Low-melting-point quinary mixed molten salt heat transfer and heat storage material and preparation method

Abstract

The present invention discloses a low-melting-point five-component mixed molten salt heat transfer and heat storage material and a preparation method thereof. The material is composed of KNO3, NaNO3, Ca(NO3)2, NaNO2 and KNO2. The specific proportions are as follows: the mass percentage of KNO3 is 8-17%, the mass percentage of NaNO3 is 13-26%, the mass percentage of Ca(NO3)2 is 6-20%, the mass percentage of NaNO2 is 12-21%, and the mass percentage of KNO2 is 31-45%. The measured melting point of the low-melting-point five-component mixed molten salt heat transfer and heat storage material provided by the present invention is about 100 °C, the decomposition temperature is greater than 615 °C, the average specific heat in the liquid state is about 1.5 J / (g·K), the cost is low, the corrosion is small, the fluidity is good, it has a large specific heat capacity and a wide working temperature range, and has good heat transfer and heat storage performance, and is very suitable for heat transfer and heat storage in the fields of 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 flexible transformation of thermal power plants.

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 mixed molten salt heat transfer and heat storage material with low melting point, low cost, low corrosivity, high specific heat, high decomposition temperature and wide operating temperature range. Background Art

[0002] Solar thermal power generation has the same quality as conventional thermal power generation and is the best configured low-carbon power generation technology for long-distance transmission of renewable energy power generation in large-scale wind-solar bases. Molten salt, as an excellent heat transfer and energy storage medium, has been successfully applied in various solar power stations, realizing the efficient utilization of solar energy. At present, in solar thermal power generation systems, the main heat transfer and heat storage medium is mixed nitrate. For example, both the Solar Two in California, USA and the Andasol solar power station in Spain use Solar Salt (60wt% NaNO3 + 40wt% KNO3) as the heat transfer and heat storage medium. This kind of mixed molten salt has good thermal stability and low cost, but its melting point is as high as 220°C and the decomposition temperature is 586°C. The relatively high melting point shortens the normal operating liquid temperature range. Hitec salt composed of 53wt% KNO3, 7wt% NaNO3 and 4wt% NaNO2 has a melting temperature of about 142°C, which is about 80°C lower than that of solar salt. However, the long-term thermal stability of Hitec salt is only 454°C and can reach 535°C in short-term use, and the upper limit temperature for use is limited, making it difficult to meet the high-temperature requirements of solar thermal power generation. Research shows that adding Ca(NO3)2 to the NaNO3-KNO3 mixed molten salt can reduce the melting point of the mixed molten salt to below 150°C. Among them, the most representative formula is Hitec XL salt (48wt% Ca(NO3)2 + 7wt% NaNO3 + 45wt% KNO3), with a melting point of 130°C, but the upper limit temperature for use is only 500°C. The heat transfer and heat storage performance of this mixed molten salt has been successfully experimentally verified in the PSA and Themi solar thermal power stations, but due to the relatively low upper limit temperature of the molten salt, the inlet steam parameters of the solar thermal power generation steam turbine are low, resulting in low solar thermal power generation efficiency.

[0003] In order to improve the efficiency of solar thermal power generation, the third-generation solar thermal power generation technology realizes the power generation process with a carbon dioxide Brayton cycle, requiring the gas turbine inlet temperature to be greater than 600 °C. This requires a higher upper limit temperature for the heat transfer and thermal storage molten salt that matches it. However, the publicly disclosed molten salt heat transfer and thermal storage formulations are difficult to meet the requirements of the third-generation solar thermal power generation for high-temperature molten salt. The mixed molten salts reported in the open literature with a use temperature greater than 600 °C are mostly mixed carbonates and chlorides, such as ternary eutectic carbonate (34.5 wt% K2CO3 + 32.2 wt% Li2CO3 + 33.3 wt% Na2CO3), mixed chloride (68.2 wt% MgCl2 + 14.0 wt% NaCl + 17.8 wt% KCl). However, the melting points of mixed carbonates and chlorides are generally relatively high. The melting point of ternary eutectic carbonate is 396 °C, and the melting point of mixed chloride is 385 °C. The melting point of the mixed salt is too high, which easily leads to the risk of pipeline freezing and blockage.

[0004] On the other hand, for trough and linear Fresnel solar concentrating and heat collection systems, due to the long loop, one loop is up to 1000 meters long, and the temperature difference of the molten salt from the inlet to the outlet of the pipeline is relatively large. To prevent pipeline freezing and blockage, it is generally desired that the lowest use temperature of the molten salt is as low as possible, less than 160 °C, and the highest use temperature is as high as possible, greater than 600 °C. The currently disclosed mixed molten salts are difficult to meet this condition. Therefore, developing a mixed molten salt material with a low melting point, a low lower use temperature, and a high upper use temperature is an urgent need for solar thermal power generation technology.

[0005] In addition, in order to scale the accommodation of wind and solar power generation, compressed air energy storage is an important large-scale energy storage method. In order to improve the compressed air electricity-electricity conversion efficiency, it is generally necessary to store the compressed heat, and the temperature of the compressed heat is generally relatively low, with a high requirement for the lower use temperature, and it is desired that the lower use temperature is as low as possible. Extraction steam heat storage is a key technology for the flexibility regulation of thermal power plants, and the heat storage materials for the flexibility regulation of thermal power plants also require a wider normal liquid temperature range.

[0006] In summary, developing low-cost mixed molten salt materials with a low melting point and a high decomposition temperature has very important application value for large-scale energy storage systems such as solar thermal power generation, compressed air energy storage, and the flexibility transformation of thermal power plants. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to preferably select a low-melting-point mixed molten salt with a melting point of about 100 °C, a decomposition temperature greater than 600 °C, and no precious metal salts therein, to reduce the melting point of the mixed molten salt while increasing its use temperature range and stability, thereby reducing the industrial heat storage cost.

[0008] To solve the above problems, the present invention discloses a formulation of a low-melting-point five-component mixed molten salt heat transfer and energy storage material. Based on the KNO3-NaNO3 binary mixed salt, nitrites (NaNO2 and KNO2) and calcium nitrate are added to form a new five-component mixed salt. By changing the component ratios of the mixed salt multiple times, the physical properties of the mixed salt are modified and optimized, so as to achieve the purpose of reducing the melting point of the mixed molten salt material while enhancing its stability.

[0009] The low-melting-point five-component mixed molten salt heat transfer and energy storage material described in the present invention is characterized in that the energy storage material is composed of KNO3, NaNO3, Ca(NO3)2, NaNO2 and KNO2. In the mixed molten salt material, the mass percentage of KNO3 is 8-17%, the mass percentage of NaNO3 is 13-26%, the mass percentage of Ca(NO3)2 is 6-20%, the mass percentage of NaNO2 is 12-21%, and the mass percentage of KNO2 is 31-45%.

[0010] To achieve the above invention purpose, the preparation method of the low-melting-point five-component mixed molten salt heat transfer and energy storage material described in the present invention includes the following steps:

[0011] (1) According to the component ratios of the five-component mixed molten salt material, use a high-precision balance to weigh the corresponding mass of elemental salts, put them into a corundum crucible, mix and grind, and stir evenly to form a solid mixed molten salt;

[0012] (2) Place the solid mixed molten salt in a constant-temperature drying oven for constant-temperature drying;

[0013] (3) Take out the solid mixed molten salt after constant-temperature drying, quickly place it in a high-temperature muffle furnace, heat the mixed molten salt to the set temperature, and keep it at a constant temperature for a period of time to completely melt the mixed molten salt to obtain a molten salt;

[0014] (4) Take out the molten mixed molten salt from the high-temperature muffle furnace and place it in the constant-temperature drying oven to cool naturally to crystallization;

[0015] (5) Place the cooled solid mixed molten salt in an ultrafine pulverizer for pulverization to obtain a mixed molten salt powder;

[0016] (6) Place the pulverized mixed molten salt powder in the constant-temperature drying oven for constant-temperature drying treatment for subsequent experimental use.

[0017] Further, the temperature of the constant-temperature drying oven is set at 90-105°C, and the constant-temperature drying time is more than 36h.

[0018] Further, the heating rate of the high-temperature muffle furnace is set at 10-15K / min.

[0019] Furthermore, the heating temperature of the high-temperature muffle furnace is set to be 150 - 250 °C above the melting point of the mixed molten salt, and the constant-temperature heating time is 10 h or more.

[0020] Furthermore, the fineness of the mixed molten salt sample after pulverization is 30 - 300 mesh.

[0021] Furthermore, for the low-melting-point five-component mixed molten salt heat transfer and heat storage material, the initial melting and filling temperature of the molten salt should be greater than 360 °C.

[0022] The beneficial effects of the present invention are as follows:

[0023] (1) The low-melting-point five-component mixed molten salt heat transfer and heat storage material of the present invention is a new type of mixed molten salt heat transfer and heat storage material with a melting point of about 100 °C, which is about 110 °C lower than solar salt and about 40 °C lower than Hitec salt; the decomposition temperature of the low-melting-point five-component mixed molten salt is greater than 615 °C, with a wide liquid temperature range, large heat storage density, and obvious anti-freezing and blocking effect.

[0024] (2) The low-melting-point five-component mixed molten salt heat transfer and heat storage material of the present invention does not contain precious metal salts, greatly reducing the cost of the heat transfer and heat storage system.

[0025] (3) The low-melting-point five-component mixed molten salt heat transfer and heat storage material of the present invention forms a multi-component eutectic salt by mixing nitrates and nitrites, with low corrosivity, low viscosity, and good fluidity. The preparation process is simple, with low requirements for equipment performance, and is suitable for large-scale production.

[0026] (4) The low-melting-point five-component mixed molten salt heat transfer and heat storage material of the present invention can be applied in fields such as 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 flexible transformation of thermal power plants, which can increase the stability of the safe operation of the entire system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below in conjunction with embodiments and the accompanying drawings, in which:

[0028] Figure 1 is the DSC curve of the mixed molten salt sample prepared in Example 1;

[0029] Figure 2 is the TG curve of the mixed molten salt sample prepared in Example 1;

[0030] Figure 3 is the specific heat curve of the mixed molten salt sample prepared in Example 1;

[0031] Figure 4 is the DSC curve of the mixed molten salt sample prepared in Example 2;

[0032] Figure 5 is the TG curve of the mixed molten salt sample prepared in Example 2;

[0033] Figure 6 is the specific heat curve of the mixed molten salt sample prepared in Example 2;

[0034] Figure 7 is the DSC curve of the mixed molten salt sample prepared in Example 3;

[0035] Figure 8 is the TG curve of the mixed molten salt sample prepared in Example 3;

[0036] Figure 9 is the specific heat curve of the mixed molten salt sample prepared in Example 3;

[0037] Figure 10 is the DSC curve of the mixed molten salt sample prepared in Example 4;

[0038] Figure 11 is the TG curve of the mixed molten salt sample prepared in Example 4;

[0039] Figure 12 is the specific heat curve of the mixed molten salt sample prepared in Example 4. Detailed Embodiments

[0040] The present invention will be further described in detail below in conjunction with specific embodiments.

[0041] 1. The present invention will be further described below in conjunction with Example 1. The low-melting-point five-component mixed molten salt heat transfer and energy storage material described in Example 1 is composed of 11.5 wt% KNO3, 21 wt% NaNO3, 10.5 wt% Ca(NO3)2, 17 wt% NaNO2, and 40 wt% KNO2.

[0042] The preparation method of the low-melting-point five-component mixed molten salt heat transfer and energy storage material described in Example 1 includes the following steps:

[0043] (1) Weigh 11.5 wt% KNO3, 21 wt% NaNO3, 10.5 wt% Ca(NO3)2, 17 wt% NaNO2, and 40 wt% KNO2 using a high-precision balance, put them into a corundum crucible, mix and grind, and stir evenly to form a solid mixed molten salt;

[0044] (2) Place the solid mixed molten salt in an oven at 105 °C for constant-temperature drying for 48 h;

[0045] (3) Take out the solid mixed molten salt after constant-temperature drying, quickly place it in a high-temperature muffle furnace, heat up the mixed molten salt to 350 °C at a heating rate of 10.5 K / min, and keep it heated at a constant temperature for 12 h to completely melt the mixed molten salt and obtain a molten salt;

[0046] (4) Take out the molten mixed molten salt from the high-temperature muffle furnace and place it in the constant-temperature drying oven at 90 °C to cool naturally until crystallization;

[0047] (5) Place the cooled solid mixed molten salt in an ultrafine pulverizer for pulverization to obtain a mixed molten salt powder with a sample fineness of 200 mesh;

[0048] (6) Place the pulverized mixed molten salt powder in the constant-temperature drying oven at 95 °C for constant-temperature drying treatment for 48 h for subsequent experimental use.

[0049] Thermophysical property tests were carried out on the low-melting-point five-component mixed molten salt heat transfer and energy storage material obtained in Example 1, 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 104.74 °C, the melting end point is 134.38 °C, the phase change latent heat is 70.94 J / g, and the primary crystallization point is 122.49 °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 624.3 °C (calculated according to a 3% mass loss); as Figure 3 shown is the specific heat curve of the mixed molten salt sample prepared in Example 3. The liquid specific heat of the mixed molten salt at 200 - 400 °C is 1.38 - 1.53 J / (g·K).

[0050] 2. The present invention will be further described below in conjunction with Example 2. The low-melting-point five-component mixed molten salt heat transfer and energy storage material described in Example 2 is composed of 9.5 wt% KNO3, 13 wt% NaNO3, 19 wt% Ca(NO3)2, 20 wt% NaNO2, and 38.5 wt% KNO2.

[0051] The preparation method of the low-melting-point five-component mixed molten salt heat transfer and energy storage material described in Example 2 includes the following steps:

[0052] (1) Weigh 9.5 wt% KNO3, 13 wt% NaNO3, 19 wt% Ca(NO3)2, 20 wt% NaNO2, and 38.5 wt% KNO2 using a high-precision balance, put them into a corundum crucible, mix and grind, and stir evenly to form a solid five-component mixed molten salt; Steps (2) - (6) are the same as those in Example 1.

[0053] The low-melting-point five-component mixed molten salt heat transfer and energy storage material obtained in Example 2 was subjected to thermal property tests, and the results are as follows. As Figure 4 shown is the DSC curve of the mixed molten salt sample prepared in Example 2. The melting point of the mixed molten salt is 106.72 °C, the melting end point is 136.19 °C, the phase change latent heat is 80.64 J / g, and the primary crystallization point is 132.22 °C; As Figure 5 shown is the TG curve of the mixed molten salt sample prepared in Example 2. The decomposition temperature of the mixed molten salt is 618.0 °C (calculated according to a 3% mass loss); As Figure 6 shown is the specific heat curve of the mixed molten salt sample prepared in Example 2. The liquid specific heat of the mixed molten salt at 200-400 °C is 1.28-1.62 J / (g·K).

[0054] 3. The present invention will be further described below in conjunction with Example 3. The low-melting-point five-component mixed molten salt heat transfer and energy storage material described in Example 3 is composed of 13 wt% KNO3, 15 wt% NaNO3, 11 wt% Ca(NO3)2, 18 wt% NaNO2, and 43 wt% KNO2.

[0055] The preparation method of the low-melting-point five-component mixed molten salt heat transfer and energy storage material described in Example 3 includes the following steps:

[0056] (1) Weigh 13 wt% KNO3, 15 wt% NaNO3, 11 wt% Ca(NO3)2, 18 wt% NaNO2, and 43 wt% KNO2 using a high-precision balance, put them into a corundum crucible, mix and grind, and stir evenly to form a solid five-component mixed molten salt; Steps (2)-(6) are the same as those in Example 1.

[0057] The low-melting-point five-component mixed molten salt heat transfer and energy storage material obtained in Example 3 was subjected to thermal property tests, and the results are as follows. As Figure 7 shown is the DSC curve of the mixed molten salt sample prepared in Example 3. The melting point of the mixed molten salt is 102.73 °C, the melting end point is 134.04 °C, the phase change latent heat is 70.03 J / g, and the primary crystallization point is 124.39 °C; As Figure 8 shown is the TG curve of the mixed molten salt sample prepared in Example 3. The decomposition temperature of the mixed molten salt is 615.1 °C (calculated according to a 3% mass loss); As Figure 9 shown is the specific heat curve of the mixed molten salt sample prepared in Example 3. The liquid specific heat of the mixed molten salt at 200-400 °C is 1.32-1.56 J / (g·K).

[0058] 4. The present invention will be further described below in conjunction with Example 4. The low-melting-point quinary mixed molten salt heat transfer and heat storage material described in Example 4 is composed of 17 wt% KNO3, 26 wt% NaNO3, 8 wt% Ca(NO3)2, 15 wt% NaNO2, and 34 wt% KNO2.

[0059] The preparation method of the low-melting-point quinary mixed molten salt heat transfer and heat storage material described in Example 4 includes the following steps:

[0060] (1) Weigh 17 wt% KNO3, 26 wt% NaNO3, 8 wt% Ca(NO3)2, 15 wt% NaNO2, and 34 wt% KNO2 using a high-precision balance, put them into a corundum crucible, mix and grind, and stir evenly to form a solid quinary mixed molten salt; Steps (2) to (6) are the same as those in Example 1.

[0061] The thermal properties of the low-melting-point quinary mixed molten salt heat transfer and heat storage material obtained in Example 4 were tested, and the results are as follows. As Figure 10 shown is the DSC curve of the mixed molten salt sample prepared in Example 4. The melting point of the mixed molten salt is 105.07 °C, the melting end point is 132.2 °C, the phase change latent heat is 71.15 J / g, and the primary crystallization point is 128.53 °C; as Figure 11 shown is the TG curve of the mixed molten salt sample prepared in Example 4. The decomposition temperature of the mixed molten salt is 616.5 °C (calculated according to a 3% mass loss); as Figure 12 shown is the specific heat curve of the mixed molten salt sample prepared in Example 4. The liquid specific heat of the mixed molten salt at 200 - 400 °C is 1.35 - 1.61 J / (g·K).

[0062] 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-mentioned 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. The content not described in detail in the present invention is all conventional technical content.

Claims

1. A low-melting-point five-component mixed molten salt heat transfer and heat storage material, characterized in that, This material is composed of KNO3, NaNO3, Ca(NO3)2, NaNO2 and KNO2; In the said mixed molten salt material, the mass percentage of KNO3 is 8 - 17%, the mass percentage of NaNO3 is 13 - 26%, the mass percentage of Ca(NO3)2 is 6 - 20%, the mass percentage of NaNO2 is 12 - 21%, and the mass percentage of KNO2 is 31 - 45%; the initial melting and filling temperature of the molten salt is greater than 360 °C; The melting point of this material is around 100 °C, and the decomposition temperature is greater than 615 °C; The said mixed molten salt is applicable to 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 for heating, and flexibility transformation of thermal power plants.

2. The preparation method of the low-melting-point five-component mixed molten salt heat transfer and heat storage material according to claim 1, characterized in that This method is based on a KNO3 - NaNO3 binary mixed salt, adding nitrites and calcium nitrate, where the nitrites are NaNO2 and KNO2, and is achieved by changing the component ratio of the mixed salt multiple times. It specifically consists of the following steps: (1) According to the component ratio of the five-component mixed molten salt material, use a high-precision balance to weigh the corresponding mass of elemental salts, put them into a corundum crucible, mix and grind, and stir evenly to form a solid mixed molten salt; (2) Place the said solid mixed molten salt in a constant-temperature drying oven for constant-temperature drying; (3) Take out the solid mixed molten salt after constant-temperature drying, quickly place it in a high-temperature muffle furnace, heat the mixed molten salt to the set temperature, and then keep it at a constant temperature for a certain period of time to completely melt the mixed molten salt to obtain a molten salt; (4) Take out the molten mixed molten salt from the high-temperature muffle furnace and place it in the constant-temperature drying oven to cool naturally until crystallization; (5) Place the cooled solid mixed molten salt in an ultrafine pulverizer for pulverization to obtain a mixed molten salt powder; (6) Place the pulverized mixed molten salt powder in the constant-temperature drying oven for constant-temperature drying treatment for subsequent experiments.

3. The preparation method of the low-melting-point five-component mixed molten salt heat transfer and energy storage material according to claim 2, characterized in that, The temperature of the said constant-temperature drying oven is set at 90 - 105 °C, and the constant-temperature drying time is more than 36 h.

4. The preparation method of the low-melting-point five-component mixed molten salt heat transfer and energy storage material according to claim 2, characterized in that, The heating rate of the said high-temperature muffle furnace is set at 10 - 15 K / min.

5. The preparation method of the low-melting-point five-component mixed molten salt heat transfer and energy storage material according to claim 2, wherein The heating temperature of the said high-temperature muffle furnace is set at 150 - 250 °C above the melting point of the mixed molten salt, and the constant-temperature heating time is more than 10 h.

6. The preparation method of the low-melting-point five-component mixed molten salt heat transfer and heat storage material according to claim 2, characterized in that, The fineness of the sample of the pulverized mixed molten salt is 30 - 300 mesh.

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