A ternary mixed molten salt heat transfer and heat storage medium and preparation method thereof

Abstract

The present invention belongs to the field of heat storage and energy storage technology of solar thermal power generation system, and discloses a ternary mixed molten salt heat transfer and heat storage medium and a preparation method thereof, wherein the heat transfer and heat storage medium is composed of potassium nitrate, sodium nitrite and sodium carbonate. The heat transfer and heat storage medium can be obtained by a method of mixed grinding, drying, melting, cooling and crystallization, and the obtained heat transfer and heat storage medium does not contain precious metal salts, thus reducing costs, and has a lower melting point and a higher thermal stability decomposition temperature.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat storage and energy storage of solar thermal power generation systems, and in particular relates to a ternary mixed molten salt heat transfer and heat storage medium and a preparation method thereof. Background Art

[0002] In the process of solar energy utilization, it has the disadvantages of discontinuity and changes with geographical location, environment, and weather conditions. In order to overcome this disadvantage, most solar energy technologies now use energy storage technology to store part of the unused energy using energy storage materials. The combination of these two technologies can have more advantages in terms of capacity buffering, scheduling and time shifting, improvement of annual utilization rate, stability of power output, and efficient full-load operation. At present, after a long period of practical research, sensible heat storage technology has been found to have the characteristics of mature technology, simple and easy-to-understand principles, easy-to-purchase raw materials and low cost, becoming one of the key technologies for current research and application. The selection of energy storage materials is the key to this technology. How to choose the heat storage material will have an important impact on the power generation efficiency of the entire system, the optimal design of the pipelines in the system, and the cost.

[0003] Compared with other energy storage materials, molten salt has attracted widespread attention due to its wide operating temperature, low vapor pressure, large heat capacity, low viscosity, good stability and many other characteristics. Using molten salt as a heat transfer medium can improve the power generation efficiency and reliability of the entire system, while significantly reducing costs, which is the main trend of the future development of solar thermal utilization technology; and using molten salt as a heat transfer and storage material can not only achieve large-scale energy storage at a low cost, but also ensure that solar thermal power generation outputs high-quality electricity. Therefore, using molten salt as a heat transfer and storage material is a necessary way for the future development of solar thermal utilization technology.

[0004] At present, molten salt has been used as a heat transfer and heat storage medium in solar thermal power generation systems. Solar Two in California, USA and Andasol Solar Power Station in Spain both use solar salt (60wt% NaNO3 + 40wt% KNO3) as a heat transfer and heat storage medium. This mixed molten salt has good thermal stability and low cost, but its melting point is as high as 220℃, which puts a test on the safety and stability of the system. Hitec salt (7wt% NaNO3 + 53wt% KNO3 + 40wt% NaNO2) with a lower melting point (143℃) is used in industrial heat transfer. This mixed molten salt has thermal stability below 454℃ and can run to 538℃ for a short time, and the decomposition temperature is relatively low. The cost of the developed KNO3-NaNO3-LiNO3-Ca(NO3)2 molten salt system has greatly increased with the sharp rise in the price of lithium nitrate on the market. Compared with thermal oil with a melting point of 13°C, the main disadvantages of the currently used molten salt are its relatively high melting point and low decomposition temperature. When molten salt is used in a solar heat transfer and storage system, protective measures must be taken to prevent the molten salt from freezing. In addition, the currently available molten salt formula also contains precious metal salts (lithium), which makes its cost relatively high. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a ternary mixed molten salt heat transfer and heat storage medium and a preparation method thereof. The heat transfer and heat storage medium does not contain precious metal salts and has a lower melting point and a higher thermal stability decomposition temperature.

[0006] To achieve the above object, the technical solution of the present invention is:

[0007] A ternary mixed molten salt heat transfer and heat storage medium consists of potassium nitrate, sodium nitrite and sodium carbonate.

[0008] Furthermore, the heat transfer and heat storage medium is composed of 40-60wt% potassium nitrate, 30-50wt% sodium nitrite and 5-20wt% sodium carbonate. If any component is higher or lower than this range, the mixed molten salt will cause problems such as non-eutectic, too low decomposition temperature, and excessive viscosity.

[0009] Preferably, the heat transfer and heat storage medium consists of 51.15 wt % potassium nitrate, 41.85 wt % sodium nitrite and 7 wt % sodium carbonate.

[0010] The present invention also provides a method for preparing a ternary mixed molten salt heat transfer and heat storage medium, comprising the following steps:

[0011] S1: Mixing, grinding and drying: Mixing and grinding potassium nitrate, sodium nitrite and sodium carbonate according to a preset ratio, and then drying;

[0012] S2: Melting: Completely melting the mixture dried in step S1, and then cooling and crystallizing;

[0013] S3: Crushing and drying: Crushing and drying the crystallized salt obtained in step S2 to obtain the heat transfer and heat storage medium.

[0014] Specifically, the step S1 is as follows: 40-60wt% potassium nitrate, 30-50wt% sodium nitrite and 5-20wt% sodium carbonate are weighed, mixed and ground, and then placed in a drying oven for constant temperature drying, with the set temperature being 100°C-130°C and the time being 24h-96h.

[0015] Specifically, the step S2 is as follows: placing the dried mixture in a high temperature furnace and heating it to 400° C.-550° C. for 12 h-24 h, and cooling it in the furnace.

[0016] Specifically, the step S3 is as follows: using an ultrafine grinder to grind the crystallized salt to a fineness of 20-200 meshes, and then drying the crushed crystallized salt at a constant temperature.

[0017] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art:

[0018] The heat transfer and heat storage medium of the present invention comprises potassium nitrate, sodium nitrite and sodium carbonate. The decomposition temperature of the single component nitrate is too low. After adding carbonate with a high decomposition temperature to the nitrate to form a eutectic mixed molten salt, its decomposition temperature can be significantly increased. The eutectic mixed molten salt can ensure stable and uniform thermal properties of phases and components within a wide operating temperature range. Therefore, the decomposition temperature of the heat transfer and heat storage medium provided by the present invention is significantly increased (the decomposition temperature is 705-730°C), and its application in a solar thermal power generation system broadens the applicable temperature range of the molten salt and improves thermal stability.

[0019] The heat transfer and heat storage medium of the present invention has a greatly increased decomposition temperature compared to conventional heat transfer and heat storage media (Solar salt, Hitec salt, KNO3-NaNO2-Na2CO3), and a lower melting point compared to Solar salt. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 TG curve of the mixed molten salt of Example 1 of the present invention;

[0021] Figure 2 is the DSC curve of the mixed molten salt of Example 1 of the present invention;

[0022] Figure 3 The specific heat measurement result of the mixed molten salt of Example 1 of the present invention;

[0023] Figure 4 The density measurement result of the mixed molten salt in Example 1 of the present invention;

[0024] Figure 5 The thermal diffusion coefficient measurement result of the mixed molten salt of Example 1 of the present invention;

[0025] Figure 6 The thermal conductivity measurement result of the mixed molten salt of Example 1 of the present invention;

[0026] Figure 7 The viscosity measurement result of the mixed molten salt of Example 1 of the present invention;

[0027] Figure 8 is the DSC curve of the mixed molten salt of Example 2 of the present invention;

[0028] Fig. 9 TG curve of the mixed molten salt of Example 2 of the present invention;

[0029] Fig.10 This is the DSC curve of the mixed molten salt of Example 3 of the present invention;

[0030] Fig.11 TG curve of the mixed molten salt of Example 3 of the present invention;

[0031] Fig.12 The DSC curve of the mixed molten salt of Example 4 of the present invention;

[0032] Fig.13 This is the TG curve of the mixed molten salt of Example 4 of the present invention. DETAILED DESCRIPTION

[0033] The following is a further detailed description of a ternary mixed molten salt heat transfer and heat storage medium and a preparation method thereof proposed by the present invention in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description and claims.

[0034] A ternary mixed molten salt heat transfer and heat storage medium, the heat transfer and heat storage medium is composed of 40-60wt% potassium nitrate, 30-50wt% sodium nitrite and 5-20wt% sodium carbonate. It can be prepared by the following steps:

[0035] S1: Mixing, grinding and drying: According to the above mass component ratio, the three components are weighed using a high-precision analytical balance, fully mixed and ground, and then placed in a drying oven for constant temperature drying, with the set temperature being 100°C-130°C and the time being 24h-96h, so that the water contained in the mixture overflows;

[0036] S2: Melting: Place the dried mixture in step S1 in a muffle furnace and heat it to 400°C-550°C for 12h-24h to completely melt the mixture and mix it evenly, and cool it in the furnace to form a hard solid crystalline salt;

[0037] S3: Crushing and drying: The crystallized salt obtained in step S2 is crushed by an ultrafine grinder to a fineness of 20-200 meshes, and then the crushed crystallized salt is placed in a drying oven for constant temperature drying.

[0038] Example 1

[0039] The ternary mixed molten salt heat transfer and heat storage medium is composed of 51.15wt% potassium nitrate, 41.85wt% sodium nitrite, and 7wt% sodium carbonate.

[0040] It is prepared by the following steps,

[0041] S1: Mixing, grinding and drying: According to the above mass component ratio, the three components are weighed using a high-precision analytical balance, fully mixed and ground, and then placed in a drying oven for constant temperature drying. The set temperature is 150°C and the time is 96 hours, so that the water contained in the mixture overflows;

[0042] S2: Melting: Place the dried mixture in step S1 in a muffle furnace and heat it to 450°C for 12 hours to completely melt the mixture and mix it evenly, and cool it in the furnace to form a hard solid crystalline salt;

[0043] S3: Crushing and drying: The crystallized salt obtained in step S2 is crushed by an ultrafine grinder to a fineness of 20-200 meshes, and then the crushed crystallized salt is placed in a drying oven for constant temperature drying.

[0044] The melting point of the mixed molten salt was analyzed by DSC (differential scanning calorimetry). Figure 2 As shown, from Figure 2 It can be concluded that the melting point is 144.5℃. Through TG (thermogravimetric analysis), the results are as follows Figure 1 As shown, the decomposition temperature is 727.8℃. Then the specific heat capacity, density, diffusion coefficient, thermal conductivity and viscosity of the mixed molten salt are measured respectively. The results are as follows Figure 3-7 The performance of the mixed molten salt of Example 1 is compared with the mainstream molten salt on the market. The results are shown in Table 1.

[0045] Table 1 Properties of mixed molten salt

[0046]

[0047] Compared with the commonly used Solar Salt, the melting point of the ternary mixed salt is reduced by about 75°C, and the decomposition temperature is increased by about 151°C. Compared with the Hitec salt, the melting point is 1.5°C higher, and the decomposition temperature is increased by about 273°C. The specific heat capacity is lower than that of the two commonly used salts, and the viscosity is larger. Its heat storage cost is 2.92 yuan more expensive than Solar Salt and 19.17 yuan cheaper than Hitec salt. The ternary mixed salt obtained in this embodiment can solve the risk of freezing and blocking of pipelines in the solar heat transfer and storage system with its wider operating temperature range and lower melting point, thereby reducing costs.

[0048] Example 2

[0049] The ternary mixed molten salt heat transfer and heat storage medium is composed of 50.05wt% potassium nitrate, 40.95wt% sodium nitrite, and 9wt% sodium carbonate.

[0050] It is prepared by the following steps,

[0051] S1: Mixing, grinding and drying: According to the above mass component ratio, the three components are weighed using a high-precision analytical balance, mixed and ground thoroughly, and then placed in a drying oven for constant temperature drying at 120°C for 48 hours to allow the water in the mixture to overflow;

[0052] S2: Melting: Place the dried mixture in step S1 in a muffle furnace and heat it to 450°C for 12 hours to completely melt the mixture and mix it evenly, and cool it in the furnace to form a hard solid crystalline salt;

[0053] S3: Crushing and drying: The crystallized salt obtained in step S2 is crushed by an ultrafine grinder to a fineness of 20-200 meshes, and then the crushed crystallized salt is placed in a drying oven for constant temperature drying.

[0054] The melting point of the mixed molten salt obtained by DSC test analysis is as follows Figure 8 As shown, from Figure 8 It can be concluded that the melting point is 142.3℃. Through TG analysis, the results are as follows Fig. 9 As shown, the decomposition temperature is 722.1°C.

[0055] Table 2

[0056]

[0057] Example 3

[0058] The ternary mixed molten salt heat transfer and heat storage medium is composed of 44wt% potassium nitrate, 36wt% sodium nitrite and 20wt% sodium carbonate.

[0059] It is prepared by the following steps,

[0060] S1: Mixing, grinding and drying: According to the above mass component ratio, the three components are weighed using a high-precision analytical balance, mixed and ground thoroughly, and then placed in a drying oven for constant temperature drying at 120°C for 48 hours to allow the water in the mixture to overflow;

[0061] S2: Melting: Place the dried mixture in step S1 in a muffle furnace and heat it to 450°C for 12 hours to completely melt the mixture and mix it evenly, and cool it in the furnace to form a hard solid crystalline salt;

[0062] S3: Crushing and drying: The crystallized salt obtained in step S2 is crushed by an ultrafine grinder to a fineness of 20-200 meshes, and then the crushed crystallized salt is placed in a drying oven for constant temperature drying.

[0063] Table 3

[0064]

[0065] Example 4

[0066] The ternary mixed molten salt heat transfer and heat storage medium is composed of 52.25wt% potassium nitrate, 42.75wt% sodium nitrite and 5wt% sodium carbonate.

[0067] It is prepared by the following steps,

[0068] S1: Mixing, grinding and drying: According to the above mass component ratio, the three components are weighed using a high-precision analytical balance, mixed and ground thoroughly, and then placed in a drying oven for constant temperature drying at 120°C for 48 hours to allow the water in the mixture to overflow;

[0069] S2: Melting: Place the dried mixture in step S1 in a muffle furnace and heat it to 450°C for 12 hours to completely melt the mixture and mix it evenly, and cool it in the furnace to form a hard solid crystalline salt;

[0070] S3: Crushing and drying: The crystallized salt obtained in step S2 is crushed by an ultrafine grinder to a fineness of 20-200 meshes, and then the crushed crystallized salt is placed in a drying oven for constant temperature drying.

[0071] Table 4

[0072]

[0073] The embodiments of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Even if various changes are made to the present invention, if these changes fall within the scope of the claims of the present invention and their equivalents, they still fall within the protection scope of the present invention.

Claims

1. A ternary mixed molten salt heat transfer and heat storage medium, characterized in that: The heat transfer and heat storage medium consists of 40-60wt% potassium nitrate, 30-50wt% sodium nitrite and 5-20wt% sodium carbonate.

2. The ternary mixed molten salt heat transfer and heat storage medium according to claim 1, characterized in that: The heat transfer and heat storage medium consists of 51.15wt% potassium nitrate, 41.85wt% sodium nitrite and 7wt% sodium carbonate.

3. A method for preparing the ternary mixed molten salt heat transfer and heat storage medium as claimed in claim 1, characterized in that: The following steps are involved: S1: Mixing, grinding and drying: Weigh 40-60wt% potassium nitrate, 30-50wt% sodium nitrite and 5-20wt% sodium carbonate, mix and grind them, and then place them in a drying oven for constant temperature drying, with the set temperature being 100°C-130°C and the time being 24h-96h; S2: Melting: Completely melting the mixture dried in step S1, and then cooling and crystallizing; S3: Crushing and drying: In order to subsequently measure various thermal properties, the crystallized salt obtained in step S2 is crushed and dried to obtain the heat transfer and heat storage medium.

4. The method for preparing the ternary mixed molten salt heat transfer and heat storage medium according to claim 3, characterized in that: The step S2 specifically includes: placing the dried mixture in a high temperature furnace and heating it to 400° C.-550° C. for 12 h-24 h, and cooling it in the furnace.

5. The method for preparing the ternary mixed molten salt heat transfer and heat storage medium according to claim 3, characterized in that: The step S3 specifically comprises: using an ultrafine pulverizer to pulverize the crystallized salt to a fineness of 20-200 meshes, and then subjecting the pulverized crystallized salt to a constant temperature drying process.

Images