high density polyethylene composite phase change materials with carbon fiber as shape-stabilized thermal storage materials

Applied Energy 200 (2017) 19–27

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Microstructure and thermal properties of cetyl alcohol/high density polyethylene composite phase change materials with carbon fiber as shape-stabilized thermal storage materials Xiang Huang, Guruprasad Alva, Lingkun Liu, Guiyin Fang ⇑ School of Physics, Nanjing University, Nanjing 210093, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


The cetyl alcohol/high density

polyethylene/carbon fiber composites were synthesized for improving thermal properties.
Microstructure and chemical structure analysis of the CPCMs were displayed and presented.
Thermal properties and thermal reliability of the CPCMs were investigated and discussed.
Thermal conductivity of the CPCMs with the CF in liquid state is 1.25 times higher than that of the pure CPCMs.

a r t i c l e

i n f o

Article history: Received 8 April 2017 Received in revised form 28 April 2017 Accepted 6 May 2017

Keywords: Thermal properties Shape-stabilized Composite phase change materials Carbon fiber Thermal energy storage

⇑ Corresponding author. E-mail address: [email protected] (G. Fang). http://dx.doi.org/10.1016/j.apenergy.2017.05.074 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved.

a b s t r a c t This work presents an experiment on thermal properties of organic cetyl alcohol phase change materials (PCMs) incorporated with high density polyethylene (HDPE). Mass proportions of PCMs ranged from 70 wt% to 90 wt%. Cetyl alcohol (CtA) was chosen as the solid–liquid PCM and HDPE worked as the supporting material. While CtA performed as thermal energy storage medium, at the same time the leakage of the PCM was resolved by HDPE. The novel shape-stabilized composite phase change materials (CPCMs) were fabricated via impregnation of CtA into HDPE. In addition, the thermal conductivity of CPCMs was enhanced by carbon fiber (CF). The microstructure, crystalline phase and chemical structure were determined by scanning electronic microscope (SEM), X-ray diffractometer (XRD) and Fourier transformation infrared spectroscope (FT–IR). The results demonstrated that CtA was well impregnated into the HDPE. Differential scanning calorimeter (DSC) was utilized to analyze thermal properties of the composite phase change materials (CPCMs), the outcome indicated that the CPCMs nearly melted at around 50 °C with a latent heat of 149.02–212.42 kJ kg1. Thermal gravimetric analyzer (TGA) confirmed that the CPCMs have an improved thermal reliability and the addition of CF contributed to a significant decrease in the leakage of CtA. The thermal conductivity meter (TCM) determined that the thermal conductivity of CPCM with 5 wt% CF was 0.33 W/(m K) and 0.47 W/(m K) in liquid and solid state respectively, which was 1.25

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and 1.22 times higher than that of original CPCM without CF. The experimental results indicate that the prepared CPCMs have prospects in thermal energy storage field. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Today, due to serious environmental issues, renewable energy sources and industrial waste heat recovery have gained increased importance. Thermal energy storage system (TESS) is the best choice to resolve the imbalance between energy utility and supply gap [1]. There are many methods of storing thermal energy, classified into three different categories like sensible heat storage (SHS), latent heat storage (LHS) and chemical energy storage [2]. SHS, which is most widely used type, uses the rise in temperature to store energy. In the LHS method, an isothermal or near–isothermal phase change process stores or releases thermal energy. Compared to SHS materials, the LHS materials not only provide the merit of ample energy storage density, but they also have other advantages like simpler devices, smaller volume, flexible design, easier to operate and maintain [3,4]. Especially, the near–constant temperature during phase change process helps to achieve the purpose of controlling temperature of the system. Chemical energy storage systems use reversible chemical reactions to store heat. However, the systems are too complex to use. Solar energy, geothermal energy and industrial waste heat all have issues like discontinuity in their availability. Thus, there is a strong demand for efficient thermal storage systems. Attributing to their high dense in thermal storage density and near constant temperature in energy storage process, latent heat storage has become a dominant method for thermal energy storage [5,6]. The phase change materials can store (or discharge) a lot of thermal energy to realize thermal energy storage, which is of great help in solving the inconsistency between energy demand and supply. According to their operating temperature range, PCMs can be divided into high, medium and low temperature classes [7,8]. According to the phase change mechanisms, PCMs can be classified into solid–solid, solid–liquid, solid–gas, and liquid–gas classes [2,9,10]. According to the chemical constituents, inorganic, organic and organic–inorganic composites are three main categories of the PCM [11]. There is a wide variety of inorganic PCMs, including crystal water and salt, molten salts, metal and its alloys. Among them crystal water and salt such as Na2SO410H2O, CaCl26H2O and MgCl26H2O are typical. In organic PCMs, advanced hydrocarbons, fatty acids, alcohols, arene and some polymers like polyolefin hydrocarbon, polyols, poly alcohols and polyamides are in common use. Compared to inorganic PCMs, organic PCMs have advantages like low corrosivity, no supercooling and no phase separation. However, organic PCMs are limited by their low density and low thermal conductivity. Energy stored in organic PCMs is relatively minor in terms of per unit volume [12,13]. Cai et al. [14] examined the influence of expanded graphite on thermal performance of CPCMs. Attributing to the porous structure of the EG, the fatty acid absorption capacity of nanofibrous mats increased. In addition, heat enthalpies of CPCMs increased with the addition of EG. Sari [15] prepared CPCMs by incorporation of the PCM within expanded perlite. Experiments showed that CPCMs still possessed better thermal and chemical stability after 1000 thermal cycle. Besides, the thermal conductivity increased by approximately 86%. What’s more, vermiculite [16], silicon dioxide [17], diatomite [18] and polymers [19] were employed as supporting materials. A few polymers like linear low density polyethylene (LLDPE), low density polyethylene (LDPE) and high density polyethylene (HDPE) were also employed as supporting materials.

Many works have been done to analyze their characteristics and applications [20–23]. Based on HDPE and paraffin, Mu et al. [24] fabricated shape-stabilized PCMs. The paraffin was uniformly dispersed into the polymer matrix and had a strong plasticizing effect on the HDPE. The melting point of the CPCM was improved. Chen and Wolcott [25] surveyed CPCMs prepared by polyethylene and paraffin, the results revealed that continuous structure resulted in leakage of paraffin. Meanwhile, HDPE was found to be better than others in crystal perfection. Ye and Ge [26] described paraffin–HDPE compound as form-stable PCM, the results showed the composites superiority for application in low-temperature heat storage. Poly (methyl methacrylate) (PMMA) was blended with fatty acid [27], the shapes of composites maintained well with 80 wt% PMMA. Polyethylene glycol (PEG) were blended with acrylic polymers [28], PEG was distributed in acrylic polymers as self–organized rather than chain–like structures. Experiments of a series of poly (ethylene oxide) (PEO)/stearic acid blends were carried out by Pielichowski and Flejtuch [29]. Melting temperature of composites ranged from 55.7 °C to 75.28 °C with enthalpy values of 202.8–252.3 kJ kg1. Usually, various additives are added to modify the thermal conductivity of CPCMs. Johansen et al. [30] prepared sodium acetate trihydrate mixtures and graphite powder. Results indicated that thermal conductivity of CPCMs with 1 wt% graphite powder can increase by 39%. Stearic acid/carbon nanotubes composites were prepared with only 3 wt% of CNTs [31], and the thermal conductivity was improved by 5.7%. Zhong et al. [32] fabricated CPCMs based on octadecanoic acid and graphene aerogel, the thermal conductivity was 14 times higher than that of the OA. Paraffin–based CPCMs incorporated by the cooper foam with 88.99% porosity achieved 44 times higher in thermal conductivities [33]. Moreover, Jana et al. [34] prepared CPCMs with Cu/Au nanoparticle, the thermal conductivity of CPCM with Cu nanoparticle showed a 74% increase. Tian et al. [35] used a hybrid of expanded graphite and CF to prepare paraffin–based CPCMs. The experiment showed a synergistic improvement in thermal conductivity. There are many applications for PCMs, like solar energy installations [36–38], textiles [39], biomaterials [40,41] and building industry [38,42,43]. Passive storage systems [44,45], active storage systems [46,47] and heating/cooling of water [48,49] are studied in building areas. In solar cooling [50,51] and solar thermal storage system [52,53], these studies are focused on poor heat transfer rate in PCMs and the performance improvement of heat storage units. In textiles, PCMs were incorporated into fibers to improve thermal properties of garment fabrics [54,55], especially for outdoor wear [56,57]. PCMs are recognized as a promising material in biomedical fields, which require thermal protection [58]. Thermal management is vital for electronic devices to operate continuously for longer periods, thus PCM-based materials are required to maintain the temperature [59,60]. Moreover, PCMs are also being investigated for space applications [61] and food industry [62]. Organic fatty acids and their eutectics predominate among PCMs used in previous works. However, organic alcohols are rarely investigated. In this work, CtA is used as the PCM, thermal properties of form-stable composites based on solid–liquid PCM were investigated. Phase change temperature and latent heat capacity are analyzed. Instead of traditional materials like expanded graphite, carbon nanotubes, and silica matrix, the supporting material used in this work is HDPE, whose structure is able to envelop the

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2.3.2. Thermal properties The thermal characteristics of the CPCMs were determined by differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin–Elmer). Each specimen was heated/cooled at the rate of 5 K/min. The operating temperature range was 5–100 °C, the accuracy of temperature measurement and enthalpy was ±0.2 °C and ±5% respectively. Thermogravimetry (Pyris 1 TGA, Perkin–Elmer) was utilized for thermal reliability test. Operating temperature range was 20–700 °C, heating rate was 20 K/min.

PCM and CF. This work is aimed at looking into the impact of the CF on the thermal performances of CPCMs. CF is an inorganic fibrous carbon compound with a thermal conductivity that can reach as high as 900 W/(m K) along the in-plane direction. The percolating network of CF acted as high thermal conductivity filler [63]. Thermal property, morphology, chemical characteristics, crystalline structure, thermal conductivity and thermal stability of CPCMs were studied.

2. Experimental 2.1. Materials Cetyl alcohol (C16H34O, 1-hexadecanol, melting point: 47–50 °C, Analytical reagent) was purchased from Sinopharm Chemical Reagent Co., Ltd. High density polyethylene (HDPE, softening point: 125–135 °C, density: 0.940–0.976 g/cm3, Industrial pure) was obtained from Dongguan Huangjiang Co., Ltd. Carbon fiber (CF, diameter: 200–600 nm, length: 5–50 lm. mpurity > 70%) was supplied by XFNANO Corporation.

2.2. Preparation of the CtA/HDPE/CF composites The experiment was split up into 8 groups, the ratios of CtA/ HDPE/CF are listed in Table 1. Firstly, the samples were divided into three groups to inquire into the effect of the CF on the leakage of the CtA. There were CPCM1/CPCM4 (the proportion of CtA to HDPE is 7:3), CPCM2/CPCM5 (proportion of CtA: HDPE = 8:2) and CPCM3/CPCM6 (proportion of CtA: HDPE = 9:1). CtA/HDPE mixtures were heated at the temperature of 160 °C agitated by a magnetic stirrer spinning 1000 rpm for 30 min. For CF doped composites, the CF was blended with the CtA in water bath at 65 °C stirring at the rate of 1500 rpm for 30 min. Then the CtA/CF composites were added into molten HDPE at 160 °C agitating at the speed of 1000 rpm for 30 min. Secondly, the effects of CF on thermal conductivity were verified by comparison of CPCM5 and CPCM7-8 where the mass ratio of the CtA to HDPE were 8:2.

Fig. 1. FT–IR spectrum of (a)–(h) CPCM1–8, (i) CF, (j) HDPE and (k) CtA.

2.3. Characterization of the CPCMs 2.3.1. Chemical characterization Fourier transformation infrared spectroscope (FT–IR, Nicolet Nexus 870, spectra range: 400–4000 cm1 with 2 cm1 resolution using KBr pellets) was used to analyze the chemical structures of the CPCMs, each spectrum received over 16 scans on average. At least three spectra of each specimen were collected. The crystalline structure of composites was determined by an X–ray diffractometer (XRD, D/MAX–Ultima III, Rigaku Corporation, Japan) at a rate of 5° (2h)/min with (2h) 1–60° range. The fixed operating conditions are 40 kV and 40 mA.

Fig. 2. XRD pattern of (a)–(h) CPCM1–8, (i) CF, (j) HDPE and (k) CtA.

Table 1 The compositions of the CtA/HDPE/CF composite PCMs. Samples

CtA (g)

HDPE (g)

CF (g)

Mass fraction of CtA

Mass fraction of CF

CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6 CCPM7 CPCM8

35 40 45 34.65 39.6 44.55 38.8 38

15 10 5 14.85 9.9 4.95 9.7 9.5

– – – 0.5 0.5 0.5 1.5 2.5

0.7 0.8 0.9 0.7 0.8 0.9 0.8 0.8

– – – 0.01 0.01 0.01 0.03 0.05

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Thermal conductivity instrument (TC 3020, Xiatech Electronic Technology co., Ltd.,) with precision of ±2.0% was utilized for thermal conductivity test via the hot-wire technique. After being heated at 160 °C in stainless-steel container, each sample was put into two identical watch-glasses. Two pieces of the sample with diameter of 5 cm and thickness of 0.5 cm were obtained. Afterwards, all samples were cooled down to the room temperature. Sand paper was used to polish the surfaces of the samples to ensure close contact between two halves and the hot-wire detector. Afterwards, the detector was placed between the two pieces of each sample. Thermal conductivity test was performed under two conditions: room temperature (28 °C) and waterbathing (65 °C). Thermal conductivity of each sample took the average of five measurements, in order to insure good accuracy.

2.3.3. Microstructure characteristics The morphologies and microstructures of CPCMs were gauged by a scanning electron microscope (SEM, S-3400NII, Hitachi Inc., Japan) at an operating voltage of 3 kV. Specimens were put on 25 mm  1 mm aluminum discs and were coated on gold with the thickness of 10–20 nm for SEM examination.

3. Results and discussion 3.1. FT–IR analysis FT–IR was used to confirm that the blends of CtA/HDPE and CF doped composites were physical mixes, their FT–IR patterns are

Fig. 3. SEM photographs of (a) HDPE, (b) CF, (c) CPCM2 and (d) CPCM8. (I: 10 lm, 100 lm.)

X. Huang et al. / Applied Energy 200 (2017) 19–27

demonstrated in Fig. 1, characterizing chemical structures of the CtA/HDPE/CF composites. Fig. 1k shows five major IR bands of the CtA, the absorption peaks were recorded at approximately 2910 cm1, 2850 cm1, 1470 cm1, 1100 cm1 and 717 cm1. The absorption peaks at 2910 cm1 and 1100 cm1 are result of antisymmetric stretching vibration of ACH2 group [64,65], meanwhile the peak at 2850 cm1 matching with symmetric stretching vibration of ACH2 group [66]. Absorption peaks at 1470 cm1 and 717 cm1 accounts for the rocking vibration of A(CH2)4 group chain [67]. The spectrum of the HDPE is drawn in Fig. 1j, four main peaks at 2910 cm1, 2850 cm1, 1470 cm1, and 717 cm1 are observed, which are assigned to ACH2 antisymmetric stretching, ACH2 stretching, ACH2 rocking and ACH2 rocking, respectively. Fig. 1i depicts a flat line because of the non-polarity of the CF. Fig. 1a–h represent the FT–IR spectrums of CPCM1-8, the curves present all the characteristic peaks of CtA and HDPE, which explains that peaks remain unchanged in the composites and no new peaks emerge as well. The experiment proves that there is no chemical interaction between CtA, HDPE and CF.

3.2. XRD analysis The crystalline structure of the CPCMs was analyzed using XRD diffractorgrams as shown in Fig. 2. Fig. 2k shows the XRD pattern of CtA, there are two reflection peaks at 22.1°and 25.0°. In Fig. 2j, there are the two prominent diffraction peaks at 21.7°and 24.1° capturing the crystallization of the HDPE, which is assigned to the (1 1 0) and (2 0 0) basal planes of the orthorhombic crystal form of HDPE [68]. There is a peak at 25.6°in Fig. 2i, indexed as (0 0 2) plane, representing the crystallization of the CF. Fig. 2a–h exhibits the crystal structure of CPCM1-8. Affected by the co-crystallization of CtA and HDPE, Fig. 2a–h shows broadening peaks near 22°and 25°. There appears to be variations in main peaks near 22°, which is the result of different proportion of CtA to HDPE, ranging from 7:3 to 9:1. CF also acts as the nucleation agent in whole crystallization process of CtA/HDPE/CF composites. However, the CF doped CPCMs have not displayed the typical peak of CF at 25.6°in

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Fig 2d–f, g and h, where the loadings of CF were 1, 3, 5 wt% respectively, which is too small to display the crystallization of CF. The result validates that the CtA/HDPE/CF are physically combined and their crystal structures remain unchanged. 3.3. Microstructure analysis Surface morphology of the HDPE, CF, CPCM2 and CPCM8 are shown in Fig. 3a–d respectively. The mass ratio of the CtA to HDPE in CPCM2 and CPCM8 is both 8:2, 5 wt% CF was added in CPCM8. The SEM photograph of the HDPE is flat with a layered structure. Fig. 3b depicts the SEM image of CF, some fibers are perpendicular to the screen, which accounts for white dots in sight. The other fibers differ in orientations, forming an irregular filament structure. The CtA is uniformly wrapped by HDPE in case of the leakage, as shown in Fig. 3c. Unlike the flaky framework shown in Fig. 3c, each fiber in Fig. 3d looks like a cylinder with a smooth surface. Attributing to the compatibility of the CF with HDPE, CFs mix well with the CtA/HDPE mixture, which lead to a good dispersion of CF in the composites. Before thermal conductivity test, the re-melting process was carried out to shape the composite into two identical specimens in order to ensure close surface contact of the two samples with the thermal conductivity detector. The morphologies of CPCM8 samples before and after the re-melting procedure were examined, and the dispersion of CtA is given in Fig. 4 across different length scales. Compared with Fig. 4a-I, b-I exhibits similar smooth surface with cylindrical structure. Fig. 4a-II and b-II depict the framework of the composite at another magnification. It can be seen that CtA is uniformly dispersed in the layered frame. 3.4. Thermal energy storage capacity DSC experiments were used to study energy storage capacity of the CPCMs in terms of melting/solidifying temperature and latent heat capacity. DSC curves in the charging and discharging process are described in Fig. 5a and b respectively. One peak is observed in

Fig. 4. SEM photographs of CPCM8 (a) before and (b) after the re-melting process. (I: 10 lm, II: 100 lm.)

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each charging DSC curve within the temperature range of 25–75 °C, which is caused by the melting of the CtA. As seen in Table 2, the CtA melts at 49 °C and solidifies at 48 °C. In CPCMs, the onset melting and solidifying temperature of each sample has declined in contrast with the pure CtA, which is the result of decreased interaction between the CtA molecules and addictives. Two absorbing peaks are shown in Fig. 5b, and the larger peak represents liquid–solid (L–S) solidification procedure, which is employed to measure latent heat value in discharging process. The minor peak denotes solid–solid (S–S) phase change, which is

Fig. 5. DSC thermograms of the CtA and CPCMs in the (a) melting and (b) solidifying state.

the result of phase transition from a disordered phase into greater ordered rotator phase after the solidification. As the content of CtA decreases, the melting and solidifying latent heat of CPCMs decreases accordingly. Only CtA functions as thermal energy storage medium. Latent heat rises as the dosage of CtA increases from 70 wt% in CPCM1 to 90 wt% in CPCM3, which also occurs from CPCM4 to CPCM6. The latent heats of CPCM4–6 are relatively lower than those of CPCM1–3, the dosage of CF leads to reduction in the melting/solidification latent heat between groups shared the same proportion of CtA to HDPE. As the content of CF in CPCM8 is increased to 5 wt%, the latent heat of CPCM8 reaches the minimum value. The relationship expression of melt-

Fig. 6. TGA and DTG graphs of CtA, HDPE and CPCMs. (a) TGA and (b) DTG.

Table 2 DSC data of the CtA and CPCMs. Samples

CtA CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6 CPCM7 CPCM8

Melting

Solidifying

Onset temperature (°C)

Peak temperature (°C)

Latent heat (kJ kg1)

Onset temperature for S–S/L–S (°C)

Peak temperature for S–S/L–S (°C)

Latent heat (kJ kg1)

49 49 49 49 48 48 49 48 49

52 51 52 52 51 51 53 51 51

237 149 188 212 149 182 208 173 171

43/48 42/48 42/48 42/48 42/48 42/48 42/48 42/48 42/48

42/47 41/47 41/46 41/46 39/47 40/47 41/46 40/47 40/47

86/137 38/82 65/108 74/120 40/81 57/102 74/122 54/99 50/96

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ing/solidifying latent heat and the mass fraction of the CtA can be written as follows:

DHCPCM ¼ gDHCtA

ð1Þ

where DHCPCM and DHCtA notes the melting/solidification latent heat of CtA/HDPE/CF composites and original CtA, respectively. g represents the mass ratio of CtA. Values acquired from this experiment are slightly lower than the theoretical values, which is ascribed to the evaporation of the CtA when the experiment temperature at 160 °C.

Table 4 Thermal cycling data of CPCM1–CPCM8. Samples

Weight before cycling (g)

Weight after cycling (g)

Leakage rate (%)

CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6 CPCM7 CPCM8

48.54 47.85 47.76 47.33 47.73 47.80 48.63 48.68

48.33 46.79 46.12 47.27 47.26 47.31 48.21 48.35

0.43 2.22 3.43 0.13 0.98 1.03 0.86 0.68

3.5. Thermal stability and leakage rate analysis TGA and DTG graphs of CPCMs are depicted in Fig. 6a and b, respectively. As shown in Table 3, Tpeak represents the temperature of maximum thermal degradation rate and DW is the percentage of mass loss. Tpeak of CtA and HDPE are 237.73 °C and 506.90 °C without charred residue. As seen from Fig. 6a, the thermal degradation of CPCMs is a two–stage process. The first step arises at the temperature between 240 and 260 °C, matching with the thermal degradation of CtA. The second step occurs between 490 and 510 °C, representing the thermal degradation of the HDPE. Fig. 6b exhibits the maximum thermal degradation rate of CtA, HDPE and CPCMs. Two peaks observed in CPCMs refer to the decomposition of the CtA and HDPE respectively. Tpeak1 in CPCMs are very close to that of pure CtA, Tpeak2 in CPCMs are similar to that of pure HDPE, indicating that the composites are provided with good stability with the addition of HDPE and CF. Besides, DW1 and DW2 roughly match the proportions of the CtA and HDPE in each sample, which means that the CtA is mixed successfully with the HDPE. The residue is recognized as the CF in modified composites, the value accords with actual usage. 500 times of heating–cooling cycling were conducted to test the leakage rate of CPCMs, the weight of composites before and after thermal cycling are recorded in Table 4. The cycling started from 28 °C and ended at 65 °C. It can be seen from CPCM1-3 that as the loading of HDPE increases, the leakage rate reduces obviously. Comparing the leakage rate of CPCM2, CPCM5, CPCM7 and CPCM8, whose content of CtA are 80 wt%, the leakage rate dropped gradually as the dosage of CF rises. Meanwhile CF has a positive effect on the leakage rate of CPCMs, as shown in Fig. 7. 3.6. Thermal conductivity analysis Thermal conductivity has become an important factor to the performance of CPCMs, which affects the heat transfer rate of thermal systems. Experiments were carried out to measure the thermal conductivity of CPCM2, CPCM5 (1 wt% CF), CPCM7 (3 wt% CF) and CPCM8 (5 wt% CF) at 28 °C (solidifying state) and 65 °C (melting state). The relevant data are listed in Table 5, the values and errors are depicted in Fig. 8. Compared with CPCM2, the thermal conduc-

Fig. 7. Leakage rate of CPCM with different loading of CF when mass fraction of CtA is 80 wt%.

Table 5 Thermal conductivity of CtA, HDPE, CPCM2, CPCM5, CPCM7 and CPCM8 with different mass fraction of the CF in solid and liquid state. Samples

Thermal conductivity (liquid) (W/(m K))

Thermal conductivity (solid) (W/(m K))

CtA HDPE CPCM2 CPCM5 (1 wt% CF) CPCM7 (3 wt% CF) CPCM8 (5 wt% CF)

0.1309 0.1834 0.1433 0.1743 0.2808 0.3220

0.1323 0.2023 0.2125 0.2596 0.4259 0.4719

tivity of CPCM8 in liquid state (65 °C) is 0.3220 W/(m K), which has increased by 1.25 times; in solid state (28 °C), the thermal conductivity of CPCM8 is 0.4719 W/(m K), which has increased by 1.22 times, suggesting that CPCMs have an excellent energy storage efficiency.

Table 3 TGA data of the CtA, HDPE and CPCMs. Samples

Tpeak1 (°C)

Dw1 (%)

Tpeak2 (°C)

Dw2 (%)

Residue (%) (700 °C)

CtA HDPE CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6 CPCM7 CPCM8

237.73 – 243.26 255.72 250.82 254.72 245.27 245.39 245.22 250.78

100 – 68.09 80.76 90.57 66.58 78.97 90.33 77.79 76.01

– 506.90 489.03 491.10 484.21 505.44 496.43 474.15 494.32 502.45

– 100 31.91 19.24 9.43 32.66 20.17 8.90 20.11 19.71

– 0 0 0 0 0.76 0.86 0.77 2.10 4.28

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171.04 kJ kg1, and solidifies at 41.51 °C with latent heat of 96.44 kJ kg1. TGA analysis verified that the prepared composites possess favorable thermal stability. Compared with the CPCM2, thermal conductivities of the CPCM5, CPCM7 and CPCM8 were remarkably increased by 0.22, 0.96 and 1.25 times respectively in liquid state, and were increased by 0.22, 1.00 and 1.22 times respectively in solid state. However, the rise in the thermal conductivity reduces the latent heat of the composites to some extent. To summarize, the modified CPCMs have advantages like stable shape, good thermal stability and preferable thermal conductivity, which is a promising material in thermal storage application. Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant nos. 51376087, 51676095). The authors also wish to thank the reviewers and editor for kindly giving revising suggestions. Fig. 8. Thermal conductivities of CPCM2, CPCM 5, CPCM 7 and CPCM 8 with different mass fractions of the CF in liquid and solid state.

Table 6 Comparison of the thermal conductivity of CPCMs with results of the previous literature. Samples

Thermal conductivity (W/(m K))

Reference

Paraffin + HDPE + EG Paraffin + HDPE Paraffin + HDPE + EG Cetyl alcohol + HDPE + CF

0.24 0.31 0.32 0.4719

[69] [70] [71] Present study

Table 6 provides some example data from previous literature for thermal conductivity enhanced by CF. The CPCMs prepared in this work has achieved a good thermal conductivity improvement. The FT–IR and XRD results show that the form-stable composites possess all the features of CtA and HDPE, which indicates that the CtA, HDPE and CF were synthesized without chemical interaction. The layered structure of HDPE makes it possible to envelop CtA. CF is uniformly dispersed in the CtA/HDPE mixtures for thermal conductivity enhancement. The melting temperature of composite approaches to 50 °C with high latent heat at least 171 kJ kg1. Although CtA acts as the energy storage medium, the loading of CtA should be controlled to certain value in case of the leakage in solid-liquid transition. Experiments show that when the dosage of CtA, HDPE and CF is 80, 15 and 5 wt% respectively, the composite not only has high latent heat but also low leakage rate. The thermal conductivity of composite with 5 wt% CF was 0.33 W/(m K) and 0.47 W/(m K) in liquid and solid state respectively, 1.25 and 1.22 times higher than that of original composite without CF. The HDPE-based composites with improved thermal conductivity, superior heat storage, good compatibility and thermal stability have promising potential in practical applications like solar energy, buildings and heat recovery systems. 4. Conclusions In this work, the CtA was blended into HDPE to constitute shape-stabilized composites. The CtA was employed as PCM and HDPE performed as the supporting matrix. The addition of CF succeeded in preventing the CtA from leakage and enhancing the thermal conductivity of composites with different mass ratio (1 wt%, 3 wt% and 5 wt%). The CtA was uniformly dispersed into HDPE without chemical interaction. The mixture of CtA/HDPE/CF was physically combined and their crystal structures remained unchanged. CPCM8 melts at 48.55 °C with latent heat of

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