paraffin composite phase change materials for advanced thermal energy storage and management

Chemical Engineering Journal 385 (2020) 123958

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Shape-stabilized hydrated salt/paraffin composite phase change materials for advanced thermal energy storage and management

T

Chuanfei Shena, Xiang Lia, Guoqing Yanga, Yanbin Wanga, Lunyu Zhaoa, Zhiping Maoa,b,c, ⁎ ⁎ Bijia Wanga,c, Xueling Fenga,b,c, , Xiaofeng Suia,c, a

Key Lab of Science and Technology of Eco-textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, People’s Republic of China b National Engineering Research Center for Dyeing and Finishing of Textiles, Donghua University, Shanghai 201620, People’s Republic of China c Innovation Center for Textile Science and Technology of DHU, Donghua University, Shanghai 201620, People’s Republic of China

HIGHLIGHTS

salt based, shape-stabilized PCMs were fabricated by simple method. • Hydrated transition enthalpy (227.3 J/g) was obtained. • AThehighhybrid materials exhibited excellent thermal stability, low supercooling degree. • ARTICLE INFO

ABSTRACT

Keywords: Hydrated salt Paraffin Phase change materials Thermal stability Supercooling degree

Thermal energy storage and management have attracted considerable interest in the field of sustainable control and utilization of energy. Thermal energy storage materials with excellent thermal properties and shape stability are in high demand. Herein, we developed a simple and effective method to fabricate hydrated salt / paraffin composite (HPC) shape-stabilized phase change materials (SSPCMs). Hydrated salt was emulsified into paraffin by an inverse emulsion template method to obtain HPC. Owing to its low volatility, paraffin enhanced the thermal stability of the hydrated salt by preventing its direct contact with the environment. Furthermore, after its crystallization, paraffin provided nucleation sites and functioned as a nucleating agent to promote the crystallization of the hydrated salt. The HPC was then simultaneously impregnated into cellulose sponge (CS), forming the SSPCMs, which exhibited excellent thermal stability, high energy storage density with a phase transition enthalpy of 227.3 J/g, and a reduced supercooling degree. Besides, there was negligible leakage during the test. The efficiency of the SSPCMs as temperature management materials was then tested by using them as a lining in a fully enclosed protective clothing.

1. Introduction Thermal energy storage and management have been considered as a prospective technology for sustainable control and utilization of energy. The increase in energy demand and awareness regarding climate change caused by energy usage has stimulated the search for efficient and advanced energy management systems [1]. Phase change materials (PCMs), the latent heat energy storage materials, can store and release large amounts of waste thermal energy during their phase transition; thus, they have tremendous potential for efficient utilization of heat energy [2–5]. Based on their phase change temperature, PCMs have been employed in areas such as building construction for cooling and

⁎

heating, solar energy systems for heat storage and release, and textiles or astronautics for thermal management [6,7]. Hydrated salts as inorganic PCMs exhibit characteristics such as high thermal conductivity, high energy storage density, and nonflammability; besides, these are non-toxic and low-cost materials [8–12]. However, the applications of hydrated salts in thermal energy management systems are often restricted owing to their disadvantages, which include problems related to supercooling, phase segregation (especially water evaporation), shape instability, and their corrosive effect on the metals used for heat transfer [13]. To solve the abovementioned limitations of the inorganic PCMs, several methods for encapsulating hydrated salts were developed [2,3,14–18]. Many studies

Corresponding authors at: No. 2999 North Renmin Road, Shanghai 201620, People’s Republic of China. E-mail addresses: [email protected] (X. Feng), [email protected] (X. Sui).

https://doi.org/10.1016/j.cej.2019.123958 Received 16 September 2019; Received in revised form 23 December 2019; Accepted 26 December 2019 Available online 27 December 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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on organic PCMs have demonstrated that encapsulation improved the thermophysical performance of PCMs by providing a large heat transfer area and reducing their reactivity with the outside environment [19–28]. However, compared with the organic PCMs, encapsulation of inorganic PCMs, such as hydrated salts is difficult because of their inherent properties. Therefore, different materials have been used to encapsulate PCMs to form core–shell or shape-stabilized PCMs. Stabilizing hydrated salts in porous materials could reduce their supercooling degree because of the stronger restriction of the nanocontainers. For instance, several examples have been reported in the literatures:[29–41] Zhang et al. impregnated magnesium nitrate hexahydrate into hydrophilic expanded graphite (EG), to obtain a latent heat density of 135.12 J/g and a decrease in the supercooling temperature from 20.3 °C to 16.91 °C [29]. Xie et al. fully filled eutectic hydrated salt into expanded vermiculite (EV) and reported that the supercooling degree decreased from 15.43 °C to 9.55 °C [35]. Rao et al. used expanded perlite (EP) as supporting materials, and reported that the composite PCMs exhibited a low supercooling degree and a higher latent heat density of 151 J/g [36]. Han et al. restricted eutectic hydrated salts in the pores of boron nitride foams (BNFs), and observed a decrease in the supercooling degree from 16.08 °C to 8.48 °C [41]. Deng et al. prepared disodium hydrogen phosphate dodecahydrate / expanded vermiculite (EV) shape-stabilized PCMs (SSPCMs), which exhibited suppressed supercooling [42]. Moreover, water evaporation must also be considered for the practical application of hydrated salts: Wu et al. developed paraffin wax coated hydrated salt / EG composites, which inhibited phase segregation of the hydrated salts and reduced the supercooling. Furthermore, the composites exhibited outstanding thermal stability between 25 °C and 50 °C [34]. Zhang et al. fabricated calcium chloride hexahydrate/ diatomite/paraffin composites PCMs and observed that the paraffin coating inhibited the supercooling of the system and increased the crystallization temperatures from −10 °C to 3.2 °C [38]. Therefore, novel carrier materials / stabilizers and facile methods to encapsulate hydrated salts remain a crucial challenge for developing economic thermal energy storage and management systems with advanced performances, such as higher storage density, suppressed supercooling effect, and better shape stability. This study proposes a facile and green strategy to fabricate SSPCMs with an enhanced thermal performance for advanced thermal energy storage and management. The latent heat of the resulting composite SSPCMs depends on the load capacity and nanoconfinement effects of the carrier materials [43]; hence, the choice of carrier materials is crucial. In this study, lightweighted cellulose sponge (CS) was employed as the carrier material. The 3D interconnected networks of the CS served as a supporting skeleton to improve the shape stability of the PCMs during phase transition, while maintaining their high weight fraction. The hydrated salt was first emulsified and isolated by paraffin; The hydrated salt / paraffin composites (HPC) could then be easily absorbed and stabilized in the hydrophobically modified CS, forming the SSPCMs, which suppressed the supercooling of the hydrated salt. The emulsification of hydrated salt with paraffin played an important role in the thermal performance enhancement of the SSPCMs. As shown in Scheme 1, paraffin as a continuous phase of the emulsion not only prevented water evaporation from the hydrated salt, but also promoted its crystallization and reduced the supercooling. Moreover, the emulsification also altered the hydrophilicity of the hydrated salt, facilitating the shape-stabilization by the porous CS. The shape-stabilizing and leakage-preventing CS occupied only a small weight fraction (7.3%) of the SSPCMs. The resultant composites exhibited an extraordinary thermal energy storage performance (ΔHm = 227.3 J/g), reduced supercooling from 34.2 °C to 19.5 °C, increased thermal stability, and excellent shape stability. To the best of our knowledge, this is a relatively high energy storage density obtained for encapsulated PCMs. The application of the SSPCMs as a lining for a fully enclosed protective

clothing was also demonstrated, establishing their excellent potential as a temperature management system. 2. Experimental 2.1. Materials Disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O, AR) was adopted as the hydrated salt PCM (Sinopharm Chemical Reagent Co., Ltd.); n-eicosane (purity greater than 99%) was selected as the paraffin PCM (Thermo Fisher Scientific Co., Ltd.). Sorbitan monooleate (Span 80) was used as the emulsifier (Sinopharm Chemical Reagent Co., Ltd.); 2 wt% of pulp-derived cellulose nanofibers (CNFs) suspension (30 nm in diameter, several μm in length) was obtained from Tianjin Haojia Cellulose Co., Ltd. (Tianjin, China); vinyltrimethoxysilane (VTMO, purity greater than 98%) was purchased from Adamas Reagent Co., Ltd.; and the remaining chemical reagents and solvents used in this work were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used as received. 2.2. Preparation of HPC by the inverse emulsion template method Paraffin (10 g) and Span 80 (2 g) were mixed and melted at 60 °C in a constant temperature bath. Then, 20 g of melted hydrated salt was added into the melted paraffin / Span 80 system. The mixture was emulsified by a homogenizer (IKA T-18 Ultra Turrax Digital Homogenizer, Germany) at a speed of 10,000 rpm for 5 min, to prepare the HPC. 2.3. Synthesis of hydrophobically modified cellulose sponge To 50 g of CNFs suspension, 1 g of VTMO was added, and hydrochloric acid was used to maintain the pH between 3.5 and 4.0. Then, the mixture was mixed well by mechanical stirring at room temperature (25 °C) for 2 h and freeze-dried at −50 °C for 24 h on a DGJT-10 freezedryer (Songyuan Huaxing Technology Develop Co., Ltd., China). Ultimately, the sponge was maintained at 110 °C for 1 h to promote cross-linking between the cellulose and VTMO [44]. 2.4. Fabrication of SSPCMs The SSPCMs were fabricated by the impregnation method [23]. The CS was completely impregnated with the melted HPC and the container was kept shaking for 20 min. This procedure ensured that the HPC were evenly infused into the sponge. Finally, the SSPCMs were obtained. The emulsion on the surface of SSPCMs was wiped off. Scheme 1 shows a schematic diagram of the forming mechanism of the SSPCMs. 2.5. Characterization The obtained emulsion was observed using an optical microscope (ECLIPSE 80i, NIKON Corporation, Japan). The morphologies of the sponge and the SSPCMs were characterized using SEM (Hitachi TM1000, Japan). The samples were ion sputtered with a Au target, using an ion-sputtering system (SBC-12 Sputter Coater, KYKY Technology Development Ltd., China) under vacuum at an acceleration voltage of 15 kV. The thermal properties of the samples were measured using differential scanning calorimetry (DSC 214 Polyma, NETZSCH, Germany). The test was performed under a nitrogen atmosphere in the scanning range from −20 °C to 70 °C at a scanning rate of 10 °C min−1. The thermal properties of the samples were evaluated by thermogravimetric analysis (TGA 209F3, NETZSCH, Germany). Under nitrogen purge, the samples were heated from 30 °C to 600 °C at a rate of 10 °C min−1. Fourier transform infrared (FTIR) analyses were performed using a 2

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Scheme 1. Schematic illustration of the preparation process of SSPCMs.

of the hydrated salt / paraffin inverse emulsion (Fig. 1a) and the ImageJ software was used to calculate its average particle size, which was approximately 300 nm as indicated by the distribution shown in the inset to Fig. 1a. Moreover, the particle size distribution of the emulsion is observed to be very uniform, allowing an even absorption of the emulsion by the sponge. As shown in Fig. 1b, the SSPCMs can be processed into various shapes, indicating the tailorable properties of the composites. Fig. 1c shows the SEM image of the cross-section of CS, which

FT-IR Spectrometer (Spectrum Two, PerkinElmer, USA) equipped with an attenuated total reflectance (ATR) accessory. All spectra were recorded in the 4000 – 400 cm−1 range and at a resolution of 4 cm−1. 3. Results and discussion 3.1. Morphology and chemical composition of SSPCMs The optical microscope was used to investigate the microstructure

Fig. 1. Morphology of the SSPCMs. (a) Optical microscope image of the hydrated salt / paraffin inverse emulsion, inset shows the size distribution of more than 200 emulsion droplets. (b) Digital picture of the SSPCMs. (c, d, e, f) SEM images of the cross-section of CS, SSPCMs, paraffin / CS, and a partial enlargement of Fig. 1d, respectively. 3

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Fig. 2. FTIR spectra of (a) hydrated salt, paraffin and HPC, (b) CNF, CS, HPC, and SSPCMs.

demonstrates its porous structure. The pore size is about 50–100 µm. Because of the capillary force provided by these pores, the emulsion can easily be sucked into the CS. Fig. 1e shows a cross-section of the CS filled with pure paraffin (paraffin / CS). Clearly, the pores are almost filled with paraffin, indicating the absorption capacity of the CS. The dense layer structure in the figure is due to the crystallization of paraffin. Fig. 1d and f show the SEM images of the cross-section of CSabsorbed HPC. We observe that the crystallized paraffin enwraps the hydrated salt particles. The holes observed in the figures are due to the volumetric shrinkage during paraffin crystallization, which primarily promotes the crystallization of the hydrated salt. Fig. 2 displays the FTIR spectra of the hydrated salt, paraffin, HPC, CNF, CS, and SSPCMs. The FTIR spectra of Na2HPO4·12H2O shown in Fig. 2a is similar to the result reported in an earlier study [45]. The wide peak between 3000 and 3700 cm−1 mainly belongs to the stretching band of O–H, while the peak at 1668 cm−1 is attributed to the vibration of H-O–H. Meanwhile, the main characteristic peak of PO43- occurs at 1075 cm−1. The FTIR spectra of n-eicosane is similar to the reported result in an earlier study [46]. The peaks at 2962, 2910, and 2848 cm−1 are the C–H stretching peaks while those at 1468 and 715 cm−1 are characteristic for n-eicosane. It is observed that the characteristic absorption peaks of the HPC are a mixture of the spectra of the hydrated salt and paraffin. Fig. 2b shows the FTIR spectra of CNF and CS. We observe that the FTIR spectrum of the modified CS shows new peaks at 1601, 1048, and 1276 cm−1, all of which are attributed to the vibrations of the vinyl groups in VTMO [44]. Because the weight fraction of CS is too small in the SSPCMs, the spectrum of the SSPCMs is the same as that of HPC.

paraffin wax can retard the water evaporation rate, as the water droplets are completely enwrapped by the inverse emulsion template method [48,49]. Therefore, after the emulsification, the HPC achieved excellent thermal stability at temperatures below 80 °C: the HPC is an emulsion system with a 2:1 hydrated salt / paraffin ratio, with a water content of 40 wt%, and approximately 26 wt% residual salt content. The TGA curve shows that the theoretical value is consistent with the experimental value. After the emulsion was absorbed by CS, the water evaporation rate is observed to be slightly higher than that in HPC, between 80 and 120 °C. This is mostly attributed to the capillary action and intermolecular forces between the CS and paraffin, which reduce the protective effect of paraffin on water. However, the CS also blocks the water molecules from evaporating outward macroscopically; so, the evaporation rate of water is slightly lower than that in HPC between 120 and 150 °C. The final residual mass of SSPCMs is 10% less than that of HPC, which is attributed to the addition and decomposition of the CS. Emulsions with different hydrated salt / paraffin ratios (1:1, 3:1, 4:1) were also prepared, which exhibited excellent thermal performance. However, those with ratios of 3:1 and 4:1 could not be uniformly impregnated into the sponge because of the increased viscosity of the emulsion. Fig. 3b shows the TGA curves of the emulsion composites with different hydrated salt / paraffin ratios, which further confirm the excellent thermal stability of the emulsion system. 3.3. Phase transition behaviors of SSPCMs The phase transition behaviors of the composite PCMs were investigated by differential scanning calorimetry (DSC). All specific data are presented in Table 1, and Fig. 4a shows the melting and cooling DSC curves of the hydrated salt, paraffin, HPC, and SSPCMs. The hydrated salt exhibits a wide melting peak and a sharp crystallization peak. It has a very high melting enthalpy, but the supercooling degree (ΔT) is 34.2 °C, which indicates that the practical application of the hydrated salt is very difficult. Paraffin exhibits a sharp melting peak, while its melting temperature is very close to that of the hydrated salt. The supercooling degree of paraffin is 5.6 °C, which is much lower than that of the DHPD. When the hydrated salt was emulsified by paraffin, HPC was obtained. As illustrated in Fig. 4a, HPC exhibits a complete melting peak, indicating the consistent melting process of hydrated salt and paraffin in the emulsion. The supercooling degree of the hydrated salt fraction in HPC is remarkably improved, and its crystallization temperature is increased from 2.4 °C to 17.0 °C. This is because paraffin crystallizes before the hydrated salt, after which it promotes the crystallization of the hydrated salt by providing nucleation positions on its large specific surface areas; thus, paraffin functions as a nucleating agent [34,38]. The choice of n-eicosane as the paraffin

3.2. Thermal stability of SSPCMs Fig. 3a shows the thermogravimetric analysis (TGA) curves of pure hydrated salt, pure paraffin, HPC, and SSPCMs at a heating rate of 10 °C min−1. The hydrated salt we used in this study was disodium hydrogen phosphate dodecahydrate (DHPD, Na2HPO4·12H2O), which exhibits incongruent melting during its solid–liquid transformation [47]. The water content of pure DHPD is about 60 wt%, and its water loss due to evaporation begins at 30 °C, as shown in Fig. 3a. At 70 °C, DHPD exhibits a weight loss of 25 wt%, which is the weight equal to approximately 5 crystal water. When the temperature increases beyond 100 °C, DHPD loses another 7 crystal water. This could be fatal to the thermal stability of the hydrated salts owing to the extensive water evaporation around the phase transition temperature, which significantly limits the subsequent applications of the hydrated salt in exposed environments. Compared with pure DHPD, pure paraffin has an excellent thermal stability at temperatures below 200 °C. Moreover, it is well known that 4

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Fig. 3. (a) TGA thermograms of pure hydrated salt, pure paraffin, HPC, and SSPCMs. (b) TGA thermograms of the hydrated salt / paraffin inverse emulsion with different ratios.

was significant as it crystallizes between the melting temperature of the HPC and the crystallization temperature of the hydrated salt. Therefore, the hydrated salt crystallizes immediately after the early crystallization of paraffin, which significantly alleviates the supercooling of the hydrated salt. Finally, we observe that HPC exhibits a supercooling degree of 19.7 °C. After being absorbed by the CS, its melting temperature slightly shifts toward the left. The supercooling degree of the SSPCMs is further reduced to 19.5 °C, which may be due to the capillary action and intermolecular forces between the CS and HPC. Fig. 4b shows no significant improvement in the supercooling of the emulsion composites with a 1:1 hydrated salt / paraffin ratio. This might be because too much paraffin wax has stronger nanoconfinement effect on the hydrated salt, which limits the improvement in the supercooling [43]. Fig. S1 shows the DSC curves of the SSPCMs with emulsion composites of different ratios. The data of these DSC curves are summarized in Table S1. For SSPCMs with hydrated salt / paraffin ratio of 3:1 and 4:1, the exothermic enthalpy of the paraffin fraction is higher than the enthalpy of the corresponding hydrated salt / paraffin emulsion. This further indicates that the emulsion composites with hydrated salt / paraffin ratios of 3:1 and 4:1 could not be uniformly impregnated into the sponge, owing to the increased viscosity of the emulsion. Fig. 4c demonstrates the thermal cycle performance of the SSPCMs: We observe no obvious change in the phase change temperature even after 50 heating and cooling cycles. The SSPCMs maintain an excellent phase transition performance with increasing number of cycles. Fig. 4d shows the phase transition enthalpy of the hydrated salt, paraffin, HPC, SSPCMs, and SSPCMs after 50 cycles. Their latent heats (ΔHm: melting enthalpy) are 275.2, 261.4, 245.3, 227.3, and 227.0 J/g, respectively.

Theoretically, the melting enthalpy of HPC is 253.4 J/g, which is calculated by Eq. (1), using the properties of pure hydrated salt, paraffin, and span 80:

( Hm )HPC =

mHydratedsalt

mHydratedsalt + mParaffin + mSpan80 × ( Hm )Hydratedsalt +

mParaffin mHydratedsalt + mParaffin + mSpan80

× ( Hm)Paraffin (1)

The differences between the theoretical and experimental values may be due to the nanoconfinement of the paraffin and hydrated salts [34,43]. The melting enthalpy of the SSPCMs is 7.2% lower than that of HPC, because of the existence of CS. However, there is no significant drop in the melting enthalpy of the SSPCMs after 50 heating and cooling cycles. The recyclability of the SSPCMs also shows that the phase separation of the hydrated salts is effectively controlled. Thus, we observe that the SSPCMs significantly improved the defects of the hydrated salt. Table 2 summarizes the recent literature on the shape stability of the PCMs. The thermal stability of the hydrated salts was evaluated by the TGA curve, which indicates by the mass loss of PCMs obtained at 80 °C. Thus, we observe that the SSPCMs proposed in this work exhibited excellent thermal performance. The encapsulation efficiency (η) of these composite PCMs was calculated using Eq. (2):

=

( Hm)CompositePCMs ( Hm)PurePCMs

× 100%

(2)

Table 1 DSC heating and cooling characteristics of composite PCMsa. PCMs

Melting

Crystallization

ΔT (°C)

Paraffin

Hydrated salt Paraffin HPC (2H:1P)b SSPCMs SSPCMs after 50 cycles 1H:1P 3H:1P 4H:1P

Hydrated salt

Tm (°C)

ΔHm (J/g)

Tc (°C)

ΔHc (J/g)

Tc (°C)

ΔHc (J/g)

36.6 36.3 36.7 35.2 36.0 36.2 36.3 36.2

275.2 261.4 245.3 227.3 227.0 244.3 246.2 248.6

– 30.7 30.0 30.0 28.9 28.5 28.7 28.7

– 257.7 77.1 71.0 63.8 118.2 60.0 48.7

2.4 – 17.0 15.7 15.5 8.8 15.2 17.5

212.9 – 145.2 120.6 120.3 105.8 140.6 142.4

34.2 – 19.7 19.5 20.5 27.4 21.1 18.7

a: Tm: Melting temperature, ΔHm: Melting enthalpy, Tc: Crystallization temperature, ΔHc: Crystallization enthalpy, ΔT: Supercooling degree, and the value of ΔT is the absolute value of the difference between Tm and Tc. b: H represents hydrated salt, and P represents paraffin. 5

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Fig. 4. Phase transition behaviors of composite PCMs: (a) DSC curves of the pure hydrated salt, pure paraffin, and HPC. (b) DSC curves of emulsion composites with different hydrated salt / paraffin ratios. (c) DSC curves of HPC, SSPCMs, and SSPCMs after 50 heating and cooling cycles. (d) Phase transition enthalpy of the hydrated salt, paraffin, HPC, SSPCMs, and SSPCMs after 50 heating and cooling cycles.

Table 2 Comparison between the thermal performance of SSPCMs investigated in the present and previous studies. Year

PCM

This work

Composite PCMs

2018

Inorganic PCMs

2019 2018 2018 2018 2018 2018 2018 2017 2016 2015 2014 2019 2018 2018 2018

Organic PCMs

Stabilizer

ΔHm (J/g)

Efficiency

ΔT (°C)

Stabilitya

Ref.

Na2HPO4·12H2O n-Eicosane

CS

227.3

84.1%

19.5

1.9%

This work

Na2SO4·10H2O Na2CO3·10H2O MgNO3·6H2O Na2SO4·10H2O(50%) Na2HPO4·12H2O(50%) CaCl2·6H2O Na2CO3·10H2O(40%) Na2HPO4·12H2O(60%) CH3COONa·3H2O MgNO3·6H2O Na2CO4·10H2O(50%) Na2HPO4·12H2O(50%) MgCl2·6H2O(30%) NH4Al(SO4)2·12H2O(70%) Na2CO3·10H2O Na2HPO4·12H2O Na2SO4·10H2O(50%) Na2HPO4·12H2O(50%) Na2SO4·10H2O Na2HPO4·12H2O Stearic acid PEG8000 PEG10000 Paraffin wax

EV

110.3

56.5%

9.6

22.5%

[35]

HEG Halloysite Diatomite/Paraffin EG EGO EG Diatomite EP EG GO/PAAAM

135.1 142.0 108.2 182.7 195.0 194.8 99.2 151.7 157.8 200.3

86.4% 67.0% 56.9% 83.6% 89.5% 68.0% 66.9% 76.4% 82.1% 90.8%

16.9 34.8 25.7 13.5 8.9 7.8 18.2 – – 8.0

4.9% – 15.2% – – – – 15.1% 15.0% –

[29] [37] [38] [31]

EG/Paraffin Silica/PVP MOF/CQD 3D porous carbon GO/BN GF/HGA

172.3 106.2 132.0 160.3 107.4 145.2

76.0% 46.8% 64.7 83.2% 66.1% 88.3%

14.9 – 25.7 24.5 24.1 9.7

28.5% 22.2% – – – –

[34] [51] [28] [27] [52] [53]

a: Mass loss of the PCMs obtained at 80 °C. 6

[32] [39] [36] [33] [50]

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Fig. 5. Shape stability test of SSPCMs, (a, b, c) represent HPC, paraffin / CS, and SSPCMs, respectively. The upper, middle, and lower figures illustrate the gradual melting process at 60 °C. The image at the right demonstrates the mechanical strength of SSPCMs at 60 °C.

where Hm is the melting enthalpy. 3.4. Shape stability and application of SSPCMs The shape stability of the PCMs is an important property that prevents leakage and maintains a fixed shape during the phase transition process. The shape stability of HPC, paraffin / CS, and SSPCMs was tested on a polypropylene cardboard placed in an oven at a constant temperature of 60 °C. The dimensions of the tested samples were 2 × 2 × 0.5 cm cubes. Fig. 5 shows that while HPC gradually melted into a pool of liquid, the pure paraffin and HPC absorbed into the CS maintained an excellent shape. The image at the right exhibits the mechanical strength of the SSPCMs: they retained their original shape with negligible liquid leakage even when loaded with 200 g weight for 10 min, which is almost 100 times their own weight at 60 °C. Table 3 lists the weight data of HPC, paraffin / CS, SSPCMs, and SSPCMs (loaded with 200 g weight) before and after heating. SSPCMs have a negligible leak. Thus, these tests demonstrated the excellent shape stability of the SSPCMs. Thermal management is a desirable function for SSPCMs. A 90 g cube (15 × 10 × 0.5 cm) of the prepared SSPCM was used as a lining for a jacket. The temperature management performance of the clothing was tested, and the results are shown in Fig. 6. A model clothed in a dark and airtight jacket was exposed to an ambient temperature of 35 °C on a sunny day. An infrared thermal imager was used to record the real-time temperature of the clothing surface by time-lapse photography. Fig. 6a shows the 3D structure diagram of the SSPCM lining inside the cloth. The higher heat storage density provided by the asprepared composites reduces the volume of the materials required in the thermal energy management systems, thereby enhancing the comfort and flexibility of the cloth. Compared with pure hydrated salt, the SSPCM lining exhibits better thermal stability and lower supercooling degree. Fig. 6b shows the infrared thermography image under working conditions. Obviously, the area with the SSPCM lining in the jacket maintained a lower temperature than the areas without the lining. Fig. 6c shows that the SSPCM lining exhibited excellent and enduring thermal adjustment performance: with the lining, the surface temperature can be maintained at 36 °C for more than 25 min. In contrast,

Fig. 6. Temperature adjustment test of a fully enclosed protective model clothing. a) 3D structure diagram of the SSPCM lining inside the cloth. b) Infrared thermography image of a person wearing the temperature adjustment clothing. c) Surface temperature of the cloth with SSPCM lining vs. the temperature without the lining.

the surface temperature of the clothing without the lining exceeded 45 °C within 200 s. 4. Conclusion In summary, hydrated salt / paraffin composite SSPCMs were designed and prepared, which exhibited enhanced thermal performance. The HPC (hydrated salt and paraffin in a 2:1 ratio) was first prepared by using the inverse emulsion template method. The hydrated salt was emulsified into paraffin, which completely prevented the water evaporation in the hydrated salt by encapsulation. Moreover, paraffin promoted the crystallization of the hydrated salt in the composites by functioning as a nucleation agent: the crystallization temperature of the hydrated salt was elevated from 2.4 °C to 15.7 °C while its supercooling degree was suppressed from 34.2 °C to 19.5 °C. The phase transition latent heat of the as-prepared SSPCMs was measured to be 227.3 J/g, and, even after 50 heating and cooling cycles, it was maintained at 227.0 J/g, showing an insignificant decrease. All these results verified the excellent thermal performance and shape stability of the SSPCMs, which makes these a promising material in the field of energy storage and management. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21805031) and Fundamental Research Funds for the Central Universities (No. 2232019G-04).

Table 3 Weight data of samples before and after shape stability testing.

mbefore (g) mafter (g) Mass loss

Appendix A. Supplementary data

HPC

Paraffin / CS

SSPCMs

SSPCMs (loaded with 200 g)

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123958.

2.41 – 100.00%

1.86 1.83 1.61%

2.16 2.12 1.85%

2.14 2.06 3.74%

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