applied sciences

Article Thermal Performance Evaluation of and Paraffin Based Mixed SSPCMs Using Exfoliated Graphite Nanoplatelets (xGnP)

Hwayoung Lee, Su-Gwang Jeong, Seong Jin Chang, Yujin Kang, Seunghwan Wi and Sumin Kim *

Building Environment & Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Korea; [email protected] (H.L.); [email protected] (S.-G.J.); [email protected] (S.J.C.); [email protected] (Y.K.); [email protected] (S.W.) * Correspondence: [email protected]; Tel.: +82-2-820-0665; Fax: +82-2-816-3354

Academic Editor: Mohammed Mehdi Farid Received: 22 February 2016; Accepted: 4 April 2016; Published: 12 April 2016

Abstract: Phase change materials (PCMs) have been used in various fields including the materials of buildings. In this research, mixed shape-stabilized PCMs (Mixed SSPCMs) were prepared by impregnating coconut oil and n-hexadecane into exfoliated graphite nanoplatelets (xGnP) through a vacuum impregnate method. Coconut oil is fatty acid ester PCM which is relatively economical in comparison to other PCMs, and n-hexadecane is paraffin PCM that has high latent heat capacity. Drawbacks include leakage in a liquid state and low thermal conductivity resolved by xGnP. When preparing Mixed SSPCMs, coconut oil and n-hexadecane were impregnated at different proportions, namely 70:30, 50:50, 30:70 wt %. Mixed SSPCMs were analyzed through SEM, FT-IR, DSC, TGA and TCi. As a result, we confirmed the microstructure, chemical stability, thermal properties, thermal stability and thermal conductivity of Mixed SSPCMs. Latent heat capacity of Mixed SSPCMs were 89.06, 104.30 and 124.50 J/g while those of SSPCMs containing single PCM were 82.34 and 96.40 J/g. Thermal conductivity of Mixed SSPCMs was more than 284% higher than that of pure coconut oil and n-hexadecane. Finally, we confirmed that coconut oil and n-hexadecane were impregnated into xGnP, and the Mixed SSPCMs have high thermal durability.

Keywords: mixed SSPCM; fatty acid ester PCM; paraffin PCM; thermal performance; thermal storage; xGnP; impregnation

1. Introduction Thermal energy storage (TES) for space heating and cooling of buildings is becoming more and more important due to the rising cost of fossil fuels and environmental concerns. It saves premium fuels and leads to the system being more cost effective by reducing the waste of energy and capital costs [1,2]. Sensible heat storage (SHS) using thermal mass of the structural elements is the most common way to storage thermal energy in buildings. It can be used for the storage and release of thermal energy in a passive way. However, in comparison to latent heat storage, a much larger volume of material is required to store the same amount of energy by changing the phase of a storage material. Therefore, an effective way to reduce the building’s energy consumption for heating and cooling is by incorporating phase change materials (PCMs) in latent heat thermal energy storage (LHTES) systems of various building elements such as walls, windows, ceilings and floors [3,4]. PCMs are generally divided into organic PCM, inorganic PCM, and eutectic PCM. Typical inorganic PCMs include water, aqueous solution, salt hydrate, and molten salt, while organic PCMs include paraffin, fatty acid, ester and sugar . Besides, eutectic mixtures of two or more miscible pure

Appl. Sci. 2016, 6, 106; doi:10.3390/app6040106 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 106 2 of 10

PCM constituents that solidify simultaneously out of the liquid at a minimum freezing point have been widely developed to enrich the diversity of PCMs [5]. Commonly, inorganic PCMs suffer from decomposition and supercooling, which can further affect their phase change properties [6]. In contrast, organic PCMs are more stable and incorrodible than inorganic PCMs in the surrounding container [7]. In particular, paraffin PCMs are more widely used for application in building because they possess large latent heat capacity, low vapor pressure, and good thermal stability [8,9]. Also, many fatty acid ester PCMs have lower latent heat capacity than paraffin PCMs, but most of them are more commercial and economical than paraffin PCMs [10]. However, fatty acid ester and paraffin PCMs also have low thermal conductivity and leakage of liquid problems that disturb their application to energy storage [11–13]. Manufacturing shape-stabilized PCM (SSPCM) by impregnating the PCMs into high thermal conductivity materials such as carbon materials with porous structure resolves these problems. Exfoliated graphite nanoplatelets (xGnP) which are porous and nano-sized carbon materials are suitable [14–18]. Additionally, they have a layered structure and are very cost effective. The low coefficient of thermal expansion of xGnP, is a desirable property for composite structural applications [19–22]. In this research, we prepared Mixed SSPCMs by impregnating two types of PCMs, coconut oil and n-hexadecane into the xGnP at different proportions to improve thermal performance and investigate the difference between their performances. We used a vacuum impregnation process that guarantees high heat storage of fatty acid ester PCM and paraffinic PCM due to capillary forces and surface tension forces during the incorporation process [23]. Further, we analyzed the microstructure, chemical stability, thermal storage performance, thermal stability, and thermal conductivity of the Mixed SSPCMs by using SEM, FT-IR, DSC, TGA, and TCi.

2. Experimental

2.1. Materials In this research, we used fatty acid ester PCM and paraffin PCM, which possess different melting points. Fatty acid ester PCM is coconut oil and paraffin PCM is n-hexadecane in this research. The melting point and latent heat capacity at solid-liquid melting of pure coconut oil are 26.78 ˝C and 110.4 J/g, and those of pure n-hexadecane are 20.84 ˝C and 254.7 J/g [23,24]. The coconut oil is obtained from the Korea Similac Corporation in Gyeonggi, South Korea, and the n-hexadecane is obtained from the Celsius Korea Corporation in Seoul, South Korea. The exfoliated graphite nanoplatelets (xGnP) were arranged from sulfuric acid-intercalated expandable graphite (3772) obtained from Asbury Graphite Mills, Inc., in Asbury, NJ, USA by applying a cost- and time-effective exfoliation process initially proposed by Drzal's group [19]. Table1 shows the physical properties of coconut oil and n-hexadecane, and Table2 shows those of xGnP.

Table 1. Physical properties of coconut oil and n-hexadecane [23,24].

Value Property Coconut Oil n-Hexadecane Melting point (˝C) 26.78 20.84 Latent heat of melting (J/g) 110.4 254.7 Thermal conductivity (W/mK) 0.321 0.154

Table 2. Physical properties of xGnP [14].

Property Value Surface area (m2/g) 20.41 Bulk density (g/cm3) 0.0053–0.010 Pore volume (cm3/g) 0.081 Thermal conductivity (W/mK) 2–300 Specific heat capacity (J/kgK) 710 Appl. Sci. 2016, 6, 106 3 of 10

2.2. Preparation We prepared the Mixed SSPCMs by impregnating the coconut oil and n-hexadecane into xGnP at different proportions: 70:30 wt %, 50:50 wt %, and 30:70 wt %. To prepare the Mixed SSPCMs, we mixed the coconut oil and n-hexadecane in each proportion at room temperature and strongly stirred. The xGnP was then dried at 105 ˝C for 24 h in a vacuum oven. Next, we used the vacuum impregnation process according to following procedure. The xGnP was put inside a filtering flask, which was connected to a water tromp apparatus to evacuate air from the porous structure. Then, a valve between the flask and a container with a total of 100 g of liquid coconut oil and n-hexadecane was opened, to allow the liquid PCMs to flow into the flask and cover the nanoparticles of xGnP. After the vacuum process was continued for 90 min, the air was allowed to enter the flask again, to force the liquid PCMs to penetrate the porous structure of the xGnP. In this process, excess PCMs which were not impregnated remained in the flask. Those were removed through a filtering process. The·coconut oil + n-hexadecane Mixed SSPCMs in the colloidal state were filtered through 1 µm filter paper until a granule type of sample appeared on the filter paper, and then those were dried in a vacuum drier at 80 ˝C for 48 h.

2.3. Characterization Techniques The microstructure of the Mixed SSPCMs were analyzed through scanning electron microscopy (SEM, JSM-6360A, JEOL, Tokyo, Japan) at room temperature. The SEM images of the samples were obtained with an accelerating voltage of 12 kV and a working distance of 12 mm, and the samples were gold coated at a few nanometers in thickness to increase their electrical conductivity [19]. Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Scientific, Waltham, MA, USA) was used to confirm the change of chemical groups of the Mixed SSPCMs at room temperature. Clear potassium bromide (KBr) disks were made from powder and used as backgrounds. The Mixed SSPCMs were analyzed over the range of 650–4000 cm´1, with a spectrum resolution of 8 cm´1. All spectra were averaged over 32 scans. This analysis of the Mixed SSPCMs was performed by point-to-point contact with a pressure device [25]. Thermal properties of the Mixed SSPCMs, such as melting and freezing point and latent heat capacities, were analyzed by the differential scanning calorimetry (DSC, DSC-Q1000, TA instrument, New Castle, PA, USA). The melting and freezing point were measured by drawing a line at the point of maximum slope of the leading edge of the peak and extrapolating to the base line, and the latent heats of the Mixed SSPCMs were analyzed as total by numerical integration of the area under the peaks that represents the solid-solid and solid-liquid phase transition [17]. The DSC measurements were performed at a 3 ˝C/min heating and cooling rate and a temperature range of ´20 to 80 ˝C. Thermal stability of the Mixed SSPCMs was analyzed by thermogravimetric analysis (TGA, Q-5000 IR, TA Instruments, New Castle, PA, USA) on approximately 11–14 mg of the samples within the range from 20 to 600 ˝C at a heating rate of 10 ˝C/min under a nitrogen flow of 20 mL/min. The TGA was measured with the composites placed in a high quality nitrogen (99.5% nitrogen, 0.5% oxygen content) atmosphere to prevent unwanted oxidation [26]. The thermal conductivity of the Mixed SSPCMs were measured by TCi thermal conductivity analyzer (C-Therm Technologies Ltd., Fredericton, NB, Canada) at the temperature of 10 ˝C. The TCi is a device for conveniently measuring the thermal conductivity of a small sample by using the Modified Transient Plane Source (MTPS) method. It is the advantage of the TCi that can measure the thermal conductivity of materials in the states of solid, liquid, powder, and mixed [27].

3. Results and Discussion

3.1. Microstructure of the Fatty Acid Ester and Paraffin Based Mixed SSPCMs Scanning electron microscopy (SEM) observations were performed for the coconut oil + n-hexadecane Mixed SSPCMs. Figure1 shows the microstructure of the Mixed SSPCMs and xGnP that were investigated by SEM analysis. Figure1a–c show the Mixed SSPCMs and Figure1d shows the porous Appl. Sci. 2016, 6, 106 4 of 10

structureAppl. Sci. of the2016, xGnP. 6, 106 We found some nano particles of xGnP in Figure1d. Through the SEM4 analysis,of 10 we confirmed that the coconut oil and n-hexadecane were fully filled into the xGnP. As shown in Figureanalysis,1a–c, liquid we confirmed PCMs were that well the impregnatedcoconut oil and into n‐hexadecane the each layer were of fully the xGnP.filled into The the porous xGnP. structure As shown in Figure 1a–c, liquid PCMs were well impregnated into the each layer of the xGnP. The of the xGnP is good for impregnation of the PCMs. The microstructure of the Mixed SSPCMs are porous structure of the xGnP is good for impregnation of the PCMs. The microstructure of the Mixed similarSSPCMs to one are another, similar regardless to one another, of the regardless ratio of the of the PCMs. ratio Consequently, of the PCMs. Consequently, heat storage heat of the storage PCMs are maintainedof the PCMs in the are Mixed maintained SSPCMs in the despite Mixed loading SSPCMs the despite xGnP. loading the xGnP.

(a) (b)

(c) (d)

Figure 1. SEM images of (a) 70 wt % coconut oil + 30 wt % n‐hexadecane Mixed shape‐stabilized phase Figure 1. SEM images of (a) 70 wt % coconut oil + 30 wt % n-hexadecane Mixed shape-stabilized phase change materials (SSPCM), (b) 50 wt % coconut oil + 50 wt % n‐hexadecane Mixed SSPCM, (c) 30 wt change% materialscoconut oil (SSPCM, + 70 wt (%b )n 50‐hexadecane wt % coconut Mixed oil SSPCM, + 50 wt and % n -hexadecane(d) exfoliated Mixedgraphite SSPCM, nanoplatelets (c) 30 wt % coconut(xGnP). oil + 70 wt % n-hexadecane Mixed SSPCM, and (d) exfoliated graphite nanoplatelets (xGnP).

3.2. FT-IR3.2. FT Analysis‐IR Analysis of the of the Fatty Fatty Acid Acid Ester Ester and and Paraffin Paraffin Based Mixed Mixed SSPCMs SSPCMs MostMost of the of ediblethe edible oils, oils, including including coconut coconut oil, oil, are are composed composed of . triglycerides. As As shown shown in Figure in Figure 2, a triglyceride2, a is a fatty is a fatty acid acid ester ester formed formed by by esterifing esterifing glycerol with with three three fatty fatty acids. acids. Coconut Coconut oil oil consists of medium‐chain triglycerides (MCTs), which are triglycerides that possess 8 to 12 carbons consists of medium-chain triglycerides (MCTs), which are triglycerides that possess 8 to 12 carbons of of fatty acids. So the coconut oil contains –CH3, –CH2, C=O, and C–O bonding. As shown in Figure 3, fattythe acids. FT‐IR So absorption the coconut spectrum oil contains of the Mixed –CH3 SSPCMs, –CH2, appears C=O, and with C–O nearly bonding. the same As absorption shown in peaks. Figure 3, the FT-IRThere absorption are absorption spectrum peaks ofof the2919–2920 Mixed SSPCMscm−1 and appears2850–2851 with cm− nearly1 occurred the sameby –CH absorption3 and –CH peaks.2 ´1 ´1 Therebonding, are absorption and 1735–1744 peaks cm of−1 2919–2920and 1149–1160 cm cm−1and occurred 2850–2851 by C=O, cm and C–Ooccurred bonding by of –CHester 3groups.and –CH 2 bonding,However, and 1735–1744the 30 wt % cm coconut´1 and oil 1149–1160 + 70 wt % cmn‐hexadecane´1 occurred Mixed by C=O, SSPCM and that C–O contains bonding a small of ester groups.percentage However, of coconut the 30 oil wt gives % coconut rise to small oil peaks + 70 wtin C=O, % n -hexadecaneand C–O bonding Mixed of ester SSPCM groups. that The contains n‐ a smallhexadecane percentage is a of paraffin coconut PCM oil that gives is riseconsists to small of 16 peaks carbons in and C=O, 34 andhydrogens C–O bonding of single of chain. ester The groups. The ncondensed-hexadecane structure is a paraffin formula PCMof that that is CH is3 consists(CH2)14CH of3. 16So the carbons n‐hexadecane and 34 hydrogensalso generate of the single peaks chain. of –CH3 and –CH2 bonding. The FT‐IR absorption peaks of the Mixed SSPCMs are shown in Table 3. The condensed structure formula of that is CH3(CH2)14CH3. So the n-hexadecane also generate the In this experiment, we mixed two types of PCMs and impregnated the mixed PCMs into xGnP peaks of –CH and –CH bonding. The FT-IR absorption peaks of the Mixed SSPCMs are shown to enhance3 thermal performance.2 Through the FT‐IR analysis, we confirmed the chemical stability of in Tablethe3 Mixed. SSPCMs. As shown the result, the both absorption peaks of coconut oil and n‐hexadecane Inare this appeared experiment, in all Mixed we mixed SSPCMs. two So types we reason of PCMs that the and coconut impregnated oil and n the‐hexadecane mixed PCMs did not into join xGnP to enhancetogether thermal chemically. performance. In addition, Through by reviewing the FT-IR previous analysis, work we [24,28], confirmed we confirmed the chemical the chemical stability of the Mixed SSPCMs. As shown the result, the both absorption peaks of coconut oil and n-hexadecane are appeared in all Mixed SSPCMs. So we reason that the coconut oil and n-hexadecane did not join Appl. Sci. 2016, 6, 106 5 of 10

togetherAppl. Sci. chemically. 2016, 6, 106 In addition, by reviewing previous work [24,28], we confirmed the chemical5 of 10 stabilityAppl. Sci. of the2016, PCMs6, 106 in xGnP. According to [24,28], the FT-IR spectrum of coconut oil/xGnP5 of 10 and stability of the PCMs in xGnP. According to [24,28], the FT‐IR spectrum of coconut oil/xGnP and n‐ n-hexadecane/xGnPstability of the PCMs SSPCMs in xGnP. is According the same to as [24,28], the each the FT pure‐IR spectrum PCMs. This of coconut means oil/xGnP that there and aren‐ not hexadecane/xGnP SSPCMs is the same as the each pure PCMs. This means that there are not chemical chemicalhexadecane/xGnP incorporations SSPCMs between is the coconut same as the oil each and pure xGnP, PCMs. and Thisn-hexadecane means that there and xGnP. are not Therefore, chemical we reasonablyincorporationsincorporations measured betweenbetween that the coconutcoconut coconut oiloil oil and + n -hexadecanexGnP, andand nn‐ Mixedhexadecane‐hexadecane SSPCMs and and arexGnP. xGnP. chemically Therefore, Therefore, stable. we we reasonablyreasonably measured measured that that the the coconut coconut oil + n‐‐hexadecanehexadecane Mixed Mixed SSPCMs SSPCMs are are chemically chemically stable. stable.

FigureFigureFigure 2. 2.Chemical 2. Chemical Chemical formation formation formation of of triglyceride triglyceride by by esterification esterificationesterification of of ofglycerol glycerol glycerol with with with fatty fatty fatty acids. acids. acids.

-C H / -CH -C H 3 / -CH2 3 2 70wt% coconut oil + 30wt% n-hexadecane Mixed SSPCM 70wt% coconut oil + 30wt% n-hexadecane Mixed SSPCM 50wt% coconut oil + 50wt% n-hexadecane Mixed SSPCM 30wt%50wt% coconut coconut oil oil + +70wt% 50wt% n-hexadecane n-hexadecane Mixed Mixed SSPCM SSPCM 30wt% coconut oil + 70wt% n-hexadecane Mixed SSPCM C= O C-O C= O C-O Absorbance Absorbance

4000 3000 2000 1000 -1 4000 3000Wavenumbers 2000 (cm ) 1000 -1 Wavenumbers (cm ) FigureFigure 3. Fourier 3. Fourier transform transform infrared infrared spectroscopy spectroscopy (FT (FT-IR)‐IR) absorption absorption spectra spectra of Mixed of Mixed SSPCMs. SSPCMs. Figure 3. Fourier transform infrared spectroscopy (FT‐IR) absorption spectra of Mixed SSPCMs. TableTable 3. 3.FT-IR FT‐IR absorption absorption spectra peaks peaks of of Mixed Mixed SSPCMs. SSPCMs. BondingTable 3. FT‐IR absorption spectra peaks ofWave Mixed number SSPCMs. range (cm−1) Bonding Wave Number Range (cm´1) –CH3 2919–2920 Bonding Wave number range (cm−1) –CH2 –CH3 2919–2920 2850–2851 –CH3 2919–2920 C=O –CH2 2850–2851 1735–1744 –CH2 2850–2851 C–O C=O 1735–1744 1149–1160 C=O C–O 1149–1160 1735–1744 3.3. Thermal Properties AnalysisC–O 1149–1160 3.3. Thermal Properties Analysis 3.3. ThermalFigure Properties 4 shows theAnalysis heating and freezing curves from DSC measurements of the coconut oil + n‐ hexadecaneFigure4 shows Mixed the SSPCMs. heating Table and freezing4 represents curves the phase from DSCtransition measurements temperature of and the latent coconut heat oil + Figure 4 shows the heating and freezing curves from DSC measurements of the coconut oil + n‐ n-hexadecanecapacity of Mixed coconut SSPCMs. oil + n‐hexadecane Table4 represents Mixed SSPCMs the phase and transition Table 5 represents temperature those and of coconut latent heat hexadecaneoil/xGnP and Mixed n‐hexadecane/xGnP SSPCMs. Table SSPCMs. 4 represents As shown the phasein Figure transition 4, the phase temperature transition andof the latent Mixed heat capacity of coconut oil + n-hexadecane Mixed SSPCMs and Table5 represents those of coconut capacitySSPCMs of during coconut heating oil + andn‐hexadecane melting occurred Mixed atSSPCMs the lower and temperature Table 5 represents of the each those melting of coconut and oil/xGnPoil/xGnPfreezing and and pointn -hexadecane/xGnPn‐ hexadecane/xGnPof the coconut oil and SSPCMs. n‐hexadecane. As As shown shown The in inMixed Figure Figure SSPCMs 4,4, the the phasedisplay phase transition transition a tendency of of thatthe the Mixedthe Mixed SSPCMsSSPCMsmore during the during Mixed heating heating SSPCM and andcontains melting melting a high occurredoccurred rate of n atat‐hexadecane, the lower temperaturethe temperature greater the of ofmelting the the each each and melting freezing melting and and freezingfreezingpoints. point pointThe of melting of the the coconut coconuttemperatures oil oil and and atn peakn-hexadecane.‐hexadecane. points of the The Mixed Mixed SSPCMs SSPCMs SSPCMs were display display 12.53 a°C, atendency tendency14.51 °C, that and that the the moremore16.92 the the Mixed°C, Mixed while SSPCM theSSPCM freezing contains contains temperatures a a high high rate rate at peak ofof nn -hexadecane,points‐hexadecane, of them thewere the greater greater 5.55 °C, the the 6.54 melting melting °C, and and 10.31 and freezing freezing°C. ˝ ˝ points.points.The The difference The melting melting between temperatures temperatures melting and at at peak freezingpeak points points points ofof of thethe the Mixed Mixed SSPCMs SSPCMs wereis were because 12.53 12.53 of °C, supercoolingC, 14.51 14.51 °C,C, and and 16.9216.92of˝ C,PCMs. °C, while while the the freezing freezing temperatures temperatures at at peak peak pointspoints of them were were 5.55 5.55 °C,˝C, 6.54 6.54 °C,˝C, and and 10.31 10.31 °C.˝ C. The difference between melting and freezing points of the Mixed SSPCMs is because of supercooling of PCMs. Appl. Sci. 2016, 6, 106 6 of 10

The noticeable result in this research is the latent heat capacity of Mixed SSPCMs. The latent heat capacities of the Mixed SSPCMs were 89.06, 104.30, and 124.50 J/g during melting, and those of during Appl.freezing Sci. 2016 were, 6, 106 92.82, 106.20, and 123.40 J/g. Although these values are lower than pure PCMs because6 of 10 3‐dimentional network structure of xGnP disturbs the heat flow of PCMs, these are the high values in consideration of the latent heat capacities of coconut oil/xGnP and n‐hexadecane/xGnP SSPCMs Thethat difference contain single between PCM. melting The latent and heat freezing capacities points of ofthe the Mixed Mixed SSPCMs SSPCMs also is appear because to follow of supercooling the law ofthat PCMs. the more the Mixed SSPCM contains a high rate of n‐hexadecane, the greater the latent heat capacity.

70wt% coconut oil + 30wt% n-hexadecane Mixed SSPCM 50wt% coconut oil + 50wt% n-hexadecane Mixed SSPCM 30wt% coconut oil + 70wt% n-hexadecane Mixed SSPCM

1

Cooling Curve

0

Heating Curve Heat Flow(W/g) Heat

-1

-20 0 20 40 60 80 o Temperature( C)

FigureFigure 4. 4.Differential Differential scanning scanning calorimetrycalorimetry (DSC) (DSC) graphs graphs of of Mixed Mixed SSPCMs. SSPCMs.

TableTable 4. 4.DSC DSC analysisanalysis of Mixed SSPCMs. SSPCMs.

Mixed SSPCM (coconut oil: Latent Heat (J/g) Mixed SSPCM (Coconut Melting point˝ (°C) Freezing point˝ (°C) Latent Heat (J/g) n‐hexadecane) Melting Point ( C) Freezing Point ( C) Melting Freezing Oil: n-hexadecane) Melting Freezing 70:30 (wt %) 12.53 5.55 89.06 92.82 50:5070:30 (wt %)%) 12.5314.51 5.556.54 89.06104.30 92.82106.20 50:50 (wt %) 14.51 6.54 104.30 106.20 30:70 (wt %) 16.92 10.31 124.50 123.40 30:70 (wt %) 16.92 10.31 124.50 123.40

Table 5. DSC analysis of coconut oil/xGnP and n‐hexadecane/xGnP SSPCMs [24,28]. Table 5. DSC analysis of coconut oil/xGnP and n-hexadecane/xGnP SSPCMs [24,28]. Latent Heat (J/g) SSPCM Melting point (°C) Freezing point (°C) LatentMelting Heat (J/g)Freezing SSPCM Melting Point (˝C) Freezing Point (˝C) Coconut oil/xGnP 26.93 14.95 Melting82.34 Freezing77.64 n‐hexadecane/xGnP 21.80 14.60 96.40 94.80 Coconut oil/xGnP 26.93 14.95 82.34 77.64 n-hexadecane/xGnP 21.80 14.60 96.40 94.80 3.4. Thermal Stability Analysis

TheTGA noticeable analysis result was inperformed this research for thermal is the latent stability heat of capacity the coconut of Mixed oil + SSPCMs. n‐hexadecane The latent Mixed heat capacitiesSSPCMs of at the temperatures Mixed SSPCMs of 20 were to 600 89.06, °C. Because 104.30, andxGnP 124.50 is a carbon J/g during material melting, that andhas high those thermal of during durability, it does not decompose at temperatures under 600 °C. So only the coconut oil and n‐ freezing were 92.82, 106.20, and 123.40 J/g. Although these values are lower than pure PCMs because hexadecane of the Mixed SSPCMs decompose in the test temperature range. Figure 5 and Table 6 3-dimentional network structure of xGnP disturbs the heat flow of PCMs, these are the high values in show the TGA results of the Mixed SSPCMs. As shown in Figure 5, the TGA graphs of the all Mixed consideration of the latent heat capacities of coconut oil/xGnP and n-hexadecane/xGnP SSPCMs that SSPCMs appear 2 degradation curves caused by thermal decomposition. The first degradation curves contain single PCM. The latent heat capacities of the Mixed SSPCMs also appear to follow the law that of the graphs represent the decomposition of n‐hexadecane. The first degradation peaks of the n‐ thehexadecane more the Mixed occurred SSPCM at the contains temperature a high rateof 165.14 of n-hexadecane, °C, 172.62 the°C, greaterand 178.38 the latent °C. The heat second capacity. degradation curves of the graphs represent the decomposition of coconut oil. The second degradation 3.4. Thermal Stability Analysis peaks of the coconut oil occurred at temperatures of 363.68 °C, 357.68 °C, and 339.79 °C. In other words,TGA the analysis coconut was oil performed has a greater for thermal stabilitydurability of than the coconut the n‐hexadecane. oil + n-hexadecane Mixed SSPCMs at temperatures of 20 to 600 ˝C. Because xGnP is a carbon material that has high thermal durability, it does not decompose at temperatures under 600 ˝C. So only the coconut oil and n-hexadecane of the Mixed SSPCMs decompose in the test temperature range. Figure5 and Table6 show the TGA results of the Mixed SSPCMs. As shown in Figure5 , the TGA graphs of the all Mixed SSPCMs appear 2 degradation curves caused by thermal decomposition. The first degradation curves of the graphs represent the Appl. Sci. 2016, 6, 106 106 7 of 10

It is also possible to compute the percentages of impregnated coconut oil and n‐hexadecane in decomposition of n-hexadecane. The first degradation peaks of the n-hexadecane occurred at the xGnP by investigating the total decomposition rates. The three Mixed SSPCMs appeared similar temperature of 165.14 ˝C, 172.62 ˝C, and 178.38 ˝C. The second degradation curves of the graphs values of total decomposition rate. The total decomposition rate were 71.59%, 70.73%, and 68.53%. In represent the decomposition of coconut oil. The second degradation peaks of the coconut oil occurred the case of the Mixed SSPCMs, xGnP remained in them although the two PCMs were fully at temperatures of 363.68 ˝C, 357.68 ˝C, and 339.79 ˝C. In other words, the coconut oil has a greater decomposed. This means that the two PCMs were impregnated into xGnP, and the Mixed SSPCMs thermal durability than the n-hexadecane. have high thermal durability.

100 70wt% coconut oil + 30wt% n-hexadecane Mixed SSPCM 50wt% coconut oil + 50wt% n-hexadecane Mixed SSPCM 30wt% coconut oil + 70wt% n-hexadecane Mixed SSPCM

80 68.53

70.73 60

71.59

Weight(%) 40 Decomposition of n-hexadecane

20 Decomposition of coconut oil

0 0 200 400 600 Temperature(oC)

Figure 5. Thermogravimetric analysis (TGA) graphs of Mixed SSPCMs.

Table 6.6. TGA analysis of Mixed SSPCMs.SSPCMs.

First peak of Second peak of MixedMixed SSPCM SSPCM (coconut (Coconut oil: First Peak of Derivative Second Peak of TotalTotal Decomposition decomposition Oil: n-hexadecane) derivativeWeight (˝ C)weight Derivativederivative Weight weight (˝C) Rate (%) n‐hexadecane) rate (%) 70:30 (wt %) 165.14(°C) 363.68(°C) 71.59 70:3050:50 (wt (wt %) %) 172.62165.14 357.68363.68 70.7371.59 30:70 (wt %) 178.38 339.79 68.53 50:50 (wt %) 172.62 357.68 70.73 30:70 (wt %) 178.38 339.79 68.53 It is also possible to compute the percentages of impregnated coconut oil and n-hexadecane in xGnP3.5. Thermal by investigating Conductivity the Analysis total decomposition rates. The three Mixed SSPCMs appeared similar values of total decomposition rate. The total decomposition rate were 71.59%, 70.73%, and 68.53%. In the The thermal conductivity analysis of the coconut oil + n‐hexadecane Mixed SSPCMs using TCi case of the Mixed SSPCMs, xGnP remained in them although the two PCMs were fully decomposed. at the temperature of 10 °C is shown in Figure 6. As shown in Figure 6, the thermal conductivities of This means that the two PCMs were impregnated into xGnP, and the Mixed SSPCMs have high the Mixed SSPCMs are 1.70, 1.61, and 1.23 W/mK. The Mixed SSPCMs appear to follow the tendency thermal durability. that the more coconut oil they contain, the greater the thermal conductivity because the pure coconut 3.5.oil has Thermal larger Conductivity thermal conductivity Analysis than pure n‐hexadecane. The thermal conductivities of pure coconut oil and n‐hexadecane are 0.321 and 0.154 W/mK, respectively [23,24]. The overall Mixed The thermal conductivity analysis of the coconut oil + n-hexadecane Mixed SSPCMs using TCi at SSPCMs have larger thermal conductivities which are 284% higher than the pure PCMs. This implies the temperature of 10 ˝C is shown in Figure6. As shown in Figure6, the thermal conductivities of the that the xGnP led to an enhancement of the thermal conductivity of coconut oil and n‐hexadecane. Mixed SSPCMs are 1.70, 1.61, and 1.23 W/mK. The Mixed SSPCMs appear to follow the tendency that Consequently, we concluded that Mixed SSPCMs can be usefully applied to various categories such the more coconut oil they contain, the greater the thermal conductivity because the pure coconut oil as buildings. has larger thermal conductivity than pure n-hexadecane. The thermal conductivities of pure coconut oil and n-hexadecane are 0.321 and 0.154 W/mK, respectively [23,24]. The overall Mixed SSPCMs have larger thermal conductivities which are 284% higher than the pure PCMs. This implies that the xGnP led to an enhancement of the thermal conductivity of coconut oil and n-hexadecane. Consequently, we concluded that Mixed SSPCMs can be usefully applied to various categories such as buildings. Appl.Appl. Sci. Sci. 20162016, ,66, ,106 106 88 of of 10 10

1.70 1.61

1.5

1.23

1.0

0.5 Thermal conductivity (W/mK) conductivity Thermal

0.0 70 : 30 (wt%) 50 : 50 (wt%) 30 : 70 (wt%) Mixed SSPCM (coconut oil : n-hexadecane)

FigureFigure 6. 6. ThermalThermal conductivity conductivity of of Mixed Mixed SSPCMs. SSPCMs.

4.4. Conclusions Conclusions WithWith the risingrising cost cost of of fossil fossil fuels fuels and and additional additional environmental environmental concerns, concerns, the importance the importance of latent of latentheat thermal heat thermal energy energy storage storage (LTES) (LTES) for space for space heating heating and cooling and cooling is increasing. is increasing. Accordingly, Accordingly, phase phasechange change materials materials (PCMs) (PCMs) have been have used been in used various in fields.various Among fields. theAmong PCMs, the fatty PCMs, acid fatty ester acid PCMs ester are PCMsrelatively are economicalrelatively economical and paraffin and PCMs paraffin have PCMs higher have latent higher heat capacities latent heat than capacities other PCMs. than Soother we PCMs.prepared So coconutwe prepared oil + n coconut-hexadecane oil + mixedn‐hexadecane shape-stabilized mixed shape PCMs‐stabilized (Mixed SSPCMs) PCMs (Mixed using exfoliatedSSPCMs) usinggraphite exfoliated nanoplatelets graphite (xGnP) nanoplatelets through (xGnP) a vacuum through impregnation a vacuum methodimpregnation to enhance method their to enhance thermal theirperformance thermal performance with supplementation with supplementation of drawbacks of ofdrawbacks PCMs such of PCMs as the such leakage as the in leakage liquid statein liquid and statelow thermaland low conductivity. thermal conductivity. When preparing When the preparing Mixed SSPCMs, the Mixed we impregnatedSSPCMs, we the impregnated coconut oil andthe coconutn-hexadecane oil and at n different‐hexadecane proportions, at different namely proportions, 70:30, 50:50, namely 30:70 70:30, wt %. 50:50, We analyzed 30:70 wt the %. microstructure, We analyzed thechemical microstructure, stability, thermal chemical properties, stability, thermal stability,properties, and thermal thermal stability, conductivity and thermal of the samples conductivity using ofSEM, the FT-IR,samples DSC, using TGA, SEM, and FT TCi.‐IR, Through DSC, TGA, the and SEM TCi. analysis, Through we confirmedthe SEM analysis, that both we of confirmed the PCMs werethat bothwell impregnatedof the PCMs intowere the well porous impregnated structure ofinto the the xGnP. porous The FT-IRstructure graphs of the show xGnP. all peaks The ofFT the‐IR bonding graphs showof both all of peaks the PCMs. of the So, bonding we confirmed of both that of the there PCMs. was no So, chemical we confirmed interaction that in there the all was Mixed no chemical SSPCMs. interactionFrom the DSC in the analysis, all Mixed we confirmed SSPCMs. thatFrom the the latent DSC heat analysis, capacity we of confirmed the coconut that oil +then-hexadecane latent heat capacityMixed SSPCMs of the coconut was higher oil + thann‐hexadecane the coconut Mixed oil/xGnP SSPCMs and wasn-hexadecane/xGnP higher than the coconut SSPCMs oil/xGnP containing and nsingle‐hexadecane/xGnP PCM. The values SSPCMs of latent containing heat during single melting PCM. were The values 89.06, 104.30, of latent and heat 124.50 during J/g whilemelting those were of 89.06,SSPCMs 104.30, containing and 124.50 single J/g PCM while were those 82.34 of andSSPCMs 96.40 containing J/g. The greater single the PCM amount were of82.34n-hexadecane, and 96.40 J/g. the Thegreater greater the latent the amount heat capacity of n‐hexadecane, becomes. Also, the wegreater analyzed the latent the decomposition heat capacity temperatures becomes. Also, and we the analyzedtotal decomposition the decomposition rates of Mixed temperatures SSPCMs throughand the TGA total analysis. decomposition As a result, rates we of confirmed Mixed SSPCMs that the throughcoconut oilTGA and analysis.n-hexadecane As a wereresult, impregnated we confirmed into xGnP,that the and coconut the Mixed oil SSPCMs and n‐hexadecane have high thermal were impregnateddurability. The into thermal xGnP, conductivities and the Mixed of the SSPCMs Mixed SSPCMs have high analyzed thermal by TCi durability. were over The 284% thermal higher conductivitiesthan those of pure of the coconut Mixed oil SSPCMsn-hexadecane. analyzed Consequently, by TCi were we over expect 284% the Mixedhigher SSPCMsthan those to be of useful pure coconutin applications oil n‐hexadecane. in various Consequently, fields due to their we highexpect thermal the Mixed performances. SSPCMs to For be useful example, in applications considering intheir various phase fields change due temperatures, to their high thermal they can performances. be applied to For improve example, energy considering loss in building their phase design change and temperatures,operation in countries they can with be aapplied contained, to improve microthermal energy climate. loss in building design and operation in countries with a contained, microthermal climate. Acknowledgments: This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No.NRF-2014R1A2A1A11053829). Acknowledgments: This work was supported by the National Research Foundation of Korea(NRF) grant fundedAuthor by Contributions: the Korea government(MSIP)Hwayoung Lee, (No.NRF Su-Gwang‐2014R1A2A1A11053829). Jeong, Seong Jin Chang, Yujin Kang and Seunghwan Wi designed and conducted the experiment. Sumin Kim designed the project. All authors contributed to the analysis Authorand conclusion. Contributions: Hwayoung Lee, Su‐Gwang Jeong, Seong Jin Chang, Yujin Kang and Seunghwan Wi designed and conducted the experiment. Sumin Kim designed the project. All authors contributed to the analysis Conflicts of Interest: The authors declare no conflict of interest. and conclusion.

Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2016, 6, 106 9 of 10

Abbreviations The following abbreviations are used in this manuscript: PCM Phase change material SSPCM Shape-stabilized phase change material xGnP Exfoliated graphite nano platelets TES Thermal energy storage SHS Sensible heat storage LHTES Latent heat thermal energy storage SEM Scanning electron microscopy FT-IR Fourier transform infrared spectroscopy DSC Differential scanning calorimetry TGA Thermogravimetric analysis

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