n-Tetradecane

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Preparation and thermal performance of silica/n-tetradecane microencapsulated phase change material for cold energy storage Yutang Fang, Hao Wei, Xianghui Liang, Shuangfeng Wang, Xin Liu, Xuenong Gao, and Zhengguo Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01799 • Publication Date (Web): 09 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

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Preparation and thermal performance of silica/n-tetradecane microencapsulated phase change material for cold energy storage

3

Yutang Fang*, Hao Wei, Xianghui Liang, Shuangfeng Wang,Xin Liu, Xuenong

4

Gao, Zhengguo Zhang

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Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry

6

of Education, South China University of Technology, Guangzhou, 510640, People’s

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Republic of China

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*Corresponding author. Tel. /fax: +86-20-87113870

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E-mail address: [email protected] (Yutang Fang)

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Abstract: A novel silica (SiO2)/n-tetradecane (Tet) microencapsulated phase change

11

material (MEPCM) was synthesized by in-situ interfacial polycondensation. The

12

influences of the amount of the composite emulsifier and the mass ratio of n-

13

tetradecane and tetraethyl silicate on the MEPCM performance were systematically

14

investigated. The morphology, chemical structure and the composition of the

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MEPCM were characterized by Scanning electron microscopy and Energy dispersive

16

X-ray spectrometer (SEM-EDS), Fourier transform infrared spectroscopy (FT-IR), X-

17

ray diffractometer (XRD), and its thermal performance and thermal stability were

18

measured by Differential scanning calorimeter (DSC) and Thermogravimetric

19

analyzer (TGA). The results showed that the n-tetradecane core material was

20

successfully encapsulated by silica shell material with encapsulation ratio of 62.04%.

21

The MEPCM had a melting enthalpy of 140.5 kJ kg-1 and thermal conductivity of

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0.139 W m-1 K-1. Because of its excellent thermal performance and thermal stability,

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silica/n-tetradecane MEPCM displays a good potential for cold energy storage.

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Keywords: Microencapsulated phase change material; Silica shell; Cold energy

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storage; Thermal performance; Thermal conductivity

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1. INTRODUCTION

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In recent years, energy problem has become the bottleneck of industrial development

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and social progress due to the imminent energy shortage and the increase of greenhouse

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emission 1. The technology of cold thermal energy storage (CTES), with the help of

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phase change material (PCM), realizes the storage and transportation of cold thermal

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energy. Because of high energy storage density and nearly isothermal behavior in phase

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transition process, the CTES technology can improve energy efficiency, achieve peak

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load shifting and reduce energy consumption 2, which has been widely applied to

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industrial fields such as recovery and utilization of industrial waste energy 3, energy

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conserving in building 4, electronic devices thermal control 5, and so on.

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As a traditional PCM, water (ice) is used for CTES system due to the advantageous

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features such as non-toxic, high energy storage density and bargain price. Tamasauskas

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6

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Compared with the heat pump system only using sensible storage, there was a 26%

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reduction in operating energy consumption. However, in order to store cold energy, air-

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conditioning system has to run below the phase change freezing point of water greatly

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decreasing the coefficient of performance (COP) of refrigerator 7. Therefore, most of

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researches for PCM focus on hydrates salt due to their more suitable phase transition

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temperature and excellent latent heat storage capacity. However, a majority of inorganic

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hydrate salts are prone to supercooling and phase separation, which limits their

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application severely 8. What’s more, only few pure hydrates are known in the required

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temperature range for air conditioning and cooling 9. Hence, the research expands to

et al. investigated the performance of a solar-assisted heat pump containing ice slurry.

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organic PCM due to their appropriate thermal properties and chemical stability. Among

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them, n-tetradecane (Tet) is an attractive alternative due to the suitable phase transition

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point in the range of 5-9 ℃ and little supercooling.

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On the other hand, leakage problems caused by direct use of phase change materials

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leading certain harm to energy storage system and environment 2. In order to prevent

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undesirable problems, several methods have been developed to prepare PCM such as

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encapsulated PCM 10 and form-stable PCM 11. Among these methods, the encapsulation,

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which can enlarge heat transfer area and reduce the interference from surroundings, is

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considered as one of the most widely used technique. Besides, organic shell materials,

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such as polystyrene 12, melamine-formaldehyde resin 13 and urea-formaldehyde resin 14,

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are widely employed for their prominent properties such as stability, durability and

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flexibility. Fang et al.

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nanoencapsulated phase change material (NEPCM) as latent functionally thermal fluid

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(LFTF) for cold thermal energy storage. The results showed that the melting and

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freezing point of the NEPCM was 4.04 and -3.43 ℃, respectively, which indicating it

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is suitable for cold thermal energy storage. Fang et al.

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nanoencapsulated n-tetradecane with urea and formaldehyde resin as shell material.

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The results demonstrated that NEPCM had a latent heat storage capacity of 134.16 kJ

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kg-1 and phase change temperature of 5.7 ℃ . Konuklu et al.

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microencapsulated caprylic acid with different organic shell materials by coacervation

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method. Qiu et al.

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butylmethacrylate (BMA)-based copolymer shells. The DSC results shows the n-

17

7

synthesized a novel polystyrene/n-tetradecane composite

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reported the synthesis of

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fabricated

prepared the microcapsules of n-octadecane with different n-

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4

70

octadecane content of the microencapsulated phase change material (MEPCM) with P

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(BMA-co-MAA) has the highest latent heat of 144.3 kJ kg-1 and both the melting and

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crystallization temperature of the PCM were altered by less than 1.0 ℃ after 1000

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thermal cycles.

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In spite of this, organic materials still have common drawbacks, such as flammability,

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low mechanical strength and poor thermal conductivity. Zhang et al. 18 investigated the

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heat storage characteristic of MEPCM slurry. The results showed that the heat storage

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capacity increased with the increasing of mass concentration, while the completion time

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increase more. So, there is an inherent requirement of high thermal conductivity shell

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material for encapsulated PCM to enhance heat transfer performance. Usually, the

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thermal conductivity of inorganic materials is significantly higher than that of the

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organic ones. Moreover, they possess higher mechanical strength compared with

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organic materials. Taguchi et al.

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hybrid as shell. The obtained phase change material showed that incorporation of

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inorganic particles in the shell could improve the thermal conductivity of the

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microcapsules. Song et al.

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phase change material for thermal energy storage. Yu et al. 21 successfully synthesized

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calcium carbonate/ n-octadecane MEPCM and its thermal conductivity (1.264 W m-1

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K-1 ) was higher than that of MEPCM with polymeric shell material (0.2 W m-1 K-1) 22.

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Based on the mentioned above, investigations on tetradecane for cold energy are still

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inadequate. Moreover, the microcapsules with inorganic shell materials for cold energy

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storage have rarely been reported. In this study, by introducing in-situ interfacial

20

19

synthesized the microcapsules with SiC/polyurea

prepared capric-stearic acid@SiO2 microencapsulated

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polycondensation strategy, we attempted to synthesize microencapsulated n-

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tetradecane with silica as shell material for cold energy storage, optimized the

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polycondensation condition, and discussed its morphology, chemical structure,

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composition and thermal performance.

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2. EXPERIMENTAL SECTION

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2.1. Materials

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n-Tetradecane (Tet, AR), tetraethyl silicate (TEOS, CP), tween-80(CP), span-80 (CP),

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polyvinyl alcohol (PVA, AR), acetic acid (HAc, AR), ethanol (AR) and sodium

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chloride (NaCl, AR) were purchased by Tianjin Kemiou Chemical Reagents, China.

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All chemicals were used as received without further purification.

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2.2. Preparation of MEPCM

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In a typical procedure, an oil phase mixture obtained by adding 10.0 g Tet and 0.75 g

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composite emulsifier composed of span-80 and tween-80 with mass ratio of 1:1, and an

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aqueous medium prepared by mixing 100 g deionized water and 1.5 g PVA together

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were even mixed in a 250 ml beaker under the water bath of 25 ℃ and the speed of

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500 rpm, then added dropwise 2 ml 2.5 mol L-1 NaCl solution for 30 min, a stable oil-

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in-water (o/w) microemulsion was formed. Then, 0.3 g 10 wt% HAc

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10.0 g TEOS added dropwise to the microemulsion, and the reaction was accomplished

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at 55 ℃ and the speed of 300 rpm for 3 h. The resulting product recorded as SiO2/Tet

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MEPCM was obtained by filtering, washing with ethyl alcohol and distilled water three

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times and drying at low temperature sequentially. The encapsulation ratio (R) can be

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determined from the following equation:

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R=

115

Where ∆Hm,

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tetradecane, respectively.

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2.3. Characterization of MEPCM

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The MEPCM was observed by a DM 2500P polarizing microscope (POM, Leica

119

Instrument Co., Germany). The morphology and surface element of the sample were

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measured with an S-3700 scanning electron microscope and energy dispersive X-ray

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spectrometer (SEM-EDS, Hitachi Co., Japan) with Au-target and accelerating voltage

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of 15 kV, and its structure was recorded with a Vector 33 Fourier transform infrared

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spectrometer (FT-IR, Bruker Instrument Co., Germany) in the range from 400 to 4000

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cm-1 using KBr pellets. X-ray diffraction pattern of the solid powder sample was

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obtained using D/max-ⅢA X-ray diffractometer (XRD, Rint Co., Japan) with Cu Kα

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radiation at the radiation tube voltage of 40 kV, tube current of 40 mA and the scan

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speed of 2° min-1. The thermal behavior of the MEPCM was carried out on Q20

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differential scanning calorimeter (DSC,TA Instrument, USA) under N2 atmosphere at

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a heating or cooling rate of 5 ℃ min-1 in the range of -30-40 ℃. About 10 mg sample

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is used each time. Thermal conductivity was measured with a 2500s thermal constant

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analyzer (Hot Disk Co., Sweden) using transient plane source (TPS) method

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microcapsules and 0.5 g water glass were filled into mold to form two equivalent thin

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films with the diameter of 2 cm and the Tet was in a liquid form. The sensor was

H m ,micro PCM  100% H m ,PCM micro-PCM,

(1)

∆Hm,

PCM

are melting enthalpy of the MEPCM and pure n-

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. 10 g

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sandwiched between two pieces of film supported by a background material. Thermal

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conductivity of the microcapsules based on the average of three tests at room

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temperature. The thermal stability of the dried MEPCM was evaluated using TG 209

137

thermogravimetric analyzer (TG, Nietzsche Co. Ltd., Germany) at a scanning rate of

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10 ℃ min-1 under a nitrogen atmosphere.

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3. RESULTS AND DISCUSSION

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3.1. Influence of polycondensation condition

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3.1.1. Amount of composite emulsifier

142 143 144

Fig. 1 Optical micrographs of the SiO2/Tet MEPCM with different amount of composite emulsifier: (a) 1.0 g, (b) 1.5 g, (c) 2.0 g and (d) 2.5 g

145

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150 120 -1

Enthalpy (kJkg )

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90 60 30 0 0.5

146 147

1.0

1.5

2.0

2.5

3.0

Amount of emulsifier (g) Fig.2 Effect of the amount of composite emulsifier on the enthalpy of SiO2/Tet MEPCM

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The emulsifier can reduce microemulsion surface tension significantly, which is in

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favor of SiO2 (shell) deposition on the oil droplets (core) surface and produces regular

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spherical MEPCM. The optical micrographs of the MEPCM with different amount of

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composite emulsifier are shown in Fig.1. When over dosage of emulsifier, composite

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emulsifier seriously influenced phase change enthalpy of MEPCM. As shown in Fig.2,

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as emulsifier of 1.5 g, the melting enthalpy of MEPCM reached 140.1 kJ kg-1, as

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emulsifier of 2.5 g, though it could produce uniform and small MEPCM, the

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microcapsules has a lower enthalpy (115.2 kJ kg-1). Therefore, the appropriate amount

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of composite emulsifier was 1.5 g.

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157

3.1.2. Mass ratio of Tet/TEOS

30/70: 40/60: 50/50: 60/40:

2.5 2.0 1.5

Heat Flow (W/g)

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77.1 kJ/kg 107.5 kJ/kg 140.1kJ/kg 151.0 kJ/kg

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -40

-30

-20

-10

0

10

20

30

40

50

Temperature (°C)

158 159

Fig. 3 Effect of the mass ratio of Tet and TEOS on the enthalpy of SiO2/Tet MEPCM

160 161

Fig. 4 Optical micrograph of SiO2/Tet MEPCM synthesized at RTet/TEOS of 30/70

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The mass ratio of Tet and TEOS (RTet/TEOS) influences phase change enthalpy of

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microcapsules. As shown in Fig. 3, the enthalpy of the microcapsules increased with

164

the increase of RTet/TEOS. When RTet/TEOS of 30/70, high content SiO2 formed by

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hydrolysis and condensation of TEOS led to some excess tiny SiO2 particles attached

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on the surface of the microcapsules (Fig.4) and the decrease of encapsulation ratio. As

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RTet/TEOS of 60/40, the melting enthalpy of MEPCM was 151.0 kJ kg-1. Since the

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obtained SiO2 shell material was too thin to encapsulate Tet core material completely,

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the stability of the microcapsules was inferior. Based on the above factors, the optimum

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mass ratio of RTet/TEOS was 50/50.

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3.2. Characterization of the SiO2/Tet MEPCM

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3.2.1. SEM observation and EDS analysis

173 174

Fig.5 SEM photo and EDS spectrum of SiO2/Tet MEPCM

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Table 1 EDS data of SiO2/Tet MEPCM Element

Weight/%

Atomic/%

C

58.09

68.21

O

28.50

25.12

Si

11.45

5.75

Others

1.96

0.91

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The morphology of SiO2/Tet MEPCM was observed using scanning electron

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microscopy (SEM). As shown in the Fig. 5(a), the synthesized MEPCM are spherical

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particles with smooth and compact surface. Its particle size is about 2 μm.

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The EDS spectrum of the SiO2/Tet MEPCM was analyzed. As shown in Fig. 5(b),

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obviously, the synthesized MEPCM contained C, O and Si element, among them, C

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atom stemmed from n-tetradecane core and Si and O atom originated from SiO2. Table

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1 shows the EDS analysis data of the SiO2/Tet MEPCM. The mass percent of C element

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was 58.09%, and the total mass percent of both Si and O element was 39.95%.

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3.2.2. Thermal performance

6

Endo

o

Tm:5.88 C

Tet

Hm:225.8kJ/kg

4

o

Tc:2.15 C

-1

Heat flow (W g )

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Hc:225.4kJ/kg

2 0

o

-2

Tm:2.39 C Hm:140.1kJ/kg

-4

MEPCM

-6

Hc:139.9kJ/kg

-20 185 186

o

Tc:-0.39 C

-10

0

10

0

20

Temperature ( C) Fig. 6 DSC curves of SiO2/Tet MEPCM and n-tetradecane

187 188 189 190

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Table 2 Comparison of encapsulation ratio of the MEPCM prepared with results in literature MEPCM

Latent heat(J/g)

Encapsulation ratio (%)

Reference

n-Tetradecan/polystyrene

98.71

44.7

7

n-Tetradecan/UF

134.16

60.0

15

n-Tetradecan /AS

113.5

51.5

24

n-Tetradecan /ABS

107.1

48.5

24

n-Tetradecan /PC

113.2

51.3

24

n-Tetradecan /SiO2

140.1

62.0

Present study

192

Fig.6 shows the DSC curves of SiO2/Tet MEPCM and pure n-tetradecane. The results

193

showed that the melting enthalpy of SiO2/Tet MEPCM and n-tetradecane was 140.1

194

and 225.8 kJ kg-1, respectively. Table 2 presents the comparison of the encapsulation of

195

MEPCM prepared in this study with different MEPCM in literature. The results indicate

196

that the of SiO2/Tet MEPCM prepared in this study are as high as those of organic shell

197

(about 120 kJ kg-1), which proving that the obtained MEPCM has good thermal storage

198

potential for cold energy storage. It could also be observed that the melting (Tm) and

199

the crystallization temperature (Tc) of the MEPCM was 2.39 and -0.39 ℃, respectively,

200

and the one of n-tetradecane was 5.88 and 2.15 ℃. Slight difference of the melting and

201

solidification points between the core material (Tet) and the MEPCM can be interpreted

202

that the thermal resistance of SiO2 shell delays the phase change process of the MEPCM.

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3.2.3. Thermal stability

0

100 MEPCM

80

-10

60

-20

40

Tet

-30

20

DTG/ %/min

203

TG/ %

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-40

0 0

100

200

300

400

500

-50

o

204

Temperature/ C

205

Fig. 7 TG and DTG curves of SiO2/Tet MEPCM

206

Fig.7 shows thermogravimetric (TG) and differential thermogravimetric (DTG) cures

207

of SiO2/Tet MEPCM and n-tetradecane. The TG results showed that n-tetradecane

208

exhibited the weight loss of 97.5% which resulted from gasification of n-tetradecane in

209

the range of 30 to 150℃, while MEPCM displayed the weight loss of 58.5%, which

210

corresponded to the core content in the range of 130 to 230℃. The initial and final

211

gasification temperature of MEPCM obviously lagged behind that of pure n-tetradecane.

212

Moreover, as shown from DTG, there existed a significant difference between their

213

gasification rates. The maximum gasification rate of n-tetradecane was -35.38% min-1

214

at 97 ℃ , while the one of MEPCM was only -15.50 % min-1 at 197 ℃ , which

215

demonstrating that silica has good protection to the n-tetradecane.

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3.2.4. Thermal conductivity

217

Table 3 Thermal conductivity of SiO2/Tet MEPCM with different mass ratio of RTet/TEOS RTet/TEOS

Thermal conductivity

g/g

/( W m-1 K-1)

100/0 a

0.098

60/40

0.125

50/50

0.139

40/60

0.144

30/70 50/50(RTet/PS) a

0.151 b

0.106

denotes pure n-tetradecane without SiO2,

b

denotes polystyrene (PS)/Tet MEPCM

218

Note:

219

synthesized at RTet/PS of 50/50 as reference 13

220

Although n-tetradecane has many advantages for cold energy storage, as organic phase

221

change material, it has inherent shortcomings such as low thermal conductivity. Table

222

3 shows the thermal conductivity of SiO2/Tet MEPCM with different mass ratio of Tet

223

and TEOS (RTet/TEOS). As shown in table 3, the thermal conductivity of pure n-

224

tetradecane was only 0.098 W m-1 K-1. When SiO2 encapsulated n-tetradecane, the

225

thermal conductivity of MEPCM markedly improved. With the decrease of the RTet/TEOS,

226

namely, the increase of the SiO2 content, thermal conductivity of MEPCM increased.

227

Such as RTet/TEOS was 50/50, the thermal conductivity of MEPCM reached 0.139 W m-

228

1

229

PS organic polymer as shell at the same content of n-tetradecane, respectively. It

230

confirms that the inorganic silica as shell plays an important role in enhancing heat

231

transfer performance of MEPCM.

K-1, which was 1.454 and 1.311 times than that of n-tetradecane, and MEPCM with

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470

2854

Transmittance

MEPCM

722

3.2.5. FT-IR analysis

1464

1084 1467 1360

3385

SiO2

4000

3500

3000

2854

Tet 2924

461

2925

721

232

1081

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2500

2000

1500

1000

500

Wavenumber (cm-1)

233 234

Fig. 8 FTIR spectra of SiO2/Tet MEPCM

235

The FT-IR spectra of MEPCM, n-tetradecane and SiO2 are shown in Fig.8. In the

236

spectrum of n-tetradecane, the absorption peaks at 1467 cm-1and 1360 cm-1 were

237

corresponded to C-H bending vibration of –CH3, the ones at 2924 cm-1 and 2854 cm-1

238

were assigned to C-H stretching vibration of CH3– and –CH2. In addition, the

239

absorption peak at 721 cm-1 was associated with –CH2 in-plane rocking vibration. In

240

the spectrum of silica, the absorption peak at 3385 cm-1 was attributed to Si-OH

241

stretching vibration, the ones at 1084 cm-1 and 461 cm-1 can be found owing to Si-O-Si

242

symmetric and asymmetric stretching vibration. In the spectrum of SiO2/Tet MEPCM, all

243

the characteristic peaks existed in n-tetradecane and silica still appeared in the MEPCM.

244

No new significant peaks observed suggesting that no chemical interaction happens

245

between the components. These results indicate that n-tetradecane is encapsulated by

246

SiO2.

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3.2.6. XRD analysis

Intensity/ (a.u)

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0

10

20

30

40

50

/ ()

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Fig.9 XRD spectrum of SiO2/Tet MEPCM

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Fig.9 displayed the XRD pattern of the SiO2/Tet MEPCM. A wide disperse peak at

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about 20°appeared which indicates that the obtained SiO2 shell displays amorphous

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structure. Since n-tetradecane is no crystal liquid under test condition, there was no

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characteristic peak in the spectrum.

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4. CONCLUSIONS

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Novel microencapsulated phase change material (MEPCM) with silica (SiO2) as shell

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and n-tetradecane (Tet) as core for cold energy storage was successfully synthesized by

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in-situ interfacial polycondensation. The obtained optimum conditions were 1.5 g

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composite emulsifier and 50/50 RTet/TEOS. According to the FT-IR, XRD, SEM-EDS

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analysis results, n-tetradecane was well encapsulated by SiO2 shell with encapsulation

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ratio of 62.04%. Based on the results of DSC and TG, the MEPCM had melting

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enthalpy of 140.1 kJ kg-1 and excellent thermal stability. Moreover, since excellent

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thermal conductively of SiO2 shell, the thermal conductively of MEPCM reached 0.139

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W m-1 K-1. It indicates that the synthesized SiO2/Tet MEPCM has considerable potential

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for cold energy storage.

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ACKNOWLEDGMENT

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This work was supported by National Natural Science Foundation of China (No.

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51536003, No.21471059).

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