A gas-solid coupled insulation material for ultra-high voltage molten salt electric heaters

Abstract

The application relates to a gas-solid coupling insulation material for an ultrahigh-voltage molten salt electric heater, which comprises solid mixed particles and a protective gas, the solid mixed particles are compounded by a nitride and magnesium oxide, the mass ratio of the nitride to the magnesium oxide is 1:1.1-5, the median particle size of the nitride ranges from 50 mu m to 400 mu m, the median particle size of the magnesium oxide ranges from 50 mu m to 400 mu m, and the protective gas is a protective gas with a dielectric strength higher than that of air. The application realizes comprehensive improvement of the insulation performance and the heat conduction performance of the filling material by regulating the spatial positional relationship of particles with different properties and introducing the protective gas, and can keep good insulation performance and heat conduction performance under different high-temperature environments (300 DEG C-800 DEG C); the mixed material can meet the requirements of the insulation material of a 6kV and above grade molten salt electric heater, and the cost of the mixed material is relatively low.

Description

Technical Field

[0001] This invention belongs to the field of molten salt thermal energy storage technology, and specifically relates to a gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters. Background Technology

[0002] Molten salt thermal energy storage technology is the preferred choice for large-scale, long-term, medium- and high-temperature thermal energy storage due to its advantages such as high energy density and good stability. Molten salt electric heaters are key equipment in the system's thermoelectric decoupling process. They can absorb excess green electricity generated from renewable energy sources, balancing power supply and demand, and can also heat molten salt to higher temperatures, significantly improving thermal storage capacity and quality.

[0003] Traditional electric heaters operate at low voltages, requiring a large number of heaters to meet megawatt-level heating demands. Currently, industrial plant voltage is typically 6 kV, while the power grid voltage is 10 kV. Traditional low-voltage heating methods necessitate step-down transformers to reduce the voltage to an even lower level before being used for molten salt heating, significantly increasing system operating costs. High-voltage electric heaters, on the other hand, can easily achieve megawatt-level heating power and can directly utilize plant voltage (6 kV) or grid voltage (10 kV), eliminating the need for the high-power transformers required in traditional low-voltage systems. Furthermore, as voltage levels increase, electricity prices decrease. Therefore, the development of high-voltage molten salt electric heating technology is urgently needed to improve system efficiency and reduce costs.

[0004] In high-voltage molten salt electric heaters, the insulation and thermal conductivity of the filler material are crucial for the stable operation of the heating element. A significant voltage difference exists between the heating wire and the metal casing, making it prone to breakdown. The excellent insulation properties of the filler powder effectively prevent this risk. Furthermore, the heat generated by the heating wire is transferred to the surface of the heating element through the filler layer; efficient thermal conductivity is key to ensuring rapid heat transfer and preventing localized overheating. Both factors together determine the safety and thermal efficiency of the equipment.

[0005] To address the issue of easy breakdown of filling materials in high-voltage heating elements, the main industrial solutions are as follows: On the one hand, electrical-grade magnesium oxide is used as the filling material for heating elements, but the effect is not ideal. Its insulation and thermal conductivity cannot meet the actual requirements of high-voltage heating elements for filling materials, making it difficult to meet the ultra-high voltage of 6 kV and above for electric heaters; on the other hand, the risk of breakdown is reduced by increasing the distance between the heating wire and the outer shell of the heating element, but this will reduce the internal thermal conductivity of the heating element, making it difficult for the heat generated by the heating wire to dissipate in time, which can easily cause melting. Summary of the Invention

[0006] To address the shortcomings of existing filler materials in terms of insulation and thermal conductivity, this invention provides a gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters. Based on the differences in median particle size (D50), particle morphology, and flowability of different filler materials, the stacking structure, electric field intensity distribution, and heat conduction path of the filler layer are synergistically designed and directionally controlled to achieve an optimal heat conduction path and the fewest potential breakdown paths. This results in a filler structure that maintains excellent insulation and thermal conductivity under different high-temperature environments.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A gas-solid coupling insulating material for an ultra-high voltage molten salt electric heater, the gas-solid coupling insulating material comprising solid mixed particles and a protective gas, wherein the solid mixed particles are composed of nitride and magnesium oxide compounded together, the mass ratio of nitride to magnesium oxide being 1:1.1 to 5, the median particle size of the nitride being 50µm to 400µm, the median particle size of the magnesium oxide being 50µm to 400µm, and the protective gas being a protective gas with a dielectric strength higher than that of air.

[0008] This invention selects metal nitrides and magnesium oxide as solid particle raw materials, and the two are compounded in a specific ratio. The high fluidity of magnesium oxide improves the packing performance of the nitrides and reduces the porosity of the filler layer. Simultaneously, the excellent insulation and thermal conductivity potential of the nitrides themselves enhances the overall performance of the mixed particles. By controlling the differences in the median particle size (D50), particle morphology, and fluidity of the two types of particles, the synergistic optimization of the filler layer packing structure, electric field intensity distribution, and heat conduction path is achieved, constructing a filler structure with optimal thermal conductivity and minimal potential breakdown paths.

[0009] A gas with a dielectric strength higher than that of air is selected as a protective gas and filled into the pores of solid mixed particles to replace the original air, thereby fundamentally suppressing the risk of air gap breakdown under high voltage; suppressing the breakdown effect of air gap under 6-10 kV high voltage, and realizing the insulation enhancement mechanism of gas-solid material coupling.

[0010] Preferably, the protective gas is carbon dioxide or sulfur hexafluoride, and the filling pressure of the protective gas is 0.5 to 5.0 bar.

[0011] Preferably, the nitride is boron nitride, aluminum nitride, or silicon nitride.

[0012] This invention quantitatively designs the filling gas pressure of the protective gas to achieve optimal synergy between insulation performance and engineering feasibility. When the porosity of the filling layer is low, the micropore volume is limited, and excessively high gas pressure can easily lead to powder rebound or affect the tube shrinking process. When the diameter of the electric heating tube is large, the equivalent electric field path length of the filling layer increases, and if the gas pressure is insufficient, it is difficult to adequately suppress air gap breakdown under high voltage. Therefore, it is necessary to combine structural parameters such as the porosity of the filling layer and the diameter of the electric heating tube to design the filling gas pressure in a targeted and reasonable manner, so as to maximize the insulation stability and thermal conductivity of the filling layer under high voltage conditions while ensuring process feasibility.

[0013] Preferably, when the porosity of the gas-solid coupling insulating material is ≤35%, the filling pressure of the protective gas is controlled to be <2.5 bar. With a low porosity (≤35%), effective gas replacement of the micropores can be achieved using a lower pressure (<2.5 bar).

[0014] Preferably, the gas-solid coupling insulating material is filled between the heating wire and the metal shell of the electric heating tube. The gas-solid coupling insulating material fills the space between the heating wire and the metal shell, ensuring that the heat generated by the heating wire is quickly transferred to the surface of the heating element without electrical breakdown, thus achieving electro-thermal conversion.

[0015] Preferably, the diameter of the electric heating tube is ≥30mm, and the filling pressure of the protective gas is controlled between 2.5 and 5.0 bar. As the diameter of the electric heating tube increases to 30mm and above, the filling pressure of the protective gas is appropriately increased (to 2.5 to 5.0 bar) to ensure an overall improvement in the dielectric strength of the air gap inside the filling layer; while meeting the requirement of increasing the breakdown voltage, the upper limit of the gas pressure is controlled to avoid adverse effects on tube shrinkage, sealing, and long-term stability.

[0016] Preferably, the porosity of the gas-solid coupling insulating material is ≤30%, the diameter of the electric heating tube is <20mm, and the filling pressure of the protective gas is controlled to be <2.0bar.

[0017] In this invention, porosity primarily affects the micropore volume of the filling layer, gas replacement efficiency, and the feasibility of the tube shrinking process; tube diameter primarily affects the equivalent electric field path length of the filling layer and the insulation requirements under high voltage. Therefore, the filling gas pressure needs to be designed synergistically, considering both porosity and tube diameter, to effectively suppress air gap breakdown and improve insulation performance while ensuring process feasibility. For structures with low porosity and small tube diameter, a lower gas pressure can be used; while for structures with larger tube diameter, the gas pressure needs to be appropriately increased.

[0018] Preferably, the mass ratio of the nitride to magnesium oxide is 1:1.4 to 4.1.

[0019] Preferably, the working voltage of the gas-solid coupling insulating material is 6 to 10 kV, and the working temperature is 300 to 800°C.

[0020] The beneficial effects of this invention are as follows: 1. Significantly improved insulation performance: By optimizing the compounding of metal nitride and magnesium oxide, the porosity of the filling layer is reduced, and the air gap is replaced by protective gas, which significantly improves the breakdown voltage of the material, meeting the requirements of 6-10kV ultra-high voltage conditions and effectively avoiding the risk of high voltage breakdown. 2. Simultaneous optimization of thermal conductivity: The dense packing of compound particles and the filling of air gaps by protective gas reduce contact thermal resistance, construct an efficient heat conduction path, and significantly improve thermal conductivity, ensuring rapid heat transfer and avoiding local overheating and melting. 3. Excellent high-temperature stability: Within a wide temperature range of 300 to 800℃, the breakdown voltage and thermal conductivity of the material change little, with the breakdown voltage difference not exceeding 13% and the thermal conductivity difference not exceeding 10%, making it suitable for the high-temperature working environment of molten salt electric heaters. 4. High feasibility of the project: The filling pressure of the protective gas is quantitatively designed according to the porosity and pipe diameter, taking into account both insulation performance and process feasibility, avoiding adverse effects on tube shrinkage, sealing and long-term stability, and the material cost is low, which makes it suitable for large-scale promotion and application. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the filling in Embodiment 1 of the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be further described in detail below through embodiments. These embodiments are illustrative of the present invention and not intended to limit the present invention. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] Unless otherwise specified, the experimental methods described in the embodiments are conventional methods; unless otherwise specified, the reagents and materials are commercially available.

[0024] Example 1 Boron nitride with a median particle size of 75µm and 35.4wt% and magnesium oxide with a median particle size of 200µm and 64.6wt% were thoroughly mixed and filled between the electric heating wire 1 and the metal shell 2. The diameter of the electric heating tube was 16mm. After the powder was compacted, CO2 protective gas was introduced at a pressure of 1.6 bar. The resulting gas-solid coupling insulating material filling structure for ultra-high voltage molten salt electric heaters is as follows. Figure 1As shown, the porosity is 29%. Breakdown tests were conducted according to GB / T 1408.1-2016, with breakdown and thermal conductivity tests performed at high temperatures of 300℃, 550℃, and 800℃. At 300℃, the material exhibited a breakdown voltage of 11.7 kV and a thermal conductivity of 1.54 W / (m·K). At 550℃, the breakdown voltage was 10.9 kV and the thermal conductivity was 1.46 W / (m·K). At 800℃, the breakdown voltage was 10.2 kV and the thermal conductivity was 1.39 W / (m·K). This invention demonstrated good thermal conductivity at representative high-temperature points (such as 300℃, 550℃, and 800℃), verifying that the material retains good thermal conductivity under high-temperature conditions.

[0025] Example 2 Boron nitride with a median particle size of 150 µm and 20.1 wt% and magnesium oxide with a median particle size of 200 µm and 79.9% were thoroughly mixed. The electric heating tube had a diameter of 40 mm. After compacting the powder, a CO2 protective gas was introduced at a pressure of 4.7 bar to obtain a gas-solid coupling insulating material with a porosity of 26% for use in ultra-high voltage molten salt electric heaters. Breakdown and thermal conductivity tests were conducted at 300 °C, 550 °C, and 800 °C. At 300 °C, the material exhibited a breakdown voltage of 11.1 kV and a thermal conductivity of 1.49 W / (m·K). At 550 °C, the breakdown voltage was 10.4 kV and the thermal conductivity was 1.36 W / (m·K). At 800 °C, the breakdown voltage was 9.8 kV and the thermal conductivity was 1.29 W / (m·K).

[0026] Example 3 30 wt% boron nitride and 70 wt% magnesium oxide were thoroughly mixed, and other parameters were as described in Example 1. The breakdown voltage and thermal conductivity were tested at a high temperature of 300°C. The material had a breakdown voltage of 10.8 kV and a thermal conductivity of 1.48 W / (m·K) at 300°C.

[0027] Example 4 40 wt% boron nitride and 60 wt% magnesium oxide were thoroughly mixed, and other parameters were as described in Example 1. The breakdown voltage and thermal conductivity were tested at a high temperature of 300°C. The material had a breakdown voltage of 11.3 kV and a thermal conductivity of 1.51 W / (m·K) at 300°C.

[0028] Example 5 50 wt% boron nitride and 50 wt% magnesium oxide were thoroughly mixed, and other parameters were as described in Example 1. The breakdown voltage and thermal conductivity were tested at a high temperature of 300°C. The material had a breakdown voltage of 7.6 kV and a thermal conductivity of 1.12 W / (m·K) at 300°C.

[0029] Example 6 The gas pressure was 1.0 bar, and other parameters were as described in Example 1. The material had a breakdown voltage of 10.1 kV and a thermal conductivity of 1.43 W / (m·K) at 300°C.

[0030] Example 7 The gas pressure was 2.0 bar, and other parameters were as described in Example 1. The material had a breakdown voltage of 10.6 kV and a thermal conductivity of 1.46 W / (m·K) at 300°C.

[0031] Comparative Example 1 Magnesium oxide was completely filled between the resistance wire and the outer shell, and the powder was compacted. Other aspects were the same as in Example 1. The porosity was 25%. Breakdown and thermal conductivity tests were conducted at a high temperature of 300°C. The material had a breakdown voltage of 5.8 kV and a thermal conductivity of 0.89 W / (m·K) at 300°C.

[0032] Comparative Example 2 Boron nitride was completely filled between the resistance wire and the outer shell, and the powder was compacted. Other aspects were the same as in Example 1. The porosity was 69%. Breakdown and thermal conductivity tests were conducted at a high temperature of 300°C. The material had a breakdown voltage of 3.9 kV and a thermal conductivity of 0.94 W / (m·K) at 300°C.

[0033] Comparative Example 3 Without protective gas filling, and otherwise referring to Example 1, the material has a breakdown voltage of 9.4 kV and a thermal conductivity of 1.37 W / (m·K) at 300°C without protective gas filling.

[0034] The materials prepared in each embodiment and comparative example were subjected to breakdown tests at 300℃ according to the standard of GB / T 1408.1-2016, and their thermal conductivity and porosity were also tested. The test results are shown in Table 1. Table 1

[0035] As can be seen from Table 1, by controlling the spatial relationship between particles with different properties, the gas-solid coupling characteristics of the composite filler material system can be significantly improved, thereby achieving synergistic optimization of insulation and thermal conductivity. The insulation performance of the material can be improved from 5.8 kV for pure magnesium oxide and 3.9 kV for nitrides to 9.4 kV, and the thermal conductivity can be improved from 0.89 W / (m·K) for pure magnesium oxide and 0.94 W / (m·K) for nitrides to 1.37 W / (m·K), while the porosity is reduced to below 29%. After the protective gas is introduced, the air gap is effectively replaced by the highly insulating gas, the interfacial electric field intensity is homogenized, and the phenomenon of local electric field concentration is effectively suppressed, resulting in a significant improvement in insulation performance to 11.7 kV, an increase of 24.5%. At the same time, it promotes the dense packing of particles inside the filling layer and the effective construction of interfacial heat flow channels, further improving the thermal conductivity by 12.4%. Moreover, it can maintain good insulation and thermal conductivity under different high-temperature environments, with breakdown voltage and thermal conductivity at 550℃ and 800℃ still reaching 10.9 kV / 1.46 W / (m·K) and 10.2 kV / 1.39 W / (m·K), respectively. W / (m·K); Comparing Examples 1, 6, 7 and Comparative Example 3, it can be found that the protective gas pressure also has a significant impact on the results, and collaborative design is necessary.

[0036] In terms of insulation performance, some nitrides (such as boron nitride and aluminum nitride) have higher breakdown voltage and higher thermal conductivity than traditional magnesium oxide when used as filler materials. However, when actually filled into electric heating elements, the poor fluidity of nitrides leads to excessive porosity, and the air gaps in the filler layer cause their insulation performance to be inferior to that of traditional magnesium oxide. In terms of thermal conductivity, the air gaps in the nitride filler layer result in a large contact thermal resistance, and their thermal conductivity is poor.

[0037] This invention employs gas-solid mixing technology. By controlling the spatial relationship of particles with different properties (designing an appropriate mixing mass ratio based on different particle sizes and performing pressurized gas replacement to allow the particles to form a controllable dense packing and contact network within the filling layer, thereby achieving positional relationship control), the insulation and thermal conductivity of the filling material are comprehensively improved. The breakdown voltage increases from 5.8kV to 9.4kV, the insulation performance increases to 1.6 times the original value, and the thermal conductivity increases from 0.89W / (m·K) to 1.37. With a W / (m·K) rating, thermal conductivity is increased to 1.5 times the original value, while porosity is reduced to below 29%. Furthermore, the introduction of protective gas replaces the original air in the pores with protective gases such as CO2 and SF6 (electrone gases), further improving insulation performance by 24.5% and thermal conductivity by 12.4%. Under different high-temperature environments (300℃~800℃), the breakdown voltage difference does not exceed 13%, and the thermal conductivity difference does not exceed 10%, maintaining excellent insulation and thermal conductivity. This meets the insulation material requirements for molten salt electric heaters of 6kV and above, and the cost of the hybrid material is relatively low.

[0038] Finally, it should be noted that the above examples are merely some specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of this invention should be considered within the scope of protection of this invention.

Claims

1. A gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters, characterized in that: The gas-solid coupling insulating material comprises solid mixed particles and a protective gas. The solid mixed particles are composed of nitride and magnesium oxide, with a mass ratio of nitride to magnesium oxide of 1:1.1 to 5. The median particle size of the nitride ranges from 50µm to 400µm, and the median particle size of the magnesium oxide ranges from 50µm to 400µm. The protective gas is a protective gas with a dielectric strength higher than that of air.

2. The gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters according to claim 1, characterized in that: The protective gas is carbon dioxide or sulfur hexafluoride, and the filling pressure of the protective gas is 0.5 to 5.0 bar.

3. The gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters according to claim 1, characterized in that: The nitrides are boron nitride, aluminum nitride, or silicon nitride.

4. The gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters according to claim 2, characterized in that: When the porosity of the gas-solid coupling insulating material is ≤35%, the filling pressure of the protective gas should be controlled to be <2.5 bar.

5. The gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters according to claim 1, characterized in that: The gas-solid coupling insulating material is filled between the heating wire of the electric heating tube and the metal shell.

6. The gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters according to claim 5, characterized in that: The diameter of the electric heating tube is ≥30mm, and the filling pressure of the protective gas is controlled between 2.5 and 5.0 bar.

7. The gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters according to claim 5, characterized in that: When the porosity of the gas-solid coupling insulating material is ≤30%, the diameter of the electric heating tube is <20mm, and the filling pressure of the protective gas is controlled to be <2.0bar.

8. The gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters according to claim 1, characterized in that: The mass ratio of the nitride to magnesium oxide is 1:1.4 to 4.

1.

9. The gas-solid coupling insulating material for ultra-high voltage molten salt electric heaters according to claim 1, characterized in that: The working voltage of the gas-solid coupling insulating material is 6 to 10 kV, and the working temperature is 300 to 800℃.

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