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Preparation, characterization and thermal properties of microencapsulated phase change material for low temperature thermal energy storage Yan Wang, Zhimin Liu, Xiaofeng Niu, and Xiang Ling Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02504 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Preparation, characterization and thermal properties of microencapsulated phase change material for low temperature thermal energy storage Yan Wang a,*, Zhimin Liua, Xiaofeng Niub and Xiang Linga aSchool of Mechanical and Power Engineering, Nanjing Tech University, Nanjing 211816, China bCollege of Urban Construction, Nanjing Tech University, Nanjing 211816, China Abstract: Microencapsulation of phase change material (PCM) has received great attention as a promising candidate for latent heat thermal energy storage application. In this study, we reported a successful synthesis of n-octadecane as the core material microencapsulated in the melamine-formaldehyde resin shell. In particular, the morphology and micro-structure of the prepared microencapsulated phase change material (MEPCM) were examined. Meanwhile, the phase change behavior and thermal storage properties of melting point, enthalpy of fusion and heat capacity were determined by Differential Scanning Calorimetry. It absorbed/released large latent heat 160.0 kJ/kg at the melting point (around 26.5 °C), which presented excellent potential for heat storage. Additionally, heating-cooling cycling tests up to 100 cycles were conducted and there was no significant thermal attenuation of enthalpy or leakage of the core material. This favorable thermal storage properties and durability of MEPCM demonstrated that our prepared MEPCM possessed excellent and stable performance suitable for its application for thermal energy storage systems. Keywords: Thermal energy storage; Microencapsulation; Thermal attenuation 1. Introduction The renewable energy utilization and the waste heat recovery have been a hot research topic at present, due to the shortage of traditional energy source and the severe environment issues by the excessive use of fossil fuels. Therefore, Thermal energy storage (TES), which could reduce the intermittency and instability of renewable energy and decreasing the asynchrony between energy supply and usage has attracted increasing attentions. The latent heat storage is one of the key techniques of TES with large energy storage density at a constant temperature and relative chemical stability [1-3], so it exhibits tremendous potential in solar energy collectors [4-6], space heating energy storage [7,8], building heating/cooling systems [9] and power and industry waste heat recovery [10,11]. The thermal performance of TES system mainly relies on the phase change material, hence the synthesis of the high performance of PCM plays a vital role in TES applications. Number of PCMs have been studied, including hydrated salt PCMs, organic compounds and eutectic PCMs [12]. Organic compounds with carbon atoms in their structure, have been widely considered due to its chemical stability, relatively large enthalpy of phase change, small super heating/cooling degree and relative low price. However, leakage is a common problem for all types of PCMs, which makes it difficult to keep thermal stability. Much effort has been devoted to solve this issue, including embedded into matrix material [13,14], using special containers [15], and encapsulation [16,17]. Microencapsulation is a process of coating the PCM with layers of shell materials, at the size of 0.1 μm-1000 μm. Both the shell and core materials can be organic and inorganic. Usually,

*

Corresponding author. E-mail: [email protected] 1

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paraffin waxes, n-alkanes, fatty acids, hydrated salts and metals can be encapsulated into the shell materials, such as silicates, clay, polymers (polyurea-formaldehyde and melamine-formaldehyde, etc.) and magnesium oxide to avoid the leakage and interactions of PCM with surroundings. The microencapsulation approaches have been investigated in the area of medical science [18] and food storage [19,20], including chemical, physico-chemical, physico-mechanical or chemical–mechanical methods [21]. In-situ polymerization is a synthesis technique to prepare the precursors (pre-polymers like oligomers) through hydrolysis of the parent chemical compound, which will be added to the continuous phase of PCM and dispersed to form the shell around the core material droplets under agitation and heating. This method is typically suitable for organic shell materials like polyurea-formaldehyde and melamine-formaldehyde. Konuklu [22], Krupa et al [23]. and other pioneering researchers [24,25] attempted to microencapsulate PCMs (such as dodecanol, decanoic acid and paraffin) into synthetic polymer material shells with in-situ polymerization method and confirmed that the MEPCMs had the good thermal performance potential for application in latent heat TES. Zhang et al. [26] prepared three different microcapsules, sodium salt of styrene-maleic anhydride copolymer (SMA/anionic), SDS and polyvinyl alcohol (PVA) encapsulated in melamine-formaldehyde by in-situ polymerization. They found SMA could had smooth shell morphology with good spherical shape, while for SDS and PVA, visible disfigurement on the shell surface or agglomeration of microcapsules were noticed. Yuan et al. [27] encapsulated paraffin into modified silica shell with graphene oxide to enhance the thermal conductivity of microcapsule. Sarier et al. [28] also successfully deposited silver nanoparticles into the urea-formaldehyde polymer shell and prepared the microcapsule of n-hexadecaneor n-octadecane as PCM. It was revealed that the silver nanoparticle could improve the thermal conductivity effectively, however the agglomeration of microcapsules cannot be ignored owing to the cohesive forces between urea-formaldehyde polymer shells incorporated with silver nanoparticles. Thermal regulation played a key role in the TES performance. Leng et al. [29] prepared NaCl-KCl binary eutectic salt microcapsule and concluded that there was 9% decrease of phase change enthalpy after 200 thermal cycles. Some efforts have been made on this issue [30,31], and also easily found the thermal attenuation of PCMs within cycles. However, there are limited previous studies directly associated the thermal regulation of the prepared MEPCMs. With this communication, this study attempted to prepare n-octadecane microcapsules enclosed in the melamine-formaldehyde resin shell by in-situ polymerization. As a representative organic PCM, n-octadecane exhibits favorable thermal properties as thermal energy storage material with relatively large latent heat 244 kJ/kg with a stable phase change at around 28 °C. The morphology was observed and characterized, and physical and thermal properties for energy storage/release efficiency, such as heat capacity, thermal conductivity, including thermal attenuation were investigated for TES applications. 2. Experimental section 2.1. Materials N-octadecane (analytical grade, AR) with favorable melting point of 28 °C was employed as the PCM for encapsulation, which was provided by China National Medicines Co. Ltd. Melamine (AR, China National Medicines Co. Ltd.) and formaldehyde (35 wt%) were applied as monomers for the synthesis of the resin shell. Sodium dodecyl sulfate (SDS), Span-80 and Tween-80 were 2


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selected as the dispersants; while triethanolamine ((HOCH2CH2)3N), acetic acid (CH3COOH) and ammonium chloride (NH4CL) were used to regulate the pH value of the solution. All the additives were in analytical grade and were used without any further purification. 2.2. Preparation of MEPCM Synthesis of the microcapsule involved preparation of the polymer shell and encapsulation of core material, and the typical procedure was described as shown in Fig. 1. At first stage, homogeneous emulsion was prepared. N-octadecane (10 g), together with SDS, Span-80 and Tween-80, was stirred in deionized water at a speed of 1000 rpm for half an hour, which was immersed in a water bath to keep the temperature at constant 80 °C. Acetic acid solution was dissolved in the solution, and the pH value was regulated between 4.5 and 5. Then stirring was continued for half an hour to obtain the emulsion phase.

Fig. 1 Preparation of MEPCM with n-octadecane core enclosed in melamine-formaldehyde resin shell through in-situ polymerization Parallel, melamine (3 g) and formaldehyde solution (6 g, 37 wt%) mixture was dissolved in deionized water (100 ml) in a beaker and was stirred at a speed of 600 rpm at 70 °C for 30 minutes with pH maintained at around 9 via tris (2-Hydroxyethyl) amine with 10% concentration. Polymerization reaction was continued with stirring and the melamine-formaldehyde pre-polymer could be obtained within 30 minutes. With both core material of n-octadecane emulsion and pre-polymer shell obtained, in situ emulsion was started. Melamine-formaldehyde pre-polymer was added drop by drop into n-octadecane emulsion and the mixture was stirred at a speed of 400 rpm at 40 °C. NH4CL solution at 10% concentration was used to control the solution pH value of 5.5. Subsequently, the 3


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mixture was heated at the rate of 2 °C/min and the temperature was kept at 65 °C for 2 hours and agitation was continued after adjusting the pH value of the mixture to 4.0 and the microencapsulation was successfully accomplished. Whereafter, the microcapsules were filtered out and washed with deionized water and ethanol several times and dried in a vacuum oven at 40 °C to get white powder of n-octadecane microcapsule with melamine-formaldehyde resin shell. 2.3. Characterization 2.3.1. Scanning Electron Microscope (SEM) The first step in the assessment of material science begins with an observation of the physical form. The scanning electron microscope (SEM) utilizes an electron beam to observe a structure down to several nm in scale, which normally detects secondary electrons to form an image for observation. Therefore, we measured the morphology and surface structure of the microcapsule samples at a magnification by scanning electron microscopy (SEM, JEOL, JSM-6510). 2.3.2. Fourier Transform Infrared Spectrometer (FTIR) The microcapsule sample structure was tested by Fourier transform infrared spectrometer (FTIR, IS10), the sample was prepared by KBr grinding and tableting. Light from the source is divided into two beams by the beam splitter. Two beams are reflected by the fixed mirror and the moving mirror respectively, and then back to the splitter. The moving mirror moves in a straight line at a constant speed, so the two beams after the splitter can form the optical path difference and cause interference. After the beam splitter is joined, the interference light that contains sample information reaches the detector and then processes the signal through the Fourier transform to obtain the infrared absorption spectrogram of transmittance or absorbance with wave number or wavelength. 2.3.3. Particle size distribution The size distribution of microcapsules was tested by Nano Particle Size & Zeta Potential Analyzer (Micromeritics, NanoPlus-3), which employs photon correlation spectroscopy and electrophoretic light scattering techniques to determine particle size and zeta potential. The NanoPlus features a particle size range of 0.1 nm to 12.3 µm with sample suspension concentrations from 0.00001% to 40%. 2.3.4. X-ray diffraction (XRD) The chemical structure of prepared n-octadecane microcapsule was investigated in the XRD (Rigaku SmartLab 2080B211) with monochromated Cu-Kα radiation. Rigaku SmartLab is a general purpose XRD system which can be used in diffraction studies of powders, thin films, and solid objects with various sizes. System has a unique five-axis θ-θ goniometer which enables many kinds of diffraction experiments. The XRD patterns were tested at a scan rate of 0.02 º/s with a 2θ range of 0 º to 50 º. 2.3.5. Thermal storage properties Thermal storage properties, including specific heat capacities, phase change temperatures and enthalpy of fusion were determined by differential scanning calorimetry (DSC, NETZSCH ， STA449C) with temperature control precision ±1 °C. The microcapsules were scanned in the temperature range of 5 °C-45 °C at various heating rates under a constant stream of nitrogen at a flow rate of 40 ml/min. Thermal conductivity of synthesized microcapsule was determined to evaluate the heat transfer property. It could be achieved by the measurement of the thermal diffusivity via laser 4


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flash diffusivity apparatus, and then thermal conductivity could be calculated as following:

(T)= (T)gC p (T)g (T)

(1)

where, λ is the thermal conductivity of the tested material, α indicates the thermal diffusivity, ρ and Cp are the density and specific heat. In this study, the thermal conductivity of prepared n-octadecane microcapsule was tested via laser flash diffusivity apparatus (LFA, NETZSCH ， 427/2/G) at the temperature range of 25 °C -50 °C. 2.3.6. Thermal cycling test Thermal durability of MEPCM were studied and thermal cyclic tests of heating-cooling processes were repeated up to 100 times via DSC to determine thermal attenuation of MEPCM. Each sample was displayed in the crucible for scanning. The cyclic tests were performed in the temperature range of 5 °C-45 °C at the constant heating rate of 5 °C/min and remained for 3 min at the temparature of 45 °C, and then cooling process was continued at the same rate. 3. Results and discussion 3.1.Morphology of prepared MECMs In this study, we attempted to prepare n-octadecane microencapsulation with melamine-formaldehyde resin shell, and the shapeable microcapsules exhibited in a white powder form and distributed homogeneously. The microstructure analysis of microcapsules was performed by SEM at various magnifications, as given in Fig. 2, which showed that the shapes of capsules were more spherical. As observed clearly, Fig. 2 SEM image of MEPCM particles the whole core material was enclosed in the shell material, suggesting successful preparation. The particle size distribution of synthesized microcapsules was presented in Fig. 3 and the particle size varied between 100 nm and 300 nm, indicating a narrow size distribution and good uniformity of particle size. While most particles (76.8%) were distributed between 100 nm and 180 nm and the mean size of the particle could be found of 142 Fig. 3 Size distribution of MEPCM particles nm. Particularly, shrinking on the surfaces of some MEPCM particles might be found owing to the smaller density of the raw materials than the resin polymers, leading to the shell structure shrink in the polymerization process [32]. 3.2. FTIR analysis Fig. 4 showed the FTIR spectrum of the microencapsulation of phase change material. There was a strong absorption peak near the wave number of 2913~2851 cm-1, caused by the C-H stretching vibration of the CH2 and CH3 groups. A moderate-strength CH2 shear-bending vibration 5


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absorption peak appeared at around 1461~1336 cm-1, and a strong -CH-in-plane rocking vibration absorption peak was found close to 810-712 cm-1, which were characteristic absorption peaks of n-octadecane. It can also be observed from the figure that a broad absorption peak appeared near 3412 cm-1, which was formed by the superposition of the stretching vibrations of O-H and N-H. The absorption peak appeared at 1568 cm-1, which was formed by the stretching vibration of C-N. The peak at 1336 cm-1 was caused by the bending vibration of C-H, while the one at 1006 cm-1 was the C-O stretching vibration. These characteristic peaks indicated that melamine reacted with formaldehyde to get the melamine resin. 3.2. Structure properties The crystallographic systems of n-octadecane, melamine-formaldehyde (MF) shell and microencapsulated PCM were evaluated by Fig. 4 FTIR spectrum of MEPCM XRD and presented in Fig. 5. It was found that the structure of the microcapsules was similar to melamine-formaldehyde shell but largely

(a)

(b)

Fig. 5 X-ray diffraction patterns of (a) n-octadecane, (b) microcapsules and MF shell different from the crystalline structure of pure n-octadecane, especially in the amorphous region. In Fig. 5 (b), the crystallographic forms of encapsulated n-octadecane showed that there was a typical sharp diffraction peak at 2θ=19.22 º, corresponding to the crystalline plane of (0 1 1). Simultaneously, the distinct diffraction peaks appeared at approximately 19.74 º, 23.28 º and 24.62 º, suggesting the crystalline plane of (0 1 2), (1 0 1) and (1 0 2), which indicated that microencapsulated n-octadecane belongs to the triclinic system. 3.3. Thermal energy storage performance According to TES application requirements, the phase change temperature and storage capacity of PCMs played the vital role in selecting suitable storage materials. The thermal scanning of MEPCM samples was carried out at various heating/cooling rates (i.e. 2K/min, 5K/min, 8K/min, 10K/min) in the temperature range of 5 °C-45 °C. Fig. 6 (a) shows the results of a test without thermal cycling. Endothermic peaks of melting appeared at around 26.5 °C, very close to the theoretical melting point; while the exothermic peaks of solidification were found at around 6


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25.2 °C, which suggested that some supercooling degree was needed and it was much more significant with larger cooling rate. The crystallized temperature dropped to 24.7 °C at 10 K/min heating rate.

(a) (b) Fig. 6 Heating and cooling DSC curves of the MEPCM as prepared at various heating/cooling rates, (a) before thermal cycles, (b) after 100 thermal cycles Enthalpy of fusion was calculate via the area of the endothermic line above an arbitrarily baseline. As seen in Fig. 6 (a), the enthalpy of the scanning sample was 160.0 J/g at the heating rate of 2 K/min. According to the mass ratio 2:1 of core material to shell of our as-prepared MEPCM, the enthalpy could be calculated as 162.7 J/g, which was very close to our scanning result. This good agreement predicted the successful microencapsulation of PCM. Fig. 6 (b) showed the results of the samples after 100 thermal cycles at various heating/cooling rates. It can be observed that the latent heat value of the MEPCM sample showed a small variation range at a large heating rate of 8 K/min or 10 K/min before or after the thermal cycles, indicating that the heating rate had little effect on the thermal performance of the MEPCM. In addition, heat capacity was also obtained during the scanning and shown in Fig. 7 to provide a better investigation of the performance of TES. The scanning was carried out at heating rate 5 K/min. The heat capacity plot exhibited a flat plot except a sharp peak at around 30 °C and the value achieved to 24 kJ/(kg·K), indicating that melting occurred and large latent heat was absorbed, beneficial to thermal energy storage. There was a slightly increase of heat capacity with temperature increasing for both crystal or liquid octadecane. The theoretical heat capacity of solid octadecane at 50 °C is 2.32 kJ/(kg·K), and the scanning result showed that it reached to 3.81 kJ/(kg·K) of microcapsule owing to the relatively larger heat capacity of the shell. Thermal conductivity plays a vital role in energy utilization efficiency since it will affect the storage and release rate of thermal energy. Fig. 7 Heat capacity of microcapsules The thermal diffusivity of our synthesized microcapsule was around 0.006 mm2/s at 30 °C. Then, the thermal conductivity could be reached to 0.14 W/(m·K) by the calculation as Equ. (1). Not surprisingly, this value was smaller than that of the pure octadecane (0.56 W/(m·K)) due to the thermal resistance at the shell-core interface and 7


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lower thermal conductivity of melamine-formaldehyde resin, which would delay the thermal response of the thermal energy storage or release. Therefore, the thermal conductivity enhancement would be a key technology in the application of latent heat storage-release system with microcapsules. 3.4 Thermal cycling tests To clarify the durability performance of MEPCM during the thermal energy storage/release processes, thermal cycling tests were conducted by repeating the heating-cooling processes up to 100 cycles in the temperature range of 5-45 °C. It was found that the melting point was still at about 26 °C, while there was a slightly decrease of latent heat. To better evaluate the heat enthalpy attenuation, it was investigated in Fig. 8. The rate of the heat enthalpy attenuation has a slightly increase at the initial 50 cycles, and then the rate of the heat enthalpy attenuation plot became flat gradually. With calculation, after 100 cycles, the enthalpy was remained 97.9% of the initial value, although the sample weight loss achieved to 6%, which suggested that the lost weight was originated from shell material and little Fig. 8 DSC analyses of the MEPCM sample with leakage of core material was found. It was thermal cycling tests demonstrated that our as-prepared MEPCM retained the good heat storage performance. 4. Conclusions In this study, n-octadecane micoencapsulated in melamine-formaldehyde resin shell with favorable heat storage/release performance and high cyclic durability was successfully prepared by in-situ polymerization. Morphology and structural characterization confirmed that the microcapsules with perfect shape and uniform size were obtained and there was no interaction between core and shell material. It presented that the as-prepared microcapsules had attracting phase change properties with enthalpy of fusion achieving to 160.0 kJ/kg. With fast cooling rate, super cooling degree can be found for solidification process and enthalpy could be decreased. Via thermal cycling tests, there was no obvious attenuation. All these results demonstrated that the synthesized microcapsules possessed good comprehensive properties for energy storage. ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (grant numbers 51406079, 51406076) and Educational Commission of Jiangsu Province, China (grant number 18KJA480003). References [1] Alva, G.; Lin, Y. X.; Fang, G. Y. An overview of thermal energy storage systems. Energy 2018, 144, 341-378. [2] Chen, X. Y.; Jin, X. G.; Liu, Z. M.; et, al. Experimental investigation on the CaO/CaCO3 thermochemical energy storage with SiO2 doping. Energy 2018, 155, 128-138. 8


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Fig. 1 Preparation of MEPCM with n-octadecane core enclosed in melamine-formaldehyde resin shell through in-situ polymerization 189x164mm (96 x 96 DPI)


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Fig. 2 SEM image of MEPCM particles 127x95mm (256 x 256 DPI)


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Fig. 3 Size distribution of MEPCM particles 51x38mm (300 x 300 DPI)


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Fig. 4 FTIR spectrum of MEPCM 69x57mm (300 x 300 DPI)


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Energy & Fuels

Fig. 5 X-ray diffraction patterns of (a) n-octadecane 69x50mm (300 x 300 DPI)


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Fig. 5 X-ray diffraction patterns of (b) microcapsules and MF shell 69x54mm (300 x 300 DPI)


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Energy & Fuels

Fig. 6 Heating and cooling DSC curves of the MEPCM as prepared at various heating/cooling rates, (a) before thermal cycles 69x54mm (300 x 300 DPI)


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Fig. 6 Heating and cooling DSC curves of the MEPCM as prepared at various heating/cooling rates, (b) after 100 thermal cycles 69x54mm (300 x 300 DPI)


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Energy & Fuels

Fig. 7 Heat capacity of microcapsules 69x53mm (300 x 300 DPI)


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Fig. 8 DSC analyses of the MEPCM sample with thermal cycling tests 69x48mm (300 x 300 DPI)


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Preparation, Characterization, and Thermal Properties of

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