Tang 2014

Accepted Manuscript

Title: Preparation and thermal properties of stearicacid/titanium dioxide composites as shape-stabilized phasechange materials for building thermal energy storage

Author: Fang Tang Lei Cao Guiyin Fang

PII: S0378-7788(14)00434-4DOI: http://dx.doi.org/doi:10.1016/j.enbuild.2014.05.030Reference: ENB 5066

To appear in: ENB

Received date: 18-1-2014Revised date: 5-4-2014Accepted date: 26-5-2014

Please cite this article as: F. Tang, L. Cao, G. Fang, Preparation and thermalproperties of stearic acid/titanium dioxide composites as shape-stabilized phasechange materials for building thermal energy storage, Energy and Buildings (2014),http://dx.doi.org/10.1016/j.enbuild.2014.05.030This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Preparation and thermal properties of stearic acid/titanium 1

dioxide composites as shape-stabilized phase change materials 2

for building thermal energy storage3

4

Fang Tang, Lei Cao, Guiyin Fang*5

School of Physics, Nanjing University, Nanjing 210093, China6

*Corresponding author. Tel: +86 25 51788228, Fax: +86 25 835937077

E-mail address: [email protected]

9

Abstract: In this study, stearic acid (SA) /titanium dioxide (TiO2) composites with 10

different mass ratios were prepared by mixing titania powder with stearic acid-water 11

emulsion. In the composites, the SA performed as phase change material for thermal 12

energy storage, and TiO2 was used as supporting material. The thermal properties of 13

the composites, such as phase change temperature and phase change latent heat, were 14

measured by differential scanning calorimetry (DSC). Fourier transformation infrared 15

spectroscope (FT-IR) analyses indicated that there was no chemical interaction during 16

the preparation process. X-ray diffractometer (XRD) and scanning electronic 17

microscope (SEM) were used to survey crystalloid phase and microstructure of the 18

SA/TiO2 composites. Besides, the thermal reliability of the composites was 19

investigated by a thermogravimetric analyzer (TGA). The satisfactory SA/TiO220

composite with 33% mass ratio of the SA melts at 53.84 with a latent heat of 21

47.82 kJ/kg and solidifies at 53.31 with a latent heat of 46.60 kJ/kg. Due to its 22

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non-inflammability, nontoxicity and good thermal stability, the composite can be used 23

as shape-stabilized phase change materials for building thermal energy storage.24

25

Keywords: Composite thermal energy storage materials; Thermal properties;26

Shape-stabilized; Stearic acid; Titanium dioxide; Building energy conservation27

28

1. Introduction29

In recent decades, the increasing energy consumption has been a big challenge 30

for the development of human society. Therefore, the investigations of new energy 31

resources and efficient ways of energy storage and recovery have attracted much 32

attention all over the world [1]. Latent heat thermal energy storage (LHTES) is one of 33

the most efficient ways to store thermal energy. In the LHTES system, thermal energy 34

is stored in phase change materials (PCMs) during a melting process while it is 35

recovered during a freezing process. It is due to the fact that the PCMs have great 36

endothermic and exothermic capacity during the phase change process [2]. The PCMs 37

can be applied in various fields according to their phase change temperatures, such as 38

air-conditioning systems [3, 4], building energy conservation [58], solar heating 39

systems [913].40

Currently, various PCMs such as inorganic salt hydrates [14], nalkanes [15], 41

fatty acids [16, 17] have been studied for LHTES application. Among all these PCMs, 42

fatty acids have advantages in proper melting temperature range, lower vapor pressure, 43

small volume change, etc. [18]. However, two disadvantages are also obvious when 44

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PCMs are used in thermal energy storage applications. One is that the low thermal 45

conductivity of most PCMs restricts the transmission of heat during phase change 46

process and the other is the liquid migration during solidliquid melting process. 47

Therefore, microencapsulation and combining PCMs with other supporting materials 48

as shapestabilized composites are developed [19, 20]. Many organic and inorganic 49

materials are chosen as supporting materials so far, such as opal [21], 50

styrenebutadienestyrene copolymer [22], expanded graphite [23], molecular sieve 51

[24] and calcium carbonate [25]. The inorganic materials are superior to organic ones 52

in buildings because most of the latter are flammable and toxic.53

The previous researches are commonly using organic polymers as supporting 54

materials [26, 27]. However, the application of PCMs with polymeric supporting55

materials is usually restricted due to their toxicity, flammability, low heat conductivity 56

and poor thermal stability [28]. So some inorganic supporting materials such as SiO257

[29] are used in composite PCMs recently, but the preparation and properties of 58

composite PCMs with titanium dioxide shells is little reported up to now.59

In recent decades, many efforts have been done to develop PCM in buildings [30, 60

31]. The aim of this work is to analyze the thermal properties of stearic acid/titanium 61

dioxide composites as shape-stabilized phase change materials for building thermal 62

energy storage.63

In this paper, the preparation and thermal properties of the SA/ TiO2 composites 64

as shape-stabilized phase change materials for building thermal energy storage are 65

presented. In the composites, the SA is used as phase change material for thermal66

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energy storage, and the TiO2 acts as supporting material. The satisfactory composite67

with 33% mass ratio of the SA melts at 53.84 with a latent heat of 47.82 kJ/kg and 68

solidifies at 53.31 with a latent heat of 46.60 kJ/kg. The composites have a good 69

thermal stability, and can be used as shape-stabilized phase change materials for70

building thermal energy storage.71

72

2. Experiments73

2.1 Materials74

Stearic acid (Reagent grade, Jiangsu Huakang Chemical Reagent Company) was 75

used as thermal storage material. Titanium dioxide (Reagent grade, Jiangsu Yonghua 76

Chemical Reagent Company) acted as the supporting material and deionized water 77

was used as solvent.78

79

2.2 Preparation of the SA/ TiO2 composites80

The SA and deionized water were mixed together in a beaker with different mass 81

ratios, as showed in Table 1. The experimental parameters used for this preparation82

process were determined according to preliminary experimental results and our 83

previous research [34]. The mixture was mixed uniformly by stirring at 75 for 3084

min with a magnetic stirrer in order to form an oil/water (O/W) emulsion. Then titania 85

powder was added into the emulsion while the stirring was kept until the emulsion 86

became viscous. Finally, the composites were dried at 48 for 24 h in vacuum oven. 87

Five kinds of the SA/ TiO2 composites were acquired, and were named as CPCM1, 88

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CPCM2, CPCM3, CPCM4 and CPCM5.89

Table 190

91

2.3 Characterization of the SA/TiO2 composites92

The chemical structure analysis of the SA/TiO2 composites was conducted on a 93

Fourier transformation infrared spectroscope (FT-IR, Nicolet Nexus 870, spectra from 94

400 to 4000 cm-1 with a resolution of 2 cm-1 using KBr pellets). The crystalloid phase 95

of the CPCMs was measured by an X-ray diffractometer (XRD, D/MAX-Ultima III, 96

Rigaku Corporation, Japan) with continuous scanning mode at a rate of 5 (2)/min 97

and operating conditions of 40 kV and 40 mA. The morphology and microstructure of 98

the SA/TiO2 composites were measured by a scanning electron microscope (SEM, 99

S-3400NII, Hitachi Inc., Japan) at room temperature condition. Before the SEM test, 100

the composites were dried in vacuum oven. During the SEM test, the sample space 101

was vacuumized. Then, the morphology and microstructure of the composites could 102

be observed. The thermal properties of the SA/TiO2 composites was determined by 103

differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin-Elmer) under a constant 104

stream of argon. The accuracy of temperature measurements was 0.2 and the 105

enthalpy accuracy was 5%. Indium was used as standard for temperature calibration. 106

The samples were put in aluminium pans that were hermetically sealed before being 107

placed on the calorimeter thermocouples. The sample space was cooled by a 108

two-stage compression refrigeration system. The cooling rate was 5 /min from 90109

to 10 , the heating rate was 5 /min from 10 to 90 . At first, the DSC 110

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cell containing a CPCM sample was cooled to a lower temperature than the melting 111

temperature of the sample. The heating block was heated at a constant rate, the 112

temperature of the reference sample pan also increased at a constant rate. If there was 113

no phase change in the CPCM sample pan, the temperature difference between the 114

CPCM sample and the reference sample pan produced an almost horizontal straight 115

line. If there was a phase change in the CPCM sample pan, the temperature difference 116

between the two pans followed a curve that deviated from the straight line. In DSC 117

test, the melting and solidifying temperatures and their corresponding latent heats of 118

the CPCMs are not affected by heating and cooling rate. The heating and cooling rate119

with 10 /min or 20 /min is usually used. Therefore, the heating and cooling rate 120

with 5 /min is appropriate in this experiment, which can ensure the accuracy of 121

measurement. Lower heating and cooling rate such as for 2 /min or 0.5 /min 122

may improve DSC resolution, but the testing time is longer. The thermal stability of 123

the SA/TiO2 composites was investigated by a thermogravimetric analyzer (Pyris 1 124

TGA, Perkin-Elmer) from 25 to 700 with a linear heating rate of 20/min 125

under a constant stream of nitrogen.126

127

3. Results and discussion128

3.1 FT-IR analysis of the SA/TiO2 composites129

Fig. 1 displays the FT-IR spectra of the SA, TiO2, CPCM1, CPCM2, CPCM3, 130

CPCM4 and CPCM5. The spectrum of the SA is showed in Fig. 1a, and two strong 131

absorption peaks at 2917cm-1 and 2849cm-1 are attributed to the asymmetric and 132

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symmetric stretching vibrations of its CH2 group. The stretching vibration of C=O 133

group corresponds to the strong absorption peak at 1700cm-1. The absorption peaks 134

around 1471cm-1 presents the bending vibration of CH3 groups. The absorption peaks 135

from 1400cm-1 to 1180cm-1 present a series of long-chain aliphatic characteristic136

peaks of the rocking vibration of CH2 group. In Fig. 1b, the characteristic absorption 137

peak at around 500cm-1 belongs to the Ti-O-Ti functional group. The spectra of the 138

CPCMs in Fig. 1 include all the characteristic peaks of the SA and TiO2. The results139

indicate that there is no chemical interaction between the SA and TiO2.140

Fig. 1141

142

3.2 XRD analyses of the SA/TiO2 composites143

Fig. 2 presents XRD patterns of the SA, TiO2 and CPCMs. Peaks at 21.5, 23.9144

of the SA in Fig. 2a are due to its regular crystallization. In Fig. 2b, peaks at 25.3, 145

37.0, 37.8, 38.6, 48.0, 53.8 and 55.0 represent the characteristic peaks of anatase 146

titanium dioxide. As shown in Fig. 2, XRD patterns of the CPCMs contain both the 147

peaks of the SA and TiO2.This result signifies the crystal structure of the SA and TiO2148

remains unchanged.149

Fig. 2150

151

3.3 Microstructure of the SA/TiO2 composites152

Fig. 3 shows SEM photographs of the TiO2, CPCM1, CPCM2, CPCM3, CPCM4 153

and CPCM5. The structure of the TiO2 is clearly observed in Fig. 3a. It is obvious that 154

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they are submicron particles. As shown in Fig. 3b-3d, the SA with different mass 155

ratios (5:1, 4:1 and 3:1) is uniformly dispersed in TiO2. It is because of the high 156

wetting ability of the SA. The TiO2 as supporting material prevents the leakage of 157

melted SA. In Fig. 3e and Fig. 3f, the leakage of the SA is because the mass of the 158

TiO2 is relatively small and the electric beam makes the SA melt when carrying out 159

SEM.160

Fig. 3161

162

3.4 Thermal properties of the SA/TiO2 composites163

Figs. 4 and 5 and Table 2 illustrate DSC results of the SA, CPCM1, CPCM2, 164

CPCM3, CPCM4 and CPCM5. In order to determine the influence of the supporting 165

materials on the latent heat of the composites, the latent heat data of the CPCMs are 166

compared with calculated ones according to Eq. (1). CPCMH and SAH represent 167

calculated latent heat of the CPCMs and measured latent heat of the SA, respectively. 168

is the mass fraction of the SA in the composites. The latent heat of the CPCMs 169

could be computed from the obtained DSC curves. The area between the straight line 170

and the DSC curve represents the energy consumed for the phase change, which is 171

integrated numerically by a program built into the DSC. Both the phase transition 172

temperature and the latent heat of phase change were recorded during a heating scan 173

by the DSC. The melting temperature of the CPCMs can be determined by drawing a 174

tangent through the point of maximum slope on the melting front peak. The 175

intersection of the tangent and the extrapolated baseline is the melting point, and the 176

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freezing temperature of the CPCMs can be also obtained by drawing a tangent 177

through the point of maximum slope on the freezing front peak. The intersection of 178

the tangent and the extrapolated baseline is the freezing point. The experimental latent 179

heat data of the CPCMs accord with calculated ones with the discrepancy no more 180

than 5%.181

SACPCM HH

(1)182

As shown in Table 2, the differences of melting temperature between SA and 183

CPCMs are less than 1.75%, and the differences of solidifying temperature between 184

SA and CPCMs are less than 1.24%. The results indicate the CPCMs and SA have 185

similar phase transition characteristics. Besides, the differences between melting186

temperature and solidifying temperature of the SA, CPCM1, CPCM2, CPCM3, 187

CPCM4 and CPCM5 are 1.06 , 1.21 , 0.48 , 0.53 , 0.13 and 0.32 . It 188

means that the TiO2 used as supporting material decreases the supercooling degree of 189

the CPCM2, CPCM3, CPCM4 and CPCM5 compared with SA during the190

solidification process.191

In latent heat thermal energy storage, the phase change latent heat can be utilized 192

during phase change process. In this composite, since only SA absorbs and releases 193

thermal energy during the phase change process, the larger mass ratio of the SA in the 194

composites will increase the thermal energy storage capacity of the composites.195

However, as shown in Fig. 3e and Fig. 3f, the leakage of the SA in CPCM4 and 196

CPCM5 indicates that the CPCM4 and CPCM5 are not ideal as thermal energy storage 197

material. So CPCM3 with 33% mass ratio of the SA is chosen as a satisfactory one. 198

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The melting temperature of the CPCM3 is 53.84 with a latent heat of 47.82 kJ/kg 199

and the solidifying temperature of CPCM3 is 53.31 with a latent heat of 46.60200

kJ/kg.201

The CPCM3 can be utilized in under-floor heating systems for thermal energy 202

storage in buildings. The under-floor heating systems can achieve significant 203

economic benefits because they can be charged by using cheap night-time electricity 204

and discharge the heat stored at the daytime to achieve the shift of electricity 205

consumption from peak period to off-peak period. The CPCM3 can also be used as 206

thermal energy storage material in exterior wallboard of buildings, which can absorb 207

heat of exterior wallboard from the surrounding air and solar radiation heat during 208

summer day, then release heat of exterior wallboard to the surrounding air at night. 209

Therefore, the cooling load of air-conditioning system may be decreased [32]. In 210

addition, the CPCM3 may be used as part of the buildings for heating applications in 211

solar-aided latent heat storage system.212

The CPCM3 floor can be achieved by incorporating CPCM3 into a hollow213

concrete floor panel. In order to obtain the CPCM3 wallboard, the SA is impregnated 214

into the TiO2 using fusion adsorption method, the obtained composite PCMs are then 215

incorporated into construction materials such as gypsum, cement, concrete and bricks.216

The incorporated PCMs can increase the thermal mass of the wall so as to decrease 217

the indoor temperature swings to increase the thermal comfort.218

Table 3 presents the comparison of the SA/TiO2 composites with results of other 219

supporting material in literature. The difference between melting temperature and 220

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solidifying temperature of the SA/TiO2 composites is much smaller than the others, so 221

the composites have a much smaller supercooling degree. Although the melting latent 222

heat of the CPCM3 is smaller than that of other PCM composites in Table 3, the 223

charging temperature (melting point) of the CPCM3 is nearly close to the discharging 224

temperature (solidifying point). It is helpful for temperature control and regulation of 225

the under-floor heating systems and PCM wallboards. This result indicates that TiO2226

as supporting material can improve the nucleation characteristics of the CPCM3 by 227

depressing its supercooling degree. So, the TiO2 is also good nucleation agents for 228

PCM during phase change process.229

Fig. 4230

Fig. 5231

Table 2232

Table 3233

234

3.5 Thermal stability of the SA/TiO2 composites235

Figs. 6 and 7 display TGA and DTG curves of the SA and CPCMs. Table 4236

exhibits the temperature of maximum weight loss and the charred residue amount of 237

the SA and CPCMs. The difference of the charred residue amount among the SA and 238

CPCMs is due to the fact that the phase change material content in the CPCMs is 239

different. As shown in Fig. 7, the only one thermal decomposition process of the SA 240

and CPCMs from 150 to 300 is due to thermal decomposition of the SA. 241

According to Fig. 6 and Fig. 7, the velocity of weight loss of the SA is much higher 242

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compared with CPCMs. This is because the TiO2 as supporting material builds up a 243

protective barrier on the surface of the composites. The barrier makes it harder for the244

flammable molecules to become gas phase, and blocks the transmission of heat flow 245

and diffusion of oxygen to the condensed phase [36]. This indicates that the 246

composites have a good thermal stability.247

It is found from these results that the TiO2 as supporting material can improve the 248

thermal stability of the CPCMs and prevent the leakage of the melted SA. So, the 249

CPCMs have good durability. The thermal cycles of the CPCMs are also important for 250

thermal energy storage applications in buildings. The thermal cycles of the CPCMs 251

will be investigated in practical building thermal energy storage.252

Fig. 6253

Fig. 7254

Table 4255

256

4. Conclusions257

Preparation and thermal properties of the SA/ TiO2 composites as phase change 258

materials for building thermal energy storage are reported. In the composites, the SA259

is used as PCM for thermal energy storage, and the TiO2 acts as supporting material. 260

The SA can be uniformly dispersed in TiO2 without chemical interaction, it is due to 261

the high wetting ability of the SA. In the composites, the crystal structure of the SA262

and TiO2 remains unchanged. The satisfactory CPCM with 33% mass ratio of the SA263

melts at 53.84 with a latent heat of 47.82 kJ/kg and solidifies at 53.31 with a 264

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latent heat of 46.60 kJ/kg. The composites have a good thermal stability, and can be 265

used as shape-stabilized phase change materials for building thermal energy storage.266

267

Acknowledgements268

This work was supported by the National Natural Science Foundation of China 269

(Grant no. 51376087) and the Priority Academic Program Development of Jiangsu 270

Higher Education Institutions.271

272

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391

Figure captions392

Fig. 1 FT-IR spectra of the (a) SA, (b) TiO2, (c) CPCM1, (d) CPCM2, (e) CPCM3, (f)393

CPCM4 and (g) CPCM5.394

Fig. 2 XRD patterns of the (a) SA, (b) TiO2, (c) CPCM1, (d) CPCM2, (e) CPCM3, (f)395

CPCM4 and (g) CPCM5.396

Fig. 3 SEM images of the (a) TiO2 (5k), (b) CPCM1 (5k), (c) CPCM2 (5k), (d) 397

CPCM3 (5k), (e) CPCM4 (5k) and (f) CPCM5 (5k).398

Fig. 4 The melting DSC curves of the SA and CPCM1-CPCM5.399

Fig. 5 The solidifying DSC curves of the SA and CPCM1-CPCM5.400

Fig. 6 TGA curves of the SA and CPCM1-CPCM5.401

Fig. 7 DTG curves of the SA and CPCM1-CPCM5.402

403

404

Table captions405

Table 1 The compositions of the SA/TiO2 composites.406

Table 2 DSC data of the SA, CPCM1, CPCM2, CPCM3, CPCM4 and CPCM5.407

Table 3 Comparison of the SA/TiO2 composites with results of other supporting 408

material in literature.409

Table 4 TGA data of the SA, CPCM1, CPCM2, CPCM3, CPCM4 and CPCM5.410

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Tables with Captions411

412

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Table 1 The compositions of the SA/TiO2 composites.414

Samples Compositions

CPCM1 40 g SA + 500g deionized water + 200g TiO2





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Table 2 DSC data of the SA, CPCM1, CPCM2, CPCM3, CPCM4 and CPCM5.418

Melting SolidifyingSample name SA and TiO2

mass ratio Temperature

()Latent heat

(kJ/kg)

Temperature

()Latent heat

(kJ/kg)

CPCM1 1:5 53.780.2 28.611.4 52.570.2 26.561.3

CPCM2 1:4 53.960.2 36.051.8 53.480.2 33.491.7

CPCM3 1:3 53.840.2 47.822.4 53.310.2 45.602.3

CPCM4 1:2 53.340.2 62.193.1 53.470.2 60.603.0

CPCM5 1:1 53.810.2 91.474.6 53.490.2 89.474.5

SA 1:0 54.290.2 188.289.4 53.230.2 180.079.0

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Table 3 Comparison of the SA/TiO2 composites with results of other supporting 423

material in literature.424

Composites Melting point

()

Solidifying

point ()

Melting latent

heat (kJ/kg)

Reference

stearic acid(60wt%)/halloysite

nanotube composite

53.46 49.21 93.97 [33]

stearic acid(83wt%)/expanded

graphite composite

53.12 54.28 155.50 [34]

stearic acid(46wt%)/silica

fume composite

58.8 52.4 82.53 [35]

stearic acid(25wt%)/TiO2 53.84 53.31 47.82 present

study

425

Table 4 TGA data of the SA, CPCM1, CPCM2, CPCM3, CPCM4 and CPCM5.426

Samples Temperature of maximum

weight loss ()Charred residue amount (%)

(700 )CPCM1 476.2 84.4

CPCM2 405.1 80.9

CPCM3 402.5 76.4

CPCM4 464.0 68.3

CPCM5 407.5 50.1

SA 653.0 0

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Highlights

The stearic acid/TiO2 composites with different mass ratios were prepared.

Chemical structure, crystalloid phase and microstructure of the CPCMs were analyzed.

Thermal properties and stability of the CPCMs were investigated.

The TiO2 can improve thermal stability of the CPCMs.

*Highlights (for review)

Documents

Tang 2014