Composite heat storage material and preparation method thereof

Abstract

The invention provides a composite heat storage material and a preparation method thereof, and relates to the technical field of phase change heat storage materials. The composite heat storage material comprises the following raw materials in percentage by mole: 10-70% of sodium hydroxide, 1-5% of potassium hydroxide, 10-60% of sodium nitrate and 10-70% of sodium nitrite. The prepared composite heat storage material has the characteristics of stable thermal cycle performance, high phase change heat storage density and the like, and is suitable for the field of medium-temperature heat storage.

Description

Technical Field

[0001] The present invention relates to the technical field of phase change heat storage materials, and in particular to a composite heat storage material and a preparation method thereof. Background Art

[0002] As energy sources such as coal, oil, and natural gas become increasingly depleted, increasing energy utilization is a pressing technical challenge facing many scientists. In the energy conversion and utilization process, inconsistent supply and demand in time or space often arise, such as peaks and troughs in electricity demand, and the discontinuity of solar, wind, and ocean energy. Energy storage technology can utilize thermal storage materials to store energy, enabling conversion between different energies and separation of energy in time and space. This allows for the conversion of previously dispersed, intermittent, and low-density energy into controllable, adjustable, and usable energy, thereby achieving a balance between energy supply and demand. Therefore, thermal storage technology is an important way to improve energy efficiency.

[0003] Common energy storage methods include thermochemical, sensible, and latent. Thermochemical storage materials, such as CaSO₄ and CaC₂O₄, have the advantages of high energy storage density and long-distance transport. However, their disadvantages are their high technical complexity and relatively high cost, which may limit their application in certain fields. Sensible storage materials, such as H₂O, metals, and concrete, store and release thermal energy by increasing or decreasing the temperature of the material. These materials are abundant and low-cost, but their disadvantages are that the storage and release processes vary with ambient temperature, meaning the storage process is not constant. Furthermore, they suffer from low heat storage density and the large mass and volume of material required to store a given amount of energy, impacting the economics and applicability of the storage system. Latent storage materials, also known as phase change materials, such as nitrates and paraffin, utilize the absorption or release of energy during phase changes to store and release heat. Their advantages include high heat storage density, easily controllable phase change processes, and constant phase change temperatures. However, some materials also suffer from low thermal conductivity, easy decomposition at high temperatures, and high supercooling.

[0004] Therefore, there is an urgent need to develop a composite material using low-cost conventional materials with low phase transition temperature and high heat storage density. Summary of the Invention

[0005] In view of this, the present invention proposes a composite heat storage material with sodium hydroxide as a matrix material and a fixed component ratio, and a preparation method thereof.

[0006] In a first aspect, the present invention provides a composite heat storage material, which comprises the following raw materials, measured by molar percentage: 10-70% sodium hydroxide (NaOH), 1-5% potassium hydroxide (KOH), 10-60% sodium nitrate (NaNO3) and 10-70% sodium nitrite (NaNO2).

[0007] On the basis of the above technical solution, preferably, the composition comprises the following raw materials in molar percentage: 20-70% sodium hydroxide, 1-5% potassium hydroxide, 10-30% sodium nitrate and 10-40% sodium nitrite.

[0008] On the basis of the above technical solution, preferably, the mass purity of the constituent raw materials is greater than 99%.

[0009] On the basis of the above technical solution, preferably, the phase change temperature of the composite heat storage material is 180-260°C.

[0010] In a second aspect, the present invention relates to a method for preparing the above-mentioned composite heat storage material, comprising the following steps: preparing the constituent raw materials in molar percentage, grinding after mixing, heating and raising the temperature, keeping the temperature under first temperature conditions and second temperature conditions respectively, and cooling to obtain the composite heat storage material.

[0011] Based on the above technical solution, preferably, the heating rate is 8 to 10° C. / min.

[0012] On the basis of the above technical solution, preferably, the first temperature condition is 95-110° C., and the insulation time is 25-45 minutes.

[0013] On the basis of the above technical solution, preferably, the second temperature condition is 330-350° C., and the insulation time is 90-120 min.

[0014] In a third aspect, the present invention provides applications of composite heat storage materials in the field of industrial waste heat storage or solar thermal utilization.

[0015] The composite heat storage material and preparation method provided by the present invention have the following advantages over the prior art:

[0016] 1. The composite heat storage material prepared by the present invention is suitable for the field of medium-temperature heat storage (about 120-300°C), and has good thermal cycle performance and stability, as well as high phase change heat storage density and other characteristics.

[0017] 2. The composite heat storage material of the present invention can be applied to the fields of industrial waste heat storage, solar thermal utilization, etc.

[0018] 3. The composite heat storage material provided by the present invention is prepared by a melting method using four raw materials of sodium hydroxide, potassium hydroxide, sodium nitrate and sodium nitrite in a specific molar ratio. After DSC curve analysis, it is confirmed that the composite heat storage material prepared by the present invention has the advantages of high latent heat density, clear phase change temperature, and specific application temperature range. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 This is a DSC curve diagram of the eutectic composite heat storage material prepared in Example 1 of the present invention;

[0021] Figure 2 This is a DSC curve diagram of the eutectic composite heat storage material prepared in Example 2 of the present invention;

[0022] Figure 3 This is a DSC curve diagram of the eutectic composite heat storage material prepared in Example 3 of the present invention;

[0023] Figure 4 This is a DSC curve diagram of the eutectic composite heat storage material prepared in Example 4 of the present invention;

[0024] Figure 5 This is a DSC curve diagram of the eutectic composite heat storage material prepared in Example 5 of the present invention;

[0025] Figure 6 This is a DSC curve diagram of the non-eutectic composite heat storage material prepared in Example 6 of the present invention;

[0026] Figure 7 This is a DSC curve diagram of the non-eutectic composite heat storage material prepared in Example 7 of the present invention;

[0027] Figure 8 This is a DSC curve diagram of the non-eutectic composite heat storage material prepared in Comparative Example 1 of the present invention;

[0028] Figure 9 This is a DSC curve diagram of the non-eutectic composite heat storage material prepared in Comparative Example 2 of the present invention;

[0029] Figure 10 This is a DSC curve diagram of the non-eutectic composite heat storage material prepared in Comparative Example 3 of the present invention;

[0030] Figure 11 This is a DSC curve diagram of the non-eutectic composite heat storage material prepared in Comparative Example 4 of the present invention;

[0031] Figure 12 This is a DSC curve diagram of the eutectic composite heat storage material prepared in Comparative Example 5 of the present invention;

[0032] Figure 13This is a thermal cycle offset diagram of the eutectic composite heat storage material prepared in Example 4 of the present invention;

[0033] Figure 14 This is a thermal cycle overlay diagram of the eutectic composite heat storage material prepared in Example 4 of the present invention. DETAILED DESCRIPTION

[0034] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0035] Common alkaline solid-liquid phase change thermal storage materials include NaOH, KOH, and LiOH, while common nitrate solid-liquid phase change thermal storage materials include NaNO3, LiNO3, and KNO3. These materials all have the characteristics of high phase change thermal storage density, constant thermal storage density, and easy management. The phase change temperatures of NaOH, KOH, and LiOH are 318°C, 380°C, and 471°C, respectively, and the phase change enthalpies are 165 J / g, 150 J / g, and 380 J / g, respectively. In contrast, the phase change temperatures of sodium nitrate, lithium nitrate, and potassium nitrate are approximately 308°C, 264°C, and 334°C, respectively, and the phase change enthalpies are approximately 199 J / g, 370 J / g, and 110 J / g, respectively. While these single salts or alkaline materials have the advantage of high thermal storage density, they still suffer from high procurement costs and high phase change temperatures.

[0036] Binary eutectic salts can lower the phase transition temperature of a material without significantly affecting the phase transition enthalpy. For example, a NaOH-KOH composite material with a molar ratio of 1:1 has a phase transition temperature of 170°C and a phase transition enthalpy of 200 J / g; a LiOH-NaOH composite material with a molar ratio of 3:7 has a phase transition temperature of 210°C and a phase transition enthalpy of 270 J / g; and a LiNO3-KNO3 composite phase change material with a molar ratio of 3:7 has a phase transition temperature of 133°C and a phase transition enthalpy of 130 J / g. While these binary eutectic salts can indeed lower the phase transition temperature of a material, the cost of binary eutectic salts with high thermal storage density is still relatively high.

[0037] As the existing technologies are not as good as the inventors' expectations, the inventors made the present invention through further research.

[0038] Through research, the inventors discovered that sodium hydroxide heat storage materials are currently widely used in the preparation of composite molten salt phase change heat storage materials due to their good chemical stability, low phase change temperature, and low cost. For example, Chinese patent CN108003842 (a method for preparing a hexadecanol composite phase change heat storage material) and Chinese patent CN108467712 (a molten salt heat storage material) are also widely used. Nitrate-based phase change heat storage materials are also widely used, such as Chinese patent CN118853097 (a hexavalent nitrate-based molten salt heat storage medium, its preparation method, and application) and Chinese patent CN116656325 (a nanofluid quaternary nitrate molten salt heat storage medium, its preparation method, and application). These preparation methods all have the characteristics of high phase change latent heat, narrow phase change temperature range, and low phase change temperature.

[0039] Therefore, the inventors aim to propose a NaOH-KOH-NaNO3-NaNO2 phase change heat storage material with sodium hydroxide as the matrix material and a fixed composition ratio, so as to be applied in the fields of industrial heat recovery and utilization, solar thermal utilization, etc.

[0040] The present invention provides a low-cost, low-melting-point, high-heat-storage-density composite thermal storage material comprising the following raw materials, measured by molar percentage: 10-70% sodium hydroxide, 1-5% potassium hydroxide, 10-60% sodium nitrate, and 10-70% sodium nitrite. Within this ratio range, the inventors discovered that the resulting composite thermal storage material exhibits both non-eutectic and eutectic states. However, while non-eutectic materials possess phase-change thermal storage capabilities, they suffer from a wide phase-change temperature range, an unclear application temperature range, and a high degree of supercooling, which can lead to untimely heat absorption and release.

[0041] To further develop a composite thermal storage material with high latent heat density, a well-defined phase transition temperature, and a specific application temperature range, the inventors experimentally determined that the four materials described above, at specific composition ratios, form a solid-phase mixture with the lowest melting point, a eutectic composite thermal storage material. The eutectic mechanism of materials lies in the interaction between the particles of the materials. The ions of the molten salt or alkali interfere with each other, altering the original lattice structure. At a certain ratio, this interference reaches an optimal state, resulting in the lowest lattice energy and a eutectic structure with a lower melting point. This solid-phase mixture lowers the melting point of the original material due to the hybridization effect between the components. The interference between the different materials in the molten state weakens the strong interionic electrostatic forces in a single salt crystal, making the crystal structure of the eutectic mixture more easily broken down than that of a single salt, resulting in a lower melting temperature. By further optimizing the above range, the resulting eutectic composite thermal storage material comprises, by mole percentage, 20-70% sodium hydroxide, 1-5% potassium hydroxide, 10-30% sodium nitrate, and 10-40% sodium nitrite.

[0042] The preparation method of the composite heat storage material of the present invention comprises the following steps: preparing the component raw materials by mole percentage, grinding after mixing, heating, keeping warm under first temperature condition and second temperature condition respectively, and cooling to obtain the composite heat storage material.

[0043] Among them, in the above steps, the heating rate is 8-10°C / min; the first temperature condition is 95-110°C, and the insulation time is 25-45 minutes; the second temperature condition is 330-350°C, and the insulation time is 90-120 minutes.

[0044] In the present invention, the post-mixing grinding step helps to evenly disperse the components and also refine the particle size. This not only improves the uniformity of the material, but also increases the phase interface area, which is beneficial to improving the heat conduction efficiency and reaction activity. At a heating rate of 8 to 10°C / min, it ensures that the energy input during the heating process will not be too fast to cause local overheating, and it also improves production efficiency. The heat preservation treatment under the first temperature condition can promote the preliminary reaction or dehydration of the precursor and other pretreatment processes, ensuring the effect of subsequent high-temperature treatment. Long-term heat preservation under the second temperature condition provides the composite material with sufficient energy to complete the required phase change, crystallization or other thermal activation processes, which is crucial to the formation of a stable composite heat storage material structure. The combined effect of the above steps helps to improve the thermal cycle stability of the final composite heat storage material, so that it maintains stable performance during multiple endothermic-exothermic processes.

[0045] The technical solution of the present invention is described in detail below with reference to specific embodiments. Unless otherwise specified, the sources of materials involved in the embodiments of the present invention are conventional commercial products.

[0046] Example 1

[0047] The composite heat storage material of this embodiment comprises the following raw materials, measured by molar percentage: 20% sodium hydroxide, 3% potassium hydroxide, 30% sodium nitrate, and 48% sodium nitrite. The preparation of the composite heat storage material comprises the following steps:

[0048] After drying the above raw materials, weigh the corresponding amounts of chemicals according to molar percentage and mix them thoroughly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min for 30 minutes to remove any water molecules absorbed by the sodium hydroxide during mixing with the other samples. After this temperature was reached, the temperature was raised to 340°C for 120 minutes. After cooling, composite thermal storage material A1 was obtained.

[0049] The composite heat storage material A1 prepared in this embodiment was tested using a synchronous thermal analyzer. The test results are as follows: Figure 1As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material A1 is 193°C, the phase transition enthalpy is 192 J / g, the DSC curve shows a single peak, and the analysis is a eutectic curve.

[0050] Example 2

[0051] The composite heat storage material of this embodiment comprises the following raw materials by molar percentage: 69% sodium hydroxide, 3% potassium hydroxide, 10% sodium nitrate, and 18% sodium nitrite. The preparation of the composite heat storage material comprises the following steps:

[0052] After drying the above raw materials, weigh the corresponding amounts of chemicals according to molar percentage and mix them thoroughly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min for 30 minutes. After this temperature was reached, the temperature was raised to 340°C for 120 minutes. After cooling, composite thermal storage material A2 was obtained.

[0053] The composite heat storage material A2 prepared in this embodiment was tested using a synchronous thermal analyzer. The test results are as follows: Figure 2 As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material A2 is 239°C, the phase transition enthalpy is 253 J / g, the DSC curve shows a single peak, and the analysis is a eutectic curve.

[0054] Example 3

[0055] The composite heat storage material of this embodiment comprises the following raw materials, by molar percentage: 48% sodium hydroxide, 4% potassium hydroxide, 28% sodium nitrate, and 20% sodium nitrite. The preparation of the composite heat storage material comprises the following steps:

[0056] After drying the above raw materials, weigh the corresponding amount of chemicals according to molar percentage and mix them evenly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min, holding for 30 minutes. After the heating period, the temperature was raised to 340°C and held for 120 minutes. After cooling, composite thermal storage material A3 was obtained.

[0057] The composite heat storage material A3 prepared in this embodiment was tested using a synchronous thermal analyzer. The test results are as follows: Figure 3 As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material A3 is 259°C, the phase transition enthalpy is 221J / g, the DSC curve shows a single peak, and the analysis is a eutectic curve.

[0058] Example 4

[0059] The composite heat storage material of this embodiment comprises the following raw materials, measured by molar percentage: 48% sodium hydroxide, 2% potassium hydroxide, 20% sodium nitrate, and 30% sodium nitrite. The preparation of the composite heat storage material comprises the following steps:

[0060] After drying the above raw materials, weigh the corresponding amounts of chemicals according to molar percentage and mix them thoroughly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min, holding for 30 minutes. After this temperature was reached, the temperature was raised to 340°C and held for 120 minutes. After cooling, composite thermal storage material A4 was obtained.

[0061] The composite heat storage material A4 prepared in this embodiment was tested using a synchronous thermal analyzer. The test results are as follows: Figure 4 As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material A4 is 258°C, the phase transition enthalpy is 212 J / g, the DSC curve shows a single peak, and the analysis is a eutectic curve.

[0062] Example 5

[0063] The composite heat storage material of this embodiment comprises the following raw materials by molar percentage: 50% sodium hydroxide, 2% potassium hydroxide, 10% sodium nitrate, and 38% sodium nitrite. The preparation of the composite heat storage material comprises the following steps:

[0064] After drying the above raw materials, weigh the corresponding amounts of chemicals according to molar percentage and mix them thoroughly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min, holding for 30 minutes. After this temperature was reached, the temperature was raised to 340°C and held for 120 minutes. After cooling, composite thermal storage material A5 was obtained.

[0065] The composite heat storage material A5 prepared in this embodiment was tested using a synchronous thermal analyzer. The test results are as follows: Figure 5 As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material A5 is 261°C, the phase transition enthalpy is 217 J / g, the DSC curve shows a single peak, and the analysis is a eutectic curve.

[0066] Example 6

[0067] The composite heat storage material of this embodiment comprises the following raw materials by molar percentage: 20% sodium hydroxide, 3% potassium hydroxide, 10% sodium nitrate, and 67% sodium nitrite. The preparation of the composite heat storage material comprises the following steps:

[0068] After drying the above raw materials, weigh the corresponding amount of chemicals according to molar percentage and mix them evenly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 110°C at a rate of 10°C / min for 25 minutes. After the heating period, the temperature was raised to 330°C for 120 minutes. After cooling, the composite thermal storage material A6 was obtained.

[0069] The composite heat storage material A6 prepared in this embodiment was tested using a synchronous thermal analyzer. The test results are as follows: Figure 6 As shown in the figure, it can be seen from the analysis that the phase transition temperature of the composite heat storage material A6 is 188-229°C, the phase transition enthalpy is 198J / g, the DSC curve shows a broad peak and multi-peak characteristics, and the analysis result is a non-eutectic curve.

[0070] Example 7

[0071] The composite heat storage material of this embodiment comprises the following raw materials by molar percentage: 20% sodium hydroxide, 2% potassium hydroxide, 58% sodium nitrate, and 20% sodium nitrite. The preparation of the composite heat storage material comprises the following steps:

[0072] After drying the above raw materials, weigh the corresponding amounts of chemicals according to molar percentage and mix them thoroughly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 95°C at a rate of 10°C / min for 45 minutes. After this temperature was reached, the temperature was raised to 350°C and held for 90 minutes. After cooling, the composite thermal storage material A7 was obtained.

[0073] The composite heat storage material A7 prepared in this embodiment was tested using a synchronous thermal analyzer. The test results are as follows: Figure 7 As shown in the figure, it can be seen from the analysis that the phase transition temperature of the composite heat storage material A7 is 185-232°C, the phase transition enthalpy is 197J / g, the DSC curve shows a broad peak and multi-peak characteristics, and the analysis result is a non-eutectic curve.

[0074] Combining Examples 1 to 7, it can be found that: First, although the raw materials are all sodium hydroxide, potassium hydroxide, sodium nitrate, and sodium nitrite, due to different component ratios, the phase change materials prepared using the same process are not necessarily all eutectic materials. Composite heat storage material A1-5 can be seen to be a eutectic material, and the eutectic point and phase change enthalpy of the eutectic material are also different. Therefore, whether the material is a eutectic material, its eutectic point and phase change temperature are all determined by the specific material properties and composition. Therefore, when Examples 1 to 5 further control the raw material ratio of the composite heat storage material to 20-70% sodium hydroxide, 1-5% potassium hydroxide, 10-30% sodium nitrate, and 10-40% sodium nitrite, the materials are all eutectic materials. Although Examples 6 and 7 have better phase change heat storage capacity, the phase change temperature range is too large, the heat storage density is relatively low, the supercooling is large, and the material application temperature range is not specific, which easily leads to the heat storage material absorbing and releasing heat in a timely manner and failing to store heat efficiently.

[0075] Secondly, DSC curve analysis shows that the eutectic salts prepared in Examples 1-5 exhibit more stable phase transition temperatures, high heat capacity, and high efficiency, enabling more efficient heat storage and release, making them suitable for scenarios requiring efficient heat storage and release. Furthermore, the eutectic salts exhibit a relatively uniform and rapid phase transition, facilitating rapid heat storage and release.

[0076] Comparative Example 1

[0077] The composite heat storage material of this comparative example comprises the following raw materials, calculated by molar percentage: 38% sodium hydroxide, 2% potassium hydroxide, and 60% sodium nitrate. The preparation thereof comprises the following steps:

[0078] After drying the above raw materials, weigh the corresponding amount of chemicals according to molar percentage and mix them evenly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min for 30 minutes. After the heating period, the temperature was raised to 340°C for 120 minutes. After cooling, composite thermal storage material B was obtained.

[0079] The DSC curve of the composite heat storage material B prepared in this comparative example was tested using a synchronous thermal analyzer. The test results are as follows: Figure 8 As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material B is 243-271°C, the phase transition enthalpy is 226 J / g, the DSC curve shows a broad peak multi-peak feature, and the analysis result is a non-eutectic curve.

[0080] Comparative Example 2

[0081] The composite heat storage material of this comparative example comprises the following raw materials, calculated by molar percentage: 40% sodium hydroxide and 60% sodium nitrite. Its preparation comprises the following steps:

[0082] After drying the above raw materials, the corresponding mass of the reagents was weighed according to molar percentage and mixed evenly in a nickel crucible. The crucible containing the reagents was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min and held for 30 minutes. After the holding period, the temperature was raised to 320°C and held for 120 minutes. After cooling, composite thermal storage material C was obtained.

[0083] The DSC curve of the composite heat storage material C prepared in this comparative example was tested using a synchronous thermal analyzer. The test results are as follows: Figure 9 As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material C is 231-269°C, the phase transition enthalpy is 266.4 J / g, the DSC curve shows a broad peak multi-peak feature, and the analysis result is a non-eutectic curve.

[0084] Comparative Example 3

[0085] The composite heat storage material of the comparative example comprises the following raw materials, calculated by molar percentage: 58% sodium hydroxide, 4% potassium hydroxide, and 38% sodium nitrite. The preparation thereof comprises the following steps:

[0086] After drying the above raw materials, the corresponding amounts of chemicals were weighed according to molar percentage and mixed thoroughly in a nickel crucible. The crucible containing the chemicals was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min for 30 minutes. After this temperature was reached, the temperature was raised to 340°C for 120 minutes. After cooling, composite thermal storage material D1 was obtained.

[0087] The DSC curve of the composite heat storage material D1 prepared in this comparative example was tested using a synchronous thermal analyzer. The test results are as follows: Figure 10 As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material D is 234-270°C, the phase transition enthalpy is 289 / g, the DSC curve shows a broad peak multi-peak feature, and the analysis result is a non-eutectic curve.

[0088] Comparative Example 4

[0089] The composite heat storage material of the comparative example comprises the following raw materials, calculated by molar percentage: 78% sodium hydroxide, 4% potassium hydroxide, and 18% sodium nitrite. The preparation thereof comprises the following steps:

[0090] After drying the above raw materials, the corresponding mass of the reagents was weighed according to molar percentage and mixed thoroughly in a nickel crucible. The crucible containing the reagents was placed in a KF1100 box furnace and heated from room temperature to 105°C at a rate of 8°C / min, then held for 30 minutes. After the holding period, the temperature was raised to 320°C and held for 120 minutes. After cooling, the composite thermal storage material D2 was obtained.

[0091] The DSC curve of the composite heat storage material D2 prepared in this comparative example was tested using a synchronous thermal analyzer. The test results are as follows: Figure 11 As shown in the figure, it can be seen that the phase transition temperature of the composite heat storage material D2 is 230-248°C, the phase transition enthalpy is 251J / g, the DSC curve shows a broad peak multi-peak feature, and the analysis result is a non-eutectic curve.

[0092] Combining Comparative Examples 1 to 4, it can be found that: the composite heat storage materials B, C, D1, and D2 can be analyzed to show that although the raw materials are two or three of sodium hydroxide, potassium hydroxide, sodium nitrate, and sodium nitrite, due to their different compositions and proportions, the DSC curve is a multi-peak curve, and the material prepared under this composition is a non-eutectic material.

[0093] By comparing Examples 1 to 5 and Comparative Examples 1 to 4, it can be seen that the phase change temperature range of the eutectic composite heat storage materials prepared in Examples 1 to 5 can be in the range of 190-260°C, and a suitable phase change material can be selected according to the specific application environment. However, the phase change temperature range of the non-eutectic materials prepared in Comparative Examples 1 to 4 is in the range of 240-270°C, the application temperature range is small, and the specific application temperature range of a single material is unclear, which can easily lead to untimely heat absorption and heat release processes of the heat storage material, inability to efficiently store heat, and easy to cause large heat loss.

[0094] Taking the eutectic composite heat storage material prepared in Example 1 as an example, it has the following advantages compared to the composite heat storage materials prepared in Comparative Examples 1 to 4: First, the phase change temperature of 190°C is suitable for medium and low temperature heat storage needs, such as solar water heating systems, industrial waste heat recovery and other applications. These systems usually operate in a lower temperature range, so the materials in this temperature range can effectively store and release heat. Second, the 190°C material has a lower operating temperature than high-temperature materials, which reduces the heat loss of the system and the thermal stress of the equipment, thereby increasing safety and service life. Third, the phase change temperature of 190°C is lower, and it can absorb and store system heat earlier in application, which can maximize the utilization of thermal energy, especially for industrial processes applied in this temperature range.

[0095] The inventors took Example 4 as an example and conducted a stability test on the composite heat storage material prepared in Example 4. Based on the phase transition temperature of the composite heat storage material A4 being 260°C, the stability test method thereof included: a DSC test, with a heating process from 220°C to 290°C and a cooling process from 220°C to 220°C, and a heating and cooling process as a thermal cycle. The test results are shown in FIG. Figure 13 、 14 shown.

[0096] DSC curves after 100 thermal cycles show little difference in the phase transition temperature and enthalpy before and after the thermal cycles. The DSC-calculated enthalpy of the material was 190.6 J / g before the cycles and 189.3 J / g after the cycles. The heat loss before and after the cycles was only 0.6%, and the eutectic structure remained. This confirms that the NaOH-KOH-NaNO3-NaNO2 eutectic composite thermal storage material prepared in this invention possesses excellent thermal cycling stability.

[0097] In summary, the eutectic composite heat storage material prepared by the present invention has better structural stability under certain conditions, is not prone to phase separation, and has slow performance degradation after long-term use. It has the characteristics of stable heat storage performance and a certain phase transition temperature. The eutectic composite heat storage material prepared by the present invention can be widely used in the field of medium-temperature heat storage, and has obvious advantages in many fields such as solar thermal energy storage, industrial waste heat recovery, building energy conservation and temperature control, power systems and energy supply, heat exchange and transmission, and has good application prospects.

[0098] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A composite heat storage material, characterized in that: The invention comprises the following raw materials in terms of molar percentage: 10-70% of sodium hydroxide, 1-5% of potassium hydroxide, 10-60% of sodium nitrate and 10-70% of sodium nitrite.

2. The composite heat storage material according to claim 1, characterized in that The invention comprises the following raw materials in terms of molar percentage: 20-70% of sodium hydroxide, 1-5% of potassium hydroxide, 10-30% of sodium nitrate and 10-40% of sodium nitrite.

3. The composite heat storage material according to claim 2, characterized in that: The mass purity of the constituent raw materials is greater than 99%.

4. The composite heat storage material according to claim 3, characterized in that The phase change temperature of the composite heat storage material is 180-260°C.

5. A method for preparing the composite heat storage material according to any one of claims 1 to 4, characterized in that: The following steps are involved: The component raw materials are prepared in molar percentage, ground after mixing, heated, kept warm under a first temperature condition and a second temperature condition respectively, and cooled to obtain a composite heat storage material.

6. The method for preparing the composite heat storage material according to claim 5, wherein: The heating rate is 8-10°C / min.

7. The method for preparing the composite heat storage material according to claim 5, wherein: The first temperature condition is 95-110° C., and the insulation time is 25-45 minutes.

8. The method for preparing the composite heat storage material according to claim 5, wherein: The second temperature condition is 330-350° C., and the holding time is 90-120 minutes.

9. Use of the composite heat storage material according to any one of claims 1 to 4 in the field of industrial waste heat storage or solar thermal utilization.

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