Ternary mixed nitric acid molten salt heat transfer and storage medium and preparation method thereof

Abstract

The invention discloses a ternary mixed nitric acid molten salt heat transfer and storage medium which comprises the following components in percentage by mass: 40-75wt% of potassium nitrate, 20-50wt% of sodium nitrite and 0.01-25wt% of lithium nitrate. The melting point of the ternary mixed nitric acid molten salt heat transfer and storage medium is lower than 130 DEG C, the lowest melting point is only 87.24 DEG C, meanwhile, the decomposition temperature is higher than 620 DEG C, the ternary mixed nitric acid molten salt heat transfer and storage medium is applied to a solar thermal power generation system, the applicable temperature range of molten salt is widened, and the thermal stability is improved.

Description

Technical Field

[0001] This invention relates to the field of physical heat transfer and energy storage technology, specifically to a ternary mixed nitrate molten salt heat transfer and storage medium and its preparation method. Background Technology

[0002] By 2030, non-fossil energy will account for approximately 25% of primary energy consumption, with total installed capacity of wind and solar power exceeding 1.2 billion kilowatts. The proportion of non-fossil energy is projected to surpass 50% by around 2040, becoming the main source of energy supply. The transition to renewable energy is essential for achieving the "dual carbon" goals. By 2060, non-fossil energy is expected to account for approximately 80% of primary energy consumption. Wind and solar power are constrained by natural conditions, exhibiting unpredictability, intermittency, and volatility, making stable power output increasingly urgent. Therefore, to achieve a high proportion of renewable energy in my country, sufficient energy storage capacity must be deployed on the generation, grid, and demand sides. Energy storage has become a key supporting technology for solving the problem of large-scale renewable energy consumption and driving the energy revolution, serving as a core element and fundamental equipment for building a new power system.

[0003] For wind power and photovoltaic power generation, the reasonable proportion of energy storage power stations is 15% to 20%. It is estimated that by 2030, the reasonable installed capacity of energy storage power stations in my country will be more than 180 to 240 million kilowatts. However, due to the need for special terrain conditions, pumped storage alone cannot meet the demand for a high proportion of renewable energy. There is an urgent need to develop new long-term and large-capacity power storage technologies other than pumped storage.

[0004] Molten salt thermal energy storage is a relatively safe and low-cost energy storage method. Its working principle utilizes molten salt as a heat transfer medium, completing the storage and release of energy through the heat storage and release cycle of the molten salt, thereby achieving efficient energy transfer. Due to its advantages such as good energy storage density, wide operating temperature range, and low cost, molten salt is widely used in solar thermal power generation and renewable energy peak shaving. As a high-temperature thermal energy storage and heat transfer medium, molten salt has advantages such as excellent heat transfer performance, low system pressure, high operating temperature, low price, non-toxicity, and safety and reliability.

[0005] Currently, over thirty integrated large-capacity solar thermal power plants with thermal energy storage have achieved commercial operation globally, with a total installed capacity exceeding 3 million kilowatts. The longest-operating of these is a molten salt thermal energy storage solar thermal power plant, which has been successfully operating for 18 years. In recent years, within China, several demonstration projects focusing on molten salt thermal energy storage for peak shaving in thermal power plants have been successively built, including Huaneng Weijiamou, Guoxin Jingjiang, Huaneng Haimen, and Dezhou in Shandong. Simultaneously, several heating and steam supply demonstration projects centered on molten salt thermal energy storage have been constructed in Hebei, Beijing, Zhejiang, and other regions. Furthermore, Liaohe Oilfield has completed the construction and commissioned a test station for electrothermal molten salt energy storage steam injection. Companies such as Zhejiang Green Storage and State Power Investment Corporation are currently constructing molten salt heat pump thermal energy storage demonstration and verification systems. Guoneng Suzhou Power Plant and Longshan Power Plant are also advancing the construction of molten salt thermal energy storage peak shaving demonstration projects based on thermal power plant steam extraction heating to further verify the engineering applicability and technical reliability of molten salt thermal energy storage technology in the flexible retrofitting of thermal power units.

[0006] Improving the thermal storage density of molten salt is one of the main development directions of molten salt sensible heat storage technology. Based on the principle of sensible heat storage, the amount of heat stored is linearly positively correlated with the effective temperature range. Therefore, expanding the operable temperature range of molten salt in the liquid phase can significantly increase its thermal storage density per unit volume / unit mass without increasing the amount of molten salt used. To achieve this goal, the molten salt must simultaneously possess the following characteristics: (a) The lowest possible melting point to reduce the risk of solidification and expand the application boundaries at low temperatures; (b) The highest possible thermal decomposition temperature to broaden the upper limit of high-temperature applications.

[0007] This allows for a wide liquid temperature range, enabling the design of high energy density and long-term stable operation of molten salt thermal storage systems.

[0008] Lowering the minimum operating temperature of molten salt increases the effective heat exchange temperature difference between it and the heat source, maximizing heat recovery under the same heat transfer load; it also ensures the lower limit of the working fluid temperature requirement for the power cycle of the molten salt thermal storage coupled energy system. Lowering the melting point can significantly reduce the risk and cost of freezing in molten salt thermal storage systems, improving their operational reliability and economy.

[0009] Increasing the heat storage / release temperature of molten salt has become a key technical means to enhance the energy conversion efficiency and economy of electrothermal molten salt thermal storage peak shaving combined heat and power, heat pump energy storage, deep peak shaving of thermal power units, and solar thermal power generation systems. In particular, the development of supercritical carbon dioxide power generation and ultra-supercritical Rankine cycle power generation technologies urgently requires high-temperature molten salt thermal storage materials, equipment, and systems with high decomposition temperatures to break through the upper limits of existing solar salts and Hitec salts and provide usable working fluids for high-temperature sources.

[0010] Currently, the commercially available molten salts are mainly binary nitrates (Solarsalt) and ternary nitrates (Hitec). Solarsalt has a melting point of about 220℃ and an operating temperature range of 290-565℃. However, it is prone to freezing and clogging of pipelines, has a small heat storage temperature difference, and low heat storage density. Hitec salt has a melting point of about 142℃ and an operating temperature range of 190-450℃, but it has the disadvantages of low decomposition temperature (nitrite begins to oxidize above 450℃) and low heat storage density. To mitigate the risk of freezing and blockage in molten salt thermal storage systems, researchers both domestically and internationally have been developing low-melting-point molten salts in recent years. Beijing University of Technology, in particular, has developed low-melting-point binary, ternary, and quaternary salts. These mixed molten salts have melting points reduced to approximately 90-140℃, decomposition temperatures between 560-630℃, and long-term minimum operating temperatures reduced to 150-200℃, with maximum operating temperatures between 500-600℃. This significantly improves the thermal storage temperature difference and density, and substantially reduces the cost of thermal storage materials and systems. High-temperature mixed carbonates have also been developed domestically and internationally, but these formulations often contain high levels of precious metal lithium salts, resulting in high costs.

[0011] In summary, the decomposition temperatures of molten salts currently being developed and used both domestically and internationally cannot meet the requirements of high-parameter power generation such as supercritical carbon dioxide power generation and ultra-supercritical steam power generation, which require high-temperature molten salts with a wide liquid temperature range of ≥600℃ and a melting point ≤100℃. Therefore, it is essential to develop molten salt thermal storage media with low melting points and high decomposition temperatures across a wide temperature range. Summary of the Invention

[0012] To address the shortcomings of existing technologies, this invention provides a ternary mixed nitrate molten salt heat transfer and storage medium and its preparation method, which can reduce the content of precious metal salts and has a low melting point and high thermal stability decomposition temperature.

[0013] This invention discloses a ternary mixed nitrate molten salt heat transfer and heat storage medium, which, by mass percentage, comprises: 40~75wt% potassium nitrate, 20~50wt% sodium nitrite and 0.01~25wt% lithium nitrate.

[0014] This invention also discloses a method for preparing a ternary mixed nitrate molten salt heat transfer and storage medium, comprising: Weigh potassium nitrate, sodium nitrite and lithium nitrate according to the preset component ratio, mix, grind and stir evenly to obtain solid mixed molten salt; The solid mixed molten salt was placed in a drying oven for drying. The dried solid mixed molten salt is placed in a muffle furnace and heated to the set temperature at a preset heating rate to melt the mixed molten salt. After standing, molten salt is obtained. Remove the molten salt from the muffle furnace and place it in a drying oven to cool naturally; The cooled molten salt is placed in a pulverizer and pulverized to obtain mixed molten salt powder; The mixed molten salt powder was placed in a drying oven for drying to obtain a ternary mixed nitrate molten salt heat transfer and heat storage medium.

[0015] As a further improvement of the present invention, the solid mixed molten salt is placed in a drying oven at 90~110°C and dried at a constant temperature for more than 10 hours to allow the moisture contained therein to escape.

[0016] As a further improvement of the present invention, the dried solid mixed molten salt is placed in a muffle furnace and heated to 350~450°C and kept at the temperature for 10~15 hours to completely melt the molten salt, fully mix the components, and completely remove the water of crystallization.

[0017] As a further improvement of the present invention, the particle size of the pulverized mixed molten salt powder is 20~200 mesh.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: The decomposition temperature of the ternary mixed nitrate molten salt heat transfer and storage medium of the present invention is significantly higher than that of common heat storage materials Solarsalt and Hitec salt, while its melting point is significantly lower. The melting point of the ternary mixed nitrate molten salt heat transfer and storage medium of the present invention is below 130°C, with the lowest melting point being only 87.24°C, while its decomposition temperature is above 620°C. Its application in solar thermal power generation systems broadens the applicable temperature range of molten salt and improves thermal stability. Attached Figure Description

[0019] Figure 1 The DSC curve of the ternary mixed nitrate molten salt in Example 1; Figure 2 The TG analysis curve of the ternary mixed nitrate molten salt in Example 1; Figure 3 The specific heat capacity of the ternary mixed nitrate molten salt in Example 1; Figure 4 The DSC curve of the ternary mixed nitrate molten salt in Example 2; Figure 5 The DSC curve of the ternary mixed nitrate molten salt in Example 3; Figure 6 The DSC curve of the ternary mixed nitrate molten salt in Example 4; Figure 7 The DSC curve of the ternary mixed nitrate molten salt in Example 5; Figure 8 The DSC curve of the ternary mixed nitrate molten salt in Example 6; Figure 9The DSC curve is for the ternary mixed nitrate molten salt of Example 7. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The present invention will now be described in further detail with reference to the accompanying drawings: Example 1:

[0022] A ternary mixed nitrate molten salt heat transfer and storage medium, comprising, by mass percentage: 56.5 wt% potassium nitrate, 33.5% sodium nitrite, and 10 wt% lithium nitrate.

[0023] The preparation method of this ternary mixed nitrate molten salt heat transfer and storage medium includes: Molten salt samples were prepared using a high-temperature melting method, with strict control over moisture content and mixing uniformity throughout the process to ensure the reliability of subsequent experiments. First, a high-precision analytical balance was used for weighing, and the molten salts were mixed according to a specific mass ratio. The mixed molten salt was then placed in a drying oven at 100℃ for 12 hours to allow the moisture to evaporate. After drying, the mixed molten salt was placed in a muffle furnace and heated to 400℃, held at that temperature for 12 hours to ensure complete melting, thorough mixing of components, and complete removal of water of crystallization. After cooling to room temperature, the mixed molten salt was pulverized using an ultrafine pulverizer. Finally, the pulverized molten salt was placed in a drying oven for constant temperature drying, yielding a well-dispersed molten salt sample ready for experimental use.

[0024] The method for measuring the melting point of this ternary mixed nitrate molten salt includes: The experiment used a STA449F3 simultaneous thermal analyzer manufactured by Netzsch GmbH, Germany, with a dedicated DSC stand. The measurement software, protective gas, and pre-measured baseline were sequentially turned on. 5-15 mg of the sample to be tested was weighed and placed in an aluminum crucible. The heating program was set to 35-450℃ at a heating rate of 10 K·min. -1 Cooling program: 450~35℃, cooling rate: 10K·min -1 Nitrogen was used as both the protective and purging gas, with a gas flow rate of 30 mL / min. -1The experiment was conducted in an N2 atmosphere. After the experiment, the melting initiation temperature Tonset, peak temperature Tpeak, enthalpy of fusion ΔHfus, and primary crystallization point Tsolidus were automatically calculated using Proteus software, thus obtaining the melting point, melting termination point, latent heat of fusion, and primary crystallization point of the sample.

[0025] The method for measuring the decomposition temperature of the ternary mixed nitrate molten salt includes: The experiment used a STA449F3 simultaneous thermal analyzer manufactured by Netzsch GmbH, Germany, with a dedicated TG stand. The program was edited to measure the baseline. The protective gas, software, and baseline were turned on sequentially. 5–15 mg of the sample was weighed and placed in an Al₂O₃ crucible. The heating program was set to 30–850 °C at a heating rate of 10 K·min. -1 Before the experiment, the furnace was evacuated, and nitrogen was introduced as both a protective and purging gas. The experiment was conducted in an inert environment with a gas flow rate of 50 mL / min. -1 and 20 mL·min -1 After the experiment, the temperature corresponding to a 5% mass loss is automatically calculated using Proteus software, thus obtaining the decomposition temperature of the sample.

[0026] The method for measuring the specific heat capacity of this ternary mixed nitrate molten salt includes: The specific heat of the mixed molten salt was measured using a STA449F3 simultaneous thermal analyzer manufactured by Netzsch AG, Germany. Sapphire was used as the standard sample, and the calculation formula is as follows:

[0027] In the formula: C p —Specific heat of standard or sample, J·(g·K) -1 ; m — the mass of the standard or sample, in grams; H – DSC signal difference between the standard sample and the reference empty crucible, μV; h — the difference in DSC signal between the sample and the reference empty crucible, in μV.

[0028] Specific heat measurement using DSC: First, run a blank baseline, then test the sapphire standard and the sample to be tested under the same conditions. Set the temperature program: isothermal-heating-isothermal. Calculate the isobaric specific heat C of the sample to be tested at the corresponding temperature by converting the heat flux difference and mass ratio between the two. p .

[0029] like Figure 1 As shown, the melting point of the ternary mixed nitrate molten salt measured in Example 1 is 87.24℃; Figure 2As shown, the decomposition temperature of the ternary mixed nitrate molten salt measured in Example 1 was 627.05℃. Figure 3 As shown, the specific heat capacity of the ternary mixed nitrate molten salt measured in Example 1 is 1.28 J / (g·K).

[0030] Compared to Solar Salt, this formulation has a melting point that is about 150°C lower and a decomposition temperature that is about 30°C higher; compared to Hitec Salt, its melting point is about 50°C lower and its decomposition temperature is about 130°C higher, resulting in a wider operating temperature range. Compared to common thermal storage materials, its average specific heat is significantly higher than the energy storage density of molten salt thermal storage media such as Solar Salt and Hitec. Example 2:

[0031] A ternary mixed nitrate molten salt heat transfer and storage medium, comprising, by mass percentage: 62.5 wt% potassium nitrate, 36.5% sodium nitrite, and 1 wt% lithium nitrate.

[0032] The preparation method of this ternary mixed nitrate molten salt heat transfer and heat storage medium is the same as in Example 1.

[0033] The method for testing the melting point and decomposition temperature of the ternary mixed nitrate molten salt heat transfer and storage medium is the same as in Example 1.

[0034] like Figure 4 As shown, the melting point of the ternary mixed nitrate molten salt measured in Example 2 is 125.67℃. Example 3:

[0035] A ternary mixed nitrate molten salt heat transfer and storage medium, comprising, by mass percentage: 61.5 wt% potassium nitrate, 36.5% sodium nitrite, and 2 wt% lithium nitrate.

[0036] The preparation method of this ternary mixed nitrate molten salt heat transfer and heat storage medium is the same as in Example 1.

[0037] The method for testing the melting point and decomposition temperature of the ternary mixed nitrate molten salt heat transfer and storage medium is the same as in Example 1.

[0038] like Figure 5 As shown, the melting point of the ternary mixed nitrate molten salt measured in Example 3 is 121.87℃. Example 4:

[0039] A ternary mixed nitrate molten salt heat transfer and storage medium, comprising, by mass percentage: 61.5 wt% potassium nitrate, 35.5% sodium nitrite and 3 wt% lithium nitrate.

[0040] The preparation method of this ternary mixed nitrate molten salt heat transfer and heat storage medium is the same as in Example 1.

[0041] The method for testing the melting point and decomposition temperature of the ternary mixed nitrate molten salt heat transfer and storage medium is the same as in Example 1.

[0042] like Figure 6 As shown, the melting point of the ternary mixed nitrate molten salt measured in Example 4 is 120.72℃. Example 5:

[0043] A ternary mixed nitrate molten salt heat transfer and storage medium, comprising, by mass percentage: 60.5 wt% potassium nitrate, 35.5% sodium nitrite, and 4 wt% lithium nitrate.

[0044] The preparation method of this ternary mixed nitrate molten salt heat transfer and heat storage medium is the same as in Example 1.

[0045] The method for testing the melting point and decomposition temperature of the ternary mixed nitrate molten salt heat transfer and storage medium is the same as in Example 1.

[0046] like Figure 7 As shown, the melting point of the ternary mixed nitrate molten salt measured in Example 5 was 116.49℃. Example 6:

[0047] A ternary mixed nitrate molten salt heat transfer and storage medium, comprising, by mass percentage: 59.5 wt% potassium nitrate, 35.5% sodium nitrite and 5 wt% lithium nitrate.

[0048] The preparation method of this ternary mixed nitrate molten salt heat transfer and heat storage medium is the same as in Example 1.

[0049] The method for testing the melting point and decomposition temperature of the ternary mixed nitrate molten salt heat transfer and storage medium is the same as in Example 1.

[0050] like Figure 8 As shown, the melting point of the ternary mixed nitrate molten salt measured in Example 6 was 92.43℃. Example 7:

[0051] A ternary mixed nitrate molten salt heat transfer and storage medium, comprising, by mass percentage: 57.5 wt% potassium nitrate, 32.5% sodium nitrite, and 10 wt% lithium nitrate.

[0052] The preparation method of this ternary mixed nitrate molten salt heat transfer and heat storage medium is the same as in Example 1.

[0053] The method for testing the melting point and decomposition temperature of the ternary mixed nitrate molten salt heat transfer and storage medium is the same as in Example 1.

[0054] like Figure 9 As shown, the melting point of the ternary mixed nitrate molten salt measured in Example 7 was 89.54℃.

[0055] Comparative Example 1: Solar Salt, a binary mixed molten salt heat transfer and storage medium, is composed of 40 wt% potassium nitrate and 60 wt% sodium nitrate. The melting point of Solar Salt is approximately 220°C; its decomposition temperature is approximately 600°C.

[0056] Comparative Example 2: Hitec, a ternary mixed molten salt heat transfer and storage medium, is composed of 53 wt% potassium nitrate, 7 wt% sodium nitrate, and 40 wt% sodium nitrite. Hitec salt has a melting point of approximately 142°C and a decomposition temperature of approximately 538°C.

[0057] The mechanism by which the ternary mixed nitrate molten salt heat transfer and storage medium of the present invention improves the decomposition temperature is as follows: Single-component molten salts have relatively low decomposition temperatures due to their inherent structural limitations. By forming a eutectic mixed molten salt using different nitrates and nitrites, the overall thermal decomposition temperature can be significantly increased while maintaining the advantage of a low melting point. The eutectic mixed molten salt maintains a single liquid phase over a wide operating temperature range, with uniform phase composition and component distribution. Its thermal properties (melting point, specific heat capacity, thermal conductivity, viscosity, and high-temperature stability) exhibit continuous and repeatable characteristics, ensuring the long-term stable operation of the thermal storage system. The thermal properties of the mixed molten salt vary with its composition and mass ratio. During preparation, it is necessary to comprehensively consider constraints such as the target temperature range, heat density, economy, and system processes to optimally determine the component types and ratio range.

[0058] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A ternary mixed nitrate molten salt heat transfer and heat storage medium, characterized in that, By mass percentage, it includes: 40-75 wt% potassium nitrate, 20-50 wt% sodium nitrite, and 0.01-25 wt% lithium nitrate.

2. A method for preparing a ternary mixed nitrate molten salt heat transfer and storage medium, characterized in that, include: Weigh potassium nitrate, sodium nitrite and lithium nitrate according to the preset component ratio, mix, grind and stir evenly to obtain a solid mixed molten salt; The solid mixed molten salt was placed in a drying oven for drying. The dried solid mixed molten salt is placed in a muffle furnace and heated to the set temperature at a preset heating rate to melt the mixed molten salt. After standing, molten salt is obtained. Remove the molten salt from the muffle furnace and place it in a drying oven to cool naturally; The cooled molten salt is placed in a pulverizer and pulverized to obtain mixed molten salt powder; The mixed molten salt powder was placed in a drying oven for drying to obtain a ternary mixed nitrate molten salt heat transfer and heat storage medium.

3. The preparation method of the ternary mixed nitrate molten salt heat transfer and heat storage medium as described in claim 2, characterized in that, The solid mixed molten salt was placed in a drying oven at 90~110℃ and dried at a constant temperature for more than 10 hours to allow the moisture it contained to escape.

4. The preparation method of the ternary mixed nitrate molten salt heat transfer and heat storage medium as described in claim 2, characterized in that, The dried solid mixed molten salt is placed in a muffle furnace and heated to 350~450℃ and kept at that temperature for 10~15 hours to completely melt the molten salt, fully mix the components, and completely remove the water of crystallization.

5. The preparation method of the ternary mixed nitrate molten salt heat transfer and heat storage medium as described in claim 2, characterized in that, The particle size of the pulverized mixed molten salt powder is 20~200 mesh.

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