drawings

[0026] The present invention will be further described below in conjunction with embodiments and the drawings. In the drawings:

[0027] Figure 1 is the DSC curve of the hybrid molten salt sample prepared in Example 1;

[0028] Figure 2 is the TG curve of the hybrid molten salt sample prepared in Example 1;

[0029] Figure 3 is the specific heat measurement result of the hybrid molten salt sample prepared in Example 1;

[0030] Figure 4 is the density measurement result of the hybrid molten salt sample prepared in Example 1;

[0031] Figure 5 is the thermal diffusivity measurement result of the hybrid molten salt sample prepared in Example 1;

[0032] Figure 6 is the thermal conductivity measurement result of the hybrid molten salt sample prepared in Example 1;

[0033] Figure 7 is the DSC curve of the hybrid molten salt sample prepared in Example 2;

[0034] Figure 8 is the DSC curve of the hybrid molten salt sample prepared in Example 3;

[0035] Figure 9 is the DSC curve of the hybrid molten salt sample prepared in Example 4. Detailed Embodiments

[0036] The present invention will be further described in detail below in conjunction with the specific embodiments.

[0037] The present invention discloses a formulation of a low-melting-point and wide-temperature-range hybrid molten salt heat transfer and energy storage material. The screening method of this material is based on the phase diagram thermodynamics theory (the total Gibbs free energy of the system is minimized), and the sub-regular solution model is used to solve the phase diagram of the KNO3-NaNO2-KNO2 ternary system. Finally, it is determined that the composition range of the lowest eutectic point of this system is: the mass percentage of KNO3 is 38-55%, the mass percentage of NaNO2 is 35-48%, and the mass percentage of KNO2 is 2.5-18%. According to the calculated composition range of the components of the lowest eutectic point of the hybrid molten salt material, any point is selected for testing. Through a series of thermal analysis experiments, a hybrid molten salt formulation with the lowest melting point and good eutectic can be optimized.

[0038] The preparation method of the described low-melting-point and wide-temperature-range hybrid molten salt heat transfer and energy storage material includes the following steps:

[0039] S1. According to the composition of the components of the lowest eutectic point of the ternary hybrid molten salt material calculated based on the phase diagram thermodynamics theory, use a high-precision balance to weigh three elemental salts, place them in a crucible and mix them evenly to form a solid hybrid molten salt;

[0040] S2. Place the solid hybrid molten salt in an oven at 110°C for constant-temperature drying treatment for 48 h;

[0041] S3. Transfer the dried solid hybrid molten salt to a muffle furnace, set the heating program, and heat the hybrid molten salt to 400°C at a heating rate of 8.5°C / min, and keep it at a constant temperature for 15 h to completely melt the hybrid molten salt to obtain a molten salt;

[0042] S4. Take out the molten salt from the muffle furnace and place it in the oven at 100°C until the molten salt is completely cooled;

[0043] S5. Use an ultra-fine pulverizer to pulverize the cooled hybrid molten salt into a hybrid molten salt powder with a fineness of 100 mesh;

[0044] S6. Place the hybrid molten salt powder in an evaporating dish and place it in the oven at 110°C for constant-temperature drying treatment for 72 h for subsequent experimental use.

[0045] According to the composition range of the components of the lowest eutectic point of the KNO3-NaNO2-KNO2 hybrid molten salt material calculated based on the phase diagram thermodynamics theory, any 4 groups of samples are taken for thermal analysis experiments, and the experimental results are listed in Table 1. Thus, it can be found that the hybrid salt corresponding to Example 1 has good eutectic and the lowest melting point.

[0046] Table 1 DSC analysis of hybrid salts

[0047] <![CDATA[KNO3:NaNO2:KNO2 (mass ratio wt%)]]> Melting point (°C) Initial crystallization point (°C) Example 1 49.0:39.5:11.5 139.9 139.8 Example 2 54.0:43.0:3.0 142.9 141.8 Example 3 46.0:45.0:9.0 141.8 143.5 Example 4 40.0:42.0:18.0 141.2 145.9

[0048] The present invention will be further described below in conjunction with Example 1. The mixed molten salt heat transfer and heat storage material described in Example 1 is composed of 49.0 wt% KNO3, 39.5 wt% NaNO2 and 11.5 wt% KNO2.

[0049] The preparation method of the mixed molten salt heat transfer and heat storage material described in Example 1 includes the following steps:

[0050] S1. Weigh 49.0 wt% KNO3, 39.5 wt% NaNO2 and 11.5 wt% KNO2 using a high-precision balance, place them in a crucible and mix evenly to form a solid mixed molten salt.

[0051] S2. Place the solid mixed molten salt in a drying oven at 110 °C for constant-temperature drying treatment for 48 h.

[0052] S3. Transfer the dried solid mixed molten salt to a muffle furnace, set the heating program, and heat the mixed molten salt to 400 °C at a heating rate of 8.5 °C / min, and keep it at a constant temperature for 15 h to completely melt the mixed molten salt and obtain a molten salt.

[0053] S4. Take out the molten salt from the muffle furnace and place it in the drying oven at 100 °C until the molten salt is completely cooled.

[0054] S5. Use an ultra-fine pulverizer to pulverize the cooled mixed molten salt into a mixed molten salt powder with a fineness of 100 mesh.

[0055] S6. Place the mixed molten salt powder in an evaporating dish and place it in the drying oven at 110 °C for constant-temperature drying treatment for 72 h for subsequent experiments.

[0056] The thermal properties of the mixed molten salt heat transfer and heat storage material obtained in Example 1 were tested, and the results are as follows. As Figure 1 shown is the DSC curve of the mixed molten salt sample prepared in Example 1. The melting point of the mixed molten salt is 139.9 °C, the melting peak point is 143.5 °C, the melting end point is 146.6 °C, the phase change latent heat is 75.3 J / g, and the primary crystallization point is 139.8 °C; as Figure 2 shown is the TG curve of the mixed molten salt sample prepared in Example 1. The decomposition temperature of the mixed molten salt is 651.6 °C (calculated according to a 3% mass reduction); as Figure 3 shown is the specific heat measurement result of the mixed molten salt sample prepared in Example 1. The specific heat of the mixed molten salt in the liquid state at 200 - 450 °C is 1.44 - 1.54 J / (g·K), and the average specific heat in the liquid state is about 1.48 J / (g·K); as Figure 4Shown are the density measurement results of the mixed molten salt sample prepared in Example 1. The density of the mixed molten salt ranges from 1.84 to 2.07 g / cm³ in the range of 150 to 500 °C. 3 , and the average density is about 1.95 g / cm³. 3 ; As Figure 5 Shown are the thermal diffusivity measurement results of the mixed molten salt sample prepared in Example 1. The thermal diffusivity of the mixed molten salt ranges from 0.08 to 0.24 mm² / s in the range of 200 to 450 °C. 2 / s, and the average thermal diffusivity is about 0.14 mm² / s. 2 ; As Figure 6 Shown are the thermal conductivity measurement results of the mixed molten salt sample prepared in Example 1. The thermal conductivity of the mixed molten salt ranges from 0.24 to 0.64 W / (m·K) in the range of 200 to 450 °C, and the average thermal conductivity is about 0.4 W / (m·K).

[0057] The present invention will be further described below in conjunction with Example 2. The mixed molten salt heat transfer and energy storage material described in Example 2 is composed of 54.0 wt% KNO₃, 43.0 wt% NaNO₂, and 3.0 wt% KNO₂.

[0058] The preparation method of the mixed molten salt heat transfer and energy storage material described in Example 2 includes the following steps:

[0059] S1. Weigh 54.0 wt% KNO₃, 43.0 wt% NaNO₂, and 3.0 wt% KNO₂ using a high-precision balance, place them in a crucible and mix evenly to form a solid mixed molten salt; Steps S2 - S6 are the same as those in Example 1.

[0060] Thermophysical property tests were carried out on the mixed molten salt heat transfer and energy storage material obtained in Example 2, and the results are as follows. As Figure 7 Shown is the DSC curve of the mixed molten salt sample prepared in Example 2. The melting point of the mixed molten salt is 142.9 °C, the melting peak point is 144.9 °C, the melting end point is 147.3 °C, the latent heat of phase change is 77.8 J / g, and the primary crystallization point is 141.8 °C.

[0061] The present invention will be further described below in conjunction with Example 3. The mixed molten salt heat transfer and energy storage material described in Example 3 is composed of 46.0 wt% KNO₃, 45.0 wt% NaNO₂, and 9.0 wt% KNO₂.

[0062] The preparation method of the mixed molten salt heat transfer and energy storage material described in Example 3 includes the following steps:

[0063] S1. Weigh 46.0 wt% KNO3, 45.0 wt% NaNO2, and 9.0 wt% KNO2 using a high-precision balance, place them in a crucible and mix evenly to form a solid mixed molten salt; Steps S2 - S6 are the same as those in Example 1.

[0064] The thermal physical properties of the mixed molten salt heat transfer and energy storage material obtained in Example 3 were tested, and the results are as follows. As Figure 8 shown in the DSC curve of the mixed molten salt sample prepared in Example 3, the melting point of the mixed molten salt is 141.8 °C, the melting peak point is 146.9 °C, the melting end point is 149.2 °C, the phase change latent heat is 75.9 J / g, and the primary crystallization point is 143.5 °C.

[0065] The following further illustrates the present invention in conjunction with Example 4. The mixed molten salt heat transfer and energy storage material described in Example 4 is composed of 40.0 wt% KNO3, 42.0 wt% NaNO2, and 18.0 wt% KNO2.

[0066] The preparation method of the mixed molten salt heat transfer and energy storage material described in Example 4 includes the following steps:

[0067] S1. Weigh 40.0 wt% KNO3, 42.0 wt% NaNO2, and 18.0 wt% KNO2 using a high-precision balance, place them in a crucible and mix evenly to form a solid mixed molten salt; Steps S2 - S6 are the same as those in Example 1.

[0068] The thermal physical properties of the mixed molten salt heat transfer and energy storage material obtained in Example 4 were tested, and the results are as follows. As Figure 9 shown in the DSC curve of the mixed molten salt sample prepared in Example 4, the melting point of the mixed molten salt is 141.2 °C, the melting peak point is 147.4 °C, the melting end point is 154.6 °C, the phase change latent heat is 71.4 J / g, and the primary crystallization point is 145.9 °C.

[0069] The above-mentioned are only the preferred embodiments of the present invention, and are not used to limit the scope of the present invention. Various changes can be made to the above embodiments of the present invention. That is, all simple, equivalent changes and modifications made according to the claims and the content of the specification of the present invention application belong to the scope of protection of the claims of the present invention patent. Those not described in detail in the present invention are all conventional technical contents.

Claims

1. A low-melting-point ternary mixed molten salt heat transfer and heat storage material, characterized in that, This material is composed of KNO3, NaNO2 and KNO2; the mass percentage of KNO3 is 38 - 55%, the mass percentage of NaNO2 is 35 - 48%, and the mass percentage of KNO2 is 2.5 - 18%; The screening method of this molten salt is based on the phase diagram thermodynamics theory. The sub-regular solution model is used to solve the phase diagram of the KNO3-NaNO2-KNO2 ternary system, so as to determine the component composition of the system's eutectic point, and the calculation results are verified by experiments; The melting point of the mixed salt is lower than 150 °C, and the heat storage density of the mixed molten salt is greater than 680 kJ / kg, which is suitable for heat transfer and heat storage in solar thermal power generation, compressed air energy storage, industrial waste heat recovery, molten salt heat pump heat storage, off-peak electricity heat storage heating, and thermal power plant transformation.

2. The preparation method of a low-melting-point ternary mixed molten salt heat transfer and energy storage material according to claim 1, characterized in that Specifically, it consists of the following steps: S1. According to the component ratio of the ternary mixed molten salt material obtained in Claim 1, use a high-precision balance to weigh three single salts, place them in a crucible and mix evenly to form a solid mixed molten salt; S2. Place the solid mixed molten salt in a drying oven for constant temperature drying treatment; S3. Transfer the dried solid mixed molten salt to a muffle furnace, set the heating program, heat it to the set temperature at a suitable heating rate, and maintain constant temperature heating for a certain time to completely melt the mixed molten salt to obtain a molten salt; S4. Take out the molten salt from the muffle furnace and place it in the drying oven until the molten salt cools completely; S5. Use an ultra-fine pulverizer to crush the cooled mixed molten salt into mixed molten salt powder; S6. Place the mixed molten salt powder in an evaporating dish and place it in the drying oven for constant temperature drying treatment for use.

3. The preparation method of a low-melting-point ternary mixed molten salt heat transfer and energy storage material according to claim 2, characterized in that, The temperature of the drying oven is set at 100 - 110 °C, and the constant temperature drying time is more than 48 h.

4. The preparation method of a low-melting-point ternary mixed molten salt heat transfer and heat storage material according to claim 2, characterized in that The heating rate of the muffle furnace is set at 5 - 10 °C / min.

5. The preparation method of a low-melting-point ternary mixed molten salt heat transfer and energy storage material according to claim 2, characterized in that, The heating temperature of the muffle furnace is set 250 - 300 °C above the melting point of the mixed molten salt, and the constant temperature heating time is more than 12 h.

6. The preparation method of a low-melting-point ternary mixed molten salt heat transfer and heat storage material according to claim 2, wherein The fineness of the crushed mixed molten salt powder is 20 - 250 mesh.

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