Experimental study on the influence of preparation parameters on strengthening stability of phase change materials (PCMs)

Renewable Energy 146 (2020) 1867e1878

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Renewable Energy journal homepage: www.elsevier.com/locate/renene

Review

Experimental study on the influence of preparation parameters on strengthening stability of phase change materials (PCMs) Bin Yang, Jiemei Liu*, Yawei Song, Ning Wang, Han Li School of Energy and Environmental Engineering, Hebei University of Technology, Beichen District, Tianjin, 300401, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 March 2019 Received in revised form 6 August 2019 Accepted 8 August 2019 Available online 11 August 2019

The preparation process of three common nanocomposite phase change materials (NCPCMs) was studied by using the single-factor test method, and the influences of drying treatment, additives, ultrasonic power and time, base liquid volume and particle concentration on the stability of NCPCMs were analyzed. The results are as follows: (1) Drying treatment had the most significant effect on the stability of ZnO NCPCMs. Additive treatment has the most significant effect on the stability of Al2O3, and the improvement degree is about 9 times of CuO and 2 times of ZnO. (2) The ultrasonic power treatment method has the most significant effect on the stability of Al2O3 NCPCMs. For the sample with the worst preparation effect, the relative absorbance of the uniformly dispersed Al2O3 NCPCMs increased by 20.30%. (3) The stability of ZnO NCPCMs was significantly affected by ultrasonic time treatment and base liquid volume. Under the two preparation conditions, the relative absorbance of the uniformly dispersed ZnO NCPCMs increased by 65.67% and 38.24%, respectively, relative to the worst-performing samples. (4) The particle concentration factor has the most significant effect on the stability of CuO NCPCMs. At the same time, the optimal preparation scheme of NCPCMs was obtained. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Synthesis parameters Stability Energy consumption Nanocomposite phase change materials

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1868 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1869 2.1. Materials and instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1869 2.2. Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1869 2.3. Analytical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1869 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1870 3.1. Effect of drying treatment on stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1870 3.2. Effect of additives on stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1870 3.3. Effect of ultrasonic power on stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1871 3.4. Effect of ultrasonic time on stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1872 3.5. Effect of base fluid volume on stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1872 3.6. Effect of particle concentration on stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1874 3.7. Energy consumption analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1875 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1876 Author contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1877 Funding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1877 Conflicts of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1877 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1877 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1877

* Corresponding author. Tel.: 13920962527. E-mail addresses: [email protected] (B. Yang), [email protected] (J. Liu). https://doi.org/10.1016/j.renene.2019.08.052 0960-1481/© 2019 Elsevier Ltd. All rights reserved.

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1. Introduction Energy consumption problem caused by population expansion, economic growth and high quality of life is worsening. Meanwhile, environmental problems become more serious due to the massive use of fossil fuels and greenhouse gas emissions. The decarbonization of the energy sector can be made possible by integrating renewable energy resources with various thermal energy storage systems which possess round-trip efficiency of >96% [1,2]. Furthermore, thermal energy storage technology allows improved dispatch-ability of power output from concentrated solar power plants and increases the plant's annual capacity factor [3]. Therefore, the technology has become an important topic that the energy sector has to pay attention to. Latent heat energy storage is a technology that uses substances to absorb or release latent heat of phase change in the process of phase change for energy storage and release. It solves the contradiction between heat supply and demand. Due to the advantages of high energy density and constant temperature, latent heat energy storage has been widely used, such as refrigeration [4], textile [5], solar energy storage [6], building energy storage [7] and other fields. Therefore, it is of great significance to develop and utilize phase change materials to solve energy problems. Organic phase change materials have been widely considered due to their non-toxicity, low corrosively and undercooling, and good thermal reliability. Wang et al. [8] used a new UV photo induced dispersion polymerization method to prepare polymethyl methacrylate(PMMA)-based microencapsulated phase change material(MEPCM). Measured, melting point and freezing point of MEPCM were 60.4
C,50.6
C with 92.1 J/g and 95.9 J/g of the latent heat of fusion and latent heat of solidification. Zhang et al. [9] successfully prepared MECPMs by solution casting, in which the shell was made of polycarbonate and the core material was stearic acid. Cao [10] et al. prepared the MEPCM with paraffin as the core material and TiO2 as the shell by sol-gel method, and found that no leakage occurred during the phase transition. In order to improve the thermal properties of organic phase change materials, nanoparticles are often introduced. Ardekani [11] et al. studied the turbulence of Ag-water and SiO2-water NCPCMs, and constructed three different spiral coils and straight tubes. The results showed that the use of nanoparticles in straight tubes can effectively improve the heat transfer rate. Zargartalebi [12] et al. studied the heat transfer enhancement ability of nanoparticles in pin fin fin MCHS. It was found that the particle size and surface energy changed the overall situation of the heat removal process due to particle agglomeration and deposition. NCPCMs also contribute to the improvement of automotive radiators. According to the results of the theoretical analysis, the increase in convection heat transfer coefficient was determined to be 26.15% and 27.72% for 0.3%Ag -doped TiO2 NCPCMs with concentrations of 1% and 2%, respectively [13]. In order to improve thermal performance, the researchers studied techniques such as surface modification and nanoparticle suspension in the base fluid. Yagnem [14] et al. increased thermal performance and critical heat flux (CHF) by adding different concentrations of alumina and copper oxide. The thermal conductivity of a 0.1% hybrid NCPCMs at room temperature is increased by 15.72% compared to deionized water. However, due to the density difference between the particles and the base material, the NCPCMs appears to sink under the action of gravity. In addition, aggregation occurs under the action of van der Waals attraction force existing between particles, which reduces the high thermal conductivity of the NCPCMs, and the interaction between particles is the main aspect affecting the stability of the fluid. Karthikeyan [15] et al. tested the effect of NCPCMs stability on boiling heat transfer and found that only stable NCPCMs

can cool the disk faster. In addition, when a stable ethanol-based multi-walled carbon nanotube NCPCMs was used to cool a disk with a laser inscribed square column texture, a boiling heat transfer rate of 59% was observed. Therefore, in order to improve the thermal properties of NCPCMs, it is necessary to conduct an indepth study on its stability. In the preparation phase of the NCPCMs, the researchers used different ultrasonic power, ultrasonic time, particle concentration and base liquid volume to obtain samples with different properties, so these four factors directly affect the stability of the NCPCMs. Mahbubul et al. [16] reported that high-quality silicon carbide (SiC) -ethylene glycol (EG) NCPCMs can be obtained when ultrasonic processing is conducted for 2.5 h. Li et al. [17] prepared copper/ethylene glycol NCPCMs with different concentrations (1.0 wt %, 2.0 wt % and 3.8 wt %). The effect of ultrasound time (0e75min) on the stability of NCPCMs was investigated. The results showed that the lowest viscosity could be obtained by ultrasound treatment of NCPCMs with a concentration of 1.0 wt % and 2.0 wt % for 60min, and the lowest viscosity could be obtained by ultrasound treatment of NCPCMs with a concentration of 3.8 wt % for 45min. Cacua et al. [18] used fractional factor experiment 2 k-1 design to explore the effect of each factor and its coupling effect on the stability of NCPCMs. At the same time, it was found that the NCPCMs with the best stability was obtained when the concentration of 0.5 wt% Al2O3 and the surfactant CTAB were used, the ultrasonic energy (regulated by adjusting the vibration amplitude) was ranged between 30 and 50% and the ultrasonic treatment was conducted for 30min. Sarsam et al. [19] analyzed the effects of different ultrasonic time and surfactant concentration on the stability of NCPCMs, and found that the optimal ultrasonic time was 60min. Asadi et al. [20] also conducted the same study, and the results showed that the optimal stability of Mg(OH)2-distilled water could be obtained by ultrasonic treatment for 30min. Nabil et al. [21] found that the most stable NCPCMs could be obtained after ultrasonic treatment for 90min. Zou [22], Zhang [23], Tao [24], Gao [25] et al. selected ultrasonic time of 4 h, 30min, 3 h and 1 h to process the NCPCMs to obtain uniform samples. Mahbubul et Al [26]. studied the stability and rheological properties of NCPCMs and found that the optimal dispersion of Al2O3/water NCPCMs can be obtained when the ultrasonic time reaches 2 h. In the research process, Li et al. [17] not only obtained the optimal ultrasonic time, but also found that the higher concentration of NCPCMs led to the lower ultrasonic time, while the lower concentration of NCPCMs required more energy to disperse the particles to achieve the effect of low viscosity. Zhang et al. [27] studied the effect of ultrasonic power on palmitic acidstearic acid eutectic mixture (PA-SA) heat transfer rate. The results showed that the heat storage time of PCM decreases with the increase of ultrasonic power. When the ultrasonic power was 60 W, 105 W and 150 W, the heat transfer rate increased by 10.64%, 23.40% and 31.91%, respectively. It can be seen from the above studies that the preliminary preparation conditions have a great influence on the properties of the composite phase change materials. And the preparation power consumption is also high. In order to obtain the optimal preparation configuration of commonly used composite phase change materials at the present stage, reduce the preparation energy consumption and improve the CPCM performance, it is necessary to consider the comprehensive influence of the preparation parameters on the stability and total power consumption of the NCPCMs. Entropy method is an important method to assign the indicator weight [28]. In the process of environmental vulnerability assessment, it is necessary to comprehensively consider regional resources, environment, ecology and society. Zhang [29] proposed an improved entropy-based ecological environment vulnerability assessment model based on actual conditions. The results show

B. Yang et al. / Renewable Energy 146 (2020) 1867e1878

that the evaluation criteria are more reasonable. Li [30] also introduced the entropy weight method and the TOPSIS method to comprehensively evaluate the development of road transport capacity. The entropy method can profoundly reflect the utility value of the index information entropy value. At the same time, the index weight value determined by the entropy method has high credibility and precision. Single factor test method, orthogonal test method, entropy method and multi-index comprehensive evaluation method were used to study the ultrasonic preparation parameters for three common NCPCMs, and the optimal preparation scheme of NCPCMs with low energy consumption and uniform dispersion was obtained in this study.

2. Materials and methods 2.1. Materials and instruments In this study, 26# paraffin wax was selected as the matrix liquid of composite material. Span80 whose chemical component is sorbitol fatty acid ester, is a yellow viscous liquid and a surface-active dispersant. The three nanoparticles used in this study are CuO, Al2O3 and ZnO. The basic parameters of the materials provided by the manufacturer are shown in Table 1. The microstructure of the surface was analyzed by SEM, as shown in Fig. 1. The results showed that the micro shape of the CuO and Al2O3 are spherical and the micro shape of ZnO particle is flake. Most of them are strongly agglomerated together before mixing into the base fluid. The equipment used in the experiment includes a BSM220.4 electronic balance manufactured by Shanghai zhuojing electronic technology co., LTD., which is used to weigh the quantity of materials and reagents involved in the experiment, with a weighing accuracy of 0.0001 g and a maximum weighing range of 220 g. A VCY500 ultrasonic processor manufactured by Shanghai yongyan ultrasonic equipment & instrument co., ltd. is used for ultrasonic dispersion processing of composite phase-change materials, with a maximum ultrasonic power of 500 W. A thermostatic magnetic stirrer with models for 08-2 g and stirring speed of 100e1500 r/ min, provided by Shanghai meiyingpu magnetic stirrer instrument manufacturing co., LTD., is used to fully mixing materials and reagents whose liquid temperature control precision is ±1
C. A high temperature vacuum drying oven with the model for DZF6050, provided by the Shanghai boxun industry co., LTD., is used to dry material and reagent, and can realize the temperature control in the range of 5
Ce250
C. A scanning electron microscope, whose model is Phenom Pro, is used for observation and analysis of the micro-surface structure of Nano particles, provided by Phenom scientific instruments (Shanghai) co., LTD. The resolution of the equipment is better than 8 nm. A UV2800 uvevisible spectrophotometer, manufactured by Shanghai shunyu hengping scientific instrument co., LTD., is used to test the transient absorbance

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changes of phase change materials, with the allowable wavelength range of 190e1100 nm. Due to the limitations of the instrument itself, the maximum continuous measurement time is only 6000s. All beaker tubes used in the experiment were purchased from Hong sheng precision instruments. 2.2. Experimental procedure In this study, a two-step method was used to prepare NCPCMs. A certain amount of paraffin-based liquid and nanoparticles of corresponding concentration were weighed by an electronic balance. The nanoparticles were fully mixed with the base solution after stirred by magnetic force for 10min. Finally, evenly dispersed NCPCMs were obtained after certain ultrasonic treatment. The error analysis of test data directly determines the reliability of experimental results and inference. The error analysis of experimental data involved in this work is mainly the weighing error in the process of material preparation. Therefore, the mass error analysis [31] of the selected electronic balance is as follows:

dM ¼ 0.0001  1/2 ¼ 0.00005g

(1)

2.3. Analytical method At present, four methods have been developed to evaluate the dispersion of NCPCMs, such as particle size analysis, zeta potential analysis, gravity sedimentation method and spectral measurement method. In this work, the stability of the NCPCMs was measured using a UV2800 uvevisible spectrophotometer, which was used to test the transient absorbance changes of the phase-change materials. The uvevisible spectrophotometer is based on lambert beer's law. The lamp source of the instrument irradiates light of different wavelengths on the object under test. Due to the nature of the sample itself, part of the incident light energy will be absorbed, so that the beam energy passing through the sample will be reduced. The difference between the incident light energy and the transmitted light energy is inversely proportional to the concentration and thickness of the sample. During the test, due to the poor stability of the sample, the Nanoparticles agglomerates or sinks, thus causing a decrease in absorbance. Therefore, the stability of the sample is assessed by the recovery of light energy. In this work, relative absorbance is defined as the ratio of absorbed light energy Ia to incident light energy I, where the absorbance [32] is expressed as follows:

A ¼ log

I ¼ Klc I  Ia

(2)

where, A refers to the absorbance, K to the molar absorption

Table 1 Material parameter. Material Name

Purity Color

Partial Size (nm)

Specific Surface Area(m2/g)

Heat Conductivity Coefficient (W/ Density (g/ Melting Point (m ▪K)) ml) (
C)

Manufacturer

26 # paraffin CuO

98%

pale yellow 99.90% black

dd

dd

0.271

0.865

25.0

30e40

13

dd

dd

dd

ZnO

99.90% white

30e40

95

dd

dd

dd

Al2O3

99.90% white

30e40

60

dd

dd

dd

Fushun wenaixin technology co. LTD Beijing deco island gold technology co. LTD Beijing deco island gold technology co. LTD Beijing deco island gold technology co. LTD

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B. Yang et al. / Renewable Energy 146 (2020) 1867e1878

Fig. 1. Electron microscopy of nanoparticles.

coefficient of the substance, l to the absorption thickness of the test sample, and c to the concentration of the test sample.

the drying treatment has the most significant influence on the stability of ZnO.

3. Results and discussion

3.2. Effect of additives on stability

3.1. Effect of drying treatment on stability The nanoparticles and paraffin studied were divided into two groups. The first group was placed in a vacuum drying oven set at 100
C, dried for 4 h, mixed and then ultrasonically dispersed; another group was directly mixed and then ultrasonically dispersed. Fig. 2 shows the influence of drying treatment on relative absorbance of different materials. Fig. 2 (a) shows the result after drying treatment, and Fig. 2 (b) shows the result without treatment. Drying treatment has an effect on the stability of different NCPCMs. When the prepared material contains traces of water, the distribution charge of the nanoparticles changes [33]. The final performance is the aggregation and settlement of the particles. As can be seen from Fig. 2, more uniform and stable NCPCMs can be obtained after drying treatment for CuO and Al2O3; however, the drying result of ZnO is opposite. This may be because the water content causes the degree of particle surface potential changes to be different for different nanoparticles. At 4000s, the relative absorbance values of Al2O3, CuO and ZnO after drying were increased by 2.5%, 2.02% and 5.44%, respectively, compared with those before treatment. It can be seen that

The nanoparticles were divided into two groups in equal amounts. The first group was mixed with paraffin and then ultrasonically dispersed. The second group of mixed solution was added with a dispersant of SPAN80 with a mass fraction of 0.5% and then ultrasonically dispersed. Fig. 3 shows the effect of additive treatment on relative absorbance of different materials. Fig. 3 (a) shows the result after additive treatment, and Fig. 3 (b) shows the result without treatment. As can be seen from Fig. 3, for the three nanomaterials, more uniform and stable NCPCMs can be obtained after additive treatment. This is because the dispersant can prevent the particles from agglomerating by enhancing the steric resistance between them, thus enhancing the stability. At 4000s, the relative absorbance values of Al2O3, CuO and ZnO after additive treatment were 19.25%, 2.12% and 10.94% higher than those without treatment, respectively. It can be seen that the additive treatment has the most significant influence on the stability of Al2O3. Zhai [34] et Al. studied the influence of the type and the concentration of surfactant on the stability of pure Al2O3-EG NCPCMs. It was found that PVP surfactant showed the most uniform and stable NCPCMs state, while SDS surfactant showed a serious

Fig. 2. Variation of relative absorbance as a function of time at different materials. (a) Drying process (b) No drying.

Fig. 3. Variation of relative absorbance as a function of time at different materials. (a) Additive (b) Additive-free.

B. Yang et al. / Renewable Energy 146 (2020) 1867e1878

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agglomeration phenomenon. Therefore, the chemical structure and physical properties of NCPCMs should be taken into account when selecting surfactants.

3.3. Effect of ultrasonic power on stability Figs. 4e6 show the influence of ultrasonic power on the relative absorbance of Al2O3, CuO and ZnO, respectively. As can be seen from Fig. 5, with the increase of ultrasonic power, the relative absorbance increases, divided by the sample prepared at 50w. The relative absorbance curve corresponding to the preparation condition of 50 W is gentle and high. The reason may be that the energy is low and the Brownian motion is relatively weak. As a result, only a small number of dispersed particles are left, and a large number of large particles are gradually precipitated in the ultrasonic process. Therefore, the fluid in the cuvette contains a small number of evenly dispersed particles, which maintains a stable suspension state within the measurement time allowed by the instrument due to their small amount and light weight. Therefore, the relative absorbance maintains a high value during the test time. When the power is greater than 50 W, the relative absorbance increases with the increase of power. The reason may be that the diameter of the dispersed particles further decreases, which reduces the flow resistance between fluids and the viscosity. Relative absorbance increases with respect to small power (greater than 50 W). With the passage of time, the amount of small diameter particles increases and agglomeration occurs, which leads to the decrease of relative absorbance. As can be seen from Figs. 4 and 6, Al2O3 NCPCMs has the same change trend as ZnO NCPCMs. With the increase of ultrasonic power, the relative absorbance first increases and then decreases. When the ultrasonic power is greater than 150 W, the reason for the decrease of relative absorbance with the increase of power may be that the energy is large and the motion between particles is fierce, which leads to the enhancement of collision effect and the generation of agglomeration. Therefore, the relative absorbance decreases. For Al2O3 NCPCMs with ultrasonic power of 400 W, due to the most intense collision and the most serious sedimentation, the fluid in the cuvette contains a small number of evenly dispersed particles, which maintains a stable suspension state within the measurement time allowed by the instrument, due to their

Fig. 5. Variation of relative absorbance as a function of time at different ultrasonic power for CuO.

Fig. 6. Variation of relative absorbance as a function of time at different ultrasonic power for ZnO.

Fig. 4. Variation of relative absorbance as a function of time at different ultrasonic power for Al2O3.

minimal amount and light weight. Therefore, the relative absorbance was kept at a low value during the test time. For samples with power of 100 W and 50 W, the initial stabilization time is long. It is may be that the particles be in a stable state due to small diameter and low viscosity. With the passage of time, the number of small diameter particles increases, the distance between particles decreases, and the greater the power is, the more violent the particle movement is, and the faster the sedimentation speed is, resulting in a faster relative absorbance reduction. When the test time reaches 4500s, the increase in relative absorbance of CuO, Al2O3 and ZnO NCPCMs are shown in Table 2. It can be seen from the data that the ultrasonic power treatment method has the most significant influence on the stability of Al2O3 NCPCMs. The relationship between the ultrasonic power and the overall absorbance of the three NCPCMs can be clearly observed in Fig. 7. It can be seen that within the allowable range of the experimental instrument, the optimal. CuO, Al2O3 and ZnO NCPCMs can be obtained when the optimal ultrasonic power is 400 W, 150 W and 150 W, respectively.

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Table 2 The increase in relative absorbance of NCPCMs. Sample

50 W

100 W

150 W

200 W

300 W

400 W

CuO NCPCMs Al2O3 NCPCMs ZnO NCPCMs

2.62%

0%

1.82%

2.26%

2.61%

2.62%

16.95%

20.30%

0%

4.89%

6.71%

8.59%

11.07%

15.40%

0%

1.72%

2.77%

6.96%

Fig. 8. Variation of relative absorbance as a function of time at different ultrasonic time for Al2O3.

Fig. 7. Variation of overall absorbance as a function of ultrasonic power at different materials.

Mahbubul et al. [35] reached a similar conclusion. They found that within the ultrasonic time range under investigation, samples with ultrasonic treatment at 50% amplitude had a slightly larger zeta (z) potential than samples with ultrasonic treatment at 25% amplitude. In other words, with the increase of ultrasonic amplitude, a more stable NCPCMs can be obtained. 3.4. Effect of ultrasonic time on stability Figs. 8e10 shows the effect of ultrasonic time on the relative absorbance of Al2O3, CuO and ZnO, respectively. As can be seen from Fig. 9, with the increase of ultrasonic time, the relative absorbance first increases and then decreases. By using ultrasonic energy to break the agglomerated particles and disperse them evenly in the fluid base, the stability of the phase change material is improved, so the relative absorbance is improved. With the increase of ultrasonic time, the ultrasonic energy released from the acoustic wave will cause the temperature of the phase-change base solution to continue to rise. Once the ultrasonic time is too long, the nanoparticles in the high-temperature base liquid will move rapidly, which will enhance the collision effect and promote the agglomeration effect, which is not conducive to the stability of the NCPCMs phase change material, so the relative absorbance decreases. It can be seen from Figs. 8e10 that Al2O3 NCPCMs has the same change trend as ZnO NCPCMs. With the increase of ultrasonic time, the relative absorbance increases. When the test time reaches 4500s, the increase in relative absorbance of CuO, Al2O3 and ZnO NCPCMs are shown in Table 3. It can be seen from the data that the ultrasonic time processing

Fig. 9. Variation of relative absorbance as a function of time at different ultrasonic time for CuO.

method has the most significant influence on the stability of ZnO NCPCMs. The relationship between ultrasonic time and total absorbance of the three NCPCMs can be clearly observed from Fig. 11. And it can be seen that, within the range allowed by the experimental instrument, the optimal CuO, Al2O3 and ZnO NCPCMs can be obtained when the optimal ultrasonic time respectively is 2 h, 5 h, 5 h. Kole et al. [36] reached a similar conclusion. They found that evenly dispersed NCPCMs could be obtained after 60min of ultrasonic treatment, but with the extension of treatment time, the particles showed a slight tendency to reunite. 3.5. Effect of base fluid volume on stability Fig. 12 14 show the influence of base liquid volume on the relative absorbance of Al2O3, CuO and ZnO, respectively. As can be seen from Fig. 13, the relative absorbance first increases and then decreases with the increase of volume. The

B. Yang et al. / Renewable Energy 146 (2020) 1867e1878

Fig. 10. Variation of relative absorbance as a function of time at different ultrasonic time for ZnO.

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Fig. 12. Variation of relative absorbance as a function of time at different volume for Al2O3.

Table 3 The increase in relative absorbance of NCPCMs. Sample

0.5 h

1h

1.5 h

2h

2.5 h

5h

CuO NCPCMs Al2O3 NCPCMs ZnO NCPCMs

3.30%

2.19%

0.18%

0%

0.18%

0.77%

0%

16.84%

19.67%

30.00%

42.95%

49.02%

0%

40.30%

59.12%

59.67%

62.96%

65.67%

relative absorbance increases with the increase of volume when the volume is lower than 45 ml. The possible reason is that under a certain ultrasonic energy, the amount of particles that can be scattered by the output energy does not reach the limit within the concentration range studied when the volume is lower than 45 ml. Therefore, it can be broken into particles of smaller diameter with the increase of volume, which reduces the viscosity and makes the

Fig. 13. Variation of relative absorbance as a function of time at different volume for CuO.

Fig. 11. Variation of overall absorbance as a function of ultrasonic time at different materials.

particles more stable in suspension in the fluid. When the volume exceeds 45 ml, the absorbance decreases with the increase of the volume. The possible reason is that the energy reaches saturation at this time, and the ability of dispersing into small diameter is weakened, the number of small diameter particles decreases, the deposition is serious, and the relative absorbance is lower. The reason why the relative absorbance of 15 ml sample is lower than that of other samples may be that most of the particles in 15 ml sample are suspended in the fluid. When the volume increased, the amount of particles in the fluid dispersed increased. Meanwhile, the distance between molecules in each volume was the same, and the distance between molecules was greater than that in 15 ml. Therefore, the relative absorbance was greater than that of the sample with a volume of 15 ml. It can be seen from Figs. 12e14 that Al2O3 NCPCMs has the same change trend as ZnO NCPCMs. The relative absorbance decreases first and then increases with the increase of ultrasonic time. Taking Al2O3 NCPCMs as an example, the change trend of relative absorbance was analyzed. The relative absorbance decreases with the

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Fig. 14. Variation of relative absorbance as a function of time at different volume for ZnO.

increase of volume when the volume is lower than 30 ml. When the volume exceeds 30 ml, the relative absorbance increases with the increase of volume. At this point, the energy has reached saturation, that is, the output energy value is constant, the number of scattered particles is certain, and the diameter is the same. As the total volume of paraffin wax increases, the distance between particles increases, so the stable time increases and the relative absorbance increases. In the later stage of the test, there were two abnormal phenomena in the two NCPCMs, namely, the sedimentation velocity of 60 ml Al2O3 NCPCMs was slower than that of 75 ml, and the sedimentation velocity of 45 ml Al2O3 NCPCMs was slowed down. The reason may be that most of the particles have been deposited in the early stage. The suspended particles in the supernatant fluid have small concentration and viscosity, and are less likely to agglomerate compared with the large volume fluid. Therefore, the deposition speed is slowed down. When the test time reaches 4500s, the increase in relative absorbance of CuO, Al2O3 and ZnO NCPCMs are shown in Table 4. It can be seen from the data that the base liquid volume factor has the most significant influence on the stability of ZnO NCPCMs. From Fig. 15, we can clearly observe the relationship between the volume of three NCPCMs base liquids and the overall absorbance. And it can be seen that, within the allowed range of the experimental instrument, the optimal CuO, Al2O3 and ZnO NCPCMs can be obtained when the optimal base liquid volume is 45 ml, 15 ml and 75 ml respectively. 3.6. Effect of particle concentration on stability Figs. 16e18 show the influence of particle concentration on relative absorbance of Al2O3, CuO and ZnO, respectively. Table 4 The increase in relative absorbance of NCPCMs. Sample

15 ml

30 ml

45 ml

60 ml

75 ml

CuO NCPCMs Al2O3 NCPCMs ZnO NCPCMs

2.55%

0.48%

0%

0.36%

0.59%

4.95%

0%

2.46%

1.48%

1.94%

27.49%

28.93%

0%

33.92%

38.24%

Fig. 15. Variation of overall absorbance as a function of volume at different materials.

Fig. 16. Variation of relative absorbance as a function of time at different concentration for Al2O3.

It can be seen from Figs. 16e18 that the three NCPCMs have the same variation trend. As the concentration increases, the absorbance per concentration decreases. Taking the CuO NCPCMs as an example, the relative absorbance variation trend was analyzed. The reason for the above decreasing trend may be that the amount of particles in the fluid becomes more and more with the increase of concentration. In a certain space, the distance between particles decreases. Under the action of ultrasound, the particles collide violently, entangle and agglomerate, causing sedimentation. Therefore, the greater the concentration, the smaller the distance between particles, the more serious the collision phenomenon, and the lower the absorbance per concentration. We can also observe that the absorbance of the initial per concentration gradually decreases with the increase of the concentration, which may be because the larger the particle size, the more serious the particle aggregation and the more obvious the subsidence in the ultrasonic process. At the same time, it can be seen that the higher the concentration is, the longer the stabilization time is, and the faster the falling speed is in the later stage.

B. Yang et al. / Renewable Energy 146 (2020) 1867e1878

Fig. 17. Variation of relative absorbance as a function of time at different concentration for CuO.

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Fig. 19. Variation of overall absorbance as a function of concentration at different materials.

can be obtained when the optimal particle concentration is 0.1%. Ilyas et al. [37] also reached similar conclusions. They found that high concentration NCPCMs had higher average particle size, agglomerate size increased and then agglomeration occurred by studying the stability of nanotubes in hot oil at different concentrations. 3.7. Energy consumption analysis The above work only considers the influence of each factor on the stability of NCPCMs and the degree of influence on the stability of different materials. In order to obtain the optimal configuration, economic factors should also be considered. In this section, copper oxide NCPCMs are taken as an example to conduct this study by using the method of orthogonal experiment. At the same time, when evaluating the comprehensive effect of factors in the preparation stage on the stability and total power consumption of NCPCMs, multi-index assessment is used, which is represented by the comprehensive score. Among them, the flow chart of the multiindex comprehensive evaluation method is shown in Fig. 20. The specific implementation mode is shown in the following formula: Fig. 18. Variation of relative absorbance as a function of time at different concentration for ZnO.

When the test time reaches 4500s, the increase in relative absorbance of CuO, Al2O3 and ZnO NCPCMs are shown in Table 5. It can be seen from the data that the particle concentration factor has the most significant influence on the stability of ZnO NCPCMs. From Fig. 19, we can clearly observe the relationship between the concentration and the overall absorbance of three kinds of NCPCMs particles. It can be seen that within the range allowed by the experimental instrument, the optimal CuO, Al2O3 and ZnO NCPCMs

Table 5 The increase in relative absorbance of NCPCMs. Sample

0.1%

0.5%

1%

2%

3%

4%

CuO NCPCMs Al2O3 NCPCMs ZnO NCPCMs

0%

90.93%

95.15%

97.73%

99.20%

99.58%

0%

87.59%

92.77%

97.84%

98.56%

99.36%

0%

87.04%

95.62%

97.67%

95.84%

92.72%

The comprehensive score of a single experiment ¼ the total absorbance membership  the weight of the total absorbance þ the total power consumption membership  the weight of the total power consumption, (3)

The membership degree of indicators ¼

index value  index minimum index maximum  index minimum

(4)

The weight of the total absorbance and total power consumption was determined by the entropy method, and the calculation formula is as follows:

0 s1 B x11 M ¼ /B @ « sm xm1

/ « /

1 x1n C « C A xmn

(5)

sm Refers to the test number. Create a new matrix P with Pij ¼ Pxmij , which is the contribution of si , where, si is i-th scheme x i¼1 ij

under the j-th attribute, Then the weight f of the index is calculated according to the following formula:

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contribution of all schemes to the attribute Xj as follows:

Ej ¼ 

m   1 X pij ln pij lmn

(7)

i¼1

The weight of total absorbance and total power consumption of nanoparticles was calculated to be 0.000271 and 0.999783, respectively. The results of orthogonal experiment and comprehensive score are shown in Table 6. As can be observed from Table 6, the overall absorbance and total power consumption are used as the assessment indicators in this work. By analyzing the range R of each factor and each level, the order of influence on the above two indicators was as follows: ultrasonic power > ultrasonic time > particle concentration > base fluid volume. At the same time, the best scheme of copper oxide NCPCMs, obtained by using the comprehensive scoring method was as follows: The concentration was 0.1%, the ultrasonic power was 50 W, the ultrasonic time was 0.5 h, and the volume was 15 ml. The comprehensive score of the scheme was 1.0000. In order to select the experimental scheme with higher overall absorbance and lower total power consumption as much as possible, Fig. 20 is drawn. Fig. 20 shows the orthogonal experiment results, in which the right 1 axis is expanded by 7000 times to draw the right - 2 axis, so as to better observe and analyze the relationship between the total power consumption and the total absorbance. It is not difficult to find that the experimental scheme of the sixteenth group best meets our requirements, and the difference between the total absorbance and the total power consumption is the largest. The final optimal solution is consistent with the one selected by the multi-indicator method, which also demonstrates the accuracy of the analytical method we use. 4. Conclusions

Fig. 20. Flow chart of the evaluation method.

Fig. 21. Orthogonal experimental result.

dj f ¼P dj

(6)

In the formula, dj ¼ 1  Ej Where, Ej represents the total

In order to obtain the optimal preparation configuration of commonly used CPCMs at the present stage, reduce the preparation energy consumption and improve the CPCM performance, this work studied the preparation process of three common NCPCMs by using the single-factor test method. The effects of drying treatment, additives, ultrasonic power, ultrasonic time, base liquid volume, particle concentration and other factors on the stability of NCPCMs were analyzed. Meanwhile, orthogonal test method, multi-index comprehensive evaluation method and entropy method were used to explore the comprehensive influence of various factors on the stability and total power consumption of the copper oxide/ paraffin NCPCMs at the preparation stage, and the following conclusions were drawn: The drying treatment had the most significant effect on the stability of ZnO NCPCMs. Additive treatment had the most significant effect on the stability of Al2O3, and the improvement was about 9 times of CuO and 2 times of ZnO. The ultrasonic power treatment method had the most significant influence on the stability of Al2O3 NCPCMs. Within the range allowed by the experimental instrument, the optimal CuO, Al2O3 and ZnO NCPCMs can be obtained when the optimal ultrasonic power was 400 W, 150 W and 150 W respectively. And the relative absorbance of the three materials with uniform dispersion was increased by 2.62%, 20.30%, and 15.40%, respectively, relative to the sample with the worst preparation effect. The effect of ultrasonic time treatment on the stability of ZnO NCPCMs was most significant. Within the allowable range of the experimental instrument, the optimal CuO, Al2O3, ZnO NCPCMs can be obtained when the optimal ultrasonic time was 2 h, 5 h, 5 h, respectively. And the relative absorbance of the three materials with uniform dispersion was increased by 3.30%, 49.02%, and

B. Yang et al. / Renewable Energy 146 (2020) 1867e1878

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Table 6 Orthogonal experimental result. Test number

Ultrasonic Ultrasonic Concentration Volume Overall Total power Total absorbance time power absorbance consumption kW ▪ h membership

1 2 300 0.1 2 2 150 0.5 3 1 150 2 4 0.5 150 3 5 1 300 1 6 2 100 3 7 1 200 0.1 8 1 100 0.5 9 1.5 150 1 10 1.5 200 3 11 2.5 150 0.1 12 1.5 50 2 13 1 50 3 14 0.5 100 1 15 2 200 2 16 0.5 50 0.1 17 2.5 300 3 18 2.5 200 1 19 1.5 300 0.5 20 1.5 100 0.1 21 2.5 50 0.5 22 0.5 300 2 23 2.5 100 2 24 2 50 1 25 0.5 200 0.5 K1 4.621 4.6552 3.3108 K2 4.0691 4.1384 3.69 K3 3.5178 3.621 3.5522 K4 2.9659 3.1041 3.7591 K5 2.4145 2.0696 3.2761 Range R 2.2065 2.5856 0.483 Factor primary and P > t > C > V secondary Optimal C ¼ 0.1%,P ¼ 50 W,t ¼ 0.5 h,V ¼ 15 ml; Scheme Comprehensive evaluation:1.0000

30 45 15 75 75 15 45 60 30 60 60 45 30 45 75 15 45 15 15 75 75 60 30 60 30 3.3454 3.4829 3.2762 3.7591 3.7246 0.4829

4761.55 4729.28 5231.83 5169.12 5167.99 5229.1 5235.11 5206.91 5236.61 5236.49 5157.64 5080.44 4477.18 5231.26 5236.8 5235.95 5058.2 5128.77 5224.11 5227.21 5228.99 5235.84 5225.63 5211.86 5236.5

65.67%, respectively, relative to the sample with the worst preparation effect. The stability of CuO NCPCMs was most affected by the base liquid volume factor. Within the range allowed by the experimental instrument, the optimal CuO, Al2O3 and ZnO NCPCMs can be obtained when the optimal base liquid volume was 45 ml, 15 ml and 75 ml respectively. And the relative absorbance of the three materials with uniform dispersion was increased by 2.55%, 4.95%, and 38.24%, respectively, relative to the sample with the worst preparation effect. The stability of ZnO NCPCMs was most affected by particle concentration. Within the range allowed by the experimental instrument, the optimal CuO, Al2O3 and ZnO NCPCMs can be obtained when the optimal particle concentration is 0.1%. Compared with the samples with the worst preparation results, the relative absorbances of the three materials with uniform dispersion were increased by 99.58%, 99.36%, and 92.72%, respectively. The weight of total absorbance and total power consumption of nanoparticles is 0.000271 and 0.999783, respectively. The optimal preparation scheme of NCPCMs with low energy consumption and uniform dispersion was obtained: the concentration was 0.1%, the ultrasonic power was 50 W, the ultrasonic time was 0.5 h, and the volume was 15 ml. Author contributions Conceptualization, Jiemei Liu; Methodology, Jiemei Liu and Han Li; Validation, Ning Wang and Yawei Song; Writing e original draft, Jiemei Liu; Writing e review & editing, Jiemei Liu and Bin Yang.

0.6 0.3 0.15 0.075 0.3 0.2 0.2 0.1 0.225 0.3 0.375 0.075 0.05 0.05 0.4 0.025 0.75 0.5 0.45 0.15 0.125 0.15 0.25 0.1 0.1

0.3744 0.3319 0.9935 0.9109 0.9094 0.9899 0.9978 0.9607 0.9997 0.9996 0.8958 0.7942 0.0000 0.9927 1.0000 0.9989 0.7649 0.8578 0.9833 0.9874 0.9897 0.9987 0.9853 0.9672 0.9996

Total power consumption membership

Comprehensive evaluation

0.2069 0.6207 0.8276 0.9310 0.6207 0.7586 0.7586 0.8966 0.7241 0.6207 0.5172 0.9310 0.9655 0.9655 0.4828 1.0000 0.0000 0.3448 0.4138 0.8276 0.8621 0.8276 0.6897 0.8966 0.8966

0.2070 0.6206 0.8277 0.9311 0.6208 0.7587 0.7587 0.8966 0.7243 0.6208 0.5174 0.9310 0.9653 0.9656 0.4829 1.0000 0.0002 0.3450 0.4140 0.8277 0.8622 0.8277 0.6898 0.8966 0.8966

Funding This research was supported by the Natural Science Foundation of Hebei Province grant number [E2019202089]; Science and technology project of the Ministry of Housing and Urban-Rural Construction of China grant number [2018-K1-009]; Tianjin Key Research and Development Program grant number [18YFHBZC00030] and the National Key Research and Development Project of China grant number [2016YFC0700707]. Conflicts of interest The authors declare no conflict of interest. Acknowledgments This work was supported by the Natural Science Foundation of Hebei Province, Science and technology project of the Ministry of Housing and Urban-Rural Construction of China, Tianjin Key Research and Development Program, and the National Key Research and Development Project of China. References [1] K.M. Powell, J.S. Kim, W.J. Cole, K. Kapoor, J.L. Mojica, J.D. Hedengren, T.F. Edgar, Thermal energy storage to minimize cost and improve efficiency of a polygeneration district energy system in a real-time electricity market, Energy 113 (2016) 52e63. [2] I. Sarbu, C. Sebarchievici, A comprehensive review of thermal energy storage, Sustainability 10 (1) (2018) 191.

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