Palygorskite composite phase change materials for thermal energy storage

Palygorskite composite phase change materials for thermal energy storage

Solar Energy Materials & Solar Cells 144 (2016) 228–234 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homep...

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Solar Energy Materials & Solar Cells 144 (2016) 228–234

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells journal homepage:

Paraffin/Palygorskite composite phase change materials for thermal energy storage Dan Yang a,b, Silan Shi a,c, Lian Xiong a,b, Haijun Guo a,b, Hairong Zhang a,b, Xuefang Chen a,b, Can Wang a,b, Xinde Chen a,b,n a

Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, PR China Xuyi Center of Attapulgite Applied Technology Research Development & Industrialization, Chinese Academy of Sciences, Xuyi 211700, PR China c Graduate University of Chinese Academy of Sciences, Beijing 10049, PR China b

art ic l e i nf o

a b s t r a c t

Article history: Received 24 April 2015 Received in revised form 14 August 2015 Accepted 3 September 2015

In this study, paraffin was selected as the phase change materials (PCMs) and rude-Palygorskite (Pal), rinsed-Pal, H þ -Pal and organic-Pal were selected as porous materials. A series of paraffin/Pals composite PCMs were prepared by direct impregnation method without vacuuming. In the composite PCMs, up to 150 wt% of paraffin could be loaded in H þ -Pal. The Differential Scanning Calorimetry (DSC) tests revealed that paraffin/ H þ -Pal composite PCM had a melting temperature of 54–56 °C and latent heat of 132.18 J g  1. In addition, the composite PCMs were characterized by Scanning Electron Microscope (SEM), Brunauer, Emmett, and Teller (BET) and Fourier Transform Infrared Spectroscopy (FTIR). Good thermal stability was observed for composite PCMs by cycling test; we found that paraffin/H þ -Pal composite PCM maintained  95% of the initial latent heat after the cycling test. In conclusion, the impregnation of Pals with paraffin was a promising method for preparation of excellent PCM composites. & 2015 Elsevier B.V. All rights reserved.

Keywords: Phase changing material Palygorskite Composite Direct impregnation Thermal stability

1. Introduction The use of Petroleum has provided various materials and fuels for human lives. Nevertheless, it also causes the greenhouse effect and is responsible for global warming. And depleting reserves of petroleum-based fuels and volatility in their market price have convened a worldwide effort to develop renewable ways to accomplish sustainable development [1,2]. Thermal energy storage (TES) has been considered an important technology that provides renewable energy sources to the end users. It has received more and more worldwide attention and research interests [3–7]. By the TES system, energy can be stored thermally in the forms of sensible heat, latent heat, and reversible chemical reaction heat. Among all these ways, the TES process in latent heat form utilizing phase change material (PCM) is considered an attractive one, which is because of their obvious advantages of high latent heat storage and release capacity at a constant temperature [8]. In general, PCMs can be encapsulated in building materials such as concrete, gypsum wall board in the ceiling or floor to increase their thermal storage capacity. They can either capture solar energy directly or thermal energy through natural n Correspondence to: No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640, PR China. Tel./fax: þ86 20 37213916. E-mail address: [email protected] (X. Chen). 0927-0248/& 2015 Elsevier B.V. All rights reserved.

convection. Considerable researches on the application of PCMs in heating and cooling system have been done [9–16]. A wide variety of PCMs are available with different heat storage capacities and phase change temperature intervals. PCMs can be further classified into organic and inorganic PCM. However, most of the inorganic PCMs are rarely used in the TES systems because of their super-cooling trend, toxicity, corrosivity and ecologically harmful properties. On the contrary, most of the organic PCMs are safe, non-corrosive, chemically inert, inexpensive, and have no phase segregation. Among them, paraffin waxes are most popular due to their remarkable properties, such as high energy storage densities, little super-cooling trend, low vapor pressure in the liquid phase, non-toxicity and commercial availability at a relatively low cost. Besides, the literatures data show that commercial grade paraffin waxes and other pure paraffin have stable properties and good thermal stability after 1000–2000 cycles [14,16–21]. Moreover, paraffin is a mixture of straight chain alkanes, its melting temperature or melting temperature range and latent heat can be altered with its alkane chain length and can vary across wide ranges [22]. Nevertheless, the use of paraffin is complicated due to their undesirable leakage problem in melted state. To some extent, the drawback restrains paraffin’s wide application as a TES material. Therefore, it is needed to use them in form-stable form. Some attempts have been devoted to solve this problem using incorporating techniques, macro/microencapsulating and shapestabilizing [23–25]. Practically, the first one is more outstanding

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among these three methods, which not only overcomes the leakage problem but makes the PCM composites used easily. The incorporating techniques can be divided into two kinds: direct incorporation and immersion methods. The first method incorporates PCM during fabrication process of composite materials. The second method incorporates PCM into composite materials by means of natural immersion. It is simple and easy to use for these two kinds of methods. Numerous organic PCMs have been integrated with various porous materials, such as clay minerals, graphite, pumice sand, concrete, graphene, diatom earth, expanded fly ash granules and other wall covering materials [6,15,26–32]. Furthermore, much more attention is drawn into the PCMs, for instance, Wang et al. [33] prepared S/M-stearic acid/montmorillonite composite PCMs, the results showed that when the content of S-stearic acid and M-stearic acid reached 47.5% and 46.9%, respectively, the composites had energy storage characteristics with melting temperatures of 59.9 °C, 58.2 °C and the latent heat of fusion of 84.4 J g  1, 83.6 J g  1. Bentonite clay and pumice sand with different organic PCMs composites had been reported by Sarı et al. [19]. The melting temperatures of the form-stable composites were in the range of 20–33 °C, while they had latent heats of melting in the range of about 28–55 J g  1. Paraffin/diatomite/ multi-wall carbon nanotubes PMC composites were tailor-made by Xu et al.; the tests revealed that the composite PCM had a melting temperature of 27.12 °C and latent heat of 89.40 J g  1 [32]. Cai et al. [23,26] prepared polyethylene/paraffin composites and polyethylene/paraffin/organophilic montmorillonite composites; they selected paraffin with the latent heat of fusion of 125.27 J g  1 as the PCM for fabricating the composites, the results indicated that the best polyethylene/paraffin PCM had a latent heat of 53.92 J g; 1 the best polyethylene/paraffin/organophilic montmorillonite PCM had a latent heat of 54.58 J g  1. The diatomite/ paraffin PCM was reported by Li Xiangyu et al. [29], paraffin with the latent heat of 134 J g  1 was used as PCM, the PCM composite had the latent heat of 63.98 J g  1. Although the extensive literature references in PCMs field, the latent heat of PCM composites was not relatively excellent, which restricted the wide application in TES system. Takahiro et al. [34] studied the impregnation of a porous material (perlite, diatom earth, and gamma-alumina) with erythritol; they revealed that the impregnation of porous material with erythritol was a promising method for conserving latent heat with high thermal storage density; the expanded perlite/erythritol composite prepared by the vacuum impregnation had the largest latent heat, which was accounting for 83% of that of pure erythritol (294.4 J g  1). This showed an outstanding thermal performance; however, erythritol was obtained through microbial fermentations, and generally its yield only reached about 50%, thus material shortage prohibited the industry development prospects. Currently, developing new energy-saving composites with improved thermal performances and durability has been the research focus for promoting the applicability of PCM composites at the industrial scale. Palygorskite (Pal, alternate name attapulgite) is a hydrated magnesium aluminum silicate with the theoretical formula (Mg, Al)2 Si4O10 (OH)4 (H2O). Pal is characterized by a large specific surface area, moderate cationic exchange capacity, and pronounced adsorption properties [35]. Due to its unique structure, low cost, abundant and easy availability, Pal is widely used in the chemical industry, agriculture, surface coatings and environment purposes, as well as adsorptive support [36]. According to our studies, specific surface area of Pal is over 130 m2 g  1 and pore volume is above 0.7 cc g  1, which are far greater than these of minerals such as kaolinite, et al. PCMs can be directly absorbed into the pore of Pals with high adsorption efficiency. This hinders leakage of the PCMs in the liquid state during a phase change process. Moreover, Pals are eco-friendly, easily molding with water and other building


materials. Thus far, Song et al. [37] prepared a novel form-stable PCMs (FSPCM) for latent heat thermal energy storage, by incorporating eutectic mixture of stearic–capric acid (S–C) into activatedattapulgite, they reported that maximum mass fraction of S–C in composite without leakage was as high as 50 wt%. The phase change temperature and latent heat of FSPCM were measured to be 21.8 °C and 72.6 J g  1 for melting process respectively. But beyond that, less insight has been devoted to Pals in the production of PCMclay composites. However, PCM/Pal composites are promising materials, which simultaneously provide high thermal storage capacity and biodegradability to meet the requirements of many TES systems in an environmentally friendly manner. The new PCM/ Pal composites not only can be used for passive heating and cooling applications in building envelops, interior and exterior wall thermal insulation coatings, but also can be adopted in polyurethane foam to develop the joint advantage of thermal insulation and thermal energy storage. In addition, several studies [10,17,19,32,38] have proven the energy savings through the incorporation of PCM into mortars for application in buildings. These report results demonstrated the feasibility of hybrid PCM mortars and the stability of PCM composite system; therefore, it was speculated that the hybrid PCM mortars had good compatibility between composite PCM and cement system. This research provided a deep insight into developing new paraffin/Pal composite PCMs. Four types of Pals were integrated with paraffin via normal pressure impregnation method. The resultant Pals and PCM/Pal composites were characterized using Brunauer, Emmett, and Teller (BET), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Thermal properties and thermal stabilities of samples were determined by the Differential Scanning Calorimetry (DSC) analysis. The effects of the kinds of modified Pals on the absorption of paraffin and the latent heat of composites were investigated; to the best knowledge of the authors, this was the first experimental study. What is more, up to 150 wt% of paraffin can be loaded in H þ -Pal, and the latent heat of the Paraffin/H þ -Pal composite reached 132.18 J g  1; until now, this was the highest latent heat of paraffin-based composite phase change materials.

2. Experimental 2.1. Materials Rude Pal, from Jiangsu AoTu Bang International Co. in China, was pretreated by distilled water, hydrochloric acid and octadearyl dimethyl ammonium in our lab. The rude Pal and the three obtained materials were hereafter referred to as rude-Pal, rinsedPal, H þ -Pal and organic-Pal. Table 1 shows the basic properties of these four porous materials. Paraffin wax (also called paraffin) with melting temperature of 50–52 °C and the latent heat of fusion of 220.3 J g  1, was selected as the PCM for fabricating the composites, which was provided by Sinopharm Chemical Reagent Co., Ltd. in China. Table 1 Mean particle size, pore surface area and porosity of Pals. Material

SBET (m2/g)

Porosity (cc/g)

dMean (nm)

Rinsed-Pal H þ -Pal Organic-Pal

132.52 259.80 124.19

0.731 1.004 1.016

22.10 16.07 32.72


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Table 2 Mix proportion of Pals and Paraffin. Series



Absorption value of paraffin/g

Rude-Pal Rinsed-Pal H þ -Pal Organic-Pal

30 30 30 30

10 10 10 10

11 13 15 12

2.2. Preparation of paraffin/Pals composite PCMs The procedure of impregnation was treated at normal pressure without vacuuming according to the method documented in the literature [17]. Firstly, Pals were dried in a muffle at 110 °C for 3 h and then passed through 0.075 mm (200 mesh) sieves; a certain amount of solid paraffin was melted in a water bath at 80 °C before the paraffin impregnation procedure. Pals were physically placed and immersed in certain amount of melted paraffin (paraffin amounts were overloaded, mix proportion of Pals and paraffin were shown in Table 2) with magnetic stirring directly for 1 h at 80 °C, and were then poured into the ordinary funnel equipped with filter paper of a known weight. The leaking melted paraffin was picked up with a flat bottom flask. The paraffin/Pals mixtures were made to stand for 36 h, 48 h and 70 h in oven at 80 °C until a constant weight. Finally, the solidified paraffin/Pals composites were taken out, smashed and then passed through 0.075 mm (200 mesh) sieves. The absorption value was average of at least three samples, and its relative error limit was about 5%. Hereafter, the new paraffin/Pals composite PCMs were named as Paraffin/rawPal, Paraffin/rinsed-Pal, Paraffin/H þ -Pal and Paraffin/organic-Pal for short. 2.3. Characterization of paraffin/Pals composite PCMs Differential Scanning Calorimetry (DSC, PT-1600) was used to evaluate the latent heat and the phase change temperatures of the paraffin/Pals composite PCMs. The DSC measurements were carried out in the temperature range of 10–80 °C at 0.5 °C/min and 3 °C/min constant heating/cooling rate and under nitrogen gas atmosphere environment. Micro-morphology of Pals and the prepared paraffin/Pals composite PCMs was observed by using Scanning Electron Microscopy (SEM) instrument (Hitachi, S-4800 FESEM). To prepare the SEM samples, approximately 0.01 g of dried, powdered Pals samples were placed on standard mounts, 15 mm in diameter and 2 mm in depth, under vacuum and coated with a 1–2 nm thick conductive layer of gold to prevent charging during imaging. Brunauer, Emmett, and Teller (BET) surface areas of various Pals materials were evaluated by N2 sorption measurement using Quantachrome ST-MP-10 (USA) at 77 K. The specific surface areas (SBET) were evaluated by the BET method; the average pore diameter (dMean) and pore volume (Porosity) were calculated by Average Pore Size and Total Pore Volume method respectively. Chemical compatibility of paraffin/Pals composite PCMs was studied via Fourier Transform Infrared Spectroscopy (FTIR, TENSOR27) analysis. Moreover, cyclic tests of heating and cooling were performed on the paraffin/Pals composite PCMs to evaluate the usable latent heat.

3. Results and discussions 3.1. Absorption of paraffin The mass percentage of paraffin in composite PCMs is particularly crucial. Mix proportion of Pals and paraffin, as well as the

absorption value of paraffin are given in Table 2. Rude-Pal and organic-Pal had low absorption values of paraffin, the mass percentage of paraffin absorbed by rinsed-Pal reached 130%. The absorption with the highest mass percentage of paraffin in composites was improved by a treatment with an acid, the absorption value of paraffin reached 15 g per 10 g H þ -Pal; it showed that large quantity of paraffin could be incorporated into H þ -Pals by using direct impregnation method without vacuum treatment, which made the composites own large thermal energy storage capacity. As we all know, Pals have cements and carbonates in its structure; these impurities reduce its adsorption performance. Rude-Pal was pretreated by distilled water and hydrochloric acid, which could remove these impurities and form the porous structure in the mineral to create a higher specific surface area, as well as to make the Pals channel unblocked. However, the increased quaternary ammonium salt absorbed on the surface of Pals undoubtedly enhanced the compatibility between the Pals and paraffin. Nevertheless, this produced a contrary effect on absorption of organic-Pal and had a relative low content of paraffin. It could be because of the lower specific surface area of organic-Pal (as shown in Table 1), or due to the strong hydrophobic properties which made the paraffin occupy the whole surface of Pals, which hindered the further absorbing of paraffin into the Pals holes. However, the reasons were just speculated, further insights into the specific reasons were needed. 3.2. BET of Pals A summary of the surface properties of the samples is listed in Table 1. It was interesting to note that the BET surface area of H þ Pal increased obviously compared with rinsed-Pal and organic-Pal, the total pore volume of H þ -Pal was higher than that of rinsed-Pal. These observations clearly demonstrated that H þ -Pal had been modified by an acid and the surface properties of H þ -Pal had been changed. Additionally, from Table 1, organic-Pal showed the lowest BET surface area (SBET) of 124.19 m2/g, which indicated that the acid activation was a more useful method to increase SBET of Pals compared with the organic treatment. 3.3. Microstructure of paraffin/Pals composite PCMs Fig. 1. shows the SEM morphologies of Pals and paraffin/Pals composite PCMs. We paid attention to the morphological differences among the three Pals (rinsed-Pal, H þ -Pal and organic-Pal); the rinsed-Pal was present in the form of fiber aggregate with a smooth surface, while the H þ -Pal and the organic-Pal were present in the form of dispersive fibers. It was noteworthy that there were much more porous and channels in structures of H þ -Pal compared with that of rinsed-Pal and organic-Pal. This result was consistent with that of BET. It was because the rude-Pals had cements and carbonates in its structure, and the H þ -Pal was treated with an acid, which could remove these impurities and form the porous structure and simultaneously create much more channels. What is more, organic-Pal showed fewer porous and channels in structures; this was probably because the alkyl chain of octadearyl dimethyl ammonium occupying the porous and organic-Pal was incapable of forming much more channels. However, it was clearly seen that the morphological differences among Pals and composite were extremely visible. The microstructures of Pals were fibrous before the impregnation treatment (Fig. 1a, c and e). Compared to the numerous pores observed in Fig. 1a, c and e, there were only few visible pores left after paraffin impregnation and no extruded or broken fiber of Pals were observed (Fig. 1b, d and f). This fact demonstrated that the PCM was well impregnated into the Pals under the normal pressure, as expected.

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Fig. 1. SEM images of Pals and the paraffin/Pals composite PCMs: (a) rinsed-Pal, (b) Paraffin/rinsed-Pal, (c) H þ -Pal, (d) Paraffin/H þ -Pal, (e) organic-Pal and (f) Paraffin/ organic-Pal.

3.4. Chemical compatibility of paraffin/Pals composite PCMs Fig. 2 illustrates the FTIR spectrums of paraffin, Pals and paraffin/Pals composite PCMs. As presented in Fig. 2, Pals had their characteristic absorption bands at 3580 cm  1, 1031 cm  1, 982 cm  1 and 479 cm  1. They represented bending vibration of O–H stretching mode in Al2–O–H groups (3580 cm  1), Si–O bond stretching vibration (1031 cm  1 and 982 cm  1) and Si–O hexagonal rings vibration (479 cm  1), respectively. Characteristic absorption bands of the paraffin used were shown as 2916 cm  1, 2850 cm  1, 1465 cm  1 and 718 cm  1, which indicated the rocking vibration of –CH2 (718 cm  1), deformation vibration of –CH2 and – CH3 (1465 cm  1) and stretching vibration of –CH2 and –CH3 (2850 cm  1 and 2916 cm  1), respectively. In comparison with the FTIR spectrums of both Pals and paraffin, it could be obviously seen that the FTIR spectrum of paraffin/Pals composite PCMs included all the above-mentioned characteristic bands of both paraffin and Pals. Additionally, no new peaks were obtained in spectrum of paraffin/Pals composite PCMs, clearly showing that no

Fig. 2. FTIR spectrums of Pals, paraffin and paraffin/Pals composite PCMs.


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chemical reaction happened between paraffin and Pals. However, Fig. 2 shows that deformation vibration of –CH2 and –CH3 (1465 cm  1) characteristic bands of paraffin/Pals composite PCMs were slightly shifted in comparison with that of paraffin. This could be due to physical interactions between paraffin and Pals, which implied good compatibility between paraffin and Pals [19]. 3.5. Thermal properties of paraffin/Pals composite PCMs Fig. 3 shows the DSC curves of paraffin and paraffin/Pals composite PCMs. We carried out DSC analysis more than three times to confirm thermal properties of the samples. The paraffin/Pals composite PCMs had slightly different DSC curves in comparison to paraffin. It was found that most of the composite PCMs have two phase peaks except for paraffin/H þ -Pal composite PCM. The first phase change peak was lower and corresponds to the solid– solid phase transition of the paraffin, and the second peak was high, corresponding to the solid–liquid phase change [39]. And paraffin/H þ -Pal composite PCM had one phase peak; this might be due to the interaction between H þ -Pal and paraffin, it remained to be further research. Moreover, the detailed results of the latent heat values are also given in Fig. 3. As presented in Fig. 3, the latent heat values and the melting temperatures were found to be 220.3 J g  1, 50–52 °C for paraffin; 110.15 J g  1, 46–48 °C for Paraffin/rude-Pal; 124.52 J g  1, 50  52 °C for Paraffin/rinsed-Pal; 132.18 J g  1, 54  56 °C for Paraffin/H þ -Pal and 120.16 J g  1, 50– 52 °C for Paraffin/organic-Pal. It indicated that Paraffin/H þ -Pal composite PCM possessed the highest latent heat; on the contrary, Paraffin/rude-Pal owned the lowest latent heat. These phenomena were probably caused by the different pretreatment of Pals. Especially, for the Paraffin/organic-Pal composite PCM, the effect was quite preeminent and it had a small latent heat. This was probably because organic-Pal was capable of forming less porous and channels, and also obtained a low mass percentage of paraffin and lower latent heat value. Therefore, owing to this property, it was not suitable for TES system. And also, the heat storage of composite was reduced compared with that of paraffin. That was due to paraffin can be used for storing heat, Pal was just as the adsorption media, it was used to overcome the leakage problem and made the paraffin used easily, which did not have the ability to store the heat. Although the latent heat of the chosen composite accounted for 60% of pure paraffin, it also could be used for passive

Fig. 3. DSC curves of paraffin and paraffin/Pals composite PCMs.

heating and cooling applications. Consequently, it was economic and acceptable candidates for TES system. 0.5 °C/min and 3 °C/min heating/cooling rates were applied to the changes in simulation of the actual transition temperature during the utilization of the composite PCM, while the slower heating/cooling rate studied (0.5 °C/min) was the one close to real scenarios of daily temperature variations. The DSC curves that allow evaluating the overall hysteretic behavior in paraffin/H þ Pals composite for the different heating/cooling rates can be seen in Fig. 4. It was further interesting to remark that the overall differences in the thermograms for different rate heating/cooling were smaller as the rate decreased [38]. It was clearly seen that the melting temperature decreased from 54–56 °C to 50–52 °C, and that the observed hysteresis was much smaller at 0.5 °C /min rate of testing. However, a slight hysteretic behavior was still seen for the heating/cooling rate of 0.5 °C /min. 3.6. Cyclic test of paraffin/H þ -Pal composite PCMs The stability properties of composite PCMs should be reliable after extended number of thermal cycling because it was required for the long time service in TES systems. So, thermal reliability for new composite PCMs was one of the key parameters needed to be researched. Generally, DSC measurement under several heating– cooling cycles could provide information of the stability of composite PCMs. Fig. 5 shows changes in the latent heat of Paraffin/ H þ -Pal composite in the cyclic test of heating and cooling. We could see that the latent heat of the paraffin/H þ -Pal composite decreased by 0.6% after 100 cyclic tests, it was considered that the latent heat almost did not change. Subsequently, the latent heat of Paraffin/H þ -Pal composite decreased to 125.12 J g  1 after 500 repetitions of the cyclic test, which accounted for  95% of the initial value. This was probably arising from the leakage of paraffin attached on the surface and within the pores due to the thermal expansion during the melting. Simultaneously, the results indicated that there was a little leakage of paraffin in composite PCM and the absorption of paraffin was not excessive. What was interesting was that the melting temperature remained at 54– 56 °C, which was constant during the whole cyclic test. Accordingly, the paraffin/H þ -Pal composite PCM was outstanding and could be suitable for real thermal energy storage applications.

Fig. 4. DSC curves of paraffin/H þ -Pals composite with different rates of: 0.5 °C/min and 3 °C/min.

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Fig. 5. Latent heat of Paraffin/H þ -Pal in cyclic heating and cooling process.

4. Conclusions In this study, paraffin was used as PCM to make paraffin/Pals composite PCMs with direct impregnation method. The absorption ability of Pals to paraffin, the heat storage properties and the cyclic stability of paraffin/Pals composite PCMs were characterized, following conclusions can be drawn: (1) Direct impregnation method without vacuuming is effective in loading Pals with paraffin. Up to 150 wt% of paraffin can be loaded in H þ -Pal. What is more, its setup is quite simple, cheap and easy of scale-up. (2) The paraffin/H þ -Pal composite PCM had latent heat and a melting temperature of 132.18 J g  1 and 54–56 °C respectively. These composite PCMs were promising candidates for TES system. (3) The hysteretic DSC response was intensified as the heating/ cooling rates were increased, and the lower testing speed exhibited a slight hysteretic behavior. (4) SEM and FTIR results showed paraffin could be well absorbed in Pals pores and had good compatibility with it. In the cyclic test of heating and cooling, paraffin/H þ -Pal composite PCM maintained  95% of the initial latent heat after the cyclic test, which suggested it had good thermal reliability.

Acknowledgments The authors acknowledge the financial support of the Joint Innovation Project of HuaiAn (HAP201418), the Foundation of Xuyi Center of Attapulgite Applied Technology Research Development & Industrialization, Chinese Academy of Sciences (201403), and the Project of Jiangsu Province Science and Technology (BE2013083, BE2014101).

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