Synthesis and characterization of microencapsulated

Applied Energy 203 (2017) 677–685

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Synthesis and characterization of microencapsulated myristic acid–palmitic acid eutectic mixture as phase change material for thermal energy storage Guruprasad Alva, Xiang Huang, Lingkun Liu, Guiyin Fang ⇑ School of Physics, Nanjing University, Nanjing 210093, China

h i g h l i g h t s Myristic acid–palmitic acid eutectic was microencapsulated with silica shell. Structure, morphology of microencapsulated phase change material were investigated. Thermal capacity, stability of microencapsulated phase change material were analyzed. Silica shell improved thermal stability of microencapsulated phase change material.

a r t i c l e

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Article history: Received 1 February 2017 Received in revised form 16 June 2017 Accepted 27 June 2017

Keywords: Microencapsulation Phase change material (PCM) Myristic acid–palmitic acid eutectic (MA–PA) mixture Microencapsulated phase change material (MPCM) Thermal storage capacity Thermal energy storage (TES)

a b s t r a c t In this work microencapsulation of myristic acid–palmitic acid (MA–PA) eutectic mixture with silica shell using solgel method has been attempted. The core phase change material (PCM) for thermal energy storage was myristic acidpalmitic acid eutectic mixture and the shell material to prevent the PCM core from leakage was silica prepared from methyl triethoxysilane (MTES). Thermal properties of the microcapsules were measured by differential scanning calorimeter (DSC). The morphology and particle size of the microcapsules were examined by scanning electronic microscope (SEM). Fourier transformation infrared spectrophotometer (FT–IR) and X–ray diffractometer (XRD) were used to investigate the chemical structure and crystalloid phase of the microcapsules respectively. The DSC results indicated that microencapsulated phase change material (MPCM) melts at 46.08 °C with a latent heat of 169.69 kJ kg1 and solidifies at 44.35 °C with a latent heat of 159.59 kJ kg1. The thermal stability of the microcapsules was analyzed by a thermogravimeter (TGA). The results indicated that the MPCM has good thermal stability and is suitable for thermal energy storage application. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Energy conservation with continuous improvement in efficiency is the key to achieving humanity’s goal of harvesting and consuming clean energy with minimal environmental pollution. Thermal energy storage is an important function in the field of energy conservation. It serves the purpose of smoothening fluctuations in gap between energy availability and demand for energy [1]. Thermal energy storage comes into play at different temperature ranges. At the high temperature side it is employed in electrical power generation, industrial waste heat recovery etc. At the lower temperature side it is employed in thermal comfort applications like heating and cooling of buildings, thermal comfort textile ⇑ Corresponding author. E-mail address: [email protected] (G. Fang). http://dx.doi.org/10.1016/j.apenergy.2017.06.082 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved.

etc. Using the latent heat capacity of a class of materials called phase change materials (PCM), to store thermal energy is a common technique due to their high density of energy storage capacity at the required operating temperature range. Phase change materials have certain undesirable properties like poor thermal conductivity and leakage upon melting. Microencapsulation enhances thermal and mechanical properties of phase change materials. Microencapsulation of phase change materials has been achieved through both organic and inorganic shell materials. Currently organic shell materials are more common than inorganic shell materials. List of organic shell materials include gelatin with gum arabic as binding material [2,3] and polymers like PMMA [4–7], polystyrene [8], melamine–formaldehyde [9–11], urea– formaldehyde [12], polyurea [13], polyurethane [14], etc. There is also organic shell material which is co–polymers made up of combination of more than one organic polymer [15–19].

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Similarly, of the different class of organic phase change materials that are microencapsulated, n–alkane (paraffin) class materials are the most commonly used. Paraffin materials are chemically inert and they are insoluble in water which is favorable to microencapsulation process. Few examples of paraffin class of materials which have been microencapsulated in the past include n–octadecane [3,9,10,13,17,18], n–hexadecane [3,5,19], n–nonadecane [3], Rubitherm [4,16], n–heptadecane [6,8,19], n–eicosane [11], n–tetradecane [15,19], n–pentadecane [19] etc. Few examples of organic non–paraffin class of materials which have been microencapsulated in the past include xylitol [14], butyl stearate [20] and many fatty acids like capric acid [21], lauric acid [21], myristic acid [21], palmitic acid [22] and stearic acid [23] etc. Currently there are many different techniques used for microencapsulation with organic shell materials. These include physical methods like spray-drying [2], complex coacervation [2,3] or chemical methods like suspension polymerization [4], emulsion polymerization [5–8], in–situ polymerization [9–12], interfacial polymerization [13] etc. Organic eutectic mixtures have been microencapsulated in the past, but very rarely. One such rare example of microencapsulation of fatty acid eutectic is Sari et al. [24] in which they microencapsulated capric acid–stearic acid eutectic mixture with an organic shell material PMMA. Organic eutectic mixtures have advantages like sharp congruent melting process. They do not have issues like phase segregation and supercooling. Fatty acid eutectics have advantages like easy availability and favorable melting temperature range for low temperature thermal comfort applications. Therefore microencapsulation of fatty acid eutectic will be of great advantage to thermal energy storage. Organic shell materials are usually toxic and flammable, and compared to inorganic polymers, they have poor heat transfer performance and thermal stability. Due to these disadvantages of organic shell materials, recently inorganic polymer materials such as silica [23,25–28], titanium dioxide [22,29] etc. have got more attention as shell materials. Most common microencapsulation technique for inorganic shell materials is the sol–gel process [22,23,25–28] which is a chemical method. However there are also few examples of using physical methods like spray–drying [29]. Microencapsulation of inorganic PCM is extremely rare as inorganic PCM are soluble in water. However there are few rare examples of microencapsulating inorganic PCM [30]. Microencapsulated PCM has applications in both passive and active thermal energy storage systems. Their main applications include usage in passive thermal energy storage systems of buildings, textile, automobile interior, medical products etc. They are also used in microencapsulated phase change material slurry for use as heat transfer fluid in active systems [31]. Thermal inertia of the buildings can be increased by incorporating phase change materials. Increased thermal inertia of the buildings results in reduced temperature fluctuations inside buildings when the temperature outside fluctuates during the day and night times. MPCM can be embedded into flooring, drywalls, concrete, ceilings, panels, gypsum boards, insulation panels, wallboards etc. MPCM has an edge over direct impregnation of phase change material, because porous nature of building materials like mortar can lead to leakage issues when the temperature is above the melting point of PCM. Phase change materials like fatty acids are chemically affected by the alkalinity of some concretes due to presence of Ca(OH)2. Similarly thermo-regulating textiles also use MPCM to improve human thermal comfort in extreme cold weathers [32]. MPCM can also be embedded into polyurethane composite foams used for insulation purpose in automobile interiors and medical storage boxes etc [33]. In HVAC systems for building thermal comfort applications, typical heat transfer fluids like water has a melting point of 0 °C. This is very low to be able to utilize its latent heat for thermal stor-

age. At the human thermal comfort temperature range where most thermal comfort applications operate, water only has sensitive heat capacity which is very low compared to its latent heat. MPCM enhances the heat capacity of water at the required operating temperature and improves heat transfer coefficient. Microencapsulated PCM slurry can serve the purpose of both heat transfer medium [34] and thermal energy storage medium. The direct use of phase change material in PCM slurries can result in clogging of heat transfer ducts due to solidification and agglomeration of PCM at low temperatures. Microencapsulation resolves this problem by avoiding direct contact between different PCM droplets. In this work sol–gel method is used with myristic acid–palmitic acid eutectic mixture as core phase change material (PCM) and silica prepared from methyl triethoxysilane (MTES) as shell material. Fatty acid eutectics provide us with greater flexibility in adjusting the operating temperature through combination of different pure components. They also have sharp melting feature with almost no supercooling. They are economical and easily available. However currently there still exists a knowledge gap with respect to microencapsulation process of fatty acid eutectic mixtures. Unlike paraffin, fatty acids are chemically reactive due to their acidic nature which can affect the microencapsulation process. Moreover, eutectic PCM microencapsulation has an additional stage of eutectic preparation process which has been covered extensively in this work. This work also generates new FT–IR and XRD characterization data for myristic acid–palmitic acid eutectic mixture. Microencapsulation of fatty acid eutectic with inorganic shell material has never been attempted in the past. In the past works, inorganic silica shell has been synthesized for various PCM materials with compounds like tetraethoxysilane (TEOS) and sodium silicate. In this work a new compound, methyl triethoxysilane (MTES) was used for generating silica shell. 2. Synthesis and characterization 2.1. Materials Eutectic mixture of myristic acid (C14H28O2, tetradecanoic acid, analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) and palmitic acid (C16H32O2, hexadecanoic acid, analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) was used as thermal storage material. Methyl triethoxysilane (Reagent grade, Tokyo Chemical Industry Co., Ltd.) was used to prepare silica. Anhydrous ethanol (Reagent grade, Sinopharm Chemical Reagent Co., Ltd) and distilled water were used as the solvent. Hydrochloric acid (Reagent grade, Nanjing Chemical Reagent Co., Ltd.) was used to adjust pH value. Sodium dodecyl sulfate (SDS; Reagent grade, Shanghai Chemical Reagent Co., Ltd.) was used as the oil–water emulsifier. 2.2. Preparation of the MA–PA eutectic mixture Myristic acid (MA) and palmitic acid (PA) were blended together to form eutectic mixture. Following equation was used for theoretically estimating the eutectic point [35].

T¼

1 1 TA

A Rlnx DHA

ð1Þ

where T, T A , xA , DHA and R represent melting temperature of eutectic mixture, melting temperature of component A, molar fraction of component A, enthalpy of component A and gas constant respectively. Based on Eq. (1), the melting temperature of the MA–PA eutectic was calculated for different MA–PA mass ratios. Results are shown in Fig. 1. For ease of calculation, the molar fraction has been


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Fig. 1. MA–PA eutectic melting point curve.

transformed into mass ratio. Two curves of theoretical melting point calculation based on the Eq. (1) for myristic acid (MA) and palmitic acid (PA) intersect at a MA–PA mass ratio of 60:40 indicating a eutectic point at that mass ratio. This was then verified with experimental data for MA–PA mixture samples with different mass ratios of MA. A total of 12 data points were chosen for experimental data and samples were prepared in water bath with different mass ratios (MA: 0%, 20%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 80% and 100%) at 70 °C and the samples were stirred with a magnetic stirrer at a rate of 500 rpm for 30 min. Out of them one sample was pure palmitic acid (PA), one sample was pure myristic acid (MA), one sample was for 20% MA mass fraction and another sample was for 80% MA mass fraction. Remaining 8 sample MA mass fractions were carefully chosen at close intervals near the theoretically estimated eutectic point. The thermal properties of different mixtures were measured by the differential scanning calorimeter (DSC) and the results are shown in Fig. 2 and Table 1.The experimental data as shown in Fig. 1 indicated lowest melting temperature at the theoretical eutectic point confirming the theoretical estimation. Mariana et al. [36] in their work on different mixtures of fatty acids also arrived at similar result with closely matching eutectic point and melting temperature for MA–PA eutectic. The melting temperature and latent heat value of pure MA were 54.55 °C and 196.58 kJ kg1 respectively, while the melting temperature and latent heat of pure PA were 62.81 °C and 208.48 kJ kg1 respectively. At eutectic point, the melting temperature and latent heat of MA–PA eutectic were 45.48 °C and 176.12 kJ kg1 respectively.

2.3. Preparation of the MA–PA eutectic oil–water emulsion 10 g of eutectic mixture (MA–PA mass ratio as 60:40) and 0.8 g of the SDS were added into 170 ml of distilled water to form MA– PA eutectic–water (O/W) emulsion. The mixture was stirred by a magnetic stirrer at 75 °C for 60 min. The stirring speed was 800 rpm. Generally in sol–gel method the pH value of the oil–water emulsion would have been adjusted to 9–10 by adding an alkaline solution like ammonia [27]. However in this work since the phase change materials are fatty acids this would lead to reaction between ammonia and fatty acids forming soap solution. Therefore in this work the pH control of oil–water emulsion using an alkaline solution like ammonia was skipped. MA–PA eutectic was evenly scattered in distilled water to form a stable oil–water emulsion which was mildly acidic with a pH value around 6.

Fig. 2. DSC curves for different MA–PA mass ratio samples (a) melting and (b) solidifying.

2.4. Preparation of the MPCM In this work three different MPCM samples were prepared with varying core PCM to shell material ratio to study the impact on performance of the MPCM. As shown in Table 2, different mass ratios of the MTES, anhydrous ethanol and distilled water were mixed together in a beaker to form a mildly acidic solution with a pH value around 6. Ethanol was used as a solvent for MTES. Hydrochloric acid was added into the mixture to adjust the pH value to 2–3. The mixture was stirred constantly at 75 °C for 30 min. The stirring speed was kept at 400 rpm. In this process, the hydrolysis reaction of the MTES happens and produces an intermediate methyl silanol sol solution as microencapsulation precursor. The reaction is represented in Scheme 1 [27] as shown below. MA–PA eutectic oil–water emulsion is now maintained at a reduced stirring speed of 300 rpm and at the same temperature of 75 °C. The methyl silanol sol solution was added to the MA–PA eutectic oil–water emulsion drop by drop and then the emulsion was kept stirring for 2 h. In this process, the silica shell was formed on the surface of the MA–PA eutectic droplets by the condensation reactions of the methyl silanol. The final shell material is a siloxane product with Si-O-Si linkage. The silica shell formation reaction is represented in Scheme 2. Finally, the microcapsules were collected by filtering, and dried at 45 °C for 24 h in vacuum oven. Three different MPCM samples of microcapsules were named as MPCM1, MPCM2 and MPCM3.

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Table 1 DSC data of PA, PCM1–PCM10 and MA. Samples

PA PCM1 PCM2 PCM3 PCM4 PCM5 PCM6 PCM7 PCM8 PCM9 PCM10 MA

Melting

Solidifying

Onset temperature (°C)

Peak temperature (°C)

Latent heat (kJ kg1)




62.81 47.79 45.33 45.42 45.25 45.59 45.48 45.71 45.49 45.8 45.57 54.55

64.82 49.18 49.48 49.84 48.99 49.45 48.66 48.85 47.96 48.2 48.04 56.34

208.48 190.99 175.34 176.14 176.70 175.76 176.12 176.48 175.16 175.39 171.58 196.58

60.68 54.41 45.6 45.7 45.36 45.10 44.89 44.68 44.30 44.16 45.01 52.93

59.52 53.12 44.06 44.07 44.25 43.53 43.59 43.02 42.92 42.65 43.29 51.56

209.87 189.62 174.75 174.14 174.73 174.72 174.98 173.80 173.08 173.10 170.38 198.72

Mass ratio of MA (%)

0 20 52 54 56 58 60 62 64 66 80 100

Table 2 Composition of the MA–PA eutectic emulsion and MTES solution. Samples

MA–PA eutectic emulsion

MTES solution

MPCM1

10 g MA–PA eutectic + 0.8 g SDS + 170 mL distilled water

10 g MTES + 10 g ethanol + 25 mL distilled water 15 g MTES + 15 g ethanol + 30 mL distilled water 20 g MTES + 20 g ethanol + 40 mL distilled water

MPCM2 MPCM3

2.5. Characterization of the MPCM The chemical structure analysis of the MPCMs was performed using a Fourier transformation infrared spectrophotometer (FT– IR, Nicolet Nexus 870, spectra from 400 to 4000 cm1 with a resolution of 2 cm1 using KBr pellets). The crystalloid phase of the MPCMs was measured by X–ray diffractometer (XRD, D/MAX– Ultima III, Rigaku Corporation, Japan) with continuous scanning mode at a rate of 5 (2h) °/min and operating conditions of 40 kV and 40 mA. The morphology of the MPCMs was examined using a scanning electron microscope (SEM, S–3400NII, Hitachi Inc., Japan). The samples were first washed in anhydrous ethanol to dissolve any MA–PA eutectic with no shell formation and then dried on tinplate sheets. The thermal properties of the MPCMs were measured by differential scanning calorimeter (DSC, Pyris 1 DSC,

Fig. 3. FT–IR spectra for (a) MA–PA eutectic, (b) silica, (c) MPCM1, (d) MPCM2 and (e) MPCM3.

Perkin–Elmer) at 5 °C/min under a constant stream of argon. The DSC instrument accuracy for temperature measurements was ±0.2 °C and for enthalpy measurements was ±5%. The thermal stability of the MPCMs was measured by a thermogravimeter (Pyris 1 TGA, Perkin–Elmer) from 25 °C to 700 °C with a linear heating rate of 20 °C/min under a constant stream of nitrogen.

Scheme 1. The hydrolysis reaction of the MTES.

Scheme 2. The condensation reactions of the methyl silanol.


Fig. 4. XRD patterns for (a) MA–PA eutectic, (b) silica, (c) MPCM1, (d) MPCM2 and (e) MPCM3.

3. Results and discussion 3.1. FT–IR analysis of the MPCM Fig. 3 presents the FT–IR spectra of the MA–PA eutectic, silica, MPCM1, MPCM2 and MPCM3. In Fig. 3a, absorption bands at

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2917 cm1 to 2850 cm1 are due to the stretching vibration of the CAH bond in ACH2 and ACH3 groups of fatty acids. Absorption bands at 1471 cm1 and 1307 cm1 are attributed to the deformation vibration of CAH bond in ACH2 group of fatty acids. Absorption band at 721 cm1 belongs to the rocking vibration of CAH bond in ACH2 group. These absorption bands indicate long chain alkane structure. Absorption band at 1700 cm1 is due to in–plane bending of hydroxyl (AOH) group of fatty acids. Absorption bands at 937 cm1 and 688 cm1 are attributed to out–of– plane bending of hydroxyl (AOH) group of water molecules. Fig. 3b shows the FT–IR spectrum of the methyl triethoxysilane. Absorption bands at 2980 cm1 and 2920 cm1 represent the symmetric stretching vibration of CAH bond in ACH3 and AC2H5 groups. Absorption bands at 1410 cm1 and 1270 cm1 represent the deformation vibration of CAH bond in ACH3 and AC2H5 groups. These absorption bands indicate the presence of methyl and ethyl groups of methyl triethoxysilane. Absorption bands at 766 cm1 and 418 cm1 represent the bending vibration of the SiAO group. These absorption bands indicate the presence of silica in methyl triethoxysilane. Fig. 3c–e display the FT–IR spectra of the MPCM1, MPCM2 and MPCM3 samples. The spectra of all the three MPCM samples retained all the original characteristic absorption bands of the fatty acids and silica. This indicates that there is no chemical interaction between the fatty acids and the silica. With increase in shell material to core PCM mass ratio, it can be clearly seen that absorption band near 770 cm1 representing SiAO group becomes more dominant indicating a thicker shell.

Fig. 5. SEM images of (a) MPCM1 (5k), (b) MPCM1 (3k), (c) MPCM2 (10k), (d) MPCM2 (1k), (e) MPCM3 (10k) and (f) MPCM3 (4k).

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G. Alva et al. / Applied Energy 203 (2017) 677–685 Table 4 Comparison of the present work with results of the other MPCMs in literature. MPCMs

Melting point (°C)

Melting latent heat (kJ kg1)

Reference

Myristic acid (MA)/polystyrene Palmitic acid (PA)/titania Stearic acid/tetraethoxysilane (TEOS) CA–SA (capric–stearic) eutectic /PMMA n–Octadecane/sodium silicate Paraffin/tetraethoxysilane (TEOS) n–Octadecane/titania n–Octadecane/calcium carbonate MA–PA eutectic/silica

47.5 61.7 53.5 21.37 27.96 58.37 25.68 29.19 46.08

98.26 63.3 171 116.25 87.46 165.68 42.57 84.37 169.69

[21] [22] [23] [24] [25] [26] [37] [38] Present study

3.3. Morphology of the MPCM SEM photographs of surface morphology of the MPCM1, MPCM2 and MPCM3 samples are shown in Fig. 5. Table 2 gives PCM to shell ratios of three different MPCM samples. Surface morphology of MPCM1 is shown in Fig. 5a–b. MPCM1 has a PCM to shell ratio of 1:1. The SEM image shows microcapsules with diameters between 5 lm to 11 lm. The shell surface is smooth. MPCM1 microcapsules have a consistent spherical shape. Surface morphology of MPCM2 is shown in Fig. 5c–d. MPCM2 has a PCM to shell ratio of 2:3. The shell surface is slightly rough compared to MPCM1. However SEM image shows MPCM2 microcapsules with similar size as MPCM1 and a consistent spherical shape. Surface morphology of MPCM3 is shown in Fig. 5e–f. MPCM3 has a PCM to shell ratio of 1:2. The shell surface is very rough. Although SEM image shows MPCM3 microcapsules with similar size as MPCM1 and MPCM2, but they do not have a consistent spherical shape. Fig. 6. DSC curves of MA–PA eutectic mixture and MPCM samples (a) melting and (b) solidifying.

3.2. XRD analysis of the MPCM Fig. 4 presents the XRD patterns of the MA–PA eutectic, silica, MPCM1, MPCM2 and MPCM3. The XRD pattern of the MA–PA eutectic is shown in Fig. 4a. Intensity peaks of MA–PA eutectic can be seen at 2h values of 13.24°, 20.26°, 21.55°, 23.77°, 29.32 °and 40.72°. The peaks at 21.55° and 23.77° are of relatively high intensity, while others are relatively small intensity peaks. Fig. 4b shows the XRD curve for silica shell. Silica shell has an amorphous structure. Therefore it does not have sharp intensity peaks. In Fig. 4c–e, it can be seen that the XRD peaks of the three MPCM samples have retained all the characteristic peaks of the fatty acids. With the decrease in PCM to shell ratio, the intensity of the peaks decrease but they still remain at the same position. This indicates that there is no change in crystal structures of both MA–PA eutectic and silica.

3.4. Thermal properties of the MPCM Fig. 6 shows DSC curves for MA–PA eutectic and MPCM samples. Table 3 contains DSC data of the MA–PA eutectic and MPCM samples. Supercooling behavior of the MPCMs can be analyzed from DSC data. As shown in Table 3, the melting temperatures of the MA–PA eutectic, MPCM1, MPCM2 and MPCM3 are 45.48 °C, 46.08 °C, 45.4 °C and 45.09 °C respectively. The solidifying temperatures of the MA–PA eutectic, MPCM1, MPCM2 and MPCM3 are 44.89 °C, 44.35 °C, 44.38 °C and 44.22 °C respectively. The degree of supercooling (DT) for MA–PA eutectic is 0.59 °C. The degree of supercooling (DT) for MPCM1, MPCM2 and MPCM3 are 1.73 °C, 1.02 °C and 0.87 °C respectively. Therefore there is a slight increase in degree of supercooling after microencapsulation. The degree of supercooling (DT) decreases with increase in thickness of the shell. Overall the degree of supercooling is less than 2 degree which is within the acceptable limit. Based on DSC data analysis, a satisfactory sample can be chosen from the three different MPCM samples. Table 3 data shows that latent heat of the MPCM1 (169.69 kJ kg1) is higher than that of

Table 3 DSC data of the MA–PA eutectic and MPCMs. Samples

MPCM1 MPCM2 MPCM3 MA–PA eutectic

Melting

Solidifying







46.08 45.4 45.09 45.48

48.58 48.45 48.43 48.66

169.69 126.83 116.12 176.12

44.35 44.38 44.22 44.89

43.47 43.54 43.29 43.59

159.59 104.17 106.74 174.98

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3.5. Thermal stability of the MPCM TGA and DTG curves of MA-PA eutectic and MPCM samples are shown in Fig. 7. TGA and DTG data of MA–PA eutectic and MPCM samples are also shown in Table 5 which lists the initial decomposition temperature, maximum weight loss temperature, final decomposition temperature, maximum decomposition rate and the charred residue amount. In the TGA curve shown in Fig. 7a, the MA–PA eutectic has negligible mass loss up till 166.66 °C and a visible weight loss begins at this point. In the DTG curve shown in Fig. 7b the weight loss rate for MA–PA eutectic increases rapidly from the initial decomposition temperature of 166.66 °C, then it reaches a maximum at 259.60 °C and finally wanes after reaching final decomposition temperature of 270.49 °C. Similarly the weight loss rate for MPCM1, MPCM2 and MPCM3 also start increasing from 164.7 °C, 163.62 °C and 163.49 °C respectively and reaches a maximum at 256.30 °C, 249.79 °C and 253.09 °C respectively. It can be seen that start point of the degradation for both PCM and MPCM samples is almost same, indicating that weight loss is mainly due to evaporation of PCM and shell material is stable at that point. The silica shell is expected to remain stable up to 700 °C. In this work the expected operating temperature of MPCM1 sample is around 46.08 °C which is its melting point. At this temperature according to TGA data, the residual weight percentage of MPCM1 sample is 99.637%. Similarly at the melting point of the MPCM2 which is 45.4 °C, according to TGA data the residual weight percentage of MPCM2 sample is 99.981%. At the melting point of the MPCM3 which is 45.09 °C, according to TGA data the residual weight percentage of MPCM3 sample is 99.961%. At a higher operating limit of 80 °C the residual weight percentage of MPCM1, MPCM2 and MPCM3 samples are 99.22%, 99.758% and 99.961% respectively. There for at the expected operational temperature, mass loss for MPCM is negligible and the initial thermal degradation of the MPCM samples begins at around 164 °C which is high above the operational temperature. Therefore MPCMs have a good thermal stability in their operational temperature. Fig. 7. Thermal degradation of MA–PA eutectic and MPCM samples (a) TGA and (b) DTG.

the MPCM2 (126.83 kJ kg1) and MPCM3 (116.12 kJ kg1) but lower than that of MA–PA eutectic (176.12 kJ kg1). This is due to the fact that mass fraction of the MTES in the MPCM1 is lower than that of the MTES in the MPCM2 and MPCM3. Since only the MA–PA eutectic absorbs and releases thermal energy during the phase change process, larger the mass ratio of the MA–PA eutectic in the microcapsules, larger the thermal energy storage capacity of the MPCMs. The excess silica gel deposits on the microcapsules shells of the MPCM2 and MPCM3, and their latent heats decrease accordingly. Therefore, MPCM1 is selected as the satisfactory sample. MPCM1 has a melting temperature of 46.08 °C with a latent heat of fusion of 169.69 kJ kg1 and a solidifying temperature of 44.35 °C with a latent heat of solidification of 159.59 kJ kg1. Table 4 presents the comparison of the MPCM1 with other PCMs in literature. It can be seen that the MPCM1 has high latent heat value in comparison to the rest.

3.6. Operational parameters of the satisfactory MPCM sample From an application point of view, the operational parameters for the chosen satisfactory MPCM sample are provided in Table 6. There are many possible applications for a MPCM with a charging

Table 6 Operational parameters for the satisfactory MPCM sample. Application parameter

Unit of measurement

Value

Charging temperature Charging capacity Discharging temperature Discharging capacity Energy storage efficiency Maximum operating temperature

°C kJ kg1 °C kJ kg1 % °C

46.08 169.69 44.35 159.59 94.04 80

Remarks

per unit mass per unit mass 0.78% PCM weight loss

Table 5 TGA data of the MA–PA eutectic and MPCMs. Samples

Initial decomposition temperature (°C)

Maximum weight loss temperature (°C)

Final decomposition temperature (°C)

Maximum decomposition rate (%/min)

Charred Residue (at 700 °C) (%)

MA–PA eutectic MPCM1 MPCM2 MPCM3

166.66 164.7 163.62 163.49

259.60 256.30 249.79 253.09

270.49 290.83 302.04 286.57

48.61 37.85 25.27 24.95

0 0.966 12.896 23.855

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and discharging range near 45 °C. This MPCM can be used in thermal energy storage system for domestic solar hot water supply systems. Such systems operate in the temperature range between 40 °C and 100 °C. It can also be embedded into floor tiles [39], wallboard panels [40] etc. to keep the home warm in cold regions. Homes having such floor tiles, wallboard panels etc can reduce their electricity cost by running heaters during off-peak hours and switching off during peak hours. In industry also waste heat recovered at a low temperature range like 40–100 °C, can be used by space heating and hot water supply systems and this MPCM can be used in thermal energy storage for such systems. 4. Conclusions In this work, MA–PA eutectic was successfully microencapsulated with inorganic silica shell using sol–gel method. Silica was synthesized using methyl triethoxysilane (MTES) compound. The MA–PA eutectic was well microencapsulated inside the silica shell. SEM images showed microcapsules with diameters in 5–11 lm range. FT-IR and XRD analysis indicated that there was no chemical reaction between the MA–PA eutectic and silica. Morphology of the shell surface was smooth for an appropriate core PCM to shell material mass ratio of 1:1 and the microcapsules were consistently spherical. The satisfactory MPCM sample melts at 46.08 °C with a latent heat of 169.69 kJ kg1 and solidifies at 44.35 °C with a latent heat of 159.59 kJ kg1. Supercooling was minimal with a gap of 1.73 °C between melting and solidifying temperatures. The TGA and DTG analysis showed that the samples had good thermal stability at the expected operational temperature range. MA–PA eutectic MPCM with silica shell was found to be suitable for thermal energy storage. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant nos. 51676095, 51376087). The authors also wish to thank the reviewers and editor for kindly giving revising suggestions. References [1] Alva G, Liu LK, Huang X, Fang GY. Thermal energy storage materials and systems for solar energy applications. Renew. Sustain. Energy Rev. 2017;68:693–706. [2] Hawlader MNA, Uddin MS, Khin MM. Microencapsulated PCM thermal–energy storage system. Appl. Energy 2003;74:195–202. [3] Onder E, Sarier N, Cimen E. Encapsulation of phase change materials by complex coacervation to improve thermal performances of woven fabrics. Thermochim. Acta 2008;467:63–72. [4] Al–Shannaq R, Kurdi J, Al–Muhtaseb S, Dickinson M, Farid M. Supercooling elimination of phase change materials (PCMs) microcapsules. Energy 2015;87:654–62. [5] Alay S, Alkan C, Göde F. Synthesis and characterization of poly (methyl methacrylate)/n–hexadecane microcapsules using different cross–linkers and their application to some fabrics. Thermochim. Acta 2011;518:1–8. [6] Sari A, Alkan C, Karaipekli A. Preparation, characterization and thermal properties of PMMA/n–heptadecane microcapsules as novel solid–liquid microPCM for thermal energy storage. Appl. Energy 2010;87:1529–34. [7] Sari A, Alkan C, Karaipekli A, Uzun O. Microencapsulated n–octacosane as phase change material for thermal energy storage. Sol. Energy 2009;83:1757–63. [8] Sari A, Alkan C, Doguscu DK, Bicer A. Micro/nano–encapsulated n–heptadecane with polystyrene shell for latent heat thermal energy storage. Sol. Energy Mater. Sol. Cells 2014;126:42–50. [9] Fan YF, Zhang XX, Wang XC, Li J, Zhu QB. Super–cooling prevention of microencapsulated phase change material. Thermochim. Acta 2004;413:1–6. [10] Zhang HZ, Wang XD. Fabrication and performances of microencapsulated phase change materials based on n–octadecane core and resorcinol–modified melamine–formaldehyde shell, Colloids and Surfaces A: Physicochem. Eng. Aspects 2009;332:129–38.

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