expanded graphite composite for thermal energy storage

Energy Conversion and Management 208 (2020) 112586

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Tailored phase change behavior of Na2SO4·10H2O/expanded graphite composite for thermal energy storage

T

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Chuanchang Li , Bo Zhang, Baoshan Xie, Xinbo Zhao, Jian Chen School of Energy and Power Engineering, Changsha University of Science and Technology, Changsha 410114, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Sodium sulfate decahydrate Expanded graphite Phase change behavior Composite phase change materials Thermal energy storage

Sodium sulfate decahydrate (SSD, Na2SO4·10H2O) is a suitable candidate for thermal storage applications as its high thermal latent heat and desirable phase change temperature, but it suffers from the supercooling, phase separation, and recalescence. In this paper, the SSD-CBO synthesized by SSD, carboxymethyl cellulose (CMC), borax decahydrate, and OP-10 successfully suppressed supercooling, and phase separation. Expanded graphite (EG) was used to stabilize and prevent the recalescence of SSD-CBO as well as enhance the thermal conductivity of pure SSD. The microstructure and chemical structure of composites were further explored, indicating the obtained composites possess uniform distribution and good chemical compatibility. The DSC analysis shows that the composite with 7 wt% EG has latent heat value of 114.0 J g−1 for melting and 105.5 J g−1 for freezing, respectively. The supercooling was reduced by more than 19 °C. The thermal conductivity of SSD-CBO/EG7 was as high as 3.6 times that of SSD-CBO and reached to 1.96 W m−1 K−1. The heating and cooling curves confirmed that the heating and cooling times of SSD-CBO/EG7 were reduced by 36.2% and 44.5% compared to those of the pure SSD, further investigating the effect on heat transfer rate by adding EG into SSD-CBO. The excellent transient temperature response of SSD-CBO/EG7 was recorded by thermal infrared images. The thermal storage and release performance evaluation suggested the SSD-CBO/EG composites especially the SSD-CBO/EG7 processes the good potential for thermal energy storage in the floor heating application. Furthermore, the mechanism of the tailored phase change behavior of SSD-CBO/EG composites was investigated.

1. Introduction With the aggravation of global energy crisis and environmental pollution [1], energy-saving technology and renewable energy have received the energy researchers’ attention and becoming a worldwide focus [2]. Thermal energy storage technology is one of the effective solutions to those two issues [3] by storing energy mainly in the form of sensible heat [4], latent heat storage [5], and chemical heat [6]. Latent heat storage (LHS) [7], which uses phase change materials (PCMs) as storage medium [8] and stores heat in the form of latent heat by phase transient process [9], has been proved that its high energy storage density [10] and small temperature swing during melting and solidification [11] are two excellent superiorities compared with other energy storage technology [12]. The variable melting point of PCMs [13] can meet the different requirements is also a huge advantage in practical application [14]. LHS has acquired a great interest in sustainable energy field [15] such as energy-conservation building [16], solar energy storage [17], thermal management for electronics/batteries [18], thermal energy storage [19] and load shifting [20], hot water system

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[21], air conditioning system [22], domestic heating [23], and waste heat recovery [24]. The apportionment of PCMs inside the building envelope can highly improve the indoor thermal environment and indoor comfort [25] as the PCMs integrated into envelopes [26] can effectively store or release latent heat to control the internal gains through thermal inertia [27] and adjust the indoor temperature [28]. The PCMs incorporated into the building envelope processes low phase change temperature (in the interval of 20–30 °C) [29], and researches are mainly focused on those three types: organic PCM [30], inorganic PCM [31], and composite PCM [32]. Inorganic salt hydrate consisted of inorganic salt and water can form a typical crystalline solid with a general formula, and it can be used alone or as eutectic mixtures for lower phase change point [33]. The major advantages of inorganic salt hydrate are its high latent heat of fusion per unit volume [34], high thermal conductivity (compared with organic PCMs), a large resource available with suitable properties [35], no or limited fire risk, slightly toxic, and inexpensive [36]. Research on incorporating inorganic salt hydrate into building envelope is one of the earliest and most studies in the field of LHS application. Pure

Corresponding author. E-mail address: [email protected] (C. Li).

https://doi.org/10.1016/j.enconman.2020.112586 Received 11 November 2019; Received in revised form 5 February 2020; Accepted 6 February 2020 0196-8904/ © 2020 Elsevier Ltd. All rights reserved.

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calcium chloride hexahydrate (CaCl2·6H2O) was filled into panels and packaged as a thermal management system (TES) for building conservation by V. V. Tyagi et al. [37], and the experimental study showed the room with the TES performs well for maintaining indoor temperature compared with the commercial room. A eutectic mixture (CaCl2, NaCl, KCl, and water) with the phase change temperature of 26–28 °C was prepared by A. Pasupathy et al. [38] and integrated into the panel of the roof for thermal management study in a residential building. Sodium sulfate decahydrate, also known as Glauber's salt (Na2SO4·10H2O, melting temperature of 32.4 °C), is a kind of promising inorganic salt hydrates with large heat fusion, wide sources, and low cost. These favorable characteristics endow Na2SO4·10H2O with high value for building use. However, the problems like high degree of supercooling [39], phase separation [40], the sudden local release of latent heat caused by recalescence, leakage during phase transient process [41], and low thermal conductivity [42] greatly impaired the heat storage performance and limited its practical application [43]. Adding nucleating agents like borax [15] to address the supercooling problem and thickening or gelling agents like carboxymethyl cellulose [44] to prevent phase separation overcome are two satisfactory ways [45]. It should be noticed that the addition of those agents couldn’t improve the thermal conductivity as well as the thermal stability of the Na2SO4·10H2O but also decrease the thermal storage capacity, and the recalescence phenomenon still exists. A desirable PCMs for building envelope applications should process high thermal conductivity for heat conduction [46] and stabled shape of practical application [47]. Carbonaceous materials like graphite [48], carbon fiber [49], and carbon nanotubes [50] have been widely researched as thermal conductivity filler because of their good performance on thermal conductivity and lightweight structure enhancement, but they present problems like low interface thermal conductance and anisotropy by introducing into PCMs [51]. Expanded graphite (EG) is a kind of carbonaceous material obtained from natural flake graphite, which is treated by chemical or electrochemical intercalation and then been heated instantaneously to produce high-temperature expansion. Due to its desirable properties of high thermal conductivity [52], low density [53], low cost [53], and good chemical stability [54], EG is a promising candidate to increase the thermal conductivity of inorganic salt hydrate [55]. Moreover, the large amount of pore structure can successfully absorb and fix abundant molten PCMs by strong surface tension forces and the capillary forces and provide the mechanical strength [56] for the whole composite materials, so that improving the thermal stability of inorganic salt hydrate [57]. Those characteristics are essential for developing the practical application of inorganic salt hydrate. Researches on Na2SO4·10H2O mainly focus on preparing Na2SO4·10H2O based eutectic mixture [58], and the melting process of Na2SO4·10H2O or composite PCMs were studied in details by several methods like DSC analysis and TG. Actually, the phase change behavior of the Na2SO4·10H2O including melting and freezing process is one of the most critical characteristics, and the freezing process should also be noticed as the heat releasing phase is important for practical application. But there are few reports on the freezing process of Na2SO4·10H2O based composite PCMs especially the prevention of recalescence phenomenon. This work focused on developing the application value of Na2SO4·10H2O as phase change material in the building envelope. The key point of our study is using EG to synthesize a novelly Na2SO4·10H2O composite as well as adjust the phase change behavior of Na2SO4·10H2O. To maximize the adjusting effects, firstly, a novelty modified compound based Na2SO4·10H2O was prepared by adopting borax decahydrate and CMC, and the octyl phenol polyoxyethylene ether (OP-10) was used as a coupling agent to improve the binding energy between Na2SO4·10H2O and EG. The devised shape stabilized composite PCMs based on the modified compound and EG were synthesized by vacuum impregnation. The phase change behavior including the melting and freezing process of pure Na2SO4·10H2O and

composite PCMs were studied, and the freezing process was tailored to almost normal and recalescence phenomenon was prevented by controlling the mass ratio of several agents and EG. The infrared thermal images were applied to analyze the effect of EG on the thermal response and distribution of composite PCMs. Moreover, a compact apparatus was set up to investigate the thermal performance of the above prepared composite PMCs for radiant floor heating application. 2. Experimental 2.1. Materials The analytical reagent sodium sulfate decahydrate (Na2SO4·10H2O, SSD) and sodium tetraborate decahydrate (borax decahydrate, Na2B4O7·10H2O) were obtained from Tianjin Fuchen Chemical Reagent Factory and Tianjin Damao Chemical Reagent Factory. The carboxymethyl cellulose (C8H16NaO8, CMC) and octyl phenol polyoxyethylene ether (C34H62O11, OP-10) were supplied by Shanghai Shanpu Chemical Reagent Co., Ltd., China. And the expanded graphite (EG) with a mean size of 300 mesh was provided by Tianjin Hengxing Chemical Reagent Co., Ltd., China, and the specific surface area and porous property of EG were tested and provided in the Supplementary Information (Fig. S1 and Table S1). 2.2. Preparation of the SSD-CBO and SSD-CBO/EG composites To suppress the problems of supercooling and phase separation, a mixture SSD-CBO was obtained by compound the CMC, borax decahydrate and OP-10 into the SSD with the mass ratio of 2:3:5:90. The mixture was stirred vigorously and kept in a thermostatic water bath at 50 °C for 30 min until the SSD melted completely and then cool to room temperature. The EG was applied to support the SSD-CBO, and the SSDCBO/EG composites were synthesized via the method of vacuum impregnation, following the fabricating process as follows: a certain amount of melted SSD-CBO and EG were put into Erlenmeyer flask, which was concatenated to a suction pump. The Erlenmeyer flask created a vacuum at −0.1 MPa for 5 min, and then maintained its temperature at 50 °C for 30 min by a thermostatic water bath. After the resulting mixture cool to room temperature, the final SSD-CBO/EG composites were prepared. Furthermore, three kinds of samples with different EG content (5 wt%, 6 wt%, and 7 wt%) labeled as SSD-CBO/ EG5, SSD-CBO/EG6, and SSD-CBO/EG7 were synthesized via the above method to inquire the effect of different proportion of EG on the properties of the composites. 2.3. Characterization X-ray diffraction (XRD) analysis was performed to investigate the crystal structure of samples by using a Bruker D8 Advance (Bruker AXS GMBH, Germany) with Cu-Kα radiation at 40 kV and 40 mA, and a step size of 0.02° in the scan range of 5–70°. The chemical structure and compatibility were examined by Fourier-transform infrared spectroscopy (FTIR, Shimadzu IRTracer-100 AH, Japan) with a scanning range of 4000–400 cm−1 at room temperature and the mass fraction of samples in KBr was no more than 1%. The morphology and microstructure images were obtained by scanning electron microscope (SEM, Zeiss Sigma 500, Germany) at an accelerating voltage of 5 kV. The thermal behaviors of samples, including phase change point and latent heat, were analyzed by differential scanning calorimeter (DSC, TA Instruments Q2000, America) from 0 °C to 60 °C at a scanning rate of 5 °C min−1 under nitrogen with a flow rate of 50 mL min−1. To obtain accurate experimental results, the DSC calibration was carried out before samples experiments in following steps: the experiment starting with a cell preheat followed by an equilibration at initial temperature 20 °C, holding isothermal for 5 min, heating at 10 °C min−1 to final temperature 350 °C and holding isothermal for 10 min. The calibration 2

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Fig. 1. Schematic diagram for thermal performance evaluation device.

3. Result and discussion

experiment was done without samples or pans. A thermal conductivity test instrument (DRX-II-RW, Xiangtan Huafeng, China) was adopted to detect the thermal conductivities at the temperature of 30 °C with an accuracy of ≤ 3%. The transient temperature response or temperature distribution of the heating and cooling process was recorded by a thermal infrared camera (FLUKE TiS50, America).

3.1. Phase separation test of SSD, SSD-CBO, and SSD-CBO/EG composites In the hydrated salt, the water molecules are incorporated into the crystalline structure of the salt. Na2SO4·10H2O is an incongruently melting salt hydrate and the hydrated water in the molecules would become the free state when it comes to the melting process. As the molting process goes on, there were few Na2SO4 dissolved in the free water and most of the Na2SO4 would be deposited at the bottom. Water separation, related to poor molecular bonding is the main factor, which determines thermal stability in the molten phase. This phase separation was much more seriously, especially over repeated melting and solidification cycles, leading to the loss of the mass of energy storage material and reduction of the thermal storage density. The phase separation of pure SSD, SSD-CBO, and SSD-CBO/EG composites were studied. After heated for 30 min in a water bath with a constant temperature of 50 °C, five test tubes with different materials were removed from the water bath. Distinct phase stratification between sodium sulfate and water appeared in a tube with pure SSD shown in Fig. 2a. A large amount of white sodium sulfate removed via heating was deposited at the bottom of the test tube due to its large density. In Fig. 2a, the tube with SSD-CBO appeared much less free water at the top layer, and the SSD-CBO suspension at the bottom layer became loose and turbid. This indicates that adding CMC to synthesize SSD-CBO was very helpful to phase stratification avoidance. That is owing to the thickening agent reduced the friction between liquid hydrated salt molecules and enhanced the flocculation, in which way the solid particles can be dispersed into the base fluid uniformly to form the viscous suspension without phase stratification [59]. As for SSD-CBO/ EG composites in Fig. 2a, the samples were homogeneous and presented good shape stability, there was no phase separation that can be seen from those three tubes. Fig. 2b exhibits samples after heated and cooled naturally in the air to solidify for 24 h. The room temperature was 10–15 °C. Without any added nucleating agent or thickening agent, tube with pure SSD still retained a certain amount of free water at the top layer and lot of sodium sulfate at the bottom layer; the middle layer was mainly composed of transparent crystals of Na2SO4·10H2O as the sodium sulfate from the bottom can contact with liquid water at the interface between water and deposited sodium sulfate. For SSD-CBO, it appeared that the SSD in SSD-CBO was all form the crystalline structure in a tube with SSD-CBO after 24 h. The sodium sulfate particles, which dispersed in the base fluid uniformly by the function of CMC, can easily be contacted with water molecules and formed a hydrated salt crystalline structure

2.4. Evaluation experiment of the thermal performance of devised composite PCMs The pure SSD, SSD-CBO, and SSD-CBO/EG composites were filled in a metal container, and an experimental set-up was constructed for studying the thermal performance of the specimen plate with pure SSD, SSD-CBO, and SSD-CBO/EG composites. The experimental system (Fig. 1) was made up of an electric heating plate, the specimen plate with PCMs, and a small adiabatic room. The experimental protocol involved two tests, one is warming (loading) and the other is cooling (unloading) test. In the warming test, the thermostatic heating plate was heated to 60 °C and maintained for 1 h to heat the PCMs in the specimen. In the cooling test, the electric heating plate was turned off and the PCMs would go through a natural cooling process in the adiabatic room. In order to analyze the thermal performance of PCMs specimens, as shown in Fig. 1, three thermocouples marked T1, T2, T3 were fixed in the PCMs with different vertical depth (dv) for recording the bottom (dv = 0.5 cm), the mid (dv = 1.0 cm), and the surface (dv = 1.5 cm) temperature of the PCMs plate, respectively. The temperature of the adiabatic room (T4) and ambient (T0) were also recorded. Those five thermocouples were linked to a paperless recorder with a scanning interval of 1 s for collecting temperature evolution. The size of the metal container is 20 cm × 20 cm × 1.5 cm, and the PCMs were pressed into the container under the pressure of 0.002 MPa. The mass and density of every specimen were listed in Table 1.

Table 1 Parameters of specimens with different PCMs.

Mass (g) Density (g cm−3)

SSD

SSD-CBO

SSD-CBO/ EG5

SSD-CBO/ EG6

SSD-CBO/ EG7

608.26 1.01

581.82 0.97

227.83 0.38

204.19 0.34

188.66 0.31

3

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Fig. 2. Phase separation testing of samples: (a) after one heating and cooling cycle; (b) after cooling for 24 h.

diffraction also demonstrates only physical interaction occurred between PCM and Supporting materials.

during solidification. Not surprisingly, in Fig. 2b, the SSD-CBO/EG composites also presented excellent shape stability without any degree of phase separation after solidification for 24 h. With the EG content increasing, the white SSD crystal which can be seen in a tube with SSDCBO/EG5 was almost disappeared in a tube with SSD-CBO/EG7. It happened because the SSD-CBO/EG7 processed more EG than SSDCBO/EG5 and there was a larger amount of pores to incorporate SSD. It further indicates that the shape stability of SSD-CBO/EG7 was better than that of the other two composites. Therefore, the phase separation of pure SSD can be effectively suppressed by synthesizing SSD-CBO and shape stabled by adding EG into the SSD-CBO compound.

3.3. Morphological analysis To study the microstructure of EG and composites, their surface images were recorded via SEM analysis. Fig. 4 shows the SEM images of EG and SSD-CBO/EG7. According to Fig. 4a and b, the EG matrix had a worm-like microstructure and various pores connected each other by graphite flakes formed a network porous structure. Previous studies [64] has verified that the various pores provide a specific surface area of EG and the high active surface of graphite microcrystal contribute to superior surface activity of EG; the hydrated salts can be absorbed and fixed inside the EG instead of leaking out of the composites by the surface tension forces and the capillary forces of multiple pores of EG. It is observed in Fig. 4c and d that the SSD was successfully impregnated into the net-pores of EG, and both the pores and surface of EG were occupied by SSD. The results suggest the EG with good porous structure can be an effective support matrix for loading hydrated salts, and the shape of SSD was successfully stabilized by vacuum impregnating method.

3.2. Crystallization characteristics The XRD patterns of pure SSD, SSD-CBO, and SSD-CBO/EG composites are shown in Fig. 3. The pure SSD had a wide band in the interval of 2θ = 15° − 20° and characteristic peaks at 2θ = 18.9°, 27.3°, 30.9°, and 31.3° [60]. The characteristic peak of the EG was centered at 2θ = 26.3° [61]. Compared to the pure SSD and EG, there is a strong diffraction peak at 2θ = 16.3° appeared in the SSD-CBO and SSD-CBO/ EG composites and no similar peaks presented in the patterns of both pure SSD and EG. This is the characteristic peak of Na2B4O7·10H2O as it maintained a good crystal structure below its phase transient temperature which was higher than that of SSD [62]. The two characteristic peaks of CMC were observed at 2θ = 20.6° and 35.9° [63]. After integrating PCM into EG, it can be noted that no new diffraction peak appeared. Even though the peak intensities were weaker and moved a bit compared with that of salt hydrate and EG. It can be owing to the confinement effects invocated by surface adsorption of EG as well as be related to the network porous structure. The simple superposition of the

3.4. Chemical structure The FT-IR spectra of pure SSD, SSD-CBO, and SSD-CBO/EG composites are depicted in Fig. 5. From the spectra of pure SSD, the peaks at 1110 cm−1 and 1458 cm−1 represented the characteristic symmetric and asymmetric stretching vibration of S = O in O-SO2-O, respectively. The peaks at 1163 cm−1 and 616 cm−1 represented asymmetric S-O stretching vibration and symmetric S-O stretching vibration, respectively [61]. A narrower symmetrical band and an asymmetrical band were observed at 840 cm−1and 1637 cm−1, which corresponded to C–C skeletal vibrations in OP-10[65] and the basic characteristic functional group eCOO in CMC, respectively [66]. The peaks at 1375 cm−1 represented symmetric CH3e bending vibration which is attached to CH3(CH2)7- in OP-10. The wide absorption band in the interval of 3000–2750 cm−1 was the stretching vibration of O–H [67], and the absorption band at 3450 cm−1 was caused by air moisture during the test. The characteristic absorption peak of EG at 1631 cm−1 and characteristic absorption peaks of SSD at 1110 cm−1 and 1458 cm−1 existed analogously with no significant new absorption peaks, indicating that there was no chemical interaction between SSD, EG, and other materials. The results were supposed to the analysis of XRD patterns and SEM images that pure SSD was absorbed and fixed into the pores of EG mainly by surface tension forces and capillary effect and there was no impact on the chemical structure of SSD, EG, and other materials. 3.5. Thermal properties The DSC thermal analysis of the samples was carried out to ascertain the thermal properties of PCMs. The DSC curves are shown in Fig. 6a,

Fig. 3. XRD patterns of pure SSD, SSD-CBO, and SSD-CBO/EG composites. 4

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Fig. 4. SEM images of EG (a and b) and the SSD-CBO/EG7 (c and d).

and extrapolated end temperature (Tc) were applied to represent the phase change temperature of melting (Tm) and freezing (Tf), respectively. The freezing temperature of DSC curves with recalescence (SSD, SSD-CBO, SSD-CBO/EG5, and SSD-CBO/EG6) was determined as the onset of the recalescence that takes place right after crystal growth [39]. And the latent heat value of melting/freezing ΔHm (ΔHf) in J g−1 was calculated according to the following equation [69]:

HDSC =

∫ (ybaseline − yDSCcurve ) ∙dT

ΔHm =

dt ∫ (ybaseline − yDSCcurve ) ∙dt = ∫ (ybaseline − yDSCcurve ) ∙dT∙ dT

= HDSC ∙

dt 1 = HDSC ∙ dT θ −1

(1)

(2) −1

s is the integrated area between the where the HDSC in J °C g baseline and the DSC curve, ybaseline is the baseline of the DSC curve and the yDSCcurve is the specific heat flow. The θ in °C s−1 represents the dt temperature-rising rate (θ = dT ) [70]. The T and t are the temperature (°C) and time (s), respectively. As can be seen in Fig. 6a, the melting peaks of five samples were similar in shape, but the curves of the freezing peak had different degrees of recalescence. Pure SSD and the SSD-CBO processed heavy recalescence during DSC testing, but the later had good improvement in supercooling. The level of recalescence of all composites in the freezing process slowly receded and even disappeared when the mass fraction of

Fig. 5. FT-IR spectra of pure SSD, SSD-CBO, and SSD-CBO/EG composites.

and the detail thermal properties are listed in Table 2. The characteristic temperatures and latent heat values from DSC measurements were obtained via the baseline extrapolation method shown in Fig. S2. According to the report [68], the extrapolated peak onset temperature (Te)

Fig. 6. (a) DSC curves of the pure SSD, SSD-CBO, and SSD-CBO/EG composites; (b) DSC cycle test curves of SSD-CBO/EG7. 5

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Table 2 Phase change properties of pure SSD, SSD-CBO, and composites. Samples

Content of SSD (β, %)

Melting temperature (°C)

Freezing temperature (°C)

Latent heat of melting (ΔHm, Jg−1)

Latent heat of freezing (ΔHf, Jg−1)

Supercooling (°C)

Efficient energy per unit mass of SSD (Eef, J g−1)

pure SSD SSD-CBO SSD-CBO/EG5 SSD-CBO/EG6 SSD-CBO/EG7

100.0 90.0 85.5 84.6 83.7

32.24 28.63 31.52 31.50 31.06

4.93 22.64 23.56 21.79 23.02

139.9 126.9 123.6 117.9 114.0

117.9 124.7 121.2 100.0 105.5

27.31 5.99 7.94 9.76 8.03

139.9 141.0 144.6 139.4 136.2

T2 = 323.15 K (50 °C) were calculated according to:

EG up to 7 wt%. The proportion of EG may be contributed to that phenomenon as the carbon pores of EG have an excellent thermal capacity which helps to create a more tight combination of chemical bonding for heat pathway and formed a uniformity heat flow in composites. The DSC curves of pure SSD illustrated that its melting temperature and freezing temperature was 32.24 °C and 4.93 °C respectively. The supercooling of pure SSD was 27.31 °C. Compared to the pure SSD, the melting temperature of SSD-CBO and SSD-CBO/EG composites decreased a little. The different melting temperature of SSDCBO/EG composites was mainly caused by the interactions (including surface tension forces and capillary forces) in different degree between SSD and pore walls of EG. The freezing temperature of SSD-CBO/EG composites was obviously increased after introducing additions to SSD. The supercooling of SSD-CBO was 5.99 °C, and that of SSD-CBO/EG5, SSD-CBO/EG6, and SSD-CBO/EG7 composites were 7.94 °C, 9.76 °C, and 8.03 °C, respectively. That results indicate that the borax decahydrate can effective improved the nucleation of SSD and reduced the degree of supercooling. As shown in Table 2, the melting and freezing heat values of pure SSD were 139.9 J g−1 and 117.9 J g−1. The huge reduction of latent heat of pure SSD was agreed with previous studies that the thermal stabilization of pure SSD is unsatisfactory [43]: as the melting of Na2SO4·10H2O would separate Na2SO4 out and form a Na2SO4 supersaturated solution, and the natural sedimentation of Na2SO4 lead to severe phase separation. For composites, the differences between melting heat and freezing heat values were slighter than that of pure SSD, and the melting and freezing heat values of SSD-CBO/EG7 are 114.0 J g−1 and 105.5 J g−1. The latent heat value and the crystal water content (Fig. S3) in composites of SSD-CBO/EG6 and SSD-CBO/EG7 reduced a little compared to that of SSD-CBO/EG5, this may attribute to EG mass fraction increases. And the hydrophobic of EG could influence the penetration of the PCM particularly materials attached to H2O, it would lead to a decrease in crystal water content and latent heat value [71]. From the above results, it is acquired that the energy storage capacities of the composites were between 114 ∼ 123 J g−1. The efficient energy per unit mass of SSD (Eef) [72] was used to assess the effectiveness of SSD in different composites and defined using the following equation [73]:

Eef =

Q = Q s + Ql = Q1 + m

(∫

Tm

T1

Cps dT +

∫T

T2

m

)

Cpl dT +ΔHcomposite

(4)

where m is the mass fraction of SSD in composites, Q s and Ql are the total specific heat capacity and latent heat capacity of composite, and Q1 is the specific heat capacity of EG and other agents. Cps (J g−1 K−1) and Cpl (J g−1 K−1) are the solid and liquid specific heat capacity of SSD. Their values are listed in Table S2. The calculated results were 135.92, 130.16, 126.19 J g−1 for SSD-CBO/EG5, SSD-CBO/EG6, SSDCBO/EG7, respectively. The thermal cycling reliability was investigated by DSC heating and cooling cycle testing. The phase transient temperature difference (°C) and heat flow change percentage (%) of SSD-CBO/EG7 by comparing with the 1st thermal cycle were presented in Fig. 6b, and the DSC curves with 50 thermal cycles were exhibited in the inset of Fig. 6b. After 50 thermal cycles, the phase transient temperature for melting and freezing had a variation of −1.55 °C and 0.63 °C, respectively; and the latent heat for melting and freezing changed by 9.16% and 0.69%, respectively. During the heating and cooling cycle testing, the phase transient temperature and latent heat for freezing keep its values in a slight fluctuation range; the phase transient temperature and latent heat for melting decreased at a fairly slow rate. The results demonstrate that SSD-CBO/EG7 possessed outstanding thermal reliability after reduplicative heating and cooling cycles. In conclusion, SSD-CBO/EG7 with high thermal storage density and excellent thermal cycling reliability have the potential for practical application. 3.6. Thermal conductivities The thermal conductivities of pure SSD, SSD-CBO, and SSD-CBO/EG composites were measured with a thermal constant analyzer. Fig. 7 describes the thermal conductivities of samples. The thermal conductivity of pure SSD was 0.54 W m−1 K−1, and it was slightly

ΔHcomposite β

(3)

where ΔHcomposite was the latent heat of composite; β has represented the mass fraction of SSD in composites. The results (Table 2) reflected that SSD-CBO had a higher Eef of 141.0 J g−1 than pure SSD, resulting from that the phase separation of SSD was suppressed in SSD-CBO. After introducing EG to SSD-CBO, the Eef of SSD in SSD-CBO/EG5 reached to 144.6 J g−1 due to more SSD can work for thermal energy storage. As the EG content increased, the Eef of SSD in composites decreased. It is attributed to that the interaction between the SSD and the EG matrix of capillary and surface tension forces could affect the crystallization of the SSD, resulting in a little more SSD confined in SSD-CBO/EG7 [74]. Nevertheless, SSD in SSD-CBO/EG7 still had a considerable Eef comparable to pure SSD. Moreover, the thermal storage capability (Q, J g−1) of prepared composites in the temperature range from T1 = 293.15 K (20 °C) to

Fig. 7. Thermal conductivities of pure SSD, SSD-CBO and SSD-CBO/EG composites. 6

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– – 0.192 3.643 1.96 – Dropping 19.7% and 18.2% after 100 thermal 100.1 and 71.1 J g−1 after 100 thermal cycles Dropping 5.69% and 4.19% after 100 thermal cycles Dropping 9.16% and 0.69% after 50 thermal cycles

[78] [79] [58] [61] This study

– – 23.3 and 1.5 J g−1 after 3 thermal cycles Almost 0 after 9th/13th thermal cycles

[76] [77]

increased by synthesizing SSD-CBO. Thermal conductivities of SSDCBO/EG5, SSD-CBO/EG6, and SSD-CBO/EG7 were 1.25 W m−1 K−1, 1.73 W m−1 K−1, and 1.96 W m−1 K−1, respectively, which were obviously high comparing to the thermal conductivity of pure SSD and SSD-CBO. In other words, thermal conductivity values of SSD-CBO/EG composites with 5%, 6%, and 7% EG content were higher than that of pure SSD by 1.3 times, 2.2 times, and 2.6 times respectively. It is because of an increased EG concentration can improve the interfacial thermal transfer contact through the specific expansion of the EG structure and its carbon walls [75]. It indicates that EG is very beneficial for thermal conductivity enhancement of the pure SSD and SSDCBO. In addition, Table 3 compared the latent heats, supercooling, thermal cycling reliability, and thermal conductivity of the SSD-CBO/ EG7 with those of composites using SSD as PCM in the literature. The SSD-CBO/EG7 shows obvious advantages of enhancement on the latent heat of freezing and the thermal cycling reliability over the reported materials. This illustrates that the SSD-CBO/EG7 processes good potential for thermal energy storage. 3.7. Heat transfer performance of SSD, SSD-CBO, and SSD-CBO/EG composites To verify the excellent heat transfer rate of SSD-CBO/EG composites, a thermal heating–cooling test was carried out by using a selfmade constant temperature water bath method. The pure SSD, SSDCBO, and SSD-CBO/EG composites were weighted and load in five 20 mL test tubes, respectively. The tubes with different samples were kept in a water bath with a heating temperature of 50 °C for a period of time, then moved to the refrigerator with a constant temperature of 5 °C for cooling. The temperature evolution of the samples was recorded by thermocouples. Fig. 8 illustrates the heating and cooling curves of pure SSD, SSDCBO, and SSD-CBO/EG composites. During the heating process, the temperature of SSD-CBO/EG composites rose rapidly compared with pure SSD and SSD-CBO. As heating time increasing, the inflection point and platform of the composites PCMs emerged between 25 °C and 32 °C, illustrating that the composite PCMs exhibited solid–liquid phase change. It took 423 s, 443 s, 503 s, 663 s, and more than 870 s for SSDCBO/EG7, SSD-CBO/EG6, SSD-CBO/EG5, pure SSD, and SSD-CBO to get the heating temperature of 50 °C. The thermal storage time of SSDCBO/EG composites was absolutely shorter than pure SSD and SSD-CBO and the composite with 7 wt% EG (SSD-CBO/EG7) presented the best heat transfer performance. After carried to the refrigerator, the latent heat of composite PCMs released because of the recrystallization of SSD,

– 74.0 79.6 140.8 105.5 28.0 7.2 14.43 17.11 23.02 32.0 33.8 23.98 32.05 31.06

113.00 125.6 110.3 172.3 114.0

17.5 – 26.4 –

SSD + 1% borax SSD + 10%borax SSD + 30%borax SSD + 2% borax + 10% gelatin gel SSD + SiO2 microcapsules SSD + Na2CO3·10H2O + 40% expanded vermiculite SSD + Na2HPO4⋅12H2O + 13 wt% EG SSD + CMC + borax + OP-10 + 7%EG

97.3 –

48.2 –

8.9 16 13 4.0 26.6 9.55 14.94 8.03

Thermal conductivity Tm(°C) Samples

Table 3 Comparison of thermal properties of SSD based composites.

Tf (°C)

ΔHm (J g−1)

ΔHf (J g−1)

Supercooling (°C)

Thermal cycling reliability

Reference

C. Li, et al.

Fig. 8. Heating and cooling curves of pure SSD, SSD-CBO, and SSD-CBO/EG composites. 7

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Fig. 9. Thermal infrared images of samples at different (a) heating and (c) cooling times; Temperature response curves of samples during (b) heating and (d) cooling.

temperature so fast and approximated to 50 °C at 111 s, while the temperature increase of pure SSD and SSD-CBO were slower than SSDCBO/EG7 and spent 352 s and 217 s to reach 50 °C as Fig. 9b illustrated. According to Fig. 9b, the temperature interval of the inflection point and platform of the SSD-CBO/EG7 were lower than those of pure SSD, which is agreed with the results in DSC analysis that the phase change point decreased a little by introducing EG and other agents into the pure SSD. Besides, the difference between Tc and Ta of SSD-CBO/EG7 was intuitively smaller than that of the pure SSD and SSD-CBO, implying the SSD-CBO/EG7 had the best thermal diffusion capacity among three samples. During cooling, SSD-CBO/EG7 also exhibited the best heat transfer rate among pure SSD, SSD-CBO, and SSD-CBO/EG7. For instance, the Ta at 209 s was 27.5 °C, 17.3 °C, and 13.5 °C for pure SSD, SSD-CBO, and SSD-CBO/EG7, respectively. The higher thermal conductivity enabled the composite PCMs to absorb/release thermal energy more rapidly. It can be seen that pure SSD had a drastic recalescence at 136 s as shown in Fig. 9c which is confirmed to the results in Fig. 9d, and the supercooling of pure SSD was more than 15 °C in this case. As for SSD-CBO, the degree of recalescence was smaller than pure SSD. The supercooling problem was obviously settled in SSD-CBO/EG7 and it displayed a more steady exothermic process. This result demonstrates SSD-CBO/EG7 composite had an excellent heat transfer efficiency during the thermal storage-release process and the most sensitive responding to temperature change among pure SSD, SSD-CBO, and SSDCBO/EG7.

resulting in the appearance of another temperature growth plateau between approximately 28 °C and 25 °C, 25 °C and 20 °C, and 14 °C and 13 °C for composite PCMs, SSD-CBO and pure SSD, respectively. As the curve shows, the supercooling of SSD-CBO was effectively suppressed compared to that of pure SSD. The crystallizing temperature of pure SSD, in this case, was about 13.5 °C. At last, the SSD-CBO/EG7 and pure SSD got the cooling temperature of 5 °C at 4137 s and 7455 s, respectively. During the entire process of heating and cooling, the heating and cooling times of SSD-CBO/EG7 were 36.2% and 44.5% less than those of the pure SSD. Consequently, the heat transfer performance including storing and releasing further verified the results of thermal conductivity. It is easy to draw that the SSD-CBO/EG composites presented excellent heat transfer rate due to the addition of EG and the SSD-CBO/ EG7 with the highest content of EG performed superior among three composites. 3.8. Transient temperature response The infrared thermal images were provided to further demonstrate the transient temperature change of the composite PCMs in the heating and cooling process visually, as shown in Fig. 9. Samples were pressed into (Φ2 cm × 2 mm) cylindrical tablet which was packed in the cylindrical mold with the same size to keep relatively uniform temperature distribution during heat absorbing and releasing. Pure SSD, SSDCBO, and SSD-CBO/EG7 were kept at an initial temperature of similar color before placed on the hot platform for heating. The center temperature (Tc) and average temperature (Ta) of each sample were recorded by a thermal infrared camera to form temperature response curves. During the heating process, the SSD-CBO/EG7 increased its

3.9. Evaluation of the thermal performance of the specimen plate with devised composite PCMs Fig. 10a–e shows the temperature evolution of different depth 8

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Fig. 10. Temperature evolution of different measuring point in each plate, (a) pure SSD; (b) SSD-CBO; (c) SSD-CBO/EG5; (d) SSD-CBO/EG6; (e) SSD-CBO/EG7, and (f) the comparison of T4 during the charging phase.

would contribute to uniform heat distribution, while the thermal heat transmission in composites major by phonon propagation [58]. The temperature evolution comparison of T4 (Fig. 10f), which is mainly raised by the thermal radiation of PCMs plate, illustrated that the adiabatic room with plate packaged SSD-CBO/EG7 has the highest T4, indicating the SSD-CBO/EG7 owned the superior capacity of thermal radiation. Fig. 11a–e shows the temperature evolution of different measuring points in each PCMs plate and Fig. 11f shows the comparison of T4 during the discharging phase in the adiabatic room. It is observed that those five PCMs plates had a stable outlet temperature during the latent heat release process. The bottom temperature T1 and mid-temperature T2 of pure SSD were similar, which may be caused by the settling of the precipitated Na2SO4, and this phenomenon was improved in SSD-CBO as the difference between T1 and T2 was slightly increased. The surface temperature T3 of both SSD and SSD-CBO plate were quite different

positions in each PCMs plate and Fig. 10f shows the comparison of T4 during the charging phase in the adiabatic room. The inner temperature of the SSD and SSD-CBO plate showed a similar pattern and held at a melting plateau after heating for 1 h due to the low thermal conductivity and huge heat fusion of pure SSD and SSD-CBO plate. After the addition of EG, as shown in Fig. 10c–e, the inner temperatures of SSD-CBO/EG composites nearly reached the programmed temperature of 60 °C and the PCM melted completely. It took 1238 s and 2746 s, 991 s and 2664 s, and 956 s and 2510 s for SSD-CBO/EG5, SSD-CBO/ EG6, and SSD-CBO/EG7 to reach their phase change point and 50 °C, respectively, indicating that the thermal conductivity of SSD-CBO/EG composites were improved with the proportion of EG. The differences between T1, T2, T3 of pure SSD and SSD-CBO were smaller than those of SSD-CBO/EG composites. It may due to the different methods of thermal transport inner PCMs. The previous study has verified that intense thermal convection happened inner homogeneous liquid PCMs

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Fig. 11. Temperature evolution of different measuring point in each plate, (a) pure SSD; (b) SSD-CBO; (c) SSD-CBO/EG5; (d) SSD-CBO/EG6; (e) SSD-CBO/EG7, and (f) the comparison of T4 during the discharging phase.

3.10. The mechanism for tailored phase change behavior of SSD-CBO/EG composites

from the T1 and T2, and it seems that the heat distribution with different measuring point in the SSD and SSD-CBO was unsatisfactory during discharging phase. Compared to the SSD and SSD-CBO, the temperature difference of SSD-CBO/EG composites between T1, T2, and T3 was smaller and more uniform due to the high thermal conductivity of EG. It is notable that the temperature difference between T4 and T3 of SSDCBO/EG composites were obviously smaller than that of SSD and SSDCBO in most of the time. As shown in Fig. 11f, T4 was mainly raised due to the heat radiation of the plate and the heat source was the latent heat of the PCMs plate. The time for T4 held above 20 °C during the discharging phase was 16816 s, 12985 s, 11022 s, 8978 s, and 9787 s for SSD, SSD-CBO, SSD-CBO/EG5, SSD-CBO/EG6, and SSD-CBO/EG7, respectively. The SSD-CBO/EG7 plate with the lowest mass loadage of composite processed longer time (above 20 °C) than that of SSD-CBO/ EG6. It’s reasonable to think that the heat storage and release capacity of SSD-CBO/EG7 is remarkable comparing to the others in this case.

The above results provided strong evidence that those agents led to a great suppression of supercooling and phase separation the composite PCMs, and the expanded graphite contributed to enhance the thermal transport and to prevent the recalescence of the composite PCMs. The remarkable performance was mainly attributed to the excellent thermal conductivity of carbon skeleton and the advantage of porous network structure, which highlights the potential for serving the SSD-CBO/EG as a thermal storage medium in the field of high-efficiency dissipation and thermal energy storage. The main process and mechanism of this study were discussed as follows. As shown in Fig. 12a, before phase transition, ten water molecules were incorporated into the crystalline structure of the sodium sulfate to form a well-ordered crystal structure when it remains solid SSD. After a period time of heating, the bonds connection between salt and water molecules were destroyed and the crystal water became to free water. As the solubility of sodium sulfate in water was not high enough at the 10

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Fig. 12. (a) solid SSD with well-ordered crystal structure; (b) liquid SSD with distinct separation; (c) liquid SSD-CBO, SSD with Na2B4O7·10H2O, CMC, and OP-10; (d) SSD-CBO and DSC curve with the drastic recalescence in the freezing peak; (e) SSD-CBO/EG composite and DSC curve with no recalescence in the freezing peak.

compatibility and shape stability but also maintained relatively high thermal energy storage capability and good thermal reliability. It was proved that the adding of EG not only improved thermal conductivity of composites but also had positive effects on phase change behaviors of SSD as the abnormal recalescence occurred upon the freezing process was successfully regulated by adding of EG into composites. The thermal conductivity of SSD-CBO/EG7 was 3.6 times as high as that of pure SSD, and the heating and cooling times of SSD-CBO/EG7 were 36.2% and 44.5% less than those of the pure SSD during heating and cooling test. The SSD-CBO/EG7 possessed good transient temperature response performance and presented best on the evaluation experiment in the adiabatic room compared with pure SSD, SSD-CBO, and other two composites. Furthermore, the mechanism of tailored phase change behavior of Na2SO4·10H2O/expanded graphite composite was revealed, the porous structures of EG with good thermal transfer ability can supply the pathways of thermal heat to help SSD with transferring heat more effective during freezing phase then overcome recalescence. In conclusion, the devised SSD-CBO/EG7 composite that can efficiently suppress the supercooling and phase separation, and especially prevent the recalescence may provide a breakthrough in the inorganic salt hydrates for thermal energy storage in the floor heating system.

melting temperature of 32.24 °C, it could not dissolve all the sulfate salts in the corresponding crystal water of the decahydrate composition. Therefore the sodium sulfate will settle down to the bottom of the container as sediment due to its higher density. Then the distinct phase separation happened which was presented in Fig. 12b, in which the upper layer is the saturated solution of Na2SO4, and the lower layer is crystals of Na2SO4. Then several agents including CMC, Na2B4O7·10H2O, and OP-10 were added into SSD to synthesize SSDCBO in Fig. 12c. First, according to the report about the nucleation of Na2SO4·10H2O[80], Na2B4O7·10H2O can be used as a nucleation catalyst to suppress the supercooling of Na2SO4·10H2O as the crystallographic data of them is crystallized agree within 15%. This study further investigated that the supercooling of Na2SO4·10H2O can be dropped more than 20 °C by employing 3 wt% of Na2B4O7·10H2O. Second, as shown in Fig. 12c, the CMC processes a long hydrophobic chain which can be associated with surrounding water molecules by hydrogen bonds. It improved the fluid volume of the polymer itself and reduced the free space for Na2SO4 to move. Therefore the viscosity of the SSD-CBO system was improved and the phase separation was suppressed. Nevertheless, a drastic recalescence still appeared in the freezing peak of the DSC curve of SSD-CBO (Fig. 12d). At last, the SSDCBO was incorporated into the pores of EG to farther tailor the phase change behavior of Na2SO4·10H2O and improve its shape stability. According to the phonon propagation theory [81], the reinforcement structures can be engineered to decrease the phonon scattering at the interface so as to increase the effective thermal conductivity, and many researchers have verified the carbonaceous skeleton of EG has its specific advantages for thermal transfer [82]. As the EG has high porosity and the SSD-CBO was firmly impregnated into the net-pores of EG (shown in Fig. 4), and the recalescence disappeared in the freezing peak of DSC curve (Fig. 12e) due to a high-efficiency heat pathway was formed in the SSD-CBO/EG7.

CRediT authorship contribution statement Chuanchang Li: Conceptualization, Methodology, Resources, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Bo Zhang: Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Baoshan Xie: Investigation, Visualization. Xinbo Zhao: Investigation, Validation. Jian Chen: Resources, Project administration. Declaration of Competing Interest

4. Conclusion

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The SSD-CBO/EG composites with different mass fractions of EG were synthesized by stabilizing SSD-CBO into the EG under the vacuum impregnation process. The SSD-CBO for suppressing the supercooling and phase separation of pure SSD was synthesis by adding a certain mass ratio of CMC, borax decahydrate and OP-10 into SSD. Based on SSD-CBO, the supercooling and phase separation of SSD-CBO/EG composites were further suppressed. Thermal properties study manifested that the composite PCMs not only exhibited good chemical

Acknowledgments This work was supported by the National Natural Science Foundation of China, China (51874047, 51504041); the Changsha City Fund for Distinguished and Innovative Young Scholars (kq1802007); 11

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the Fund for University Young Core Instructors of Hunan Province, China; the Outstanding Youth Project of Hunan Provincial Department of Education, China (18B148); the Innovation Program for Postgraduate of Hunan Province, China (CX20190688); and the Hunan Province 2011 Collaborative Innovation Center of Clean Energy and Smart Grid.

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