Phase change energy storage material for solar heat storage and preparation method thereof

Abstract

The invention provides a phase change energy storage material for solar heat storage and a preparation method thereof, and belongs to the technical field of phase change energy storage materials.The phase change energy storage material for solar heat storage is prepared from, by weight, 80-85 parts of sodium acetate trihydrate, 17-18 parts of modified expanded graphite and 4-5 parts of a modified nucleating agent; the modified expanded graphite is prepared by carrying out gradient high-temperature treatment on expandable graphite; the modified nucleating agent is prepared by treating potassium fluoborate with a silane coupling agent and then dispersing the treated potassium fluoborate in ethanol. The enthalpy value of the prepared phase change energy storage material can be effectively improved, and the cycle attenuation rate of the phase change energy storage material is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of phase change energy storage materials, and particularly relates to a phase change energy storage material for solar heat storage and a preparation method thereof. Background Art

[0002] Phase change is the process in which the physical phase (solid, liquid, gas) of a substance changes. Taking water as an example, water has three states: water vapor, water, and ice, corresponding to the gas phase, liquid phase, and solid phase respectively. Gradually heating normal-temperature water, when it reaches 100 °C, the water will gradually boil, emitting a large number of bubbles until it completely evaporates. This process of liquid-phase water becoming gas-phase water is the gas-liquid phase change. Putting normal-temperature water into the freezer of a refrigerator and continuously cooling it, the water will gradually cool. When it reaches 0 °C, it starts to freeze and becomes a mixture of ice and water until it completely solidifies into a complete ice block. This process of liquid-phase water becoming solid-phase water is the liquid-solid phase change. Most of the various substances in nature exist in three aggregate states of solid, liquid, and gas, and can also undergo corresponding phase change processes.

[0003] In recent years, the utilization ratio of renewable energy and clean energy in fields such as electricity has been continuously increasing. However, the use of clean energy (such as wind energy, solar energy, biomass energy, and geothermal energy) is often restricted by supply intermittency, difficult storage, and poor stability. Phase change materials (PCMs) have characteristics such as high heat energy storage capacity and low cost, and are ideal carriers for clean energy conversion and storage. During the phase change process, the phase change material can store heat energy in the form of latent heat. Compared with other types of heat storage media (such as sensible heat and chemical reaction heat), latent heat is more efficient and stable.

[0004] The existing Chinese invention patent with publication number CN105950120A discloses a phase change energy storage material for solar heat storage, which is composed of 80 - 85% of sodium acetate trihydrate, 5 - 8% of a nucleating agent, and 7 - 15% of a shaping carrier. The nucleating agent is a mixture of ethanol and potassium fluorate salt, and the shaping carrier is expanded graphite. It can solve the problems of supercooling and phase separation existing in using sodium acetate trihydrate as a solar heat storage material. This phase change material not only uses potassium fluorate salt for crystal nucleation, but also uses the surface tension of the phase interface formed when ethanol contacts the phase change material for rapid nucleation. The combination of these two nucleation methods plays a role of mutual induction, avoiding the failure of a single nucleation method due to too high instantaneous temperature during actual use.

[0005] However, the porosity and specific surface area of expanded graphite will limit the adsorption capacity for molten sodium acetate, resulting in partial liquid leakage after cycling and too rapid attenuation of the enthalpy value. Moreover, the collapse of the expanded graphite framework will also affect the cycle stability and structural durability, resulting in too large an enthalpy decay (cycle decay rate) after cycling. Summary of the Invention

[0006] To solve the problems existing in the background art, the present invention provides a phase change energy storage material for solar heat storage and a preparation method thereof, which can effectively improve the enthalpy value of the prepared phase change energy storage material and reduce its cycle attenuation rate.

[0007] To achieve the above object, in the first aspect, the present invention provides a phase change energy storage material for solar heat storage, which comprises the following components by weight: 80-85 parts of sodium acetate trihydrate, 17-18 parts of modified expanded graphite, and 4-5 parts of modified nucleating agent;

[0008] The modified expanded graphite is prepared by subjecting expandable graphite to gradient high-temperature treatment;

[0009] The modified nucleating agent is prepared by treating potassium fluoroborate with a silane coupling agent and then dispersing it in ethanol.

[0010] Further, it comprises the following components: 82 parts of sodium acetate trihydrate, 17.5 parts of modified expanded graphite, and 4.5 parts of modified nucleating agent.

[0011] Further, the preparation method of the modified expanded graphite is as follows:

[0012] A1. Immerse the expandable graphite in a sulfuric acid solution for 2-5 minutes, then filter it to obtain acid-intercalated graphite;

[0013] A2. Under an inert atmosphere, heat the acid-intercalated graphite obtained in A1 at a rate of 8-10 °C / min to 790-810 °C, and maintain it for 20-25 minutes to promote the preliminary decomposition of the intercalation compound and form a microporous structure to obtain initially expanded graphite;

[0014] A3. Continue to heat to 1100 °C, and at the same time apply microwave irradiation of 750-800 W for 10-12 s, and rapidly cool it. Use thermal shock to expand the pore size, and use the penetrability of microwaves to heat to accelerate the release of interlayer gas to form macropores, while maintaining the connectivity of the pore structure, thus obtaining the modified expanded graphite.

[0015] Further, the expandable graphite has a specification of 100 mesh or 200 mesh.

[0016] Further, the concentration (mass fraction) of the sulfuric acid solution is 90-95%.

[0017] Further, the preparation method of the modified nucleating agent is as follows:

[0018] B1. Mix potassium fluoroborate and a silane coupling agent (KH-550) in a mass ratio of 1:(0.09-0.11), and stir at 60±2 °C to form a hydrophobic coating layer to reduce its affinity with polar sodium acetate, obtaining hydrophobic potassium fluoroborate;

[0019] B2. Mix ethanol with the hydrophobic potassium fluoroborate obtained in B1 at a mass ratio of (3.5 - 4):1, and then perform ultrasonic dispersion to break particle agglomeration and form a uniform dispersion system, thus obtaining the modified nucleating agent.

[0020] Furthermore, in B1, the stirring speed is 180 - 240 r / min, and the stirring time is 30 - 40 min.

[0021] Furthermore, in B2, the frequency of ultrasonic dispersion is 35 - 45 kHz, and the time of ultrasonic dispersion is 25 - 35 min.

[0022] In the second aspect, the present invention provides a preparation method of the above-mentioned phase change energy storage material for solar heat storage, including the following steps: heat sodium acetate trihydrate to melt it, first add the modified nucleating agent, then add the modified expanded graphite, stir evenly, and cool to obtain it.

[0023] This application has the following beneficial effects:

[0024] The modified expanded graphite of the present invention is prepared by subjecting expandable graphite to gradient high-temperature treatment, with an increase in the average pore size and an increase in the proportion of macropores; at the same time, the modified nucleating agent is prepared by subjecting potassium fluoroborate to hydrophobization treatment and then optimizing the dispersion to inhibit pore defects.

[0025] The silane coupling agent (KH-550) forms a hydrophobic layer on the surface of potassium fluoroborate, significantly reducing its affinity with polar sodium acetate; sodium acetate preferentially adsorbs into small pores through capillary action rather than being physically intercepted by macropores, thereby synergistically improving the utilization rate of the effective phase change material and synergistically increasing the enthalpy value.

[0026] The interception amount of macropores decreases, the amount of sodium acetate locked by pore collapse decreases; moreover, the hydrophobic potassium fluoroborate reduces the filling of macropores, avoiding pore collapse of macropores caused by thermal expansion differences (the collapse rate decreases), further reducing the permanently locked amount of sodium acetate, synergistically inhibiting enthalpy decay, and thus reducing the enthalpy decay rate after 1000 cycles. Description of the Drawings

[0027] Figure 1 、Comparison trend chart of the initial enthalpy value and the enthalpy value after 1000 cycles of the phase change energy storage materials prepared in Examples 1 - 3 and Comparative Examples 1 - 3 in the test examples of the present invention;

[0028] Figure 2 、Comparison trend chart of the 1000-cycle decay rate data of the phase change energy storage materials prepared in Examples 1 - 3 and Comparative Examples 1 - 3 in the test examples of the present invention. Detailed Embodiments

[0029] The following further elaborates on this application with reference to examples.

[0030] Except for special instructions, the raw materials in the examples and comparative examples of this application are all commercially available.

[0031] Example 1: (1) Preparation of modified expanded graphite, and the preparation method is as follows:

[0032] A1. Immerse expandable graphite in a sulfuric acid solution with a mass fraction of 92% for 3 minutes, then filter it out to obtain acid-intercalated graphite. The sulfuric acid solution with this concentration can ensure sufficient intercalation reaction and avoid excessive corrosion of the graphite structure. Expandable graphite (product number JT-KPZ-11, 200 mesh) is purchased from Qingdao Jintao Graphite Co., Ltd.

[0033] A2. Heat the acid-intercalated graphite obtained in A1 in an inert atmosphere to 800 °C at a rate of about 9 °C / min and maintain it for 22 minutes to promote the preliminary decomposition of the intercalation compound and form a microporous structure, obtaining primary expanded graphite.

[0034] A3. Continue to heat to 1100 °C to expand the pore size by thermal shock; at the same time, apply 780 W of microwave irradiation for 11 s to accelerate the release of interlayer gas by the penetrative heating of the microwave, forming macropores, while maintaining the connectivity of the pore structure; quickly cool (quenching with liquid nitrogen), and further expand the pore diameter and stabilize the pore structure by using the difference in thermal expansion coefficients between the graphite sheets and the gas; thus obtaining the modified expanded graphite.

[0035] (2) Preparation of modified nucleating agent, and the preparation method is as follows:

[0036] B1. Mix potassium fluoroborate and silane coupling agent (KH-550) according to a mass ratio of 1:0.1, stir at a temperature of about 60 °C, the stirring speed is 210 r / min, and the stirring time is 35 minutes to form a hydrophobic coating layer and reduce its affinity with polar sodium acetate, obtaining hydrophobic potassium fluoroborate.

[0037] Potassium fluoroborate (with a content of 99%) is purchased from Zhejiang Licheng New Materials Co., Ltd. KH-550 is purchased from Hangzhou Jessica Chemical Co., Ltd.

[0038] B2. Mix ethanol and the hydrophobic potassium fluoroborate obtained in B1 according to a mass ratio of 3.8:1, and then perform ultrasonic dispersion. The frequency of ultrasonic dispersion is 40 kHz, and the time of ultrasonic dispersion is 30 minutes to break the particle agglomeration and form a uniform dispersion system, thus obtaining the modified nucleating agent.

[0039] (3) A preparation method of a phase change energy storage material for solar thermal energy storage includes the following steps: By weight, heat 82 parts of sodium acetate trihydrate until it melts, first add 4.5 parts of the modified nucleating agent, then add 17.5 parts of the modified expanded graphite, stir evenly, and cool to obtain.

[0040] Example 2: The difference between this example and Example 1 lies in the different ratios of the raw materials sodium acetate trihydrate, modified nucleating agent, and modified expanded graphite.

[0041] Specifically, (1) Preparation of modified expanded graphite, and its preparation method is as follows:

[0042] A1. Immerse expandable graphite in a sulfuric acid solution with a mass fraction of 92% for 3 minutes, then filter it out to obtain acid-intercalated graphite. The sulfuric acid solution with this concentration can ensure sufficient intercalation reaction and avoid excessive corrosion of the graphite structure. Expandable graphite (product number JT-KPZ-11, 200 mesh) is purchased from Qingdao Jintao Graphite Co., Ltd.

[0043] A2. Heat the acid-intercalated graphite obtained in A1 in an inert atmosphere, raise the temperature to 800 °C at about 9 °C / min, and maintain it for 22 minutes to promote the preliminary decomposition of the intercalation compound and form a microporous structure, obtaining initially expanded graphite.

[0044] A3. Continue to raise the temperature to 1100 °C to expand the pore size by thermal shock; at the same time, apply 780 W of microwave irradiation for 11 s, and use the penetrability heating of the microwave to accelerate the release of the interlayer gas, forming macropores, while maintaining the connectivity of the pore structure; quickly cool (quench with liquid nitrogen), and use the difference in thermal expansion coefficients between the graphite sheets and the gas to further expand the pore diameter and stabilize the pore structure; thus, the modified expanded graphite is obtained.

[0045] (2) Preparation of modified nucleating agent, and its preparation method is as follows:

[0046] B1. Mix potassium fluoroborate and silane coupling agent (KH-550) in a mass ratio of 1:0.1, stir at a temperature of about 60 °C, the stirring speed is 210 r / min, and the stirring time is 35 minutes to form a hydrophobic coating layer and reduce its affinity with polar sodium acetate, obtaining hydrophobic potassium fluoroborate.

[0047] B2. Mix ethanol and the hydrophobic potassium fluoroborate obtained in B1 in a mass ratio of 3.8:1, and then perform ultrasonic dispersion. The frequency of ultrasonic dispersion is 40 kHz, and the ultrasonic dispersion time is 30 minutes to break the particle agglomeration and form a uniform dispersion system, thus obtaining the modified nucleating agent.

[0048] (3) A preparation method of a phase change energy storage material for solar heat storage, including the following steps: By weight, heat 80 parts of sodium acetate trihydrate until it melts, first add 4 parts of the modified nucleating agent, then add 17 parts of the modified expanded graphite, stir evenly, and cool to obtain.

[0049] Example 3: The difference between this example and Example 1 lies in the different ratios of the raw materials sodium acetate trihydrate, modified nucleating agent, and modified expanded graphite.

[0050] Specifically, (1) Prepare modified expanded graphite, and its preparation method is as follows:

[0051] A1. Immerse expandable graphite in a sulfuric acid solution with a mass fraction of 92% for 3 minutes, then filter it out to obtain acid-intercalated graphite. The sulfuric acid solution at this concentration can ensure sufficient intercalation reaction and avoid excessive corrosion of the graphite structure. Expandable graphite (product number JT-KPZ-11, 200 mesh) is purchased from Qingdao Jintao Graphite Co., Ltd.

[0052] A2. Heat the acid-intercalated graphite obtained in A1 in an inert atmosphere to 800 °C at a rate of about 9 °C / min and maintain it for 22 minutes to promote the preliminary decomposition of the intercalation compound and form a microporous structure, obtaining initially expanded graphite.

[0053] A3. Continue to heat to 1100 °C to expand the pore size by thermal shock; at the same time, apply microwave irradiation of 780 W for 11 s to accelerate the release of interlayer gas by the penetrative heating of microwaves to form macropores, while maintaining the connectivity of the pore structure; rapidly cool (quench with liquid nitrogen), and further expand the pore diameter and stabilize the pore structure by using the difference in thermal expansion coefficients between graphite sheets and gas; thus, the modified expanded graphite is obtained.

[0054] (2) Prepare modified nucleating agent, and its preparation method is as follows:

[0055] B1. Mix potassium fluoroborate and silane coupling agent (KH-550) in a mass ratio of 1:0.1, stir at a temperature of about 60 °C, with a stirring speed of 210 r / min and a stirring time of 35 minutes to form a hydrophobic coating layer and reduce its affinity with polar sodium acetate, obtaining hydrophobic potassium fluoroborate.

[0056] B2. Mix ethanol and the hydrophobic potassium fluoroborate obtained in B1 in a mass ratio of 3.8:1, and then perform ultrasonic dispersion at a frequency of 40 kHz for 30 minutes to break particle agglomeration and form a uniform dispersion system, thus obtaining the modified nucleating agent.

[0057] (3) A preparation method of a phase change energy storage material for solar heat storage includes the following steps: By weight, heat 85 parts of sodium acetate trihydrate until molten, first add 5 parts of the modified nucleating agent, then add 18 parts of the modified expanded graphite, stir evenly, and cool to obtain.

[0058] Comparative Example 1: The difference between this comparative example and Example 1 is that the modified expanded graphite is replaced by initially expanded graphite; and the modified nucleating agent is replaced by a nucleating agent (delete the silane coupling agent KH-550).

[0059] Specifically, (1) Prepare initially expanded graphite, and its preparation method is as follows:

[0060] A1. Immerse the expandable graphite in a sulfuric acid solution with a mass fraction of 92% for 3 minutes, then filter it to obtain acid-intercalated graphite. The sulfuric acid solution at this concentration can ensure sufficient intercalation reaction and avoid excessive corrosion of the graphite structure.

[0061] A2. Under an inert atmosphere, heat the acid-intercalated graphite obtained in A1 to 800 °C at a rate of about 9 °C / min and maintain it for 22 minutes to promote the preliminary decomposition of the intercalation compound and form a microporous structure, obtaining the initially expanded graphite.

[0062] (2) Prepare the nucleating agent. The preparation method is as follows: Mix ethanol and potassium fluoroborate in a mass ratio of 3.8:1, and then carry out ultrasonic dispersion. The frequency of ultrasonic dispersion is 40 kHz, and the time of ultrasonic dispersion is 30 minutes to obtain the nucleating agent.

[0063] Comparative Example 2: The difference between this comparative example and Example 1 is that the modified nucleating agent is replaced with a nucleating agent (delete the silane coupling agent KH-550).

[0064] Specifically, (2) Prepare the nucleating agent. The preparation method is as follows: Mix ethanol and potassium fluoroborate in a mass ratio of 3.8:1, and then carry out ultrasonic dispersion. The frequency of ultrasonic dispersion is 40 kHz, and the time of ultrasonic dispersion is 30 minutes to obtain the nucleating agent.

[0065] Comparative Example 3: The difference between this comparative example and Example 1 is that the modified expanded graphite is replaced with initially expanded graphite.

[0066] Specifically, (1) Prepare the initially expanded graphite. The preparation method is as follows:

[0067] A1. Immerse the expandable graphite in a sulfuric acid solution with a mass fraction of 92% for 3 minutes, then filter it to obtain acid-intercalated graphite. The sulfuric acid solution at this concentration can ensure sufficient intercalation reaction and avoid excessive corrosion of the graphite structure.

[0068] A2. Under an inert atmosphere, heat the acid-intercalated graphite obtained in A1 to 800 °C at a rate of about 9 °C / min and maintain it for 22 minutes to promote the preliminary decomposition of the intercalation compound and form a microporous structure, obtaining the initially expanded graphite.

[0069] Test Example: Test object: Phase change energy storage materials prepared in Examples 1 - 3 and Comparative Examples 1 - 3.

[0070] Test items and methods: ① Initial enthalpy value; ② 1000-cycle decay rate. Among them, ① The initial enthalpy value is the value obtained immediately after the phase change energy storage material is made; ② The 1000-cycle decay rate is the enthalpy decay rate of the phase change energy storage material after 1000 cycles.

[0071] Test results: See Table 1.

[0072] Table 1. Test Data of Test Examples

[0073]

[0074] Result Analysis: Analyze Examples 1 - 3 and combine with the data in Table 1 and Figure 1 - Figure 2 It can be seen that the enthalpy value of the phase change energy storage material prepared by the present invention (Examples 1 - 3) reaches above 261.3 J / g, and the decay rate after 1000 cycles is as low as below 1.5%.

[0075] Analyze Example 1 and Comparative Examples 1 - 3 and combine with the data in Table 1 and Figure 1 - Figure 2 , by comparing Comparative Example 1 and Comparative Example 2, it can be known that compared with Comparative Example 1, in Comparative Example 2, the virgin expanded graphite is replaced with the modified expanded graphite of the present invention alone, and as a result, the enthalpy value of the prepared phase change energy storage material significantly decreases instead of increasing, and the decay rate after 1000 cycles significantly increases instead of decreasing.

[0076] This is because compared with virgin expanded graphite, the average pore diameter of the modified expanded graphite increases, and the proportion of macropores increases. Although the porosity increases, the capillary force (inversely proportional to the pore diameter) is significantly weakened, resulting in that the molten sodium acetate is difficult to be effectively adsorbed by small pores, but is more physically intercepted by macropores rather than stably adsorbed; the retention ability of macropores for liquid sodium acetate is weak, and part of the material cannot participate in the crystallization / melting cycle during the phase change process, resulting in a decrease in the apparent enthalpy value. Moreover, the macroporous structure bears repeated thermal stresses during long-term thermal cycling (1000 times), and the weak van der Waals force between graphite lamellae is difficult to maintain the macroporous morphology, resulting in pore collapse and locking of sodium acetate; the locking ratio is positively correlated with the pore diameter, and the collapsed macropores form closed spaces, and the internal sodium acetate cannot be released, leading to an increase in the enthalpy decay rate.

[0077] By comparing Comparative Example 1 and Comparative Example 3, it can be known that compared with Comparative Example 1, in Comparative Example 3, the nucleating agent is replaced with the modified nucleating agent of the present invention (modified with silane coupling agent KH-550) alone, and as a result, the enthalpy value and the decay rate after 1000 cycles of the prepared phase change energy storage material change little.

[0078] Combined with Example 1 for comparison, it can be known that replacing the virgin expanded graphite with the modified expanded graphite of the present invention and replacing the nucleating agent with the modified nucleating agent of the present invention (modified with silane coupling agent KH-550) can produce a synergistic effect, synergistically increasing the enthalpy value of the prepared phase change energy storage material and synergistically reducing the decay rate after 1000 cycles of the prepared phase change energy storage material.

[0079] In addition, it should be noted that in the above specific embodiments, the various specific technical features described can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0080] In addition, any combination can be made among various different embodiments of the present invention, as long as it does not violate the idea of the present invention, and it should equally be regarded as the content disclosed by the present invention.

Claims

1. A phase change energy storage material for solar thermal storage, characterized in that: The composition comprises the following components by weight: 80-85 parts of sodium acetate trihydrate, 17-18 parts of modified expanded graphite and 4-5 parts of modified nucleating agent; The modified expanded graphite is prepared by subjecting expandable graphite to a gradient high temperature treatment; The modified nucleating agent is prepared by treating potassium fluoroborate with a silane coupling agent and then dispersing the treated potassium fluoroborate in ethanol.

2. The phase change energy storage material for solar thermal storage according to claim 1, characterized in that: The invention comprises the following components: 82 parts of sodium acetate trihydrate, 17.5 parts of modified expanded graphite and 4.5 parts of modified nucleating agent.

3. The phase change energy storage material for solar thermal storage according to claim 1 or 2, characterized in that: The preparation method of the modified expanded graphite is as follows: A1. Soak expandable graphite in sulfuric acid solution for 2-5 minutes, filter it out, and obtain acid intercalated graphite; A2, heating the acid intercalated graphite obtained in A1 to 790-810°C at 8-10°C / min in an inert atmosphere, and maintaining the temperature for 20-25min to obtain primary expanded graphite; A3. Continue to heat up to 1100°C, apply 750-800W microwave irradiation for 10-12s, and quickly cool to obtain modified expanded graphite.

4. The phase change energy storage material for solar thermal storage according to claim 3, characterized in that: The specification of the expandable graphite is 100 mesh or 200 mesh.

5. The phase change energy storage material for solar thermal storage according to claim 3, characterized in that: The concentration of the sulfuric acid solution is 90-95%.

6. The phase change energy storage material for solar thermal storage according to claim 1 or 2, characterized in that: The preparation method of the modified nucleating agent is as follows: B1. Mix potassium fluoroborate and silane coupling agent (KH-550) in a mass ratio of 1:(0.09-0.11), and stir at 60±2°C to obtain hydrophobic potassium fluoroborate; B2. Mix ethanol and the hydrophobic potassium fluoroborate obtained in B1 in a mass ratio of (3.5-4):1, and then perform ultrasonic dispersion to obtain a modified nucleating agent.

7. The phase change energy storage material for solar thermal storage according to claim 6, characterized in that: In B1, the stirring speed is 180-240 r / min, and the stirring time is 30-40 min.

8. The phase change energy storage material for solar thermal storage according to claim 6, characterized in that: In B2, the frequency of ultrasonic dispersion is 35-45 kHz, and the time of ultrasonic dispersion is 25-35 min.

9. A method for preparing a phase change energy storage material for solar thermal storage according to any one of claims 1 to 8, characterized in that: The method comprises the following steps: heating and melting sodium acetate trihydrate, firstly adding a modified nucleating agent, then adding modified expanded graphite, stirring evenly, and cooling to obtain the product.

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