Advanced nanocomposite phase change material based on calcium

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Advanced nanocomposite phase change material based on calcium chloride hexahydrate with aluminum oxide nanoparticles for thermal energy storage Xiang Li, Yuan Zhou, Hongen Nian, Xinxing Zhang, Ouyang Dong, Xiufeng Ren, Jinbo Zeng, Chunxi Hai, and Yue Shen Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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Advanced nanocomposite phase change material based on calcium chloride hexahydrate with aluminum oxide nanoparticles for thermal energy storage Xiang Lia, b, *, Yuan Zhoua, b, *, Hongen Niana, b , Xinxing Zhanga,b , Ouyang Donga,b , Xiufeng Rena, b, Jinbo Zenga, b, Chunxi Haia,b ,Yue Shena, b a

Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China

b

Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining, 810008, China

Abstract The present study prepared nanocomposite phase-change materials (PCMs) based on calcium chloride hexahydrate (CaCl2·6H2O) with gamma aluminum oxide (γ-Al2O3) nanoparticles to characterize phase change behavior such as the supercooling degree, phase change temperature, latent heat, thermal conductivity, and thermal stability. Results demonstrate that thermal conductivity and heat transfer of the CaCl2·6H2O/γAl2O3

nanocomposite

PCMs

are

significantly

enhanced,

supercooling

of

CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs are suppressed. Moreover, a fifty-runcycling test verifies that the CaCl2·6H2O nanocomposite PCMs contained with 1wt.% γ-Al2O3 possesses enhanced thermal behavior. The degree of supercooling is within the range of 0.3-1.1°C, the maximum reductions in the latent heat is 5.9% and no phase segregation was observed. The CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs presented acceptable thermal reliability, chemical stability, and heat transfer characteristics, thereby reflecting its acceptability for low-temperature solar thermal energy storage applications. Keywords: Phase change material (PCM); Nanocomposite; Calcium chloride hexahydrate; Aluminum oxide nanoparticle; Phase change behavior

1. Introduction Thermal energy storage technique based on phase change materials (PCMs) has

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been recognized as one of advanced energy technologies for improving utilization efficiency of renewable energy sources and protecting environment.

The inorganic

hydrated salts PCMs such as CaCl2·6H2O,1 MgCl2·6H2O,2 NaSO4·10H2O, 3,4

CH3COONa·3H2O,5Na2HPO4·12H2O,4,6Mg(NO3)2·6H2O7etc., have received great

attention in the field of energy storage technology due to higher heat storage capacity, constant phase transition temperature, nonflammable and so on. The CaCl2·6H2O PCM system is among the most preferred inorganic PCMs given their low cost, accessibility, and high thermal storage performance. 8 Nevertheless, CaCl2·6H2O PCM exhibits some problems such as supercooling, low heat transfer and so on in application of actual engineering. The most frequently used method to make the improvement is using nucleating such as SrCl2·6H2O,9 KNO3,10 Ba(OH)2·8H2O,11 borax,12 etc. Low heat transfer conditions employed methods that focused on the use of additives to enhance the heat transfer of the process while minimizing PCM side reactions. 8,13-15 The application of nanomaterials has recently been considered to enhance the performance of thermal storage systems given their notable effect on PCM thermal properties. Many of researchers have started to study the phase change performance and heat transfer characteristics of nanocomposite PCMs.16-18 In addition, several studies have been reported Al2O3 nanoparticles were implanted in the base PCMs and fluids

to

enhance

their thermal

conductivity.21-22 However, data

on

the

CaCl2·6H2O/Al2O3 nanocomposite PCMs system has been minimally reported. In the present study, the γ-Al2O3 nanoparticles were implanted into the CaCl2·6H2O PCM system to generate CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs to characterize their phase change behavior such as the supercooling degree, phase change temperature, latent heat, thermal conductivity, and thermal stability. The present study aimed to identify the mechanism that efficiently optimized the thermal properties of CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs for its successful commercialization in thermal energy storage systems.


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2. Experimental Section 2.1. Materials

Anhydrous calcium chloride (CaCl2, purity>96%, Tianjin Yongda Chemical Co., Ltd.) was used as purchased. Gamma aluminium oxide (γ-Al2O3, 50nm, purity >99.9%), silicon dioxide (SiO2, 50nm, purity >99.9%), titanium dioxide (TiO2, 50nm, purity >99.9%), and copper (Cu, 50nm, purity >99.9%) nanoparticles were purchased from Beijing Dk Nano Technology Co., Ltd. Sodium dodecylbenzenesulfonate (SDBS), cetyltrimethylammonium bromide (CTAB), and sodium dodecyl sulfate (SDS) were purchased from Sinopharm Chemical Reagent Co., Ltd. Span-80 was also purchased from Aladdin.

2.2. Preparation of nanocomposite PCMs

Industrial grade CaCl2 was dissolved and crystallized to generate the CaCl2·6H2O material. The CaCl2·6H2O nanocomposite PCMs were prepared by two step method. The surfactant was added to the CaCl2·6H2O-based PCM and SrCl2·6H2O nucleating agent mixture, and was homogeneously stirred using a magnetic stirrer at 50°C for 20 min. The nanoparticles were then added and stirring was continued for another 10 min. Finally, ultrasonic vibration was applied to the preparation processes for 30 min to improve the dispersion stability of the mixture and minimize nanoparticle aggregation. The vibrator temperature during the preparation processes was maintained above the CaCl2·6H2O melting point to preserve its liquidity. The compositions of the various CaCl2·6H2O nanocomposites are listed in Tab.1. The used method is schematized in Fig. 1.

Tab.1 Fig. 1. 2.3. Characterization

The ASTM D5470 was chosen as a guideline in the thermal equilibrium method for solid state thermal conductivity value measurements of the CaCl2·6H2O nanocomposite


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PCMs. The measurements were performed in triplicate to ascertain the correct values. The latent heat of phase change was obtained by using a differential scanning calorimeter (DSC, NETZSCH 200F3, Germany) at a heating rate of 1°C·min-1 under a constant stream of Ar at atmospheric pressure. The DSC had a temperature precision of 0.1°C, temperature repeatability of 0.01°C, calorimetric precision of 1% and a calorimetric repeatability of 0.1%. The viscosity was measured with the Brinell rotational viscometer (NDJ-8S, Shanghai Nirun Intelligent Technology Co., Ltd., China). The values of the supercooling and heat storage (release) curves were gathered using the twin-bath setup. The schematic diagram of experimental set-up was shown in Fig. 2. Transmission electron microscopy (TEM, JEM-1200EX, Japan) and X-ray diffraction (XRD, X’pert Pro, Holland) were employed to investigate the phase structure and morphology of nanoparticles. Fig. 2. The contact angle of nanoparticles was measured using the static sessile drop method with a contact angle goniometer (Dataphysics OCA40 Micro, Germany). The resolution of this instrument is ±0.1°. Nanoparticles were packed using a tablet press to approximate a flat solid surface. A drop of CaCl2 solution (20%) with a temperature of 20°C and a volume of 5μl was placed on this specially prepared surface, and the image was immediately sent via a charge-coupled device (CCD) camera to the computer to analyze the shape of the formed drop. Three measurements were made, and the average value was used.

3. Results and discussion To research an effective nanoparticle of CaCl2·6H2O nanocomposite PCMs, a heating-cooling experiments were carried out with γ-Al2O3, TiO2, Cu, and SiO2 nanoparticles, respectively, while the content of all nanoparticles was fixed at 0.5 wt.%. Fig. 3 displays the freezing curve of CaCl2·6H2O nanocomposites PCMs. It is obvious that the CaCl2·6H2O plus nucleating agent exhibits a significant supercooling degree about 5.3°C. In addition, the supercooling degree of the CaCl2·6H2O nanocomposite


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PCMs with different nanoparticles (γ-Al2O3, TiO2, Cu, and SiO2) were 0.2, 2.2, 5.4 and 9.5°C, respectively. The present study applied the uncertainty formula, which was established by Moffat,23 to calculate the supercooling degree of the 0.5 wt% CaCl2·6H2O–γ-Al2O3 sample as presented the data in Tab.2.

Fig.3

Tab.2

𝛿∆𝑇 𝛿𝑇𝑛 2 𝛿𝑇𝑚 2 = [( ) +( ) ] ∆𝑇 𝑇𝑛 𝑇𝑚 𝛿𝑇𝑚 𝑇𝑚

=

0.3 27

0.5

𝛿𝑇𝑛 0.2 = = 0.74% 𝑇𝑛 26.8

= 1.11%, and

𝛿∆𝑇 ∆𝑇

= 1.33%

(1)

where Tn is the supercooling temperature; Tm is the temperature of the plateau during crystallization; and ΔT is the supercooling degree, such that ΔT = Tm - Tn. The supercooling degree of the CaCl2·6H2O nanocomposite PCMs exhibited obvious reductions following the addition of the mixed nanoparticles, indicating the presence of heterogeneous nucleation. The energy barrier of a cluster that must be overcome prior to its irreversible crystal growth is defined as follows:[24] ∆G𝐶∗ = 𝑓(θ) =

3 16𝜋𝛾𝑆𝐿 Ω2𝑆

3(Δg)2

∙ 𝑓(𝜃)

(2+𝑐𝑜𝑠𝜃)(1−𝑐𝑜𝑠𝜃)2 4

(2) (3)

where ΩS is the volume per particle, 𝛾𝑆𝐿 is the solid-liquid interface free energy, Δg is the liquid-crystalloid free energy difference per particle, and θ is the contact angle. The 16/3 factor is related to the shape of the nucleus, which was assumed spherical. According to Eqs. (2) and (3), the θ must be reduced to minimize the ∆G𝐶∗ such that if θ is equal to 0ºthen nucleation is at its highest. The relation between θ and the surface energy is defined as: 𝑐𝑜𝑠𝜃 =

𝜎𝐿𝐵 −𝜎𝑆𝐵 𝜎𝐿𝑆

(4)

where 𝜎𝐿𝐵 is the specific surface free energy between the nanoparticle and the CaCl2


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solution; 𝜎𝑆𝐵 is the specific surface free energy between the crystal nucleus and nanoparticle; and 𝜎𝐿𝑆 is the specific surface free energy between the crystal nucleus and CaCl2 solution. A smaller value of 𝜎𝑆𝐵 generates a cosθ value closer to one and a ∆G𝐶∗ value closer to zero. The crystal nucleus can be formed nearly without supercooling degree. Fig. 4 displays the contact angle of nanoparticle and calcium chloride solution, the contact angle of nanoparticle (γ-Al2O3, TiO2, Cu, and SiO2) and calcium chloride solution were 25.46, 32.75, 35.20 and 23.28°, respectively. In theory, the order of nucleating ability should be SiO2, γ-Al2O3, TiO2, and Cu nanoparticle. However, too much hydroxyl groups on the SiO2 nanoparticle surface, which have the opposite effect for the crystallization growth of CaCl2·6H2O. Therefore, the γ-Al2O3 nanoparticles has the best performance for reducing the supercooling degree. Fig. 5 illustrates the transmission electron microscopy (TEM) photographs of the irregularly shaped Al2O3 nanoparticles, which were categorized as the nucleating agent and were approximately 50 nm in diameter. All of the peaks of Al2O3 nanoparticles were measured by XRD and compared with the peaks of data retrieved from the Joint Committee on Powder Diffraction Standards,

25

which are presented in Fig. 6. The

comparisons confirmed the presence of γ- Al2O3 in the presented material.

Fig.4 Fig.5 Fig.6

During the preparation of CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs, it is important to avoid agglomeration and segregation of nanoparticles. As shown in Fig. 7, 26

if the hydrocarbon moieties of surfactants encountered the calcium chloride solution,

small nano-aggregates will aggregate into large ones and settle down rapidly because of the strong interaction among the hydrophobic chains of surfactant molecules, which affect thermal properties of CaCl2·6H2O nanocomposite PCMs. Therefore, it is crucial to select an effective dispersant for the CaCl2·6H2O nanocomposite PCMs. The viscosity with the addition of different dispersants (SDS, SDBS, CTAB and Span-80)


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in the nanocomposite PCMs is shown in Fig. 8 while the content of all dispersants was fixed at 1.0 wt.%. As can be seen from the Fig. 8, the thickening effect of addition of CTAB and Span-80 was superior to that of SDS and SDBS addition. At the same time, the γ-Al2O3 nanoparticles dispersed completely and there was no agglomeration after addition of CTAB. In comparison, the γ-Al2O3 nanoparticles show agglomeration phenomenon after addition of Span-80. In consideration of viscosity, dispersion stability and agglomeration, CTAB was the best surfactant for CaCl2·6H2O composite PCMs containing γ-Al2O3 nanoparticles.

Fig.7. Fig.8.

The relationship between the γ-Al2O3 nanoparticles concentration and the supercooling degree of the nanocomposite PCM is shown in Fig.9. The measurements were performed in triplicate to ascertain the correct values. Tab.3 presents the measurement results and standard deviation of supercooling degree of the nanocomposite PCM. The results indicate that the supercooling degree of the CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs was within the range of 0–2°C, which is indicative of the additional latent heat released by the nanocomposite PCMs.

Fig.9.

Tab. 3

The latent heats of CaCl2·6H2O nanocomposite PCMs with respect to mass fraction of γ-Al2O3nanoparticles are illustrated in Fig. 10. Fig.10. For latent heats of the CaCl2·6H2O nanocomposite PCMs with different γ-Al2O3


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nanoparticles contents (0.5 wt.%, 1.0 wt.%, 1.5 wt.%, and 2.0 wt.%), the maximum reductions is 9%. According to the theory of mixtures, the latent heat of the CaCl2·6H2O nanocomposite PCMs is equal to the values calculated following the multiplication of the CaCl2·6H2O latent heat value with its mass fraction in the composite PCM. The calculated values are presented in Fig. 10, which indicate that the latent heat of each composite was lower than the calculated latent heat. The increase in the γ-Al2O3 nanoparticles mass fractions resulted in larger deviations given that the heat transfer performance of the nanocomposite PCMs is different from that of the conventional solid-liquid mixture.27-28 The surface and size of the nanoparticle affect its thermal characteristics.28-29 The specific heat capacity of the nanocomposite PCMs was reduced following the addition of nanoparticles. The latent heat of phase change is calculated as follows: ∆𝐻 = ∫ 𝜌𝑐𝑝 (𝑇)𝑑𝑇

(5)

where ρ is the density, C is the specific heat capacity, T is the temperature. The latent heat of phase change increases with the specific heat capacity, while the other conditions were unchanged. The specific heat capacity decreases with the increasing of the concentration of γ-Al2O3 nanoparticles, and the latent heat of the CaCl2·6H2O nanocomposite PCMs decreases and the deviation becomes larger. In addition, instrument effect also has certain influence on the deviation. Therefore, the equation for the simple solid-liquid mixture does not suit the latent heat calculations for the nanocomposite PCMs. A further increase in the amount of γ-Al2O3 nanoparticles to 2.5 wt% exhibited a latent heat capacity decrease to 114.6 J/g given that an excess of γAl2O3 nanoparticles restrain the crystallization growth of CaCl2·6H2O and reduce the ratio of the CaCl2·6H2O phase change component in the composite. 21 Meanwhile, once the concentration of γ-Al2O3 is greater than 2.0wt.%, conglomeration of γ-Al2O3 nanoparticles was observed. This phenomenon may influence the phase change performance of the CaCl2·6H2O nanocomposite PCMs. Thus, 2.0% is the maximum concentration of γ-Al2O3 in CaCl2·6H2O nanocomposite PCMs. Thermal conductivity is the primary mechanism that gauges the rate of heat storage


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and release during fusion and crystallization. A low thermal conductivity both signifies a reduction in heat storage and release, and restricts its application.30 The thermal conductivities of the CaCl2·6H2O nanocomposite PCMs with 0.5, 1.0, 1.5, and 2.0 wt% γ-Al2O3 nanoparticles demonstrated a marked increase following an increase in the amount of γ-Al2O3 nanoparticles in the composite as presented in Fig. 11. The conductivity of the CaCl2·6H2O PCM was 0.341 W/m·K. The conductivities of the CaCl2·6H2O/γ-Al2O3 (0.5, 1.0, 1.5, and 2.0 wt%) nanocomposite PCMs were 0.459 W/m·K, 0.672 W/m·K, 0.976 W/m·K and 1.373 W/m·K, increased by 34.6%, 97%, 186% and 303%, respectively, which is indicative of the primary contribution of γAl2O3 nanoparticles in obtaining a high thermal conductivity. The addition of γ-Al2O3 nanoparticles was not only beneficial in the reduction of the degree of supercooling, but also facilitated in the enhancement of the thermal conductivity. Moreover, improvements in the thermal conductivity also affected the heat storage and heat release times.

Fig.11 Fig.12 illustrates the heat storage and heat release curves of the CaCl2·6H2O/γAl2O3 nanocomposite PCMs, wherein the heat storage curve exhibited a CaCl2·6H2O equilibrium temperature increase from 15°C to 40°C after 75 min. Moreover, the temperature exhibited a slow rise with a relatively long heat storage platform at ca. 27– 29°C. The addition of the 0.5, 1.0, 1.5, and 2.0 wt% γ-Al2O3 nanoparticles presented temperature increase from 15°C to 40°C after 68, 64, 42, and 38 min, respectively. In addition, the complete melting times of the CaCl2·6H2O/γ-Al2O3 nanocomposites PCMs were reduced by 19.33%, 14.66%, 44%, and 49.33%, respectively, which indicate an improved composite heat transfer rate. The heat release curves indicate that the CaCl2·6H2O temperature dropped from 40°C to 15°C after 454 min, whereas the CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs presented the same temperature drop within only 339–410 min. The complete solidification times of CaCl2·6H2O/γ-Al2O3


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nanocomposites PCMs were reduced by 9.69%, 17.84%, 19.38%, and 25.33%. A large amount of heat is absorbed and discharged following melting and freezing, respectively. Therefore, the thermal conductivity of PCMs was enhanced by the addition of the nanoparticles. In terms of its macroscopic properties, the heat storage and release times required to obtain the equilibrium temperature were reduced in the CaCl2·6H2O/γAl2O3 nanocomposite PCMs. In addition, the heat release takes so much longer than the heat storage process. In the process of melting, the proportion of liquid PCM tends to increase with time elapsing, and the material flow of liquid PCM extend the scope of melting temperature. In the process of heat transfer of PCM at horizontal direction, natural convection of liquid PCM accelerate melting processes. Under the same conditions, the rate of melting is faster than the rate of solidifying. It is presented as the heat release takes so much longer than the heat storage process macroscopically.

Fig. 12

Although thermal cyclic operations are inherent in long-term melting and solidification operations, it is essential for PCMs to minimize or eliminate any undesirable phase change temperature and latent heat changes during the melting and solidification of composite PCMs as these changes affect energy storage and release rates.31 The CaCl2·6H2O nanocomposite PCMs with 1.0 wt% γ-Al2O3 nanoparticles as tested for ascertaining the long-term thermal stability. The selected heating and cooling temperatures for the cyclic performance tests were 40°C and 15°C, respectively, of which the results are presented in Fig. 13. The sample with 1.0 wt% γ-Al2O3 nanoparticles maintained an adequate thermal stability up to 50 cycles: the variation of the phase change temperature was within the range of 27 –29°C and the degree of supercooling was within the range of 0.3–1.1°C, which is indicative of the composite PCMs releasing relatively more latent heat.

Fig.13


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The DSC-measured latent heat is presented in Fig. 14, wherein the latent heat was calculated to be 159.6 J/g and the maximum deviation of the latent heat was -5.9%. The percentage of heat loss (η) was evaluated by following formula: η = (ΔH1 –ΔH2)/ ΔH1× 100%

(6)

where ΔH1 represents the latent heat of initial sample, ΔH2 represents the latent heat of different cycle numbers samples. The thermal properties of the CaCl2·6H2O nanocomposite PCMs with 1.0 wt% γ-Al2O3 nanoparticles after thermal cycling are presented in Tab.4. Extended cycle tests are necessary in order to assess the long-term cycling stability of the nanocomposite PCM. Fig.14 Tab.4

4. Conclusions Herein, a new nanocomposite phase change material CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs was prepared and phase change behavior were systematically studied. The addition of γ-Al2O3 nanoparticles act as a nucleating agent and enhance the thermal conductivity. When adding 0.5, 1.0, 1.5, and 2.0 wt% γ-Al2O3 nanoparticles, the supercooling degree of CaCl2·6H2O/γ-Al2O3 nanocomposite PCMs was within the range of 0 -2°C, the complete melting times of CaCl2·6H2O/γ-Al2O3 nanocomposites PCMs were reduced by 19.33%, 14.66%, 44%, and 49.33%, the complete solidification times of CaCl2·6H2O/γ-Al2O3 nanocomposites PCMs were reduced by 9.69%, 17.84%, 19.38%, and 25.33%. Moreover, the CaCl2·6H2O nanocomposite PCMs with 1.0 wt% γ-Al2O3 also shows excellent thermal cycling stability.

AUTHOR INFORMATION Corresponding Author *Telephone: +86-971-6330483. E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of Qinghai province of China (No. 2017-ZJ-938Q), National Science Foundation for Qaidam Saline Lake Chemical

Engineering Science of China (No. U1607113), and Major Science and Technology Projects of Qinghai Province of China (No. 2015-GX-A1A) are gratefully acknowledged. References [1] Li X.; Zhou Y.; Nian H.E.; Ren X. F.; Dong O.Y.; Hai C.X.; Shen Y.; Zeng J.B. Phase change behaviour of latent heat storage media based on calcium chloride hexahydrate composites containing strontium chloride hexahydrate and oxidation expandable graphite. Appl. Therm. Eng. 2016, 102:38-44. [2] Li G.; Zhang B.B.; Li X.; Zhou Y.; Sun Q.G.; Yun Q. The preparation, characterization and modification of a new phase change material: CaCl2·6H2OMgCl2·6H2O eutectic hydrate salt. Sol. Energy Mater. Sol. Cells. 2014, 126: 51-55. [3] Lei Y.; Yang S. K.; Zhao H. P. In Situ Synthesis and Phase Change Properties of Na2SO4·10H2O@SiO2 Solid Nanobowls toward Smart Heat Storage. J. Phys. Chem. C 2011,115, 20061-20066. [4]Wu Y.P.; Wang. T. Hydrated salts/expanded graphite composite with high thermal conductivity as a shape-stabilized phase change material for thermal energy storage, Energy Convers Manage. 2015,101, 164-171. [5] Li X.; Zhou Y.; Nian H.E.; Ren X.F.; Dong O.Y.; Hai C.X.; Shen Y.; Zeng J.B. Preparation and thermal energy storage studies of CH3COONa·3H2O-KCl composites salt system with enhanced phase change performance. Appl. Therm. Eng. 2016,102, 3844. [6] Liu Y.S.; Yang Y.Z. Preparation and thermal properties of Na2CO3·10H2ONa2HPO4·12H2O eutectic hydrate salt as a novel phase change material for energy storage. Appl. Therm. Eng. 2017,112, 606-609. [7] Frusteri F.; Leonardi V.; Maggio G. Numerical approach to describe the phase change of an inorganic PCM containing carbon fibres. Appl. Therm. Eng. 2006, 26:


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1883-1892. [8] N'Tsoukpoe K.E.; Rammelberg H.U.; Lele A.F.; Korhammer K.; Watts B.A.; Schmidt T.; Ruck W.K.L. A review on the use of calcium chloride in applied thermal engineering. Appl. Therm. Eng. 2014, 75,513-531. [9] Lane G.A. Adding strontium chloride or calcium hydroxide to calcium chloride hexahydrate heat storage material. Solar Energy, 1981, 1, 73-75. [10] Bilen K.; Takgil F.; Kaygusuz K. Thermal energy storage behavior of CaCl2·6H2O during melting and solidification. Energy Sources, Part A. 2008, 30, 775-787. [11] Paris J.; Jolly R. Calcium chloride hexahydrate fusion-solidification behavior. Thermochimica Acta. 1989, 2, 271-278. [12] Liu D.; Xu Y. L.; Thermoproperties research on nucleators-CaCl2·6H2O composites under distinctive systems. Acta Energiae Solaris Sinica. 2007, 7,732-738. [13] Duan Z.; Zhang H. ; Sun L.; Cao Z.; Xu F.; Zou Y.; Chu H.; Qiu S.; Xiang C.; Zhou H.CaCl2·6H2O /Expanded graphite composite as form-stable phase change materials for thermal energy storage. J Therm Anal Calorim. 2014, 115:111-117. [14] Karaipekli A.; Biçer A.; Sarı A.; Tyagi V.V. Thermal characteristics of expanded perlite/paraffin composite phase change material with enhanced thermal conductivity using carbon nanotubes. Energy Convers Manage. 2017, 134, 373-381. [15] Wu W.; Zhang G.; Ke X.; Yang X.; Wang Z.; Liu C. Preparation and thermal conductivity enhancement of composite phase change materials for electronic thermal management. Energy Convers Manage. 2015, 101, 278-284. [16]Fang, Y.T.; Wei, H.; Liang, X.H. ; Wang, S.F. ; Liu, X. ; Gao, X.N.; Zhang, Z.G. Preparation and thermal performance of silica/n-tetradecane microencapsulated phase change material for cold energy storage. Energy Fuels. 2016, 30, 9652-9657. [17]Geng, L.X. ; Wang, S.F. ; Wang, T.Y. ; Luo, R.L. Facile synthesis and thermal properties of nanoencapsulated n-dodecanol with SiO2 shell as shape-formed thermal energy storage material. Energy Fuels. 2016, 30, 6153-6160. [18]Fang, X.; Fan, L.W. ; Ding, Q.; Wang, X. ; Yao, X.L. ; Hou, J.F. ; Yu, Z.T.; Cheng, G.H. ; Hu, Y.C. ; Cen, K.F. Increased thermal conductivity of eicosane-based composite phase change materials in the presence of graphene nanoplatelets. Energy Fuels. 2013,


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Chin. Univ. 2015, 31, 452-456. [31] Harikrishnan S.; Magesh S.; Kalaiselvam S. Preparation and thermal energy storage behaviour of stearic acid-TiO2 nanofluids as a phase change material for solar heating systems. Thermochimi Acta. 2013,565, 137-145.


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Fig. 1. Schematic preparation of CaCl2·6H2O nanocomposite PCMs

Fig. 2. Schematic diagram of experimental set-up.


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Fig.3 Cooling curves of the PCMs with different nanoparticles.

Fig.4 The contact angle of nanoparticle and calcium chloride solution


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Fig.5 TEM photographs of γ-Al2O3 nanoparticles

Fig.6 XRD pattern of γ-Al2O3 nanoparticles.


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Fig.7 Reaction of surfactants to the nanoparticles. 26

Fig.8 The viscosity of the PCMs with the different dispersants.


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Fig.9 The relationship between the γ-Al2O3 nanoparticles concentration and the supercooling degree of the nanocomposite PCM

Fig.10 Latent heat of CaCl2·6H2O /γ-Al2O3 nanocomposite PCMs with different mass fractions.


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Fig.11 Thermal conductivity of CaCl2·6H2O /γ-Al2O3 nanocomposite PCMs

Fig. 12 (a) Heat storage curves and (b) heat release curves of CaCl2·6H2O /γAl2O3 nanocomposite PCMs with different mass fractions.


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Fig.13 Results of cyclic performance tests of the CaCl2·6H2O nanocomposite PCMs with 1.0wt% γ-Al2O3

Fig. 14 DSC melting curves of the CaCl2·6H2O nanocomposite PCMs with 1.0wt%γ-Al2O3 after 50 melting–freezing cycles.


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Table 1 The compositions of the CaCl2·6H2O nanocomposites Al2O3

Nanoparticles (wt%) TiO2 Cu

SiO2

SDBS

Surfactant (wt%) SDS Span-80

CTAB

No.1

0.5

—

—

—

—

—

—

—

No.2

—

0.5

—

—

—

—

—

—

No.3

—

—

0.5

—

—

—

—

—

No.4

—

—

—

0.5

—

—

—

—

No.5

0.5

—

—

—

1

—

—

—

No.6

0.5

—

—

—

—

1

—

—

No.7

0.5

—

—

—

—

—

1

—

No.8

0.5

—

—

—

—

—

—

1

No.9

1.0

—

—

—

—

—

—

1

No.10

1.5

—

—

—

—

—

—

1

No.11

2.0

—

—

—

—

—

—

1

No.12

2.5

—

—

—

—

—

—

1

Table 2 Uncertainty analysis of supercooling degree(0.5%γ-Al2O3-CaCl2·6H2O) Times

Tn (°C)

Tm (°C)

1 2 3 4 5 Mean value

26.8 26.7 27.0 26.7 26.8 26.8

27.0 26.8 27.3 27.0 26.9 27.0

Tn is the supercooling temperature; Tm is the temperature of the plateau during crystallization

Table 3 The measurement results and standard deviation of supercooling degree of the nanocomposite PCM. Supercooling degree /°C Average/°C

Standard error

5.6

5.27

0.350

0.1

0.2

0.17

0.058

0.2

0.2

0.23

0.058

Samples 1th

2nd

3rd

5.3

4.9

0.5%γ-Al2O3-CaCl2·6H2O

0.2


0.3

CaCl2·6H2O


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0.4

0.2

0.3

0.30

0.100


0.8

0.6

0.7

0.70

0.100


1.1

1.2

1.3

1.20

0.100

Table 4 Summary of the thermal properties of eutectic CaCl2·6H2O nanocomposite PCMs with 1.0wt% γ-Al2O3 nanoparticles after thermal cycling Melting temperature

Temperature deviation

Latent heat

Latent heat deviation

Initial sample

28.6 °C

__

169.6 J/g

__

10 Cycles 20 Cycles 30 Cycles 40 Cycles

28.3°C 27.7°C 28.4°C 28.3°C

-1.0% -3.1% -0.7% -1.0%

166.5 J/g 168.3 J/g 163.7 J/g 161.2 J/g

-1.8% -0.8% -3.5% -4.9%

50 Cycles

27.4°C

-4.2%

159.6 J/g

-5.9%


Advanced nanocomposite phase change material based on calcium

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