Shape-remodeled macrocapsule of phase change materials for thermal energy storage and thermal management

Applied Energy 247 (2019) 503–516

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Shape-remodeled macrocapsule of phase change materials for thermal energy storage and thermal management De-Hai Yu, Zhi-Zhu He

T

⁎

Beijing Key Laboratory of Optimized Design for Modern Agricultural Equipment, Vehicle Engineering, College of Engineering, China Agricultural University, Beijing 100083, China

H I GH L IG H T S

PCM macrocapsule was fabricated through a cast molding method. • ATheshape-remodeled flexible shell can sustain a maximum stretching of 432%. • The EBiInSn-based prepared PCM macrocapsule can be remodeled as needed to a complicated shape. • An innovative PCM-based conformal thermal control method was demonstrated. • The thermoelectric generation module based on PCM macrocapsule was developed. •

A R T I C LE I N FO

A B S T R A C T

Keywords: PCM macrocapsule Octadecanol Bi-In-Sn eutectic alloy Thermal energy storage Thermal management Thermoelectric power generation

This paper reports on a novel phase change material macrocapsule for thermal energy storage, which can be dynamically and repeatably remodeled as needed to a complicated shape with large-scale deformation. In addition, it effectively eliminates the stress mismatch, induced by the volumetric expansion (or shrink) of the phase change material during melting (or solidification), through the self-adaptative deformation of the coated flexible shell. The shape-remodeled macrocapsule, consisting of octadecanol as the core and the silicone elastomer for encapsulation, was prepared through a cast molding method. The high-concentration microparticles of lowmelting Bi-In-Sn eutectic alloys were embedded into elastic shell for significantly enhancing its latent heat storage and heat conductivity. The prepared macrocapsule has a high latent heat density of 210.1 MJ/m3, which of the contribution from the shell is about 20%. The thermal conductivity of the macrocapsule core reaches to 1.53 W/m·K with a 428% increase compared with pure octadecanol. The flexible shell attains a high thermal conductivity of 1.98 W/m·K with an 890% increase compared with pure silicone, which also remains a high stretchability with 432% strain. The performance of shape remodeling, energy storage capacity, and heat charging and discharging rates of the macrocapsule were demonstrated in detail. The applications of the prepared macrocapsule as thermal management for the flexible electronic devices and the thermal storage for thermoelectric energy harvesting were also investigated. The present study opens the way for further development of elastic phase change material capsule applications in energy storage systems and thermal control engineering.

1. Introduction Phase change material (PCM) based on the absorption and release of latent heat during the solid-liquid phase transition[1] has been widely applied in various areas[2] ranging from solar energy utilization [3,4], industrial waste-heat recovery[5], thermoelectric energy harvesting[6], to building temperature control[7,8]. The latent heat density of PCM material is substantially larger than sensible heat under small

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temperature difference. It has an attractive advantage of high thermal storage density and can be used to effectively improve the performance and reliability of the energy system. In addition, PCM could absorb a large amount of heat during solid-liquid phase transition, which can be applied as a kind of important heat dissipation technologies for electronics thermal management[9], such as a PCM used as a heat sink against high power thermal shock[10]. However, most organic[11] and inorganic[12] PCMs have low thermal conductivity[13], which

Corresponding author at: 17 Qinghua Donglu, China Agricultural University, Beijing 100083, China. E-mail address: [email protected] (Z.-Z. He).

https://doi.org/10.1016/j.apenergy.2019.04.072 Received 25 November 2018; Received in revised form 26 March 2019; Accepted 15 April 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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D.-H. Yu and Z.-Z. He

induce the large thermal stress and increase the risk of shell cracking [46]. It is also noteworthy that the void or porosity inside a macrocapsule not only decrease the energy storage density[47], but it also decreases the internal heat transfer performance due to decreasing the contact surface with shell[48]. Alam et al.[49] recently proposed a novel method to encapsulate nitrate based inorganic PCM through coating a polymer-metal shell, which can expand and contract to accommodate for the large PCM volumetric expansion on melting. The thin[50] and elliptical-shaped[51] capsules were also designed and fabricated for the application in latent heat thermal energy storage system. However, the polymer shell could delay the rates of the heat charging and discharging process due to its low thermal conductivity [45]. The thickness of the macrocapsule shell also has important impact on its energy storage density compared with nano/micro capsules. The encapsulation ratio of PCM mass to the total mass of the shell is only 61.7 wt% in PCM-HSB concrete[37], and about 74.4 wt% for molten salt PCM reported in[41], which decrease the latent heat storage density of the PCM macrocapsule. While the rigid shell of the macrocapsule can bear the external extrusion force, it is hard to make the dynamical morphologic adjustment as needed, which greatly inhibits their further applications in practical energy storage and thermal control engineering. Various geometries of PCM encapsulations have attracted significant attention to improve the energy storage efficiency[52]. Barba and Spiga[53] have investigated impact of the different macrocapsule shapes (slab, cylinder and sphere) on the discharging process of a domestic hot water tank. Navarro et al. [14] have found that PCM-spheres with low thermal conductivity make difficult to melt the PCM. Once the macrocapsule was fabricated, its shape cannot be remodeled due to the rigid shell structure, which means that the spherical macrocapsule cannot be used to application scenarios that only allow the cylindrical shape. Another important limitation of the rigid shell is that it cannot sustain the large deformation, which would cause the shell to crack and the PCM to leak. The thermal properties of the PCM macrocapsule shell have important impact on its energy storage capacity and efficiency, charging and discharging rates, while the mechanical properties of the shell determine its reliability and application area. The above analysis has indicated that the conventional PCM macrocapsules coated with rigid shell would lead to many issues, such as large voids needed to accommodate the volumetric change during solidliquid phase change, the coated shell with less contribution to latent heat storage, and the shape hard to be dynamically adjusted as needed. In order to furthermore broaden the applications of the PCM macrocapsule, this paper aims to develop a novel PCM macrocapsule through coating with flexible shell. The high-concentration microparticle of low-melting alloy (Bi31.6In48.8Sn19.6) was filled into elastic shell to significantly improve its latent heat storage density and heat conductivity, which is different from the other particle inclusions (such as carbon nanotubes, graphene and copper nanoparticle) only for thermal conductivity enhancement. The flexible shell allows not only macrocapsule shape to dynamically and repeatably remodel when needed, but also effectively eliminating the stress mismatch induced by the volumetric expansion (or shrink) of the PCM during melting (or freezing) process. To our knowledge, the shape-remodeled PCM macrocapsule developed here has not yet been reported in the literature. The rest of paper is organized as follows. In Section 2, the PCM macrocapsule was prepared through cast molding method, which is consisted of the octadecanol composite as PCM core and the silicone elastomer as flexible shell. In Section 3, the thermal properties of the macrocapsule core and shell were investigated separately. The mechanical property of the flexible shell was also tested. In addition, the performance of shape remodeling, energy storage capacity, and heat charging and discharging rates of the macrocapsule were demonstrated in detail. Furthermore, the applications of the prepared macrocapsule as conformal thermal management for the electronic devices and the thermal storage for thermoelectric energy harvesting were also investigated.

decreases the heat charging and discharging processes, and furthermore has negative effect on the energy storage efficiency[12]. In addition, there is a risk of PCM leakage from the thermal storage system during the solid-liquid phase transition[14]. Considerable efforts have been devoted to address these technical issues. It is an effective method to improve PCM heat transfer by dispersion of nano/micro particles with high thermal conductivity into the PCM matrix[15] including metal particle[16], carbon nanotubes [17,18] and graphene[19]. The shapestabilized PCMs[20] is a new type PCM composite through impregnating the PCMs within a supporting material, such as clay[21], metal foam[22], sintered copper-powder[23] and highly graphitized 3D network carbon[24]. The supporting material prevents the melted PCMs from leaking to keep the whole system in solid state[25], which could also enhance the thermal conductivity of the composite. In addition, micro/macro capsules are developed to improve the thermal performance and prevent its leakage[26]. Recently, the low melting point alloy with high thermal conductivity have also been used as PCM[27]. Encapsulation not only protects the PCM against harmful environmental reactions and prevents its leakage, but it also increases the contact area and improves the heat transfer efficiency. The PCM capsule can be classified as a nanocapsule with a size smaller than 1 μm, microcapsule with size 1–1000 μm and macrocapsule with size larger than 1 mm, respectively. In the last decades, nano/micro encapsulated PCM has attracted significant attention due to its unique feature of the large area-volume ratio[28], such as development of a novel cementitious mortars incorporating microencapsulated PCM[29]. However, the volume ratio of the encapsulation shell or support structure can reach up to 50%[30], which decreases its latent heat storage density. In addition, volume changes during solid-liquid phase transition cycles could induce the serious stresses on the shell of nano/micro capsule[31]. Thus, it is hard to guarantee that there is no leakage of the PCM in the microcapsule-based energy storage systems. In contrast, a macrocapsule has a larger volume ratio of the PCM core to shell compared with microcapsule, which has also attracted great interest in the recent years, such as the wide applications of the spherical PCM macrocapsule packed bed system[32] in energy efficient buildings[14] and concentrating solar power[33,34]. The existent PCM macrocapsules are often encapsulated with rigid shells[35], which can sustain large stresses so that it even could be used as an integrated structural-functional energy storage system. Vicente and Silva[36] reported a novel brick masonry wall by using steel encapsulating paraffin wax, which has demonstrated the ability of storing and releasing energy when needed. Cui et al.[37] recently developed a novel PCM-HSB (hollow steel balls) concrete, which is capable of reducing and deferring the peak indoor temperature and better mechanical interlocking with the mortar matrix. It is almost necessary for high temperature PCM macrocapsule applications to adopt the rigid shell such as the stainless steel or ceramics materials due to the high temperature compatibility. Ma et al.[38] investigated the Fe-shell/Cucore encapsulated metallic PCMs prepared by an aerodynamic levitation method. Zheng et al. [39] have developed an encapsulation method through applying stainless steel/carbon steel as the shell material for salts based PCMs. Zhang et al.[40] successfully prepared macrocapsules using copper as the PCM and chromium-nickel as the shell material. Li et al. [41] developed a high-temperature packed-bed thermal energy storage system using stainless steel as the macro-encapsulation of molten salt PCM. The ceramic based rigid shell may be more suitable for encapsulation of high temperature PCMs due to its excellent corrosion resistance, such as Al2O3[42], and ceramic mixture [43] comprising feldspar, kaolin, ball clay and silica. Most organic and inorganic PCMs have a high-volume expansion ratio during solid-liquid phase transition process, such as 25.58% for NaCl[42], 7.96% for 1-tetradecanol[44] and 15.28% for paraffin wax [45], which needs macrocapsule with a shell to provide enough large voids to accommodate such volumetric changes[31]. However, it is hard to introduce voids inside the nano/micro capsule, which could 504

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2. Experimental section

with an octadecanol composite to prepare the macrocapsule core. Secondly, the silicone composite gaskets with a designed thickness of 1–3 mm were molded by the spherical Mold-2 as illustrated in Fig. 2(b). Thirdly, the macrocapsule core was put into spherical Mold-2, and the gaskets were used to retain a fixed distance between the macrocapsule core and the inner surface of spherical Mold-2. The liquid silicone composite was then injected into spherical Mold-2 to fill the reserved space around the macrocapsule core as shown in Fig. 2(c). The spherical Mold-2 was subsequently put into a vacuum chamber to expel air bubbles inside. Finally, the macrocapsule was placed in vacuum drying oven for 5 h to cure at temperature 50 °C. It is observed from Fig. 2(d) that the flexible shell thickness with size of 2.5 mm is very uniform along circumference with outer diameter of 50 mm. It is because that the spherical models are accurately fabricated by 3D printer, and the gaskets are used to make sure the accurate position alignment of macrocapsule core and shell. The prepared PCM macrocapsules with different sizes based on the shells of EBiInSn-silicone and nanocopper-silicone are shown in Fig. 2(e) and (f), respectively. It is noteworthy that the PCM macrocapsule with complicated shapes according to the actual needs can also be easily designed and prepared through the developed cast molding method. For example, the shape of concentric cylinder shell may be more suitable for the thermal management of cylindrical battery. Fig. 3 shows the different shapes of macrocapsules including circle, rectangle, cylinder, polygon, and letter shapes, respectively.

2.1. Materials Octadecanol (CAS:112-92-5) with melting temperature between 55 and 60 °C was supplied by Shanghai Macklin Biochemical Co., Ltd., copper nanoparticles with average diameter of 300 nm from Beijing Deke Island Gold Technology Co., Ltd., and silicone of Ecoflex 00-30 from Smooth-On. Raw expandable graphite (mesh 80, expandable ratio of 425 ml/g) was purchased from Qingdao Graphite Co. Ltd., China. A microwave oven with a power of 1000 W is used to treat with the expandable graphite and produce the expanded graphite (EG). The eutectic Bi31.6In48.8Sn19.6 alloy (EBiInSn) is composed of 31.6 wt% bismuth, 48.8 wt% indium and 19.6 wt% tin. These metal elements were supplied by Zhuzhou Yilong Hung Industrial Co. Ltd. They were mixed together by the given proportion at 200 °C in vacuum drying oven for 4 h to prepare EBiInSn.

2.2. Preparation of PCM macrocapsule PCM used for macrocapsule core was prepared through mixing octadecanol with EG by an electric motor with 1000 rpm for 10–15 min at 70 °C, which is illustrated in Fig. 1(a). The followed vacuum treatment is used to enhance the adsorption of octadecanol by EG. Ecoflex00-30 rubbers used for the flexible shell are platinum-catalyzed silicones that are versatile and easy to use. Flexible PCM composite was prepared in three steps as shown in Fig. 1(b). Firstly, EBiInSn was mixed into parts of Ecoflex00-30 A and Ecoflex00-30 B separately using an electric mortar with 2500 rpm for 20–30 min at 70 °C. The liquid EBiInSn is easily stirred to fabricate micro liquid metal droplets with sizes about 20–50 μm. The obtained silicone composites of part A and part B are then mixed by an electric mortar with 100 rpm for 3–5 min, which were then cured in vacuum drying oven for 5 h at 50 °C. The copper nanoparticles are also used as the inclusion into silicone matrix for comparation with the EBiInSn-silicone composite. Fig. 2 illustrate the fabrication process of spherical macrocapsule through cast molding method. The spherical model consists of two plastic hemispherical shells, which are fabricated through 3D printer (Raise3D N2). Firstly, the spherical Mode-1 shown in Fig. 2(a) is filled

2.3. Characterization The solid-liquid phase change behavior of the octadecanol-EG and EBiInSn-silicone composites were characterized by differential scanning calorimetry (DSC, 200F3Maia, NETZSCH). Samples with mass of 10 mg were put into aluminum pans to cool and heat at the ramp rate of 10 °C/ min in the temperature range of −80 °C to 100 °C. We also demonstrated that the scanning rate have less impact on the melting/solidification temperature and latent heat, respectively. The melting latent heat of pure octadecanol is obtained about 241.5 kJ/kg, which agrees with 239.7 kJ/kg reported in the literature [10]. The thermal conductivities of the composites were tested by transient plane source method (TPS, 2500S, Hot Disk). The cylindrical

Fig. 1. The schematic diagram of preparation process of PCM composites: (a) octadecanol composite, optical photos of solid octadecanol, and octadecanol mixture with 2 wt% EG; (b) EBiInSn-silicone-EG composite, optical photos of liquid silicone composite with 80 wt% EBiInSn and 1% EG and the corresponding cured film. 505

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Fig. 2. The schematic diagram of preparation process of the spherical PCM macrocapsule: (a) preparation of macrocapsule core through Model-1; (b) gasket used for localization of the macrocapsule core; (c) coating the silicone elastomer to form flexible shell through Model-2; (d) optical photos of the macrocapsule core and shell; (e) optical photos of the PCM macrocapsule with EBiInSn-silicone shell and with (f) nano-copper-silicone shell.

The tension-torsion fatigue test system (MTS809, MTS Systems Corporation) was also used to test the anti-compression ability of PCM macrocapsule, where the compression rate was set as 2 mm/min. Three samples for each test were evaluated to guarantee the consistency of the results. The heat storage and release performances of the macrocapsules were tested through low/high temperature test chamber (BTH-225C, Dongguan Baer Test Equipment Co., Ltd.) with inner operation size of 500 mm × 750 mm × 600 mm. The temperature of the macrocapsule was recorded by T-type thermocouples through Agilent 34970A Data acquisition instrument. The samples were placed in the low/high temperature test chamber with heating and cooling rates about 1.5 °C/ min between 25 and 70 °C.

specimen was prepared with diameter of 20 mm and height of 5 mm, and its surface was polished to reduce the thermal contact resistance with a disk-type Kapton sensor (No.5464 with radius 3.189 mm). The thermal conductivity value is obtained through averaging the results of three samples at each test. Thermal gravimetric analysis (TGA, STA449C, Netzsch) was used to evaluate the thermal stability of the composites. The heating rate was 10 °C/min in the temperature range between 30 °C and 700 °C. The tests were repeated twice in order to verify the result consistence. The micro-morphology of the octadecanol-EG and EBiInSn-silicone composites were observed using a scanning electron microscope (SEM, S-4800, Hitachi) and a micro-computed tomography (Micro-CT, Xradia 410 Versa, Carl Zeiss). The cube-shaped sample of the composite with size of 3.7 mm was prepared to test for Micro-CT. A digital metalloscope (GMM-550P, Shanghai Optical Instrument Factory) was also used to take the microscopy images of EBiInSn-silicone composite. The samples of EBiInSn-silicone with size of 120 mm × 10 mm × 1 mm ware prepared and tested on an Instron 5567 mechanical testing machine. The experiments were run at an extension rate of 20 mm/min. The data were collected continuously throughout the experiment until failure for the strain to break samples.

3. Results and discussion 3.1. The thermal properties of the macrocapsule core Octadecanol was chosen as the PCM macrocapsule core due to its high latent heat density of 241.5 kJ/kg with a melting temperature of 55.7 °C, while it has low thermal conductivity of 0.29 W/m·K. EG with

Fig. 3. The different shapes of macrocapsules: (a) the shapes of circle, rectangle, cylinder and polygon; (b) the letter shapes for presentation of “I love CAU”. 506

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Fig. 4. The micro-morphology and thermal properties of octadecanol composite: (a) SEM micro-morphology of EG; (b) SEM micro-morphology of mixture of EG and octadecanol; (c) the melting-freezing DSC curves, (d) latent heat density, and (e) thermal conductivities of the octadecanol composites with different mass fraction of EG; (f) TGA and DTG curves of octadecanol composite.

temperature (T5%) and the temperature of maximum weight loss rate (Tmax) of EG(3 wt%)-octadecanol are 187 and 254 °C, respectively, which are higher than those (175 and 250 °C) of pure octadecanol. T5% and Tmax for EG(5 wt%)-octadecanol could furthermore increase to 190 and 259 °C, respectively. These results indicate that the addition of EG can enhance the thermal stability of pure octadecanol to some extent. EG with micro-porous structures has excellent octadecanol-adsorption capacity, which can act as protective walls to delay the evaporation and thermal decomposition of octadecanol. The observed results about the impact of EG on high thermal stability of octadecanol is similar to that reported in composite of octadecanol-GO (graphene oxide)[54]. It is also noteworthy that the residue weight closes to EG weight for all the composites, which indicate that EG has been uniformly filled into octadecanol matrix.

high thermal conductivity were added to improve the thermal conductivity of octadecanol. The surface morphologies and worm-like microstructures of EG are characterized by the SEM image shown in Fig. 4(a). The micro-porous surface structures of EG help enhance the octadecanol absorption due to a large surface area. It is observed from Fig. 4(b) that octadecanol has been impregnated and absorbed into the micro pores of the EG. It is found from the DSC curves in Fig. 4(c) that the melting of octadecanol occurred at 55.7 °C. It is concluded from Fig. 4(c) and (d) that the addition of EG has less impact on the melting and freezing temperatures and the melting latent heat of octadecanol, such as 5 wt% EG slight decreases its melting latent heat from 241.5 to 234.1 kJ/kg. However, it is observed from Fig. 4(e) that the addition of EG could significantly enhance the thermal conductivity of octadecanol, such as 5 wt% EG can improve its thermal conductivity from 0.29 to 2.24 W/m·K. Thermal stability is another crucial concerning to the PCM for practical applications. TGA and DTG curves of samples are shown in Fig. 4(f). It could be found from Fig. 4(f) that 5% weight loss 507

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Fig. 5. The micro-morphology and thermal properties of silicone composite: (a) Optical micrographs of the silicone microstructure with 50% EBiInSn and (b) with 80% EBiInSn; (c) SEM micro-morphology and (d) 3D Micro-CT image of silicone composite with 80 wt% EBiInSn and 1% EG; (e) the melting-freezing DSC curves, (f) latent heat density, and (g) thermal conductivities of the silicone composites with different mass fraction of EBiInSn; (h) TGA and DGA curves of EBiInSn-silicone composite.

EBiInSn micro-particles cannot precipitate due to its high-concentration and the stickiness effect of silicone. It can be also observed from MicroCT 3D morphology in Fig. 5(d) that EBiInSn micro-particles are uniformly filled into 3D silicone matrix without precipitation. The conventional PCM macrocapsule shell is only used for encapsulation and no heat storage effect, while the flexible shell presented here has high heat storage density. Fig. 5(e) shows the heating and cooling DSC curves of the EBiInSn-silicone composites. The DSC curve of pure EBiInSn alloy presents the melting temperature starting at 60.2 °C and the endothermic peak centered at 62.3 °C, respectively. It partially coincides with the melting endothermic range of octadecanol, which thus enhances the capacity of latent heat storage. The latent heat of EBiInSn is about 29.9 kJ/kg at mass, while it has large volumetric capacity about 240.5 MJ/m3 due to its high density of 8043 kg/m3 [10], which is higher than that of octadecanol about 215.9 MJ/m3. There are two endothermic peaks at melting process for EBiInSn-silicone composite. One is corresponding to EBiInSn alloy and another is from

3.2. The thermal and mechanical properties of the macrocapsule shell Most of the conventional PCM macrocapsules are encapsulated by rigid shell such as stainless steel, which requires a cavity to accommodate for the volumetric expansion of the PCM during melting. In this paper, a novel silicone elastomer imbedded with high-concentration EBiInSn micro-particles was prepared for the flexible shell. The optical micrographs of the EBiInSn-silicone microstructures with EBiInSn of 50 wt% and 80 wt% are given in Fig. 5(a) and (b), respectively. The high-concentration EBiInSn micro-particles with size of 20–50 μm are evenly distributed in the silicone matrix. It is important to note that the preparation of EBiInSn micro-particle presented here is easy and low cost without complicated process compared with other metal particle fabrication (such as copper particle preparation at high temperature) due to its low melting point, which only needs to be fully stirred at 70 °C. The SEM micro-morphology of EBiInSn(80 wt%)-silicone composites is shown in Fig. 5(c). It is also very interesting to note that the 508

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Fig. 6. The mechanical property of EBiInSn-silicone composite: the image shows the EBiInSn mass fraction of 50% at 0% (a) and 650% strain (b); (c) stress versus strain plot for EBiInSn-silicone composite from EBiInSn mass fraction 0% to 86 wt% tested until failure; (d) strain at break for EBiInSn-silicone composite as function of EBiInSn mass fraction.

The major issue of the silicone elastomer as the macrocapsule shell is its low thermal conductivity of about 0.2 W/m·K, which is smaller than 0.29 W/m·K of octadecanol. Thus, another contribution from EBiInSn micro-particle with 19.2 W/m·K is to improve the performance of heat conduction in silicone composite. It is observed from Fig. 5(g) that 86 wt% EBiInSn can lead to the thermal conductivity of silicone raise to 0.87 W/m·K. This improvement is still not enough due to the EBiInSn micro-particles separated by silicone matrix with low thermal conductivity, which impedes the heat conductive pathways. Copper particles with 398 W/m·K are also mixed with silicone to verify this deduction, which indicates that 80 wt% nano-copper only lead to the thermal conductivity of silicone raise to 0.95 W/m·K. It is very interesting that a small quantity of EG about 1 wt% added to EBiInSn-silicone composite can significantly improve the thermal conductivity of the composite. It can increase the thermal conductivity of EBiInSn (86 wt%)-silicone to 2.25 W/m·K with a 1025% increasing. However, pure silicone added with 1 wt% EG only has 0.3 W/m·K with a 50% increasing. The main reason behind this interesting phenomenon is that the network consisted of worm-like EG creates thermally conductive pathways through cross-linking with EBiInSn micro-particles as shown in Fig. 5(c), which significantly enhances the thermal conduction. This enhancement effect is also observed in copper-silicone composite as shown in Fig. 5(g), while its increase magnitude is evidently lower than that of EBiInSn. Compared with nanocopper-silicone composite, EBiInSn-silicone composite has a smaller viscosity at high concentration due to its liquid state during cure process, which helps to fabricate macrocapsule shell through casting molding method presented here. As shown in Fig. 5(g), the liquid silicone composite with 80 wt% EBiInSn is easy to spread out to form the cured thin film, while it is hard to prepare thin film for nanocopper-silicone with the same high concentration. The thermal stability of macrocapsule shell is crucial for preventing PCM leakage. Fig. 5(h) shows the TGA and DGA curves of EBiInSnsilicone composite. The results indicate that the thermal stability of EBiInSn-silicone is evidently better than that of octadecanol composite as show in Fig. 4(f). It is observed from Fig. 5(h) that T5% is 311 °C for pure silicone, 344 °C for EBiInSn(50 wt%)-silicone and 437 °C EBiInSn (86 wt%)-silicone, respectively. There are two degradation peaks in DTG curve for pure silicone below 700 °C shown in Fig. 5(h), one is at

Fig. 7. The spherical macrocapsule remodeled as the shapes of (a) dumbbell, (b) prolate ellipsoid, (c) concave and (d) petal.

silicone at −42.5 °C. The melting process of EBiInSn micro-particles in silicone composite agrees well with that of the bulk phase of EBiInSn alloy. It is demonstrated in Fig. 5(f) through comparing melting latent heat with its theoretical value, which is equal to the multiplication of mass fraction with the EBiInSn latent heat

LBInSn − silicon = φLLM

(1)

where LLM = 240.5 MJ/m for EBiInSn, φ is the mass fraction of EBiInSn. It is noteworthy that EBiInSn micro-particle has a larger supercooling degree compared with its bulk phase. The freezing temperatures of EBiInSn micro-particle with size 20–50 μm are mainly located at range of 20–40 °C as shown in Fig. 5(e). 3

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Fig. 8. (a) The cake-shaped macrocapsule placed on the hot plate with 120 °C restores to the original spherical shape with the heating time of 60 min; (b) the sphere diameters change along the vertical and horizontal directions after remodeling transition between the shapes of cake and sphere.

Fig. 9. Uniaxial compression test of spherical macrocapsules with stainless-steel shell (a) and EBiInSn-silicone shell (b); (c) load-displacement curves of uniaxial compression test.

3.3. The shape-remodeled and deformation performances of PCM macrocapsule

430 °C and another at 624 °C. However, only one degradation peak appears in DTG curves for both composites with addition of 50 wt% and 86 wt% EBiInSn, which are 483 °C and 467 °C, respectively. EBiInSn can act as protective walls to delay the evaporation and thermal decomposition of silicone. The main advantage of the silicone elastomer as the macrocapsule shell is its excellent flexibility as shown in Fig. 1(f). The mechanical behavior of the EBiInSn-silicone is tested under tensile loading for different mass fractions of EBiInSn. Fig. 6(a) and (b) shows the images of EBiInSn(50 wt%)-silicone composite from the initial state stretching to 650%, respectively. The results from stress-strain curves of EBiInSnsilicone in Fig. 6(c) indicate that increasing concentration of EBiInSn helps to increase the stiffness of the composites, which means that the tensile stress would become larger with increasing EBiInSn concentration for the same strain. However, the larger concentration of EBiInSn induces a smaller strain at broken as shown in Fig. 6(d). In order to balance the flexibility and latent heat density of the macrocapsule shell, EBiInSn(80 wt%)-silicone composite is chosen here for subsequent tests, which has strain of 432% at broken and melting latent heat of 192.4 MJ/m3.

The shape of the PCM macrocapsule with rigid shell is hard to be dynamically and repeatably remodeled, which is thus restricted for many applications. The prepared PCM macrocapsule has capacity of large-scale deformation for dynamical shape remodeling. A spherical macrocapsule with flexible shell composited of EBiInSn(80 wt%)-silicone can be remodeled to a variety of shapes, such as shapes of dumbbell, prolate ellipsoid, concave and petal as shown in Fig. 7, respectively. This remodeling process is convenient through deforming the macrocapsule when its core keeps at liquid state. The macrocapsule with large deformation is then to be obtained after PCM core solidification. It is noteworthy that the remodeled shape is maintained by the solid PCM core, which is dynamically and conformably encapsulated by elastic shell. The macrocapsule with large deformation can restore to the original sphere through heating without other bound. It is observed from Fig. 8(a) that the cake-shaped macrocapsule placed on the hot plate with 120 °C gradually restores to the spherical shape, which could also be carried out through soaking the macrocapsule in the hot water. In 510

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melting could be given by 1/3

⎧ R = ⎛ ρOS - S ⎞ R S ⎪ i ⎝ ρOS - L ⎠ ⎨ ⎪ Ro = (Rs + δ ) ⎡1 + ⎣ ⎩

ρOS - S − ρOS - L ρOS - L

(

3 1/3 RS ⎤ Rs + δ

)⎦

(2)

where Rs and Ri denote the radius of macrocapsule core at solid and liquid state, respectively, δ for the initial shell thickness, Ro for outer shell radius at melting state, ρOS_S about 894 kg/m3 denotes the density of solid octadecanol density, and ρOS_L about 812 kg/m3 for its liquid state. For Rs = 47.5 mm and δ = 2.5 mm, the predicted value of Ro = 51.4 mm agrees with the measured result of 51.5 mm. The pressure in macrocapsule after melting can be given as

Pi = Po +

μ 2

∫λ

λi

o

1−

(λ−2 + λ−5) dλ + λ−4 − 3)

−1 Jlim (2λ2

(3)

where λi = Ri / RS and λ o = Ro /(RS + δ ) are determined by Eq. (2), Po denotes the outer ambient pressure; and μ is the shear modulus of the silicone composite. The above equation indicates that the inner liquid PCM pressure is related to its radius stretch. It can be found that λi − λ o = 1% for λ o ≃ 1 from Eq. (2), which is much smaller than the broken-strain 432% for EBiInSn(80 wt%)-silicone composite. Eq. (3) then furthermore reduces to Pi ≃ Po + (λi − λ o ) μ , which indicates that the pressure of macrocapsule core increase about 0.01μ from solid transition to liquid state. The typical value of μ is about 105 Pa for silicone elastomer composite according to the relation of strain-stress presented in Fig. 6(c). Thus the pressure increase of macrocapsule core is about 103 Pa and has almost no effect on the macrocapsule, which is completely different from that of the rigid shell [31,46]. All the test results also indicate that the flexible shell can sustain the thermal stress induced by the volumetric expansion or shrink due to the solid-liquid phase transition.

Fig. 10. The heat storing and releasing performance of the PCM macrocapsule: (a) the average latent heat and density of the macrocapsule as the function of shell thickness ratio; (b) the heat storing/releasing test of the four samples listed in Table 1;(c) the thermal performance of cake-like PCM macrocapsulebased heat sink for the simulated heat source with heating power of 8 W.

order to verify the repeatability of remodeling transition between the shapes of cake-like and sphere, we tested 30 times and recorded the sphere diameter changes along the vertical (Dv) and horizontal (Dh) directions. The results presented in Fig. 8(b) indicate that all the deviations are smaller than 0.4 mm, which is just 1.6% of the initial sphere diameter. The PCM macrocapsule with rigid shell can sustain the large extrusion pressure, while the irreversible damage happens after suffering a large stress. The prepared macrocapsule with flexible shell, in contrast, is hard to sustain a large stress, while it could recover to the initial shape after the core broken. The macrocapsule with stainless-steel shell filled with octadecanol was prepared for uniaxial compression test as shown in Fig. 9(a), which has the outer diameter of 50 mm and the shell thickness of 1 mm. The load-displacement curves presented in Fig. 9(b) show that the loading pressure over 24.1 kN could induce the irreversible damage of stainless-steel shell, while about 0.56 kN lead to the damage of the shape-remodeled macrocapsule. Different from the rigid shell used for bearing external stress, the solid PCM core of macrocapsule with flexible shell is considered as the major force carrier. When the PCM core keeps at liquid state, however, the flexible shell could maintain the macrocapsule shape. The prepared macrocapsule with broken PCM core could recover to the original shape through only heating. This feature may be useful for the heat storage system exposed to external extrusion stress. Another advantage of EBiInSn-silicone shell is to effectively eliminate the stress mismatch induced by the volumetric expansion (or shrink) of the PCM during melting (or solidification). To quantitatively evaluate the inner expansion pressure of macrocapsule during melting process, the analytic relation of the deformation induced by volume expansion is derived in Appendix A. The main results are presented here. The radiuses of macrocapsule core and shell after completely

3.4. The heat charging and discharging performance of the PCM macrocapsule Most PCM capsules shells have less contribution to the heat storage due to the negligible sensible heat of shell material, which would decrease its latent heat storage density for large thickness. However, the prepared flexible shell consisted of EBiInSn-silicone composite effectively mitigate this weakness. The latent heat storage of the PCM macrocapsule presented here include both core and shell. The shell thickness ratio is defined as η = δ /(RS + δ ) . According to Eq. (1), the latent heat volumetric density L of the macrocapsule could be estimated by

L = (1 − η)3Los + [1 − (1 − η)3] φLLM

(4)

3

where Los = 215.9 MJ/m is the melting latent heat of octadecanol. The average density of the macrocapsule could be evaluated according to the mass conservation

ρ = (1 − η)3ρos−s + [1 − (1 − η)3]

ρsi ρLM φρsi + (1 − φ) ρLM

(5)

where ρSi is the density of silicone about 1070 kg/m , and ρLM for 3

Table 1 The material composition and thermal properties of the sample macrocapsules. No.

1 2 3 4 1

Material composition of core and shell

1

Oct : Silicone Oct: Silicone + 80 wt%EBiInSn + 1 wt%EG Oct + 2 wt%EG: Silicone + 80 wt%EBiInSn + 1 wt%EG Oct + 3 wt%EG: Silicone + 80 wt%EBiInSn + 1 wt%EG

Latent heat density (MJ/m3)

Thermal conductivity (W/mK) Core

shell

0.29 0.29 1.16 1.53

0.20 1.98 1.98 1.98

Oct denotes octadecanol. 511

168.6 213.4 211.6 210.1

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Fig. 11. The heat storing/releasing test of the four samples listed in Table 1: (a) the experiment platform; (b) the temperature change during heat storing/releasing test.

EBiInSn density about 8043 kg/m3. In order to demonstrate the impact of the shell on the latent heat density, the macrocapsule with outer diameter 50 mm and shell thickness 2.5 mm is considered here. For the shell consisted of the pure silicone, the volumetric latent heat density of the macrocapsule is estimated by Eq. (4) about 168.6 MJ/m3, while it increases to 210.1 MJ/m3 for EBiInSn(80 wt%)-silicone composite, which is responding to η = 10%. It is observed from Fig. 10 that the volumetric latent heat density is about 216.0 MJ/m3 for EBiInSn(90 wt %)-silicone, which is almost unaffected by η due to the high latent heat of EBiInSn. For η = 100%, the macrocapsule has completely become an elastic shape-stabilized PCM. The flexible PCM recently has also attracted significantly attention[55], such as the spherical macrocapsule composed of the polymeric phase change composite material[56]. However, the thermal conductivity and latent heat density leave much to be desired. The elastic shape-stabilized PCM (η = 100%) presented here has high latent heat density of 192.4 MJ/m3 and high thermal conductivity of 1.98 W/mK for EBiInSn(80 wt%)-silicone with addition of 1 wt% EG, which can also sustain the large deformation of 432%. The heat charging and discharging rates of PCM macrocapsule have important impact on the energy storage efficiency, such as the thermal storing and releasing of packed bed latent heat storage systems dependent on the heat transfer between PCM macrocapsules and the heat transfer fluid [32]. Four samples with the outer diameter of 50 mm and the shell thickness of 2.5 mm were prepared to evaluate the impact of the thermal conductivities of macrocapsule core and shell on heat charging and discharging rates, which have different material compositions and thermal properties as shown in Table 1. The experiment platform is presented in Fig. 11(a). Fig. 11(b) shows the temperatures (T1) of macrocapsules center change during two heating-cooling cycles, where the air temperature (T2) of the inner chamber was also recorded. The sample 1 is consisted of pure

octadecanol core (0.29 W/m·K) and silicone shell (0.20 W/m·K) without any additives as the control group. In order to validate the thermal effect of the macrocapsule shell, the sample 2 just changes the shell material as EBiInSn(80 wt%)-silicone with addition of 1 wt% EG compared with the sample 1. The shell thermal conductivity of sample 2 is 1.98 W/m·K, and evidently higher that of the sample 1. The test results show that increasing the thermal conductivity of shell could accelerate the heat charging and discharging rates of macrocapsule. For the macrocapsule with large diameter, such as 50 mm considered here, the effect of thermal conductivity of the PCM core has also play a key role. The sample 3 adopted the same shell used in sample 2, while the macrocapsule core is consisted of octadecanol with 2 wt% EG (1.16 W/ m·K). It is observed from Fig. 11(b) that increasing the thermal conductivity of the core could significantly reduce the heat charging and discharging times. The PCM core of sample 4 is consisted of octadecanol with 3 wt% EG (1.53 W/m·K) and the shell used in the sample 2, which could reduce the charging time from 112 mins to 53 mins and discharging time from 83 mins to 20 mins compared with that of the sample 1 without any additives, respectively. These results have indicated that increasing the thermal conductivities of the PCM core and shell could significantly improve the heat transfer during solid-liquid phase transition. The durability of the macrocapsule of sample 4 was evaluated by thermal cyclic tests performed by repeating meltingfreezing cycles. For each cycle, the test time was fixed at 350 min as shown in Fig. 11(b). The mass of the macrocapsule did not show any significant weight loss after 50 thermal cycles. In addition, there was no any significant change in the charging and discharging times.

3.5. Application examples The shape-remodeled PCM macrocapsule developed here could be 512

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Fig. 12. The PCM macrocapsule-based heat sink for the simulated heat source with the heating power of 7 W and the different shapes of (a) flat structure, (b) concave structure, (c) convex structure and (d) folded structure.

system size and a light weight[57]. The prepared spherical PCM macrocapsule can quickly absorb a large amount of thermal energy due to its high surface ratio, high heat conductivity and high latent heat density. Macrocapsule could be remodeled to plate shape such that it has a large contact surface with TEG module for thermal energy harvesting. The commercial Bi2Te3 TEG module (TEG1-142) with the size of 40 mm × 40 mm × 3.2 mm was adopted here. Fig. 13(a) shows the heat charging of the spherical PCM macrocapsule soaked in hot water, which was then quickly and tightly attached with the hot side of the TEG module for heat discharging and electric power generation. It is noteworthy that the aluminum heat sink is used on the cold side of TEG module without other power input. The center temperature of the PCM macrocapsule during heat charging and discharging processes is presented in Fig. 13(b). The liquid-solid phase transition of PCM leads to a constant temperature platform abound 35 °C on the hot side of the TEG module as shown in Fig. 13(b). The cold side temperature of the TEG module is about 25 °C due to the double effects from the PCM discharging and aluminum heat sink, where the environment temperature is 20 °C. In order to obtain a maximum electric power output, the external load was set as the same with the inner electric resistance of the TEG module[57], which is about 4.1 Ω at the temperature range of 25–35 °C. It is observed from Fig. 13(c) that both output voltage and current curves slowly drop during the PCM discharging, which could lead to a stable electric power output as shown in Fig. 13(d). It is mainly because of the constant temperature difference on both sides of the TEG module (see Fig. 13(d)). For a single PCM macrocapsule with outer diameter of 50 mm, the net electric energy of 32.1 J was obtained, which could be applied for sensor charging such as wearable electronics. The main advantage of the developed PCM macrocapsule used for

applied to many areas, such as thermal management, solar energy utilization and thermoelectric energy harvesting. Two application examples based on the spherical PCM macrocapsule of sample 4 listed in Table 1 were discussed here. The PCM-based thermal management technology is powerful in ensuring electronic devices working safely and efficiently. However, PCM encapsulated with rigid shell is hard to adapt to dynamical shape of the devices, such as the flexible electronics. A prototype thermal management system based on the shape-remodeled PCM macrocapsule was developed here. The flexible heating film with diameter of 50 mm and heating power of 7 W was used to model a deformable electronic device, which could be dynamically changed between different shapes such as the flat, convex, concave and folded structures as shown in Fig. 12. The prepared spherical PCM macrocapsule is easily to tightly contact with the curved surface through remodeling shape as illustrated in Fig. 12, such as the remodeled prolate ellipsoid (see Fig. 7(b)) used as the heat sink for the device with the concave shape ((see Fig. 12(b))). It is observed from Fig. 12 that all the temperature rise rates of the simulated power devices with different shapes are inhibited when the temperature reaches up to 60 °C. It is because of the large heat absorption from the PCM macrocapsule melting. However, the temperature rise become rapid when the melting completed. These results indicate that the prepared PCM macrocapsule as the heat sink could be easily remodeled as needed to control the temperature of the devices with complicated shape. Another application is to develop a novel thermoelectric system integrated with the shape-remodeled PCM macrocapsule. Thermoelectric generator (TEG) based on the Seebeck effect could directly convert waste heat into electricity, which has several advantages, such as long operating life, silent operation, no moving parts, a compact 513

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Fig. 13. The performance test of the thermoelectric system integrated with shape-remodeled PCM macrocapsule: (a) the experiment platform for heat charging and discharging of the PCM macrocapsule used for TEG module; (b) the temperature of the PCM macrocapsule and both sides of the TEG module; (c) the output voltage and current of the TEG module; (d) the output power and temperature difference on both sides of the TEG module.

remodeled as needed to a complicated shape with large-scale deformation, which also effectively eliminates the stress mismatch induced by the volumetric expansion/shrink of the PCM during melting/freezing process. (4) The flexible thermal management method based on the developed PCM macrocapsule was developed and demonstrated for the electronics devices with dynamically-changed shape, such as the flexible power electronics. (5) A novel PCM-based thermoelectric system was also proposed to obtain an electrical power output of 32.1 J for a single PCM macrocapsule with outer diameter of 50 mm. Its main advantage is that the macrocapsule shape could be easily remodeled to enhance the heat transfer efficiency for both charging and discharging.

TEG module is that it could be easily remodeled to enhance the heat transfer efficiency for both charging and discharging. In addition, the thermal energy could be held on the PCM macrocapsule through latent heat in advance, and it discharges heat to TEG module when needed. 4. Conclusions In summary, this paper has reported on a novel shape-remodeled PCM macrocapsule for thermal energy storage and thermal management. The main results could be concluded in the following. (1) The shape-remodeled PCM macrocapsule with complicated shape was prepare through a cast molding method, which obtains a high latent heat density of 210.1 MJ/m3, and its contribution from the shell is about 20%. The thermal conductivity of the PCM composite for macrocapsule core reaches to 1.53 W/m·K with a 428% increase through adding 3 wt% EG. (2) The silicone-based flexible shell was filled with the high-concentration microparticles of EBiInSn (80 wt%) and low-concentration EG (1 wt%) to significantly enhance its latent heat storage (192.4 MJ/m3) and thermal conductivity (1.98 W/m·K with an 890% increase compared with pure silicone), which also remains a high stretchability with 432% strain. (3) The as-prepared macrocapsule can be dynamically and repeatably

It can be concluded based on this work that the shape-remodeled macrocapsule has good potential to be applied in the heat storage system and used for the thermal management applications. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC) under Key Project No.91748206, and the Fundamental Research Funds for the Central Universities, and Beijing Special Project of Fostering and Developing Science and Technology Innovation Base.

Appendix A We consider a PCM macrocapsule with core radius RS and flexible shell thickness δ , where outer radius of macrocapsule is then equal to Rs + δ . The volume conservation of macrocapsule shell is considered due to the incompressible silicone composite. (R, Θ, Φ) and (r, θ, φ) are considered as the spherical coordinates in the undeformed and deformed configurations of macrocapsule shell, respectively. With the assumption of centrally 514

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symmetric deformation, the three principal stretches are given by

⎧ λr = dr / dR λθ = λ = r /R ⎨ λ φ = λ = r /R ⎩

(A1)

λr λ θ λ φ = 1 is meet for incompressible silicone composite. In addition, the radial stretch is given by λr = the symmetric sphere, in the absence of body forces in the radial direction, is expressed as:

dσr σ − σθ +2 r =0 dr r

λ−2 .

Then the stress equilibrium equation of

(A2)

Here σφ = σθ is assumed due to the spherical symmetry. Constant uniform radial pressure Pi and Po are applied at the inner and outer surfaces of the sphere. Therefore, the boundary conditions are:

⎧ σr (r = Ri ) = −Pi ⎨ ⎩ σr (r = Ro) = −Po

(A3)

In order to obtain the solution of the stress model described by Eqs. (A1)–(A3), the constitutive relation of elastomer adopts the Gent free-energy function [58]:

⎧ Wel =

μJlim 2

(

ln 1 −

⎨ σ (λ ) = σ − σ = r θ ⎩

J Jlim

)

λ dWel 2 dλ

(A4)

+ − 3, and Jlim = J (λlim ) is a constant related to the limiting stretch λlim , μ is shear modulus of the flexible shell. Through where J = integration of the stress equilibrium equation of Eq. (A2), and combining with Eq. (A3) and Eq. (4), we could obtain the relation [58]: λ−4

2λ2

Pi = Po +

μ 2

∫λ

λi

o

1−

(λ−2 + λ−5) dλ + λ−4 − 3)

−1 Jlim (2λ2

(A5)

where λi and λ o are the stretch at the inner and outer shell, respectively.

Heat Mass Tran. 2018;127:838–56. [16] Tian HQ, Du LC, Wei XL, Deng SY, Wang WL, Ding J. Enhanced thermal conductivity of ternary carbonate salt phase change material with Mg particles for solar thermal energy storage. Appl Energy 2017;204:525–30. [17] Qian TT, Li JH, Feng WW, Nian HE. Single-walled carbon nanotube for shape stabilization and enhanced phase change heat transfer of polyethylene glycol phase change material. Energy Convers Manage 2017;143:96–108. [18] Li A, Wang JJ, Dong C, Dong WJ, Atinafu DG, Chen X, et al. Core-sheath structural carbon materials for integrated enhancement of thermal conductivity, and capacity. Appl Energy 2018;217:369–76. [19] Zhang L, Yang WB, Jiang ZN, He FF, Zhang K, Fan JH, et al. Graphene oxidemodified microencapsulated phase change materials with high encapsulation capacity and enhanced leakage-prevention performance. Appl Energy 2017;197:354–63. [20] Huang XB, Chen X, Li A, Atinafu D, Gao HY, Dong WJ, et al. Shape-stabilized phase change materials based on porous supports for thermal energy storage applications. Chem Eng J 2019;356:641–61. [21] Lv PZ, Liu CZ, Rao ZH. Review on clay mineral-based form-stable phase change materials: Preparation, characterization and applications. Renew Sustain Energy Rev 2017;68:707–26. [22] Yang XH, Bai QS, Zhang QL, Hu WJ, Jin LW, Yan JY. Thermal and economic analysis of charging and discharging characteristics of composite phase change materials for cold storage. Appl Energy 2018;225:585–99. [23] Li ZT, Wu YX, Zhuang BS, Zhao XZ, Tang Y, Ding XR, et al. Preparation of novel copper-powder-sintered frame/paraffin form-stable phase change materials with extremely high thermal conductivity. Appl Energy 2017;206:1147–57. [24] Chen X, Gao HY, Yang M, Dong WJ, Huang XB, Li A, et al. Highly graphitized 3D network carbon for shape-stabilized composite PCMs with superior thermal energy harvesting. Nano Energy 2018;49:86–94. [25] Umair MM, Zhang Y, Iqbal K, Zhang SF, Tang BT. Novel strategies and supporting materials applied to shape-stabilize organic phase change materials for thermal energy storage-A review. Appl Energy 2019;235:846–73. [26] Shchukina EM, Graham M, Zheng Z, Shchukin DG. Nanoencapsulation of phase change materials for advanced thermal energy storage systems. Chem Soc Rev 2018;47:4156–75. [27] Ge HS, Li HY, Mei SF, Liu J. Low melting point liquid metal as a new class of phase change material: An emerging frontier in energy area. Renew Sustain Energy Rev 2013;21:331–46. [28] Liu CZ, Rao ZH, Zhao JT, Huo YT, Li YM. Review on nanoencapsulated phase change materials: Preparation, characterization and heat transfer enhancement. Nano Energy 2015;13:814–26. [29] Olivieri L, Tenorio JA, Revuelta D, Navarro L, Cabeza LF. Developing a PCM-enhanced mortar for thermally active precast walls. Constr Build Mater 2018;181:638–49. [30] Salunkhe PB, Shembekar PS. A review on effect of phase change material encapsulation on the thermal performance of a system. Renew Sustain Energy Rev 2012;16:5603–16.

References [1] Zalba B, Marin JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251–83. [2] Zhang HL, Baeyens J, Caceres G, Degreve J, Lv YQ. Thermal energy storage: recent developments and practical aspects. Prog Energy Combust 2016;53:1–40. [3] Xu B, Li PW, Chan C. Application of phase change materials for thermal energy storage in concentrated solar thermal power plants: A review to recent developments. Appl Energy 2015;160:286–307. [4] Khan MMA, Ibrahim NI, Mahbubul IM, Ali HM, Saidur R, Al-Sulaiman FA. Evaluation of solar collector designs with integrated latent heat thermal energy storage: A review. Sol Energy 2018;166:334–50. [5] Xu HX, Romagnoli A, Sze JY, Py X. Application of material assessment methodology in latent heat thermal energy storage for waste heat recovery. Appl Energy 2017;187:281–90. [6] Atouei SA, Ranjbar AA, Rezania A. Experimental investigation of two-stage thermoelectric generator system integrated with phase change materials. Appl Energy 2017;208:332–43. [7] Lizana J, Chacartegui R, Barrios-Padura A, Valverde JM. Advances in thermal energy storage materials and their applications towards zero energy buildings: A critical review. Appl Energy 2017;203:219–39. [8] Soares N, Bastos J, Pereira LD, Soares A, Amaral AR, Asadi E, et al. A review on current advances in the energy and environmental performance of buildings towards a more sustainable built environment. Renew Sustain Energy Rev 2017;77:845–60. [9] Sahoo SK, Das MK, Rath P. Application of TCE-PCM based heat sinks for cooling of electronic components: A review. Renew Sustain Energy Rev 2016;59:550–82. [10] Yang XH, Tan SC, He ZZ, Zhou YX, Liu J. Evaluation and optimization of low melting point metal PCM heat sink against ultra-high thermal shock. Appl Therm Eng 2017;119:34–41. [11] Sharma RK, Ganesan P, Tyagi VV, Metselaar HSC, Sandaran SC. Developments in organic solid-liquid phase change materials and their applications in thermal energy storage. Energy Convers Manage 2015;95:193–228. [12] Milian YE, Gutierrez A, Grageda M, Ushak S. A review on encapsulation techniques for inorganic phase change materials and the influence on their thermophysical properties. Renew Sustain Energy Rev 2017;73:983–99. [13] Cabeza LF, Barreneche C, Martorell I, Miro L, Sari-Bey S, Fois M, et al. Unconventional experimental technologies available for phase change materials (PCM) characterization. Part 1. Thermophysical properties. Renew Sustain Energy Rev 2015;43:1399–414. [14] Navarro L, Barreneche C, Castell A, Redpath DAG, Griffiths PW, Cabeza L. High density polyethylene spheres with PCM for domestic hot water applications: Water tank and laboratory scale study. J Storage Mater 2017;13:262–7. [15] Qureshi ZA, Ali HM, Khushnood S. Recent advances on thermal conductivity enhancement of phase change materials for energy storage system: A review. Int J

515

Applied Energy 247 (2019) 503–516

D.-H. Yu and Z.-Z. He

material filled in spherical capsules. Appl Therm Eng 2016;108:1200–5. [45] Li W, Li SG, Guan SK, Wang YH, Zhang X, Liu XL. Numerical study on melt fraction during melting of phase change material inside a sphere. Int J Hydrogen Energy 2017;42:18232–9. [46] Yu QH, Romagnoli A, Al-Duri B, Xie DM, Ding YL, Li YL. Heat storage performance analysis and parameter design for encapsulated phase change materials. Energy Convers Manage 2018;157:619–30. [47] Chandrasekaran P, Cheralathan M, Velraj R. Effect of fill volume on solidification characteristics of DI (deionized) water in a spherical capsule - An experimental study. Energy 2015;90:508–15. [48] Archibold AR, Gonzalez-Aguilar J, Rahman MM, Goswami DY, Romero M, Stefanakos EK. The melting process of storage materials with relatively high phase change temperatures in partially filled spherical shells. Appl Energy 2014;116:243–52. [49] Alam TE, Dhau JS, Goswami DY, Stefanakos E. Macroencapsulation and characterization of phase change materials for latent heat thermal energy storage systems. Appl Energy 2015;154:92–101. [50] Park J, Shin DH, Lee SJ, Shin Y, Karng SW. Effective latent heat thermal energy storage system using thin flexible pouches. Sustain Cities Soc 2019;45:143–50. [51] Shin DH, Park J, Choi SH, Ko HS, Karng SW, Shin Y. A new type of heat storage system using the motion of phase change materials in an elliptical-shaped capsule. Energy Convers Manage 2019;182:508–19. [52] Wei JJ, Kawaguchi Y, Hirano S, Takeuchi H. Study on a PCM heat storage system for rapid heat supply. Appl Therm Eng 2005;25:2903–20. [53] Barba A, Spiga M. Discharge mode for encapsulated PCMs in storage tanks. Sol Energy 2003;74:141–8. [54] Wang XL, Cheng XM, Li YY, Li G, Xu J. Self-assembly of three-dimensional 1-octadecanol/graphene thermal storage materials. Sol Energy 2019;179:128–34. [55] Li WW, Cheng WL, Xie B, Liu N, Zhang LS. Thermal sensitive flexible phase change materials with high thermal conductivity for thermal energy storage. Energy Convers Manage 2017;149:1–12. [56] Anghel EM, Pavel PM, Constantinescu M, Petrescu S, Atkinson I, Buixaderas E. Thermal transfer performance of a spherical encapsulated PEG 6000-based composite for thermal energy storage. Appl Energy 2017;208:1222–31. [57] Elghool A, Basrawi F, Ibrahim TK, Habib K, Ibrahim H, Idris DMND. A review on heat sink for thermo-electric power generation: Classifications and parameters affecting performance. Energy Convers Manage 2017;134:260–77. [58] Zhu J, Li TF, Cai SQ, Suo ZG. Snap-through Expansion of a Gas Bubble in an Elastomer. J Adhesion 2011;87:466–81.

[31] Blaney JJ, Neti S, Misiolek WZ, Oztekin A. Containment capsule stresses for encapsulated phase change materials. Appl Therm Eng 2013;50:555–61. [32] de Gracia A, Cabeza LF. Numerical simulation of a PCM packed bed system: A review. Renew Sustain Energy Rev 2017;69:1055–63. [33] Zhao BC, Cheng MS, Liu C, Dai ZM. Thermal performance and cost analysis of a multi-layered solid-PCM thermocline thermal energy storage for CSP tower plants. Appl Energy 2016;178:784–99. [34] Nithyanandam K, Pitchumani R, Mathur A. Analysis of a latent thermocline storage system with encapsulated phase change materials for concentrating solar power. Appl Energy 2014;113:1446–60. [35] Jacob R, Bruno F. Review on shell materials used in the encapsulation of phase change materials for high temperature thermal energy storage. Renew Sustain Energy Rev 2015;48:79–87. [36] Vicente R, Silva T. Brick masonry walls with PCM macrocapsules: An experimental approach. Appl Therm Eng 2014;67:24–34. [37] Cui HZ, Tang WC, Qin QH, Xing F, Liao WY, Wen HB. Development of structuralfunctional integrated energy storage concrete with innovative macro-encapsulated PCM by hollow steel ball. Appl Energy 2017;185:107–18. [38] Ma BQ, Li JQ, Xu Z, Peng ZJ. Fe-shell/Cu-core encapsulated metallic phase change materials prepared by aerodynamic levitation method. Appl Energy 2014;132:568–74. [39] Zheng Y, Zhao WH, Sabol JC, Tuzla K, Neti S, Oztekin A, et al. Encapsulated phase change materials for energy storage - Characterization by calorimetry. Sol Energy 2013;87:117–26. [40] Zhang GC, Li JQ, Chen YF, Xiang H, Ma BQ, Xu Z, et al. Encapsulation of copperbased phase change materials for high temperature thermal energy storage. Sol Energy Mat Sol C 2014;128:131–7. [41] Li MJ, Jin B, Ma Z, Yuan F. Experimental and numerical study on the performance of a new high-temperature packed-bed thermal energy storage system with macroencapsulation of molten salt phase change material. Appl Energy 2018;221:1–15. [42] Fukahori R, Nomura T, Zhu CY, Sheng N, Okinaka N, Akiyama T. Macro-encapsulation of metallic phase change material using cylindrical-type ceramic containers for high-temperature thermal energy storage. Appl Energy 2016;170:324–8. [43] Wickramaratne C, Dhau JS, Kamal R, Myers P, Goswami DY, Stefanakos E. Macroencapsulation and characterization of chloride based inorganic Phase change materials for high temperature thermal energy storage systems. Appl Energy 2018;221:587–96. [44] Liu MJ, Fan LW, Zhu ZQ, Feng B, Zhang HC, Zeng Y. A volume-shrinkage-based method for quantifying the inward solidification heat transfer of a phase change

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