Silicon-based nanofluid phase change latent heat energy storage medium for solar photo-thermal power station

Abstract

The invention discloses a silicon-based nanofluid phase change latent heat energy storage medium for a solar photo-thermal power station. The silicon-based nanofluid phase change latent heat energy storage medium comprises 35%-40% of high-purity 5N spherical silicon, 50%-55% of a phase change material, 3.5%-4.5% of multilayer graphene, 1.0%-1.5% of nano-copper, 5%-8% of high-purity 4N crystalline flake graphite and 0.5%-1.0% of a coupling agent. Wherein the coupling agent improves the interface bonding of the spherical silicon and the PCM; high-purity 5N spherical silicon is used as a porous framework, the high specific surface area and nanoscale pores of the high-purity 5N spherical silicon are used for adsorbing a phase change material, the problem of traditional PCM leakage is solved, and wide-temperature-range coverage is achieved through a high-temperature-section phase change material, a low-temperature-section phase change material, a temperature-zone-divided multi-stage energy storage strategy and layered loading of the spherical silicon; the multilayer graphene forms a three-dimensional heat conduction network through ultrasonic dispersion, the heat conduction performance is improved, graphene interlayer gaps are filled with nano-copper, the interface thermal resistance is reduced, the local rapid heat transfer capacity is directionally improved, a micron-sized crystalline flake graphite protection layer is formed on the surface of the composite material through chemical vapor deposition, and the oxidation resistance is improved.

Description

Technical Field

[0001] The present invention relates to the technical field related to solar thermal power plants, specifically a phase change latent heat energy storage medium for a silicon-based nanofluid solar thermal power plant. Background Art

[0002] Solar thermal power generation is a new way of utilizing solar energy, which has the advantage of providing a high-quality power supply through heat storage. The heat transfer medium exchanges heat with water to generate steam, which then drives a steam turbine to do work and generate electricity. The heat transfer medium in the evaporation system of a solar thermal power plant is generally molten salt, whose operating temperature is generally in the range of -30 to 550 °C (actual heating is required for antifreeze), and the thermal conductivity is 0.2 W / (m·K), with relatively low thermal conductivity and low energy storage density. Therefore, it is urgent to develop a phase change latent heat energy storage medium with high energy storage, high weather resistance, and wide temperature range to solve the existing problems. Summary of the Invention

[0003] To solve the defects existing in the prior art, the present invention provides a phase change latent heat energy storage medium for a silicon-based nanofluid solar thermal power plant.

[0004] To solve the above technical problems, the present invention provides the following technical solutions:

[0005] The phase change latent heat energy storage medium of the silicon-based nanofluid solar thermal power plant of the present invention is prepared from raw materials with the following mass fractions:

[0006] High-purity 5N spherical silicon 35% - 40%, phase change material 50% - 55%, multi-layer graphene 3.5% - 4.5%, nano copper 1.0% - 1.5%, high-purity 4N flake graphite 5% - 8%, and coupling agent 0.5% - 1.0%;

[0007] The phase change material includes a high-temperature phase change material and a low-temperature phase change material. The high-temperature phase change material accounts for 60% of the total mass of the phase change material, and the low-temperature phase change material accounts for 40% of the total mass of the phase change material.

[0008] As a preferred technical solution of the present invention, the high-temperature phase change material is modified paraffin, and the low-temperature phase change material is composite calcium chloride.

[0009] As a preferred technical solution of the present invention, the preparation method of the medium provided by the present invention includes the following steps:

[0010] Step 1: Pickle and activate high-purity 5N spherical silicon to obtain a spherical silicon skeleton with high porosity;

[0011] Step 2: Load the phase change material onto the spherical silicon skeleton, and use the vacuum impregnation method to stepwise fill the high-temperature / low-temperature phase change materials;

[0012] Step 3: Conduct material compounding. Graphene / nano-copper hybrid slurry (ethanol solvent) is ball-milled with the silicon matrix loaded with phase change material.

[0013] Step 4: Form a micron-scale flaky graphite protective layer on the surface of the composite material through chemical vapor deposition.

[0014] As a preferred technical solution of the present invention, the method for pickling and activating high-purity 5N spherical silicon is to soak it in a mixed solution of HNO3:HCl = 3:1 for 2 hours, rinse it with deionized water until neutral, and then dry it in vacuum at 120°C for 6 hours to obtain a spherical silicon skeleton with a porosity of 88% - 90%.

[0015] As a preferred technical solution of the present invention, the specific operation method of Step 2 is: put the pretreated spherical silicon skeleton into modified paraffin, conduct a primary vacuum impregnation, with a vacuum degree ≤ 10 -3 Pa and a temperature of 80°C to make the impregnation rate ≥ 92%, and then conduct a secondary impregnation in a composite calcium chloride hydrate, with a vacuum degree ≤ 10 -2 Pa and a temperature of 25°C, and add 3wt% nucleating agent such as BaCO3 to inhibit supercooling.

[0016] As a preferred technical solution of the present invention, the method for ball-milling and compounding the graphene / nano-copper hybrid slurry (ethanol solvent) with the silicon matrix loaded with phase change material in Step 3 is: add graphene and nano-copper into the ethanol solvent in a mass ratio of 3:1, and conduct ultrasonic treatment to form a uniform slurry. During the ultrasonic dispersion process, utilize the hydrogen bond interaction between the hydroxyl groups (-OH) on the surface of the silicon skeleton and the oxygen-containing groups (-COOH) of graphene to induce the alignment of graphene along the heat flow direction, and the thermal conductivity anisotropy ratio reaches 5:1.

[0017] As a preferred technical solution of the present invention, the specific operation of Step 4 is to introduce methane / hydrogen (CH4:H2 = 1:4) at 600°C, grow a dense graphite layer with a thickness of 1 - 2μm on the surface of the composite material, and then cool it down to 200°C under nitrogen protection.

[0018] The beneficial effects of the present invention are:

[0019] This kind of phase change latent heat energy storage medium for a silicon-based nanofluid solar thermal power station uses high-purity 5N spherical silicon as a porous framework, utilizes its high specific surface area and nano-scale pores to adsorb the phase change material, solves the problem of leakage of traditional PCM, constructs a three-dimensional network structure through the sol-gel method, and the porosity > 85%; and by setting the phase change material in the high-temperature section and the phase change material in the low-temperature section, adopts a multi-stage energy storage strategy with temperature zones, and realizes wide temperature range coverage through the layered loading of spherical silicon; multi-layer graphene forms a three-dimensional heat conduction network through ultrasonic dispersion, improving the heat conduction performance. Among them, nano-copper fills the interlayer space of graphene, reduces the interface thermal resistance, and directionally improves the local rapid heat transfer ability. A micron-scale flake graphite protective layer is formed on the surface of the composite material through chemical vapor deposition, and the ultraviolet reflectivity > 92%, and the antioxidant life is extended to more than 15 years. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings are used to provide a further understanding of the present invention, and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation to the present invention.

[0021] In the drawings:

[0022] Figure 1 is a schematic diagram of the preparation process of the phase change latent heat energy storage medium for the silicon-based nanofluid solar thermal power station of the present invention;

[0023] Figure 2 is an analysis diagram of the comparative performance of the phase change latent heat energy storage medium for the silicon-based nanofluid solar thermal power station of the present invention;

[0024] Figure 3 is a schematic diagram of the comparative effect of the phase change latent heat energy storage medium for the silicon-based nanofluid solar thermal power station of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The following describes the preferred embodiments of the present invention with reference to the drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0026] Example 1: The phase change latent heat energy storage medium for the silicon-based nanofluid solar thermal power station of the present invention is prepared from raw materials with the following mass fractions:

[0027] 35% - 40% of high-purity 5N spherical silicon, 50% - 55% of phase change material, 3.5% - 4.5% of multi-layer graphene, 1.0% - 1.5% of nano-copper, 5% - 8% of high-purity 4N flake graphite, and 0.5% - 1.0% of coupling agent; among them, the coupling agent improves the interfacial bonding between spherical silicon and PCM.

[0028] The phase change material includes a high-temperature phase change material and a low-temperature phase change material. The high-temperature phase change material accounts for 60% of the total mass of the phase change material, and the low-temperature phase change material accounts for 40% of the total mass of the phase change material. Among them, high-purity 5N spherical silicon is used as the porous skeleton to adsorb the phase change material by utilizing its high specific surface area and nano-scale pores, solving the leakage problem of traditional PCM. A three-dimensional network structure is constructed by the sol-gel method, and the porosity > 85%; and by setting the high-temperature phase change material and the low-temperature phase change material, a multi-stage energy storage strategy for different temperature zones is adopted, and wide temperature range coverage is achieved through the hierarchical loading of spherical silicon; multi-layer graphene forms a three-dimensional heat conduction network through ultrasonic dispersion, improving the heat conduction performance. Among them, nano-copper fills the interlayer voids of graphene, reducing the interfacial thermal resistance and directionally improving the local rapid heat transfer ability. A micron-scale flake graphite protective layer is formed on the surface of the composite material by chemical vapor deposition, and the ultraviolet reflectivity > 92%, and the anti-oxidation life is extended to more than 15 years.

[0029] Through the pore gradient design of spherical silicon (large pores in the upper layer are filled with low-temperature PCM, and small pores in the lower layer are filled with high-temperature PCM), automatic hierarchical storage of thermal energy by temperature zone is realized, and the charge-discharge efficiency is increased by 25%. Graphene axial heat conduction + nano-copper transverse reinforcement: The heat transfer direction is controllable, avoiding local heat accumulation caused by the isotropic nature of traditional composite materials.

[0030] Among them, the high-temperature phase change material is a modified paraffin fatty acid eutectic salt (the melting point can be adjusted to 50 - 60 °C), and the low-temperature phase change material is composite calcium chloride, with a latent heat ≥ 180 J / g and a melting point of -40 °C. High temperature: ≥ 200 J / g (including the graphene thermal conductivity enhancement effect) Low temperature: ≥ 160 J / g The heat storage density is increased by 40% - 60% compared with traditional molten salts. The composite calcium chloride phase change material breaks through the -30 °C solidification limit of traditional molten salts, solving the problem of paralysis of the energy storage system in northern winters. Modified paraffin + graphene heat dissipation network, avoiding high-temperature expansion and leakage, meeting the needs of desert / tropical regions.

[0031] Among them, as Figure 1 shown, its preparation method includes the following steps:

[0032] Step 1: Pickle and activate high-purity 5N spherical silicon to obtain a spherical silicon skeleton with high porosity; suppress the volume expansion of PCM through the elastic deformation of the spherical silicon skeleton (Young's modulus ≥ 50 GPa) (the capacity retention rate > 95% after 1000 cycles); control the pore distribution of spherical silicon by acid etching (macropores of 50 - 100 nm adsorb PCM, and micropores enhance capillary adsorption force), achieving a PCM loading rate > 92%.

[0033] Step 2: Load the phase change material onto the spherical silicon skeleton, and fill the high-temperature / low-temperature phase change materials step by step by the vacuum impregnation method;

[0034] Step 3: Conduct material compounding. Graphene / nano-copper hybrid slurry (ethanol solvent) is ball-milled with the silicon matrix loaded with phase change material; the graphene-nano-copper composite interface provides toughness support, and the fracture toughness is ≥4 MPa·m at -60°C. 1 / 2 The slurry is ball-milled and mixed with the silicon skeleton loaded with PCM (rotation speed 300 rpm, time 2 h) to form a continuous heat conduction path.

[0035] Step 4: Form a micron-scale flake graphite protective layer on the surface of the composite material by chemical vapor deposition.

[0036] Among them, the method for pickling and activating high-purity 5N spherical silicon is to soak it in a mixed solution of HNO3:HCl = 3:1 for 2 hours, rinse it with deionized water until neutral, and then dry it in vacuum at 120°C for 6 hours to obtain a spherical silicon skeleton with a porosity of 88% - 90%.

[0037] Among them, the specific operation method of Step 2 is to put the pretreated spherical silicon skeleton into the modified paraffin for a first vacuum impregnation, with a vacuum degree ≤10 -3 Pa and a temperature of 80°C to make the impregnation rate ≥92%, and then conduct a second impregnation in the composite calcium chloride hydrate, with a vacuum degree ≤10 -2 Pa and a temperature of 25°C, and add 3 wt% nucleating agent such as BaCO3 to inhibit supercooling.

[0038] Among them, the method for ball-milling and compounding the graphene / nano-copper hybrid slurry (ethanol solvent) with the silicon matrix loaded with phase change material in Step 3 is to add graphene and nano-copper to the ethanol solvent in a mass ratio of 3:1, and ultrasonically treat to form a uniform slurry. During the ultrasonic dispersion process, the hydrogen bond between the hydroxyl groups (-OH) on the surface of the silicon skeleton and the oxygen-containing groups (-COOH) of graphene is used to induce the alignment of graphene along the heat flow direction, and the thermal conductivity anisotropy ratio reaches 5:1 (axial vs radial); nano-copper is embedded between graphene layers through atomic diffusion (spacing 0.34 → 0.36 nm) to form a "pinning effect", and the interfacial thermal resistance is reduced to 10 -8 m 2 ·K / W (1 / 20 of traditional composite materials).

[0039] Among them, the specific operation of Step 4 is to introduce methane / hydrogen (CH4:H2 = 1:4) at 600°C to grow a 1 - 2 μm dense graphite layer on the surface of the composite material, and then cool it to 200°C under nitrogen protection to increase the surface ultraviolet reflectivity to 93%.

[0040] The specific formulation in this embodiment is as follows: (1) Formulation: spherical silicon 40%, graphene 4%, nano copper 1.2%, PCM 54.8%. (2) Preparation process: pickling and activation of the silicon skeleton (HNO3:HCl = 3:1); vacuum impregnation of PCM (80 °C, 10 -3 Pa); ultrasonic dispersion and compounding of graphene / nano copper. Performance data: latent heat 215 J / g, thermal conductivity 6.8 W / (m·K), capacity retention rate 96.2% after 1000 cycles; formation of a micron-scale flake graphite protective layer on the surface of the composite material by chemical vapor deposition.

[0041] Comparative example 1, formulation: silicon 40% + no graphene + no nano copper, where it is a pure silicon skeleton without thermal conductivity enhancement.

[0042] Comparative example 2, formulation: silicon 40% + graphene 4% + no nano copper, only graphene without interface optimization.

[0043] Comparative example 3, silicon 40% + no graphene + nano copper 5%, only nano copper without a dominant thermal network.

[0044] As Figure 2 shown in the data analysis, the synergistic thermal conductivity network of graphene and nano copper: graphene (high in-plane thermal conductivity) provides the main conduction path, and nano copper fills the interlayer gap to reduce the interface thermal resistance. Data shows that: the thermal conductivity of comparative example 2 (only graphene) is 2.1 W / (m·K), and that of example 1 (graphene + nano copper) is increased to 6.8 W / (m·K), with an increase of 224%. Mechanical strengthening: nano copper is embedded between graphene layers to inhibit the peeling of the sheets and improve the low-temperature toughness. Data shows that: the impact strength of comparative example 2 is 3.1 MPa·m 1 / 2 , and that of example 1 is increased to 4.5 MPa·m 1 / 2 , with an increase of 45%. B: The synergistic effect of the silicon skeleton and graphene on leakage suppression: the nano-pores of the silicon skeleton adsorb PCM, and the graphene coating layer further blocks the flow of liquid PCM. Data shows that: the leakage rate of comparative example 1 (pure silicon skeleton) is 12%, and that of example 1 (silicon + graphene) is <0.1%, a reduction of 99.2%. Conclusion: The comparative example data shows that only when silicon, graphene, and nano copper act synergistically in a specific ratio can a breakthrough effect of high thermal conductivity + zero leakage + wide temperature range be achieved, as Figure 3 shown.

[0045] Temperature range coverage:

[0046] Both traditional PCM and molten salt require external energy to maintain the temperature range (electric tracing / anti-freezing), while this solution realizes full-temperature-range self-adaptation through dual PCM hierarchical loading (calcium chloride + modified paraffin). Thermal conductivity efficiency:

[0047] The graphene-nano copper network boosts the thermal conductivity to 6.5 W / (m·K), and the charging and discharging rates are 30 times those of traditional PCMs, making it suitable for the rapid peak shaving requirements of industries and commerce.

[0048] Reliability:

[0049] The silicon skeleton has a porosity > 85% and is surface-coated with flake graphite, solving the industry problems of leakage of traditional PCMs and molten salt corrosion, and the outdoor lifespan reaches 15 years +.

[0050] Economy:

[0051] Although the material cost is higher than that of molten salt, the maintenance-free design + long lifespan reduce the total life cycle cost by 30% (calculated based on 10 years).

[0052] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. Phase change latent heat energy storage medium for a silicon-based nanofluid solar thermal power station, characterized in that, It is prepared from raw materials with the following mass fractions: High-purity 5N spherical silicon 35% - 40%, phase change material 50% - 55%, multi-layer graphene 3.5% - 4.5%, nano copper 1.0% - 1.5%, high-purity 4N flake graphite 5% - 8% and coupling agent 0.5% - 1.0%; The phase change material includes a high-temperature phase change material and a low-temperature phase change material. The high-temperature phase change material accounts for 60% of the total mass of the phase change material, and the low-temperature phase change material accounts for 40% of the total mass of the phase change material.

2. The phase change latent heat energy storage medium of the silicon-based nanofluid solar thermal power station according to claim 1, wherein The high-temperature phase change material is modified paraffin, and the low-temperature phase change material is composite calcium chloride.

3. The phase change latent heat energy storage medium for a silicon-based nanofluid solar thermal power station according to any one of claims 1-2, characterized in that, Its preparation method includes the following steps: Step 1: Pickle and activate high-purity 5N spherical silicon to obtain a spherical silicon skeleton with high porosity; Step 2: Load the phase change material onto the spherical silicon skeleton, and use the vacuum impregnation method to stepwise fill the high-temperature / low-temperature phase change materials; Step 3: Conduct material compounding. The graphene / nano copper mixed slurry is ball-milled with the silicon matrix loaded with the phase change material; during the ultrasonic dispersion process, the hydrogen bond between the hydroxyl group (-OH) on the surface of the silicon skeleton and the oxygen-containing group (-COOH) of graphene is used to induce the alignment of graphene along the heat flow direction, and the thermal conductivity anisotropy ratio reaches 5:1; Step 4: Form a micron-scale flake graphite protective layer on the surface of the composite material by chemical vapor deposition.

4. The phase change latent heat energy storage medium for a silicon-based nanofluid solar thermal power station according to claim 3, characterized in that, The method for pickling and activating high-purity 5N spherical silicon is to soak it in a mixed solution of HNO3:HCl = 3:1 for 2 hours, rinse it with deionized water until neutral, and then dry it in vacuum at 120°C for 6 hours to obtain a spherical silicon skeleton with a porosity of 88% - 90%.

5. The phase change latent heat energy storage medium of the silicon-based nanofluid solar thermal power station according to claim 3, characterized in that, The specific operation method of the said step 2 is to put the pretreated spherical silicon skeleton into modified paraffin for a primary vacuum impregnation with a vacuum degree ≤ 10 -3 Pa and a temperature of 80 °C to make the impregnation rate ≥ 92%, and then conduct a secondary impregnation in composite calcium chloride hydrate with a vacuum degree ≤ 10 -2 Pa and a temperature of 25 °C, and add 3 wt% nucleating agent such as BaCO3 to inhibit supercooling.

6. The phase change latent heat energy storage medium for a silicon-based nanofluid solar thermal power station according to claim 3, wherein The method for ball-milling the graphene / nano copper mixed slurry with the silicon matrix loaded with the phase change material in Step 3 is to add graphene and nano copper to an ethanol solvent in a mass ratio of 3:1, and ultrasonically treat it to form a uniform slurry. During the ultrasonic dispersion process, the hydrogen bond between the hydroxyl group (-OH) on the surface of the silicon skeleton and the oxygen-containing group (-COOH) of graphene is used to induce the alignment of graphene along the heat flow direction, and the thermal conductivity anisotropy ratio reaches 5:

1.

7. The phase change latent heat energy storage medium for a silicon-based nanofluid solar thermal power station according to claim 3, wherein The specific operation of Step 4 is to introduce methane / hydrogen (CH4:H2 = 1:4) at 600°C, grow a 1 - 2μm dense graphite layer on the surface of the composite material, and then cool it to 200°C under nitrogen protection.

Images