Supercooling Suppression and Thermal Conductivity Enhancement of

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Supercooling Suppression and Thermal Conductivity Enhancement of Na2HPO4·12H2O/Expanded Vermiculite Form-Stable Composite Phase Change Materials with Alumina for Heat Storage Yong Deng,† Jinhong Li,*,† Yanxi Deng,*,† Hongen Nian,‡,§ and Hua Jiang∥ †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, People’s Republic of China ‡ Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, People’s Republic of China § Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining 810008, People’s Republic of China ∥ Qinghai University, Xining 810016, People’s Republic of China ABSTRACT: In the heat storage applications, Na2HPO4·12H2O phase change materials (PCM) show significant defects including form instability, high supercooling degree and low thermal conductivity. Aiming at these drawbacks, the Na2HPO4·12H2O-alumina/expanded vermiculite (EVM) form-stable composite phase change materials (NEA fs-CPCMs) with supercooling suppression and heat transfer enhancement were prepared. The favorable wettability of NEA fs-CPCMs was beneficial to their form-stabilization (encapsulation mass fractions above 59.7 wt %). The supercooling degree of NE5.3 was reduced to 1.4 °C after adding 5.3 wt % alumina as the nucleating agent (decreased by 90%). The surface effects of EVM and alumina and high wettability of PCM-alumina were responsible for the supercooling suppression of PCM. Large specific surface area of EVM could provide nucleation sites for crystallization of PCM. The surface electronegativity of alumina increased the affinity ability and diffusion rate of ions, thus effectively increasing the probability of nucleation. The heat transfer of NEA fs-CPCMs was significantly enhanced by the alumina as the thermal conductivity enhancement filler. The thermal conductivity of NE5.3 reached 0.418 W/(m K). Thermal energy storage behavior analysis indicated that the NEA fsCPCMs showed large heat storage capacity (melting process: 97−151 J/g; solidification process: 60−89 J/g). The thermal reliability of NEA fs-CPCMs was effectively improved by coated paraffin. KEYWORDS: Phase change material, Expanded vermiculite, Alumina, Supercooling suppression, Thermal conductivity enhancement

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INTRODUCTION Recently, hydrated salt phase change materials (HSPCMs) show application potential in solar energy conservation due to their suitable melting point, large heat storage capacity and density, nonflammability, and low cost. The solar energy systems containing phase change materials (PCMs) can significantly improve the thermal inertia of the systems, thereby allowing the solar energy to be used at any time even at night without sunlight and thus bridging the demand and supply gap of the normal electrical energy consumption.1−4 However, due to liquid leakage, supercooling, and slow heat transfer, the applications of HSPCMs in solar energy conservation applications are largely restricted. Encapsulation can effectively solve the liquid leakage problem of PCMs by controlling their volume change, avoid the interaction with outer environment, allow PCMs to retain the solid shape during phase transition, and enhance the mechanical stability of the system.5−7 Lately, carrier materials including expanded graphite,8−10 expanded perlite,11,12 and silica13 have been employed to encapsulate HSPCMs into their © XXXX American Chemical Society

pores and on the surfaces by capillary force and surface tension effects to prepare form-stable composite PCMs (fs-CPCMs). Expanded vermiculite (EVM), a hydrous phyllosilicate mineral, exhibits the highly porous structure and large specific surface area, which allows high heat storage capacities of PCM. In our previous studies,14,15 the maximum encapsulation ratio of EVM-based polyethylene glycol fs-CPCMs was larger than 66 wt %. Also, with lightweight, good compatibility, chemical inertness, nonflammability and low cost, EVM are considered as suitable heat storage materials in heat storage applications in solar energy conversion. Moreover, the large specific surface area of EVM can provide numerous nucleation sites, accelerate the crystallization process of HSPCMs, and help to reduce the supercooling degree. EVM-based fs-CPCMs containing HSPCMs shows wide application potentials, but their thermophysical properties have not been reported so far. Received: February 6, 2018 Revised: March 18, 2018 Published: March 21, 2018 A

DOI: 10.1021/acssuschemeng.8b00631 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Photographs of (a) NE0 with leakage trace and (b) NE0 with good form stability.

Table 1. Components of the Prepared NEA fs-CPCMs NEA fsCPCMs

EVM (g)

Na2HPO4·12H2O (g)

Alumina (g)

EVM weight fractions (wt %)

Na2HPO4·12H2O weight fractions (wt %)

Alumina weight fractions (wt %)

NE0 NE1.3 NE2.7 NE4.0 NE5.3

3.00 3.00 3.00 3.00 3.00

5.82 5.66 5.46 5.30 5.12

0.00 0.11 0.23 0.35 0.45

34.0 34.2 34.5 34.7 35.0

66.0 64.5 62.8 61.3 59.7

0 1.3 2.7 4.0 5.3

1.5−6.7 μm) as nucleating agents. Indeed, inorganic particles proved to be effective nucleating agents, but the supercooling suppression mechanism is currently unclear and needs to be elucidated. Among the inorganic particles had been reported as nucleating agents, alumina showed the relatively high thermal conductivity (30 W/(m K)), high surface activity, stable thermal and chemical properties, and low thermal expansion coefficient. Moreover, alumina is cheaper than other inorganic particles. Several researches confirmed that the dispersion of thermal conductivity enhancement fillers could particularly improve the heat transfer of PCMs. For example, carbon nanotubes, carbon fiber, silver nanostructure and Ti4O7 were employed to enhance the low thermal conductivity of PCMs.24−27 Currently, alumina has been used as effective heat transfer enhancement additives of organic PCMs.28,29 Alumina may be beneficial and suitable to simultaneously reduce the supercooling of HSPCMs and enhance the thermal conductivity. In this study, Na2HPO4·12H2O was selected as HSPCM because it melted at 35−45 °C with a large latent heat (larger than 250 J/g) and was considered as a promising medium for heat storage applications in solar energy conversion. We designed Na2HPO4·12H2O-alumina/EVM form-stable composite phase change materials (NEA fs-CPCMs), in order to simultaneously prevent leakage, suppress supercooling, and enhance heat transfer of Na2HPO4·12H2O via the synergistic effect between EVM and alumina. The microstructure, chemical compatibility, supercooling suppression, thermal conductivity enhancement, heat storage behavior and thermal stability of NEA fs-CPCMs were analyzed in detail. The NE2.7 coated by paraffin (denoted as NE2.7C) was prepared to improve the thermal reliability of NE2.7 after large numbers of thermal cycles. The supercooling suppression mechanism of alumina as nucleating agent was also clarified. The prepared NEA fsCPCMs were expected to be a potential candidate for heat storage applications in solar energy conversion.

In order to overcome the supercooling problems of HSPCMs, several strategies have been proposed, such as adding nucleating agents or using the coldfinger. The nucleating agents serve as nucleation sites to make crystals nucleate and grow more easily. Li et al.16 reported that the supercooling degree CaCl2·6H2O PCM reduced from 13.8 to 2.8 °C after adding 3.0% SrCl2·6H2O as the nucleating agent. It is generally postulated that the nucleation is easily induced when the difference of lattice constants between PCM and nucleating agent is less than 15%,17,18 thus large numbers of experimental screenings and validations are needed. The surface properties of nucleating agents significantly affect the crystal growth of HSPCMs due to the increased surface free energy and accelerated nucleation. Therefore, the nucleation effect is strongly dependent on the affinity between nucleating agents and crystals of HSPCMs.19 The adsorption of ions or molecules onto the surfaces of nucleating agents is a critical step for crystal growth. The adsorption capacity obviously increases when the size of nucleating agents is reduced to the nanometer, submicronmeter or micrometer scale.20,21 Therefore, the nano-, submicrometer- or microsized particles are considered as suitable nucleating agents to suppress supercooling of HSPCMs. Recently, some inorganic particles have been reported as nucleating agents to reduce the supercooling degree of HSPCMs. Cui et al.19 found that the supercooling degree of CH3COONa·3H2O containing 0.5% nanocopper (particle size: 10−30 nm) was reduced to approximately 0.5 °C. Hu et al.22 reported that AlN nanoparticles (particle size: 50 nm) could prevent the supercooling of CH3COONa·3H2O significantly. Liu et al.20 investigated the effect of alumina particles (particle size: 70−200 nm) on the supercooling behavior of Na2SO4·10H2O−Na2HPO4·12H2O eutectic hydrated salt. The results showed that the alumina particles were highly effective nucleating agents for supercooling suppression. Ryu et al.23 found that the supercooling degree of Na2HPO4· 12H2O was obviously reduced below 1 °C by using copper (particle size: 1.5−2.5 μm) and carbon particles (particle size: B


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Figure 2. (a) XRD pattern and (b) SEM image of the alumina.

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Q5000) from 30 to 100 °C with a heating rate of 10 °C/min in a N2 atmosphere.

EXPERIMENTAL SECTION

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Materials. EVM (particle size: 2−3 mm) was purchased from Lingshou County, Hebei Province, China. It is mainly composed of SiO2 (43.2 wt %), MgO (24.2 wt %), and Al2O3 (12.68 wt %). Alumina (α-phase; particle size: approximately 200 nm; thermal conductivity: 30 W/(m K)) was obtained from Xuancheng Jingrui New Material Co., Ltd., Anhui Province, China. Na2HPO4·12H2O (A.R.) and paraffin (C.P.) were purchased from Xilong Chemical Reagent Beijing Co., Ltd., China. Preparation of NEA fs-CPCMs and NE2.7C. The NEA fsCPCMs were prepared via the blending and impregnation method. First, alumina was dispersed in the liquid Na2HPO4·12H2O under stirring at 70 °C for 1 h. Second, the EVM was added into the above suspension and maintained at 70 °C for 2 h. The impregnated EVM was transferred onto filter paper (Figure 1a) and obvious leakage trace was observed. Hence, the filter paper was heated to 70 °C and replaced continuously to remove the leaky PCM. Finally, the NEA fs-CPCMs with good form stability were obtained (Figure 1b). Their components are shown in Table 1. The encapsulation mass fractions (wt %) of NE0, NE1.3, NE2.7, NE4.0, and NE5.3 were respectively 66.0, 64.5, 62.8, 61.3, and 59.7 and the corresponding contents of alumina (wt %) were 0, 1.3, 2.7, 4.0, and 5.3. The NE2.7C was prepared to improve the thermal reliability of NE2.7. First, the sufficient paraffin was heated to obtain liquid paraffin. Next, the sample of NE2.7 was placed inside a filter net and immersed in the liquid paraffin and then quickly taken out. Finally, the NE2.7 coated by paraffin was transferred onto filter paper to remove the excess paraffin of surface of NE2.7 until no obvious trace was observed, then the NE2.7C was obtained. Characterization. Morphologies of alumina, EVM, and NEA fsCPCMs were observed with scanning electronic microscopy (SEM, HITACHI S-4800). All samples were coated with gold particles before testing to increase their electroconductivity. X-ray diffraction (XRD, Rigaku DMAX 2400) was used to collect the diffraction patterns at a scanning rate of 8°/min in the 2θ range of 20°−90° for alumina and 5°−60° for NEA fs-CPCMs. The contact angle was measured by using Drop Shape Analyzer (KRUSS DSA25). The liquid PCM (about 3 μL) was respectively dripped on the surface of circular sheet of EVM and alumina and then the contact angle was obtained by taking pictures after 3 s. The cooling curves of PCM and NEA fs-CPCMs were obtained by the constant temperature water bath method (Multichannels temperature recorder: TOPRIE TP720, Thermocouple: T, Record interval: 1 s). All samples were heated to 70 °C and then cooled to 18 °C. The thermal conductivity of NEA fs-CPCMs was determined with the transient hotline method (TC 3000E) at 26 °C with the accuracy within ±3%. All samples were processed into two planes (Approximate size: 3 cm × 3 cm), and then the probe was caught in the middle of the two planes. Each sample of NEA fsCPCMs was measured five times to calculate the mean value. Phase change parameters of PCM, NEA fs-CPCMs and NE2.7C were obtained with a differential scanning calorimeter (DSC, TA Q100). The testing temperature range was between 0 and 70 °C at a rate of 5 °C/min in a N2 atmosphere. Thermal stability of PCM and NEA fsCPCMs was carried out with a thermogravimetric analyzer (TGA, TA

RESULTS AND DISCUSSION Characterization of Alumina. Figure 2a shows the XRD pattern of the alumina. All the observed diffraction peaks exactly corresponded to the crystal planes of α-alumina phase (JCPDS card No. 81-1667). The SEM image of the alumina is shown in Figure 2b. The alumina particles were agglomerated together, and their size was approximately 200 nm. These submicrometer-sized particles were expected to obviously promote the heterogeneous nucleation of PCM and accelerate their crystallization rate, thereby reducing the supercooling degree. Morphology and Wettability of NEA fs-CPCMs. The EVM exhibited a nonuniform multilayered pore structure and high porosity (Figure 3a), providing large encapsulation capacity. Figure 3b,c illustrates the SEM image of NE0 and the corresponding EDS spectrum. The melted PCMs were completely adsorbed on the surfaces and in pores of EVM. The EDS spectrum confirmed the presences of Na, P, O, Si, Mg, and Al elements. Al element came from EVM and its composition on the surface of NE0 was 0.69 wt %. SEM images of NE1.3, NE2.7, NE4.0, and NE5.3 and their corresponding EDX mapping results of Al element and EDS spectra are displayed in Figures 3d−q. The NE1.3, NE2.7, NE4.0, and NE5.3 showed similar morphologies with NE0. The agglomerated alumina particles enwrapped by melted PCMs could be clearly observed when the weight fractions of alumina reached 5.3 wt % (Figure 3p,q). EDX mapping of Al element indicated that its distribution on the surface of NE1.3, NE2.7, NE4.0, and NE5.3 became dense as alumina weight fractions increased (Figure 3e,h,k,n). The EDS spectrum revealed the elemental composition of Al on the surface of NE1.3, NE2.7, NE4.0, and NE5.3 gradually increased (Figure 3f,i,l,o). The above results showed that the alumina as nucleating agent and thermal conductivity enhancement filler was effectively dispersed to enhance heat transfer and suppress the supercooling of NEA fs-CPCMs. From Figure 3p,q, the two-phase interfaces of PCM-EVM and PCM-alumina contacted compactly. The contact angles of PCM-EVM and PCM-alumina were respectively 15.03° and 5.31° (Figure 4), indicating that the PCM, EVM and alumina interacted with high wettability. The excellent wetting property could significantly enhance affinity of PCM and EVM and alumina, which allowed the alumina enwrapped by melted PCMs to be easily impregnated into the pores and surfaces of EVM. The strong physical adsorption effects of EVM and alumina on PCM were beneficial to prevent liquid leakage C


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Figure 3. SEM images of (a) EVM, (b) NE0 and (c) the corresponding EDS spectrum; SEM images of (d) NE1.3, (g) NE2.7, (j) NE4.0 and (m) NE5.3, and (e, h, k and n) their corresponding EDX mapping of Al element, and (f, i, l and o) EDS spectrum; SEM images of (p and q) NE5.3.

Figure 4. Contact angles of (a) liquid PCM and EVM, and (b) liquid PCM and alumina.

D


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ACS Sustainable Chemistry & Engineering during phase transition of PCM, thus the high wettability was conducive to the encapsulation and the form stability of NEA fs-CPCMs. Chemical Compatibility of NEA fs-CPCMs. Figure 5 shows the XRD patterns of PCM and NEA fs-CPCMs. The

Table 2. Supercooling Degree and the Corresponding Suppression Ratio of PCM and NEA fs-CPCMs Samples

Supercooling (°C)

Suppression ratio (%)

PCM NE0 NE1.3 NE2.7 NE4.0 NE5.3

14.3 10.7 10.3 8.3 5.1 1.4

25.2 28.0 42.0 64.3 90.2

numerous nucleation sites for crystallization of PCM and accelerate crystallization rate. When the component of alumina was increased up to 5.3 wt %, the supercooling degree of NE5.3 was reduced to 1.4 °C and the suppression ratio reached 90.2%, indicating that the supercooling phenomenon was nearly eliminated. The mechanism of heterogeneous nucleation could be used to explain the supercooling suppression. Based on the assumption that the shape of nucleus was spherical, the energy barrier a cluster must overcome before growing irreversibly as a crystal could be defined as30,31

Figure 5. XRD patterns of PCM and NEA fs-CPCMs.

diffraction peaks of PCM were well indexed to Na2HPO4· 12H2O (JCPDS card No. 70-0654). Some differences in relative peaks intensity between PCM and JCPDS card might be caused by the different shapes of crystals.12 The XRD patterns of NEA fs-CPCMs contained all the diffraction peaks of PCM and EVM, and no new peak appeared except the peaks of alumina. Moreover, the positions of peaks of NEA fs-CPCMs showed one-to-one corresponding relationship with PCM, EVM and alumina. The above results indicated that there was no chemical reaction in the phase transition process of NEA fsCPCMs. Therefore, NEA fs-CPCMs had the excellent chemical compatibility. Supercooling Behavior of NEA fs-CPCMs. The cooling curves of PCM and NEA fs-CPCMs are shown in Figure 6. The

ΔGC* = f (θ ) =

3 2 16πγSL V

3(Δg )2

f (θ ) (1)

(2 + cos θ )(1 − cosθ )2 4

(2)

where γSL is the solid−liquid interface free energy; V represents the volume per particle; Δg is the liquid-crystalloid free energy difference per particle; θ represents the contact angle. In this study, the contact angle between PCM and alumina was 5.31°, indicating that the value of ΔG*C was close to zero. The above analysis indicated that the crystal nucleus of PCM could be formed nearly without supercooling. Thus, it is feasible to reduce the supercooling degree with alumina as the nucleating agent. In addition, the similarity of crystal structure between PCM and nucleating agent determined whether the nucleation induction would occur. The nucleation was easily induced when the difference was less than 15%. Crystal parameters of PCM and alumina are shown in Table 3. PCM and alumina showed significant differences in the crystal form and lattice constant. The decrease in the supercooling degree could not be explained by the above significant differences. The possible reasons could be described as follows. The isoelectric point of alumina was pH = 7−8.5, which was lower than the Na2HPO4· 12H2O (pH > 9). Thus, the surfaces of alumina were negatively charged. The results were consistent with the previous report.32 The nucleation mechanism of PCM on the surfaces of alumina is shown in Figure 7. The negative charge on the surfaces of alumina strongly interacted with ions of PCM by the electrostatic effect, thus increasing the affinity ability and diffusion and combination rate of ions. Hence, the probability of nucleation and crystallization of PCM was effectively increased, thus leading to the supercooling suppression of PCM. Thermal Conductivities of NEA fs-CPCMs. Thermal conductivity of PCM determines the heat charging and discharging rates during the melting and solidification processes. Figure 8 shows the thermal conductivities of NEA fs-CPCMs with different weight fractions (wt %) of alumina (0, 1.3, 2.7, 4.0, and 5.3), and the repeated values are summarized

Figure 6. Cooling curves of PCM and NEA fs-CPCMs.

supercooling degree is defined as the difference between the temperature of plateau and the supercooling temperature of cooling curves. The supercooling degrees and corresponding suppression ratios of PCM and NEA fs-CPCMs are summarized in Table 2. The results showed that the EVM and alumina significantly affected the supercooling behavior of PCM and that the supercooling degree was decreased constantly with the increase in the alumina weight fraction. After the PCM was encapsulated into the pores of EVM, the supercooling degree of PCM was decreased from 14.3 to 10.7 °C because the large specific surface area of EVM could provide E


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ACS Sustainable Chemistry & Engineering Table 3. Crystal Parameters of PCM and Al2O3 Substance

Crystal form

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

JCPDS card no.

PCM Al2O3

Monoclinic system Trigonal system

15.718 4.760

9.018 4.760

12.769 12.993

90.00 90.00

121.39 90.00

90.00 120.00

70-0654 81-1667

Figure 7. Schematic diagram of the nucleation mechanism of PCM on the surfaces of alumina.

test. As a result, less heat was released during cooling process, thus leading to the large decrease in the latent heats of NEA fsCPCMs in solidification process. There were two endothermic peaks in the DSC curve of PCM (Figure 9a). Based on previous research,33,34 the HSPCM had different melting temperatures because attached water could not completely dissolve the salts. The solution containing undissolved solute was not in equilibrium at the melting point. The first peak at 37.68 °C was attributed to the fact that the Na2HPO4·12H2O lost 5 attached water molecules to form Na2HPO4·7H2O. The second peak at 47.18 °C was due to the fact that the Na2HPO4·7H2O further lost 5 attached water molecules to form Na2HPO4· 2H2O from the original saturated solution. The NE0 and NE1.3 exhibited the similar heat storage behavior with Na2HPO4· 12H2O. However, NE2.7, NE4.0, and NE5.3 had only one endothermic peak near 52 °C and the first peak disappeared. It was postulated that the large mass transfer area and surface effects of alumina increased the diffusion rates of Na+, HPO42−, and water molecules, thus accelerating the dissolution of Na2HPO4·7H2O. Hence, Na2HPO4·12H2O melted from two steps to one step. Moreover, it was found that both the second endothermic peaks (Figure 9a) and exothermic peaks (Figure 9b) of all NEA fs-CPCMs obviously shifted toward the highertemperature direction compared with pure PCM. The shift might be caused by the confinement effects of EVM and surface interactions of alumina.35 The influences of confinement effects on the phase change temperatures of NEA fs-CPCMs could be explained with the Clapeyron−Clausius Equation:36−38

Figure 8. Thermal conductivities of NEA fs-CPCMs measured by using transient hotline method.

in Table 4. The thermal conductivity of NEA fs-CPCMs containing 0, 1.3, 2.7, 4.0, and 5.3 wt % of alumina was respectively 0.287, 0.321, 0.347, 0.378, and 0.418 W/(m K), which were respectively 11.8%, 20.9%, 31.7%, and 45.6% higher than that of NE0. An approximately linear fit result (Figure 8) between alumina weight fractions (x) and thermal conductivity of NEA fs-CPCMs (y) is given by y = 0.03159x + 0.25524

(R2 = 0.99674)

(3)

The determination coefficient (R2 = 0.99674) indicated a high correlation between y and x. The results indicated that the thermal conductivities of NEA fs-CPCMs were greatly enhanced by alumina. The enhancement was mainly attributed to rapid heat transfer of alumina and their effective dispersion in the pores and on surfaces of EVM. Heat Storage Behavior of NEA fs-CPCMs. The DSC curves of pure PCM and NEA fs-CPCMs during the melting and solidification processes are respectively shown in Figure 9a,b and the corresponding phase change parameters are listed in Table 5. It should be noted that there was a big difference between HM and HS, probably caused by the dehydration and water evaporation of PCM during the heating process of DSC

ln

T2 ΔV = (P2 − P1) T1 ΔH

(4)

where T1 and T2 represent phase change temperatures; ΔV and ΔH respectively denotes the volume change and enthalpy change during the phase change; P1 and P2 are the ambient pressures during the phase change. When PCM melted, the volume was positive because of the volume expansion (ΔV > 0). The ΔH was also positive (ΔH > 0). Because the PCM was

Table 4. Repeated Thermal Conductivities of NEA fs-CPCMs NEA fs-CPCMs

first

second

third

fourth

fifth

Mean value (W/(m K))

NE0 NE1.3 NE2.7 NE4.0 NE5.3

0.2882 0.3204 0.3446 0.3793 0.4184

0.2873 0.3238 0.3471 0.3770 0.4188

0.2867 0.3200 0.3488 0.3783 0.4166

0.2872 0.3200 0.3460 0.3769 0.4202

0.2873 0.3202 0.3493 0.3778 0.4183

0.287 0.321 0.347 0.378 0.418

F


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Figure 9. DSC curves of PCM and NEA fs-CPCMs during the melting (a) and solidification processes (b).

to analyze its thermal reliability. The DSC curves of NE2.7 and NE2.7C during the melting and solidification processes after 10, 20, and 30 thermal cycles are shown in Figure 10, and the corresponding phase change parameters are summarized in Table 6. It was found that the phase change temperature of

Table 5. Phase Change Parameters of PCM and NEA fsCPCMs during the Melting and Solidification Processesa Melting process

Solidification process

Samples

TM (°C)

HM (J/g)

TS (°C)

HS (J/g)

PCM NE0 NE1.3 NE2.7 NE4.0 NE5.3

37.68 and 47.18 36.35 and 50.17 35.95 and 52.43 52.63 52.01 52.48

246.70 151.10 136.10 120.60 111.40 97.68

14.07 19.13 17.75 20.29 19.05 17.75

122.10 88.77 82.04 73.86 69.26 60.31

Table 6. Phase Change Parameters of NE2.7 and NE2.7C during the Melting and Solidification Processes after 10, 20, and 30 Thermal Cyclesa Melting process

a

Notes: TM and TS is respectively the phase change temperature in melting and solidification process, and HM and HS respectively represents the latent heat value.

confined to the porous spaces of EVM, the volume expansion led to the increase in ambient pressure (P 2 > P 1 ). Consequently, T2 > T1. When PCM was solidified, ΔV < 0 and P2 < P1, thus T2 > T1. The above results indicated that the phase change temperatures of NEA fs-CPCMs were higher than that of pure PCM. The phase change enthalpy of NEA fsCPCMs decreased constantly with the increase in the alumina weight fraction. The latent heat was 97−151 J/g in the melting process and 60−89 J/g in the solidification process. Based on previous research,11,12,39−43 the latent heats at least 50 J/g were needed at an affordable cost for heat storage applications of fsCPCMs, thus the decrease in latent heat of NEA fs-CPCMs was acceptable and reasonable for heat storage applications. Thermal Reliability of NEA fs-CPCMs. The latent heats of NEA fs-CPCMs decreased as the content of alumina increased (Table 5). It is necessary to achieve the balance between thermal conductivity and latent heats. Thus, the NE2.7 with relatively high thermal conductivity and latent heat was selected

Solidification process

Samples

TM (°C)

HM (J/g)

TS (°C)

HS (J/g)

NE2.7 NE2.7-C10 NE2.7-C20 NE2.7-C30 NE2.7C−C10 NE2.7C−C20 NE2.7C−C30

52.63 52.15 51.70 51.90 52.44 52.04 52.20

120.60 99.39 76.40 56.75 114.40 101.60 90.55

20.29 19.65 18.64 17.73 16.69 18.14 19.02

73.86 57.51 38.37 25.90 67.49 60.63 53.16

a

Notes: NE2.7-C10, NE2.7-C20 and NE2.7-C30 respectively represents the sample of NE2.7 after 10, 20, and 30 thermal cycles, and NE2.7C-C10, NE2.7C-C20 and NE2.7C-C30 is respectively the sample of NE2.7C after 10, 20, and 30 thermal cycles.

NE2.7 had small change after thermal cycles, but the latent heat significantly decreased with the increase of number of thermal cycles. The latent heat of NE2.7-C30 was only 56.75 and 25.90 J/g during melting and solidification process, respectively, indicating the thermal reliability of NE2.7 was not so satisfactory, which was caused by PCM dehydration and water evaporation during the thermal cycles. In fact, based on previous research,8,13,44 it was difficult to maintain high thermal reliability of HSPCMs after large numbers of thermal cycles.

Figure 10. DSC curves of NE2.7 and NE2.7C during the melting (a) and solidification processes (b) after 10, 20, and 30 thermal cycles. G


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Figure 11. TGA and DTG curves of (a) PCM, (b) NE0, (c) NE1.3, (d) NE2.7, (e) NE4.0, and (f) NE5.3.

thermal cycles. Moreover, NEA fs-CPCMs exhibited good chemical compatibility. However, their thermal stability was not so satisfactory due to PCM dehydration and water evaporation, which was effectively improved by the increased weight fractions of EVM and alumina.

In order to improve the poor thermal reliability of NE2.7, the NE2.7C was prepared to strongly prevent the escape of water molecules of PCM during the thermal cycles. Compared with NE2.7-C10, NE2.7-C20 and NE2.7-C30, the NE2.7C-C10, NE2.7C-C20 and NE2.7C-C30 showed higher latent heats both melting and solidification processes (Table 6), demonstrating the coated paraffin was effective in improving the thermal reliability of NE2.7 within at least 30 thermal cycles. The NE2.7C-C30 retained a relatively high latent heat (90.55 J/g in melting process and 53.16 J/g in solidification process). Thermal Stability of NEA fs-CPCMs. Thermal stability of PCM and NEA fs-CPCMs was evaluated by TGA and DTG (Figure 11). There were three instable weight loss platforms when the PCM was heated to 150 °C (Figure 11a). The 12 attached water molecules were completely lost at 125.5 °C to form Na2HPO4, and the total weight loss was 58.22%. The weight loss was mainly attributed to PCM dehydration and water evaporation. Na2HPO4 was further converted into (NaPO3)x at 340.0 °C. From Figure 11b−f, all the NEA fsCPCMs exhibited similar thermal stability characteristics. In the designed working temperature of 30−100 °C, the weight loss percentages of NE0, NE1.3, NE2.7, NE4.0, and NE5.3 were respectively 39.07%, 38.14%, 36.34%, 35.63%, and 33.90% and the maximum speed on weight loss occurred in the range of 60−67 °C, indicating that their thermal stability was not so satisfactory. However, the increased contents of EVM and alumina effectively improved the thermal stability of NEA fsCPCMs because the surface adsorption effects could prevent the escape of water molecules.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Jinhong Li). *E-mail: [email protected] (Yanxi Deng). ORCID

Jinhong Li: 0000-0002-0368-8184 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC Grant Nos. U1607113 and 51574209), Program of Qinghai Science and Technology Department (Grant No. 2017-HZ-805) and Fundamental Research Funds for the Central Universities (Grant No. 2652017357).

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REFERENCES

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CONCLUSIONS The NEA fs-CPCMs were prepared to overcome the drawbacks of Na2HPO4·12H2O PCM, including the form instability, high supercooling degree, and low thermal conductivity. The encapsulation capacity of NEA fs-CPCMs was larger than 59.7 wt %. The supercooling phenomenon of NEA fs-CPCMs was nearly eliminated (reduced to 1.4 °C) by the synergistic effect between EVM and alumina and their surface effects and high wettability of PCM-alumina were responsible for the supercooling suppression. Heat transfer of NEA fs-CPCMs was significantly increased by nearly 45.6% with dispersed alumina. The NEA fs-CPCMs showed the suitable phase change temperature (17−53 °C) and large thermal energy storage capacity (60−151 J/g). The thermal reliability of NE2.7 was effectively improved by coated paraffin within at least 30 H


Research Article

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