fatty acid eutectic form-stable phase change composite nanofibrous membranes for thermal energy storage

fatty acid eutectic form-stable phase change composite nanofibrous membranes for thermal energy storage

Journal Pre-proof Using co-electrospinning method to regulate phase change temperatures of fatty acid eutectic/polystyrene/fatty acid eutectic form-st...

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Journal Pre-proof Using co-electrospinning method to regulate phase change temperatures of fatty acid eutectic/polystyrene/fatty acid eutectic form-stable phase change composite nanofibrous membranes for thermal energy storage Huizhen Ke, Qufu Wei

PII:

S0040-6031(19)30287-4

DOI:

https://doi.org/10.1016/j.tca.2019.178438

Reference:

TCA 178438

To appear in:

Thermochimica Acta

Received Date:

26 March 2019

Revised Date:

17 October 2019

Accepted Date:

20 October 2019

Please cite this article as: Ke H, Wei Q, Using co-electrospinning method to regulate phase change temperatures of fatty acid eutectic/polystyrene/fatty acid eutectic form-stable phase change composite nanofibrous membranes for thermal energy storage, Thermochimica Acta (2019), doi: https://doi.org/10.1016/j.tca.2019.178438

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Using co-electrospinning method to regulate phase change temperatures of fatty acid eutectic/polystyrene/fatty acid eutectic form-stable phase change composite nanofibrous membranes for thermal energy storage

Huizhen Ke1*, Qufu Wei1,2

Fujian Key Laboratory of Novel Functional Textile Fibers and Materials, Faculty of Clothing and

Design, Minjiang University, Fuzhou, Fujian, 350108, China 2

Key Laboratory of Eco-Textiles, Ministry of Education, Jiangnan University, Wuxi, Jiangsu,

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214122, China

Corresponding Author:

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*

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Huizhen Ke, Ph.D. E-mail: [email protected]

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Qufu Wei, Ph.D. E-mail: [email protected]

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Highlights:  Co-electrospinning method was used for regulating phase change temperature.  Fatty acid eutectic/PS/fatty acid eutectic form-stable PCCFMs were developed.  Melting peak temperatures were about 17-24 oC with the 1 oC temperature interval.  Phase change enthalpies of form-stable PCCFMs were approximately 41-60 kJkg-1. 

The membranes were more suitable to use in thermo-regulating fibers and textiles.

Abstract The seven kinds of fatty acid eutectics including capric-lauric acid (CL), capric-myristic acid (CM), capric-palmitic acid (CP), capric-stearic acid (CS), capric-lauric-myristic acid (CLM), capric-lauric-

stearic acid (CLS) and capric-myristic-palmitic acid (CMP) were selected as solid-liquid PCMs. And then co-electrospinning method was used to produce and design innovative fatty acid binary eutectic/polystyrene/fatty acid binary eutectic (i.e., CL/PS/CL, CM/PS/CM, CP/PS/CP, CS/PS/CS, CL/PS/CM, CL/PS/CP, CL/PS/CS, CM/PS/CP, CM/PS/CS and CP/PS/CS) and fatty acid binary eutectic/polystyrene/fatty acid ternary eutectic (i.e., CL/PS/CLM, CM/PS/CLM, CP/PS/CLM, CS/PS/CLM, CL/PS/CLS, CM/PS/CLS, CP/PS/CLS, CS/PS/CLS, CL/PS/CMP, CM/PS/CMP,

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CP/PS/CMP and CS/PS/CMP) form-stable phase change composite fibrous membranes (PCCFMs) with the melting peak temperatures ranging from about 17 to 24 oC with the temperature interval of 1 oC. The scanning electron microscope images revealed that these electrospun PS-based form-

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stable PCCFMs exhibited well and stable nanofibrous network structure. The differential scanning calorimetry results indicated that the phase change temperatures of electrospun PS-based form-

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stable PCCFMs could be regulated to the lower temperature range by changing the type of the

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loaded fatty acid binary and ternary eutectics, and their phase change enthalpies were approximately 41-60 kJ/kg. Based on the consideration of phase change temperatures, the developed electrospun

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PS-based form-stable PCCFMs could be more suitable to use in temperature-regulated fibers and textiles as well as energy efficiency building applications.

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Keywords: co-electrospinning; phase change temperatures; polystyrene; fatty acid eutectics; phase

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change composite fibrous membranes;

1. Introduction Thermal energy storage as a reliable and efficient energy conversion and storage technology has received significant interest of researchers and energy engineers in the recent decades. Thermal

energy can be stored primarily as latent heat storage, sensible heat storage and chemical reaction energy storage [1-2]. Among the above mentioned forms, latent heat storage using phase change materials (PCMs) is one of the most promising methods to store and utilize excess solar energy and thermal energy. At present, a great deal of PCMs have been extensively researched and applied in engineering fields such as automotive applications [2] (e.g., battery thermal management systems), utilization of solar energy (e.g., solar water heaters [3] and solar air heaters [4]), building energy

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conservation [5], cooking applications [6] and thermo-regulated fibers [7] because they exhibit many excellent advantages of low cost, high energy storage density, absorbing and releasing thermal energy at a constant temperature, etc.

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Fatty acids and their derivatives (e.g., fatty acid eutectics and fatty acid esters) as organic solidliquid PCMs are typically used for thermal management of different application areas. However,

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directly realizing efficient thermal energy conversion, storage and utilization for these organic solid-

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liquid PCMs have been hindered owing to the liquid leakage problem during phase change process. A large number of researches have shown that this drawback could be solved by developing form-

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stable PCMs consisting of solid-liquid PCMs and supporting materials. Currently, different types of supporting materials such as inorganic materials (e.g., silica fume [8], diatomite [9], expanded

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perlite [10], expanded graphite [11] and expanded vermiculite [12]) and polymers (e.g., polymethyl

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methacrylate [7], hierarchical porous polymer [13], polyurethane [14], cellulose [15], polyvinylpyrrolidone [16], polyacrylonitrile [17] and polyethylene terephthalate [18]) were considered and developed to stabilize the shape of solid-liquid PCMs (e.g., fatty acids [12], eutectic mixtures [8, 15, 18], paraffin [9-10], polyethylene glycol [11, 14, 16], 1-octadecanol [13] and fatty acid esters [17]).

According to the knowledge of literatures, phase change temperatures and enthalpies are the important thermal performance indicators for the assessment of form-stable PCMs in the practical engineering applications. Generally, phase change enthalpies of form-stable PCMs depend largely on the effective mass ratio of the encapsulated solid-liquid PCMs, whereas their phase change temperatures are typically close to those of the loaded solid-liquid PCMs when there is no chemical reaction or attractive interaction between the solid-liquid PCMs and the supporting materials. For

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examples, Zhang and coworkers [10] studied the thermal properties of the pure paraffin (PA) and four type paraffin-carbon nanotubes/expanded perlite (PA-CNTs/EP) form-stable PCMs with different CNTs mass fractions. Their research results indicated that the phase change enthalpies of

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the PA-CNTs/EP form-stable PCMs decreased with increasing CNT content because only PA can store thermal energy during phase change process, whereas there was no appreciable change on the

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phase transition temperatures. Moreover, the thermal properties of the pure stearic acid (SA), the

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stearic acid/expanded vermiculite (SA/EV) and the stearic acid/expanded vermiculite-carbon (SA/EVC) form-stable PCMs were also experimentally investigated by Zhang and copartners [12].

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The results suggested that the phase transition temperatures of these form-stable PCMs were not significantly affected by the introduction of the EV and EVC acting as supporting materials, while

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the enthalpies associated with melting and freezing for the SA encapsulated in the form-stable

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PCMs were considerably reduced compared to those of the pure SA. Qu et al. [15] analyzed phase change temperatures and enthalpies of tetradecanol-myristic acid eutectic (TD-MA) and tetradecanol-myristic acid/hydroxylpropyl methyl cellulose (TD-MA/HPMC) form-stable PCMs and revealed that the melting and solidifying peak temperatures of the TD-MA/HPMC form-stable PCMs were only 0.23 oC and 1.23 oC higher than those of the pure TD-MA eutectic, respectively. Furthermore, the similar research results about the phase change temperatures and enthalpies of the

capric acid-based binary fatty acid eutectics/polyethylene terephthalate form-stable phase change composite nanofibers were also reported by my research group in the previous literature [18]. In other words, it is very difficult to regulate and obtain the innovative form-stable PCMs having the different phase change temperatures with those of the combined solid-liquid PCMs through direct vacuum impregnation [10, 12], melting blending and ultrasonic method [15] and conventional electrospinning method [18], etc.

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The innovation of this paper is to apply co-electrospinning method to regulate phase change temperatures of the polystyrene-based (PS-based) form-stable phase change composite fibrous membranes (PCCFMs) for storage and retrieval of thermal energy. A series of electrospun fatty acid

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binary eutectic/polystyrene/fatty acid binary eutectic and fatty acid binary eutectic/polystyrene/fatty acid ternary eutectic form-stable PCCFMs were developed and their morphological structures,

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thermal properties were systematically investigated and compared with each other by scanning

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electron microscope (SEM) and differential scanning calorimetry (DSC), respectively.

2. Experimental

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2.1 Materials

According to the data from my previous research papers [19], the fatty acid binary and ternary

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eutectics including capric-lauric acid (CL), capric-myristic acid (CM), capric-palmitic acid (CP),

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capric-stearic acid (CS), capric-lauric-myristic acid (CLM), capric-lauric-stearic acid (CLS) and capric-myristic-palmitic acid (CMP) were prepared as solid-liquid PCMs by using capric acid (CA), lauric acid (LA), myristic acid (MA), palmitic acid (PA) and stearic acid (SA) as raw materials that were purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China) and used without any further purification in this paper. The N,N-dimethyl formamide (DMF) and polystyrene

(PS, Mw=260,000) particles were supplied by Sinopharm Group Chemical Reagent Co., Ltd. and Scientific Polymer Products, Inc., respectively. 2.2 Fabrication of electrospun PS-based form-stable PCCFMs Firstly, the PS particles were added to DMF at the mass percent of 15 % with respect to the DMF solvent. Thereafter, the 15 wt.% PS solution was stirred at room temperature until the solution became clear. Subsequently, the fatty acid binary or ternary eutectic was respectively dissolved into

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the above PS solution to prepare the fatty acid binary eutectic/PS and fatty acid ternary eutectic/PS composite spinning solutions. The mass ratio of fatty acid eutectic/PS was fixed at 1:1. The asprepared fatty acid eutectic/PS composite spinning solutions were placed in the 20 mL syringes

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fitted with a metallic needle with an inner diameter of 0.6 mm. Figure 1 shows the schematic diagram of the co-electrospinning experimental setup. The parameters including a feed rate of 0.5

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mL/h, an applied voltage of 18 kV, and a tip-to-collector working distance of 18 cm were set for the

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co-electrospinning. The preparation scheme of electrospun binary eutectic/PS/binary eutectic (i.e., CL/PS/CL, CM/PS/CM, CP/PS/CP, CS/PS/CS, CL/PS/CM, CL/PS/CP, CL/PS/CS, CM/PS/CP,

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CM/PS/CS and CP/PS/CS) and binary eutectic/PS/ternary eutectic (i.e., CL/PS/CLM, CM/PS/CLM, CP/PS/CLM, CS/PS/CLM, CL/PS/CLS, CM/PS/CLS, CP/PS/CLS, CS/PS/CLS, CL/PS/CMP,

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respectively.

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CM/PS/CMP, CP/PS/CMP and CS/PS/CMP) form-stable PCCFMs are listed in Tables 1 and 2,

2.3 Characterizations The morphological structure of the developed electrospun binary eutectic/PS/binary eutectic and binary eutectic/PS/ternary eutectic form-stable PCCFMs was investigated using scanning electron microscopy (SEM, S-3400N, Hitachi, Japan). The phase change temperatures and enthalpies including peak onset temperatures (To), melting peak temperatures (Tm), freezing peak temperatures

(Tf), peak end temperatures (Te), melting enthalpies (ΔHm) and freezing enthalpies (ΔHf) of these form-stable PCCFMs were determined by differential scanning calorimeter (DSC214Polyma, Netzsch, Germany). Indium was used as the reference for calibration of temperature. The phase change temperatures and enthalpies were calculated based upon the areas under the peaks for solidliquid phase transitions of samples by using the thermal analysis software affiliated with the DSC equipment. The precision of measurements for calorimeter and temperature in the DSC experiments

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was ±2.0 %. The DSC measurement was carried out in the temperature range of -20-60 oC with the heating and cooling rates of 8 °C/min under continuous purging of 50 mL/min N2 inert gas. The melting and freezing enthalpies were calculated based upon the areas under the solid-liquid phase

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change peaks of samples through the thermal analysis software affiliated with the DSC equipment. The experiments were repeated three times for each sample and herein the average values were

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reported. The results acquired from the DSC measurements were reproducible with standard

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deviation typically less than ±2.0 %. The same analytical method has also been reported in my previous published literatures [17-19]. Moreover, the 100 DSC thermal cycling test was also

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conducted to investigate thermal reliability of form-stable phase change composite nanofibrous membranes with the same DSC test conditions. The similar characterization method was also used

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to evaluate the thermal reliability of poly (ethylene glycol)/boron nitride/graphene nanoplatelets

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composite phase change materials [20], PCM/poly(vinyl alcohol) phase change composite fibers [21] and the fatty acid ternary eutectics/polyacrylonitrile/nanographite form-stable phase change composite fibrous membranes [22] in the previously published research papers.

3 Results and discussion 3.1 Morphological structure As shown in Figure 1, co-electrospinning is a simple, convenient and versatile technique for

generating ultrafine fibers at room temperature. The fatty acid binary eutectic/PS and fatty acid ternary eutectic/PS phase change composite nanofibers produced respectively from left and right syringe pumps were deposited on the aluminum foil covered collector at the same time to form ternary composite form-stable PCCFMs under the same electrospinning conditions. The surface morphologies of electrospun binary eutectic/PS/binary eutectic form-stable PCCFMs including CL/PS/CL, CM/PS/CM, CP/PS/CP, CS/PS/CS, CL/PS/CM, CL/PS/CP, CL/PS/CS, CM/PS/CP,

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CM/PS/CS and CP/PS/CS were observed by SEM, as shown in Figures 2 and 3, respectively. The SEM images revealed that the average fiber diameter of electrospun fatty acid binary eutectic/PS phase change composite nanofibers were ranged between about 520 and 700 nm. The adhesion

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among nanofiber intersections could be observed occasionally. Moreover, it could be seen in Figures 2 and 3 that electrospun binary eutectic/PS/binary eutectic nanofibrous membranes

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exhibited well and stable nanofibrous network structure without any leakage of the binary eutectic

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from the composite nanofibers, indicating that electrospun PS nanofibers could be acted as a kind of supporting materials by capillary force adsorption and surface tension. Figures 4, 5 and 6 reveal

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SEM images of electrospun binary eutectic/PS/CLM (i.e., CL/PS/CLM, CM/PS/CLM, CP/PS/CLM and CS/PS/CLM), binary eutectic/PS/CLS (i.e., CL/PS/CLS, CM/PS/CLS, CP/PS/CLS and

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CS/PS/CLS), binary eutectic/PS/CMP (i.e., CL/PS/CMP, CM/PS/CMP, CP/PS/CMP and

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CS/PS/CMP) form-stable PCCFMs, respectively. Compared with the surface morphologies of electrospun binary eutectic/PS/binary eutectic form-stable PCCFMs, the SEM images of electrospun

binary

eutectic/PS/ternary

eutectic

form-stable

PCCFMs

presented

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morphological and structural characteristics. The corresponding average fiber diameters were approximately 480-870 nm. The SEM observation demonstrated that morphological structure of electrospun PS-based form-stable PCCFMs were not significantly affected by the type of the

selected fatty acid eutectics. 3.2 Thermal energy storage properties The phase change temperatures and enthalpies of fatty acid binary and ternary eutectics (i.e., CL, CM, CP, CS, CLM, CLS and CMP) have been reported in my previous published papers [19]. The corresponding thermal performance data are listed in Table 3. The thermal characteristics of the developed electrospun PS-based form-stable PCCFMs loading two identical fatty acid binary

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eutectics (i.e., CL/PS/CL, CM/PS/CM, CP/PS/CP and CS/PS/CS) and the PS-based form-stable PCCFMs loading two fatty acid different binary eutectics (i.e., CL/PS/CM, CL/PS/CP, CL/PS/CS, CM/PS/CP, CM/PS/CS and CP/PS/CS) were also characterized by DSC. The obtained DSC curves

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are plotted in Figures 7 and 8, respectively. Table 4 summarizes the phase change temperatures and enthalpies of the above electrospun binary eutectic/PS/binary eutectic form-stable PCCFMs. As

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revealed in Figures 7 and 8, the endothermic and exothermic peaks representing the solid-liquid

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phase change behaviors of binary eutectics encapsulated in form-stable PCCFMs were clearly observed in the entire temperature region of the DSC analysis, suggesting that these binary eutectics

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were successfully combined into the nanofibrous membranes by co-electrospinning method. As shown in Table 4, the peak positions of the melting temperatures of electrospun PS-based form-

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stable PCCFMs loading two identical binary eutectics were located at about 23 oC, 25 oC, 27 oC and

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30 oC for CL/PS/CL, CM/PS/CM, CP/PS/CP and CS/PS/CS, respectively, which were only 0.7 oC, 0.2 oC, 1.7 oC and 0.3 oC different from those of the corresponding binary eutectics (i.e., CL, CM, CP and CS). However, it is clearly seen from Table 4 that the melting peak temperatures of electrospun PS-based form-stable PCCFMs loading two different binary eutectics (i.e., CL/PS/CM, CL/PS/CP, CL/PS/CS, CM/PS/CP, CM/PS/CS and CP/PS/CS) were obviously lower than those of the corresponding binary eutectics encapsulated in form-stable PCCFMs. Meanwhile, their melting

peak temperatures were also shifted to the lower temperature range of approximately 19-24 oC compared with those of electrospun PS-based form-stable PCCFMs loading two identical binary eutectics due to the synergy effect of the two types of fatty acid eutectics. The DSC analysis indicated that the new phase change temperatures of electrospun PS-based form-stable PCCFMs were designed and obtained by selecting two different kinds of binary eutectics as solid-liquid PCMs during the co-electrospinning process. Additionally, the DSC curves of electrospun binary

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eutectic/PS/CLM (i.e., CL/PS/CLM, CM/PS/CLM, CP/PS/CLM and CS/PS/CLM), binary eutectic/PS/CLS (i.e., CL/PS/CLS, CM/PS/CLS, CP/PS/CLS and CS/PS/CLS) and binary eutectic/PS/CMP (i.e., CL/PS/CMP, CM/PS/CMP, CP/PS/CMP and CS/PS/CMP) form-stable

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PCCFMs are presented in Figures 9, 10 and 11, respectively. Table 5 displays the phase change temperatures (i.e., To, Tm, Tf and Te) and enthalpies (i.e., ΔHm and ΔHf) extrapolated from the DSC

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curves of the above binary eutectic/PS/ternary eutectic form-stable PCCFMs. The reversible phase

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change endothermic and exothermic peaks were also clearly observed in the DSC curves of electrospun binary eutectic/PS/ternary eutectic form-stable PCCFMs. Comparing data from Tables 3

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and 5, there was a great difference about phase change temperatures between electrospun binary eutectic/PS/ternary eutectic form-stable PCCFMs and the loaded fatty acid eutectics. In order to

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better observe the temperature difference, Figures 12 and 13 illustrate the distribution of melting

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peak temperatures of the selected fatty acid eutectics, as well as electrospun binary eutectic/PS/binary eutectic and binary eutectic/PS/ternary eutectic form-stable PCCFMs, respectively. As shown in Figures 12 and 13, the melting peak temperatures of electrospun PSbased form-stable PCCFMs ranged from about 17 oC to 24 oC, which were significantly lower than those of the selected fatty acid eutectics (about 18-31 oC).

The change of phase change

temperatures of electrospun PS-based form-stable PCCFMs could be attributed to the synergistic or

superposition effect of thermal energy storage properties of the two kinds of phase change fibers with the different thermal energy storage properties. Moreover, the eutectic effect of a part of binary and ternary eutectics distributed on the surface of fibers or scattered in the intersection of phase change fibers could also have a certain effect on the thermal energy storage properties of electrospun PS-based form-stable PCCFMs. It is concluded that controlling the types of the loaded fatty acid eutectics during co-electrospinning process can effectively regulate the phase change

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temperatures of the PS-based form-stable PCCFMs. Tables 4 and 5 reveal that the phase change enthalpies of these PS-based form-stable PCCFMs were about 41-60 kJ/kg. Furthermore, comparisons on melting and freezing peak temperatures (Tm and Tf) as well as melting and freezing

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enthalpies (ΔHm and ΔHf) of some form-stable composite PCMs reported in the literatures are listed

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in Table 6 [8, 10-13, 15-18].

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Moreover, in order to determine the thermal reliability of electrospun PS-based form-stable PCCFMs, Figures 14 and 15 display the 100 times DSC thermal cycling curves of the CL/PS/CMP

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and CM/PS/CMP form-stable PCCFMs during the melting and freezing processes. As shown in Figures 14 and 15, the endothermic and exothermic peaks of the form-stable PCCFMs almost

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remain unchanged, suggesting that there was almost no significant change observed in thermal

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energy storage properties during the process of thermal recycling. This result also demonstrate that the developed CL/PS/CMP and CM/PS/CMP form-stable PCCFMs have good thermal reliability for thermal energy storage.

4 Conclusions A series of electrospun PS-based form-stable PCCFMs including CL/PS/CL, CM/PS/CM, CP/PS/CP, CS/PS/CS, CL/PS/CM, CL/PS/CP, CL/PS/CS, CM/PS/CP, CM/PS/CS, CP/PS/CS,

CL/PS/CLM, CM/PS/CLM, CP/PS/CLM, CS/PS/CLM, CL/PS/CLS, CM/PS/CLS, CP/PS/CLS, CS/PS/CLS, CL/PS/CMP, CM/PS/CMP, CP/PS/CMP and CS/PS/CMP were systematically developed through co-electrospinning method and their melting peak temperatures were adjusted to 17-24 oC owing to the synergy effect of the two types of fatty acid eutectics. The SEM images presented that electrospun binary eutectic/PS/binary eutectic and binary eutectic/PS/ternary eutectic form-stable PCCFMs exhibited good fiber morphology. Their phase change enthalpies were

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determined to be approximately 41-60 kJ/kg. It is concluded that these electrospun PS-based formstable PCCFMs could be considered as a kind of preferred potential thermal energy storage materials for temperature-regulated fibers and energy-saving building applications because of their

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applicable phase change temperatures.

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Conflict of interest

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The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict

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of interest in connection with the work submitted.

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Acknowledgements

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This research was financially supported by the National Natural Science Foundation of China (No. 51706092), Natural Science Foundation of Fujian Province of China (No. 2018J05091), Cultivation Program for Outstanding Young Scholars in Colleges and Universities of Fujian Province of China (2018), Education Science Research Program for Young and Middle-aged Teacher of Fujian Province of China (No. JAT170445), Open Project Program of Fujian Key Laboratory of Novel

Functional Textile Fibers and Materials, Minjiang University (No. FKLTFM1904) and the Science and Technology Pre-research Program of Minjiang University (No. MJY16001).

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[16] X.G. Zhang, J.X. Qiao, H. Zhao, Z.H. Huang, Y.G. Liu, M.H. Fang, X.W. Wu, X. Min, Preparation and performance of novel polyvinylpyrrolidone/polyethylene glycol phase change materials composite fibers by centrifugal spinning, Chem. Phys. Lett. 691 (2018) 314-318. [17] H.Z. Ke, Y.G. Li, A series of electrospun fatty acid ester/polyacrylonitrile phase change composite nanofibers as novel form-stable phase change materials for storage and retrieval of thermal energy, Text. Res. J. 87 (2017) 2314-2322.

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[18] H.Z. Ke, D.W. Li, H.D. Zhang, X.L. Wang, Y.B. Cai, F.L. Huang, Q.F. Wei, Electrospun formstable phase change composite nanofibers consisting of capric acid-based binary fatty acid eutectics and polyethylene terephthalate, Fiber. Polym. 14 (2013) 89-99.

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[19] H.Z. Ke, Phase diagrams, eutectic mass ratios and thermal energy storage properties of multiple fatty acid eutectics as novel solid-liquid phase change materials for storage and retrieval of

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thermal energy, Appl. Therm. Eng. 113 (2017) 1319-1331.

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[20] J. Yang, L.S. Tang, R.Y. Bao, L. Bai, Z.Y. Liu, W. Yang, B.H. Xie, M.B. Yang, Largely enhanced thermal conductivity of poly (ethylene glycol)/boron nitride composite phase change

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materials for solar-thermal-electric energy conversion and storage with very low content of graphene nanoplatelets, Chem. Eng. J. 315 (2017) 481-490.

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[21] E. Zdraveva, J. Fang, B. Mijovic, T. Lin, Electrospun poly(vinyl alcohol)/phase change

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material fibers: morphology, heat properties, and stability, Ind. Eng. Chem. Res. 54 (2015) 87068712.

[22] H.Z. Ke. Investigation of the effects of nano-graphite on morphological structure and thermal performances of fatty acid ternary eutectics/polyacrylonitrile/nanographite form-stable phase change composite fibrous membranes for thermal energy storage, Sol. Energy 173 (2018) 11971206.

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Figure Captions Figure 1 Schematic diagram of the experimental setup for co-electrospinning

Figure 2 Representative SEM images of electrospun PS-based form-stable PCCFMs loading two

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identical binary eutectics: (a) CL/PS/CL, (b) CM/PS/CM, (c) CP/PS/CP and (d) CS/PS/CS

Figure 3 Representative SEM images of electrospun PS-based form-stable PCCFMs loading two

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different binary eutectics: (a) CL/PS/CM, (b) CL/PS/CP, (c) CL/PS/CS, (d) CM/PS/CP, (e)

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CM/PS/CS and (f) CP/PS/CS

Figure 4 Representative SEM images of electrospun binary eutectic/PS/CLM form-stable PCCFMs:

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(a) CL/PS/CLM, (b) CM/PS/CLM, (c) CP/PS/CLM and (d) CS/PS/CLM Figure 5 Representative SEM images of electrospun binary eutectic/PS/CLS form-stable PCCFMs:

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(a) CL/PS/CLS, (b) CM/PS/CLS, (c) CP/PS/CLS and (d) CS/PS/CLS

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Figure 6 Representative SEM images of electrospun binary eutectic/PS/CMP form-stable PCCFMs: (a) CL/PS/CMP, (b) CM/PS/CMP, (c) CP/PS/CMP and (d) CS/PS/CMP Figure 7 DSC curves of electrospun PS-based form-stable PCCFMs loading two identical binary eutectics during heating and cooling processes Figure 8 DSC curves of electrospun PS-based form-stable PCCFMs loading two different binary eutectics during heating and cooling processes

Figure 9 DSC curves of electrospun binary eutectic/PS/CLM form-stable PCCFMs during heating and cooling processes Figure 10 DSC curves of electrospun binary eutectic/PS/CLS form-stable PCCFMs during heating and cooling processes Figure 11 DSC curves of electrospun binary eutectic/PS/CMP form-stable PCCFMs during heating and cooling processes

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Figure 12 Distribution of melting peak temperatures of fatty acid binary and ternary eutectics Figure 13 Distribution of melting peak temperatures of electrospun binary eutectic/PS/binary eutectic and binary eutectic/PS/ternary eutectic form-stable PCCFMs

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Figure 14 DSC thermal cycling curves of the CL/PS/CMP form-stable PCCFMs during heating and cooling processes

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Figure 15 DSC thermal cycling curves of the CM/PS/CMP form-stable PCCFMs during heating and

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cooling processes

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Figure 12

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Figure 13

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Figure 14

Figure 15

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Table 1 The sample codes of electrospun fatty acid binary eutectic/PS/fatty acid binary eutectic form-stable PCCFMs Solutions (syringe 1)

Solutions (syringe 2)

form-stable PCCFMs

1 2 3 4

CL/PS CM/PS CP/PS CS/PS

CL/PS CM/PS CP/PS CS/PS

CL/PS/CL CM/PS/CM CP/PS/CP CS/PS/CS

5 6 7 8 9 10

CL/PS CL/PS CL/PS CM/PS CM/PS CP/PS

CM/PS CP/PS CS/PS CP/PS CS/PS CS/PS

CL/PS/CM CL/PS/CP CL/PS/CS CM/PS/CP CM/PS/CS CP/PS/CS

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Number

Table 2 The sample codes of electrospun fatty acid binary eutectic/PS/fatty acid ternary eutectic form-stable PCCFMs Solutions (syringe 2)

1 2 3 4 5 6 7 8 9 10 11 12

CL/PS CM/PS CP/PS CS/PS CL/PS CM/PS CP/PS CS/PS CL/PS CM/PS CP/PS CS/PS

CLM/PS CLM/PS CLM/PS CLM/PS CLS/PS CLS/PS CLS/PS CLS/PS CMP/PS CMP/PS CMP/PS CMP/PS

form-stable PCCFMs CL/PS/CLM CM/PS/CLM CP/PS/CLM CS/PS/CLM CL/PS/CLS CM/PS/CLS CP/PS/CLS CS/PS/CLS CL/PS/CMP CM/PS/CMP CP/PS/CMP CS/PS/CMP

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Solutions (syringe 1)

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Number

Table 3 The peak onset temperatures (To), peak end temperatures (Te), melting peak temperatures (Tm), freezing peak temperatures (Tf), melting enthalpies (ΔHm) and freezing enthalpies (ΔHf) of fatty acid binary and ternary eutectics fatty acid

Freezing process

To

Tm

Te

ΔHm

To

Tf

Te

ΔHf

(oC)

(oC)

(oC)

(kJ/kg)

(oC)

(oC)

(oC)

(kJ/kg)

CL

19.3±0.4

22.7±0.5

29.4±0.6

127.2±2.5

18.4±0.4

CM

20.1±0.4

26.0±0.5

31.0±0.6

155.2±3.1

17.1±0.3

CP

22.9±0.5

28.7±0.6

34.7±0.7

141.4±2.8

20.4±0.4

CS

25.7±0.5

31.2±0.6

33.4±0.7

156.8±3.1

23.6±0.5

CLM 15.8±0.3

18.7±0.4

22.1±0.4

131.4±2.6

13.4±0.3

CLS

17.9±0.4

22.8±0.5

29.5±0.6

132.5±2.7

CMP 17.6±0.4

22.3±0.5

28.5±0.6

136.8±2.7

9.4±0.2

125.2±2.5

6.5±0.1

145.9±2.9

19.3±0.4

10.7±0.2

136.1±2.7

22.9±0.5

17.3±0.3

147.1±2.9

11.1±0.2

7.6±0.2

129.7±2.6

16.9±0.3

13.1±0.3

5.1±0.1

129.7±2.6

15.2±0.3

12.8±0.3

6.8±0.1

132.6±2.7

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eutectics

Melting process

Table 4 The peak onset temperatures (To), peak end temperatures (Te), melting peak temperatures (Tm), freezing peak temperatures (Tf), melting enthalpies (ΔHm) and freezing enthalpies (ΔHf) of electrospun binary eutectic/PS/binary eutectic form-stable PCCFMs form-

Melting process

Freezing process

stable

To

Tm

Te

ΔHm

To

Tf

Te

PCCFMs

(oC)

(oC)

(oC)

(kJ/kg)

(oC)

(oC)

(oC)

23.4±0.5

28.3±0.6

51.8±1.0

17.6±0.4

CM/PS/CM

20.6±0.4

25.8±0.5

30.6±0.6 42.4±0.8

CP/PS/CP

22.4±0.5

27.0±0.5

31.5±0.6

CS/PS/CS

24.7±0.5

30.9±0.6

CL/PS/CM

16.2±0.3

CL/PS/CP

(kJ/kg)

16.3±0.3

10.1±0.2 46.5±0.9

17.3±0.4

14.6±0.3

9.4±0.2

48.7±1.0

17.3±0.4

15.1±0.3 11.9±0.2 48.1±1.0

34.8±0.7

42.1±0.8

19.1±0.4

17.1±0.3 10.7±0.2 41.4±0.8

21.5±0.4

25.5±0.5

52.1±1.0

16.4±0.3

16.0±0.3

13.2±0.3

20.0±0.4

24.7±0.5 51.4±1.0

14.4±0.3

13.5±0.3 10.9±0.2 50.8±1.0

CL/PS/CS

16.8±0.3

21.0±0.4

24.8±0.5

56.4±1.1

16.2±0.3

16.1±0.3 7.6±0.2

55.7±1.1

CM/PS/CP

18.2±0.4

23.1±0.5

27.9±0.6

49.5±1.0

15.2±0.3

13.3±0.3 7.2±0.1

47.3±0.9

CM/PS/CS

18.0±0.4

23.3±0.5

27.5±0.6

42.3±0.8

15.8±0.3

12.2±0.2 8.1±0.2

42.0±0.8

CP/PS/CS

17.1±0.3

24.7±0.5

53.7±1.1

16.4±0.3

15.8±0.3 9.4±0.2

52.8±1.1

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18.2±0.4

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CL/PS/CL

ΔHf

8.9±0.2

42.1±0.8

51.3±1.0

Table 5 The peak onset temperatures (To), peak end temperatures (Te), melting peak temperatures (Tm), freezing peak temperatures (Tf), melting enthalpies (ΔHm) and freezing enthalpies (ΔHf) of electrospun binary eutectic/PS/CLM, binary eutectic/PS/CLS and binary eutectic/PS/CMP formstable PCCFMs form-stable

Te

ΔHm

To

(oC)

(oC)

(oC)

(kJ/kg)

(oC)

15.7±0.3 13.1±0.3 13.8±0.3 15.6±0.3 19.2±0.4 15.3±0.3 10.1±0.2 11.7±0.2 14.1±0.3 11.3±0.2 13.4±0.3 16.7±0.3

19.6±0.4 21.6±0.4 20.7±0.4 22.8±0.5 23.7±0.5 19.0±0.4 17.1±0.3 19.0±0.4 18.0±0.4 21.7±0.4 19.9±0.4 21.3±0.4

23.7±0.5 25.3±0.5 24.1±0.5 27.2±0.5 27.9±0.6 23.1±0.5 22.3±0.4 23.1±0.5 21.8±0.4 25.3±0.5 23.8±0.5 23.5±0.5

45.6±0.9 56.5±1.1 42.2±0.8 45.3±0.9 45.5±0.9 48.2±1.0 43.7±0.9 41.9±0.8 53.4±1.1 50.2±1.0 52.1±1.0 60.9±1.2

15.0±0.3 14.6±0.3 11.5±0.2 15.6±0.3 17.7±0.4 11.7±0.2 12.0±0.2 13.3±0.3 10.2±0.2 16.7±0.3 13.6±0.3 12.3±0.2

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Te

ΔHf

(oC)

(oC)

(kJ/kg)

14.5±0.3 14.0±0.3 8.3±0.2 15.2±0.3 16.4±0.3 8.7±0.2 11.3±0.2 12.8±0.3 8.2±0.2 14.9±0.3 13.1±0.3 10.4±0.2

13.7±0.3 11.4±0.2 4.2±0.1 8.4±0.2 8.1±0.2 3.7±0.1 8.8±0.2 11.1±0.2 4.6±0.1 8.9±0.2 10.7±0.2 7.3±0.1

42.6±0.9 56.9±1.1 42.3±0.8 41.6±0.8 45.3±0.9 49.1±1.0 43.5±0.9 41.9±0.8 52.4±1.0 50.8±1.0 51.5±1.0 60.7±1.2

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CL/PS/CLM CM/PS/CLM CP/PS/CLM CS/PS/CLM CL/PS/CLS CM/PS/CLS CP/PS/CLS CS/PS/CLS CL/PS/CMP CM/PS/CMP CP/PS/CMP CS/PS/CMP

Freezing process

To

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PCCFMs

Melting process

Table 6 Comparisons on melting and freezing peak temperatures (Tm and Tf) and enthalpies (ΔHm and ΔHf) of some form-stable composite PCMs reported in the literatures form-stable

Melting process Tm

ΔHm

Tf

ΔHf

(oC)

(kJ/kg)

(oC)

(kJ/kg)

CA-PA (32 wt.%)/SF

25.53

48.98

18.83

paraffin (62.52 wt.%)/EP

48.51

105.39

44.15

103.85

[10]

PEG (83.33 wt.%)/EG

45.33

128.24

40.91

120.45

[11]

SA (63.12 wt.%)/EVC

67.12

134.31

66.16

135.94

[12]

1-octadecanol (75 wt.%)/HPP

58.50

169.1

TD-MA (49.46 wt.%)/HPMC

34.61

206.45

PEG (50 wt.%)/PVP

52.70

BS (45 wt.%)/PAN CA-MA (50 wt.%)/PET

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48.69

References

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[13]

31.09

204.59

[15]

81.40

37.40

81.90

[16]

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composite PCMs

Freezing process

21.20

55.10

12.61

55.13

[17]

24.98

58.06

13.88

57.62

[18]

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CA-PA: capric-palmitic acid; SF: silica fume; EP: expanded perlite; PEG: polyethylene glycol; EG: expanded graphite; SA: stearic acid; EVC: a composite of expanded vermiculite and carbon; HPP:

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hierarchical porous polymer; TD-MA: tetradecanol-myristic acid eutectic mixture; HPMC:

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hydroxylpropyl methyl cellulose; PVP: polyvinylpyrrolidone; BS: butyl stearate; PAN: polyacrylonitrile; CA-MA: capric-myristic acid; PET: polyethylene terephthalate