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modified sepiolite composite as a form-stable phase change material for thermal energy storage
Applied Clay Science 146 (2017) 14–22
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Research paper
Lauric acid/modified sepiolite composite as a form-stable phase change material for thermal energy storage
MARK
Qiang Shena, Jing Ouyanga,b, Yi Zhanga,b, Huaming Yanga,b,c,⁎ a b c
Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Hunan Key Lab for Mineral Materials and Application, Central South University, Changsha 410083, China State Key Lab of Powder Metallurgy, Central South University, Changsha 410083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Sepiolite Composite phase change materials Lauric acid Modification Thermal energy storage
A series of novel composite phase change materials (PCMs) were prepared by impregnating lauric acid (LA) into the chemically modified sepiolite (SEP) via a vacuum impregnation method. Modification strategy was developed to improve the adsorption capacity of SEP, and the effects of thermal and chemical modification on the physical and chemical properties of SEP were investigated. The loading of LA inside the acid treated SEP could reach up to 60 wt%, which was 50% higher than that of pristine SEP. The corresponding latent heats of the composite PCMs exhibited 125.2 J/g at the melting temperatures of 42.5 °C and 113.9 J/g at the freezing temperatures of 41.3 °C, respectively. The increased latent heat could be attributed to the better microstructure of the modified SEP. The thermal conductivity (0.59 W/(m·k)) of the composite PCMs was higher than that of LA. The composite PCMs presented chemical and thermal reliability after 200 thermal cycling tests. The form-stable composite PCMs could be the promising candidate material for thermal energy storage.
1. Introduction Developments in renewable and sustainable energy have been of prime significance after the oil crisis in the 1970s. Thermal energy storage (TES) has proved to be a low-cost and promising technique for energy saving efficiency improvement (Ding et al., 2016a,b; Li and Wu, 2012; Niu et al., 2016a,b), which also in turn mitigates the energy consumption. For the latent heat of TES, phase change material (PCM) has become an attractive option due to its repeatable utilization property, constant heat source temperature, high heat recovery, and high energy storage density (Rathod and Banerjee, 2013; Shu et al., 2017; Yan et al., 2017). PCM also has been widely used in many fields such as insulation clothing, building energy conservation, air condition systems, solar energy storage, and waste heat recovery (Hou et al., 2017; Kuznik et al., 2011; Peng et al., 2016a,b). In recent years, many PCMs have been widely researched for application in energy conservation buildings (Jin et al., 2017; Peng et al., 2017a, 2017b; Sharma et al., 2013). Building materials including PCMs allow the TES, thus achieving spatial and temporal transfer of energy (Pielichowska and Pielichowski, 2014). During the development of building materials including PCMs, the leakage of PCMs and the thermal transfer between the ambient environment and PCMs are the main difficulties (Chen et al., 2015; He et al., 2016; Ouyang et al.,
⁎
2016). So the form-stable PCMs have been greatly investigated in these years. The form-stable PCM could be prepared via impregnating PCM into the supports like bentonite, expanded perlite, diatomite, vermiculite, and clay mineral (Fu et al., 2017a,b; Karaipekli and Sarι, 2016; Liu and Yang, 2015; Lv et al., 2017; Memon et al., 2013; Sarι et al., 2014; Song et al., 2014a). Despite the innovations made in the previous investigations, most of researchers have continued to use paraffin as PCM in energy storage systems (Shen et al., 2016; Tang et al., 2015). Other PCMs suitable for application in low-temperature heat storage systems (40–75 °C) require further research before practical use. Among the phase transformation matrix for PCMs, lauric acid (LA) possesses a melting point in the desired operating temperature range of application in building field (melting/freezing at 41–45 °C), and exhibits little supercooling during the freezing process and large latent heat with respect to thermal cycling for thermal energy storage in the long term. Additionally, LA has reliable chemical stability with small volume changes during phase transition. So, LA has been considered as a desirable candidate for thermal energy storage systems (Liu and Yang, 2016; Liu et al., 2017; Wen et al., 2016). On the other hand, porous supports have been used in the building field in recent years. Sepiolite (SEP) is considered to be special mineral with its fibrous structure (Konuklu and Ersoy, 2016; Ma and Zhang, 2016; Yang et al., 2016). This unique crystal structure is
Corresponding author at: Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (H. Yang).
http://dx.doi.org/10.1016/j.clay.2017.05.035 Received 14 April 2017; Received in revised form 21 May 2017; Accepted 24 May 2017 0169-1317/ © 2017 Published by Elsevier B.V.
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Fig. 1. SEM images of (a) SEP, (b) CSEP, (c) NSEP and (d) HSEP. TEM images of (e) SEP and (f) HSEP.
China. Before the composite PCMs preparation, SEP was thermally treated at 400 °C for 2 h to obtain CSEP. SEP was immersed in sufficient amount of 10 wt% sodium solution at 30 °C for 2 h to obtain NSEP. SEP was acid-leached in 10 wt% HCl solution at 30 °C for 2 h to form HSEP.
responsible for its important characteristic related to adsorption active centres, dehydration, porosite, and surface area. SEP has good physical properties like non-toxicity, porous structure and fire resistance. And it has great chemical compatibility with organic PCM (Li et al., 2016; Ying and Zhang, 2016). However, SEP has its shortcoming that the pores of natural clay mineral are commonly blocked by impurities. Hence, raw SEP needs to be modified before commercial utilization. Recently, clay minerals could be functionalized upon proper modification, which increased the compatibility with matrix (Zhang et al., 2016a,b,c, 2017; Zhou et al., 2016). Currently, modified SEP supported PCMs for thermal energy storage have been little documented. In this paper, SEP was first treated by calcination, alkali leaching and hydrochloric acid treatment. Then, LA was absorbed in raw and modified SEP by the vacuum impregnation method. Finally, the characterization and properties of the composites were determined. The results indicated that the form-stable composite will be a potential candidate for thermal storage application.
2.2. Preparation of the form-stable composite PCMs The composite PCMs were prepared by vacuum impregnation method. Firstly, LA was placed in a conical flask with HSEP, the conical flask was heated at 80 °C and the vacuum was evacuated to − 0.1 MPa for 40 min. Then, the vacuum pump and air entry was closed to force LA to penetrate into HSEP, with ultrasonic heating at 80 °C for 5 min. To remove excess LA by thermal filtration, the composite PCMs were maintained at 80 °C for 48 h as the mass did not decrease longer. After cooling, the mixture was ground to obtain the composite PCMs. LA with different mass ratio (30%, 40%, 50%, and 60%) is dispersed in HSEP, and the composites were denoted as HS-LA1, HS-LA2, HS-LA3 and HSLA4, respectively.
2. Experimental 2.3. Characterization 2.1. Materials The SEM images of the samples were obtained using a JEOL JSM6360LV SEM. The TEM images were collected with a JEOL JEM-2100F TEM. FTIR (Nicolet 5700) spectra of the samples were investigated in the range of 450–4000 cm− 1. The XRD (RigakuD/max 2550) experi-
Lauric acid (LA, C12H24O2) was analytically pure and supplied by Tianjin Kemiou Chemical Reagent Co., Ltd., China. Natural sepiolite (SEP) was collected from mineral deposit located in Hunan Province in 15
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Table 1 Chemical compositions of SEP before and after modification (wt%). Samples
SiO2
CaO
MgO
Al2O3
Fe2O3
P2O3
TiO2
K2O
Na2O
F
SEP GSEP NSEP HSEP
51.28 51.45 50.58 72.62
27.27 26.20 25.80 0.07
16.56 17.44 17.06 21.23
2.87 2.87 2.77 3.83
0.96 0.91 0.94 1.23
0.17 0.14 0.15 0.01
0.13 0.15 0.10 0.15
0.13 0.14 0.14 0.21
0.10 0.13 0.28 0.09
0.41 0.43 0.49 0.51
Fig. 2. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of SEP, CSEP, NSEP and HSEP.
under N2 flow. TGA (STA8000) was investigated at a heating rate of 10 °C/min up to 600 °C in N2 atmosphere. Thermal conductivities (TC 3000) of the samples were measured by hot-wire method, and 200 accelerated thermal cycles were performed in a thermostatic chamber
ments were performed on the samples at a scan rate of 3°/min in the range of 5–80°. N2 adsorption-desorption isothermals (ASAP 2020) were performed at 77 K. DSC analysis (DSC21400A-0211-L) was investigated in the range of 30–80 °C at a heating rate of 5 °C/min
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originally clogged pores were opened. It is a good phenomenon for the porous SEP as more LA can be loaded. The changes of acid treatment indicated that metallic oxides were the main impurity which plugged up the pore. XRF was applied to confirm the observation. The main chemical compositions of raw SEP and modified SEP were listed in Table 1, SEP was mainly composed of 51.28 wt% SiO2, 27.27 wt% CaO and 16.56 wt% MgO. After the treatment of calcination and alkali leaching, it had no obvious change of the contents of ingredients. However, after the purification of acid treatment, the contents of SiO2 (72.62 wt%) and MgO (21.23 wt%) significantly increased and CaO (0.07 wt%) nearly disappeared, indicating that the hydrochloric acid treatment is an efficient way to remove the impurity which clogs the pore. Fig. 2 illustrates the N2 adsorption-desorption isotherms of raw and modified SEP, the porous textures were summarized in Table 2. The N2 adsorption-desorption isotherms of raw and modified SEP were similar (Fig. 2a), indicating that the microstructure of SEP was maintained after modification treatment. The N2 adsorption-desorption isotherms of raw and modified SEP were classified as H3 hysteresis loops. The hysteresis was related to the capillary condensation of the filling and emptying of mesopores, showing that many mesopores existed in the supports (Yu et al., 2015). In addition, the N2 adsorption quantity increased sharply, indicating the existence of micropores (Liu et al., 2012). This feature demonstrated the coexistence of mesopore and micropore structures, which was in accord with the SEM and TEM results. The specific surface area and pore volume had no obvious change after the calcination and alkali leaching treatment (Table 2). However, three parameters were improved obviously in HSEP as the micropore size was reduced and the mesopore and macropore size increased, which resulted in the increase of the surface area. The pore size distributions of HSEP showed a narrow range of 2–20 nm and the pore diameter shifted to larger diameter (Fig. 2b), which is desirable for
Table 2 Textural properties of raw SEP and modified SEP. Properties
SEP
CSEP
NSEP
HSEP
Specific surface area (m2/g) Pore volume (cc/g) Mean pore diameter (nm)
51.58 0.12 9.46
52.99 0.14 12.69
53.92 0.14 13.82
76.08 0.29 15.53
with the temperature controller. 3. Results and discussion 3.1. Characterization of modified sepiolite Sepiolite (SEP) powder is mainly composed of fibrous structure, some pores can be seen. However, the fiber bundles are agglutinated as a bed and many pores on the surface of SEP were blocked by impurities (Fig. 1a). No obvious improvement was found after calcination treatment (Fig. 1b) and the fibrous structure changed a little after alkali leaching (Fig. 1c). But after acid treatment, the fiber bundles became much more loose while the morphology of the fibrous structure was basically retained. In addition, there were fewer impurities and much more pores on the surface of SEP (Fig. 1d). TEM was applied to confirm the observations from SEM. The thick fiber bundles of raw SEP stick together (Fig. 1e), which was in accord with the SEM observation. After treatment with hydrochloric acid, the fiber bundles became more dispersed than raw SEP (Fig. 1f). Because the acid treatment produced a very strong discontinuity in both octahedral and tetrahedral sheets, causing SEP to transform phyllosilicate-like structure into an inosilicate-like structure (Yebra et al., 2003). A great number of pores can be obviously seen in Fig. 1f. Due to the reaction between metallic oxides and hydrochloric acid, pore edges were enlarged gradually and
Fig. 3. (a, b) XRD patterns and (c, d) FTIR spectra of raw and modified SEP, LA and the composite PCMs.
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Fig. 4. SEM images of (a) HS-LA1, (b) HS-LA2, (c) HS-LA3 and (d) HS-LA4 composite PCMs.
Fig. 5. TEM images of (a) HSEP and (b) HS-LA4, and (c, d) corresponding EDS analysis, respectively.
its role as a PCM carrier.
7.2° and 26.5°, indicating the modification treatment didn't change the crystalline structure of SEP. A large number of calcite crystalline phases was also found in SEP, CSEP and NSEP, but had not appeared in HSEP, which indicated that the impurity was removed after acid treatment, which was well agreement with the microstructure observation and XRF analysis. Fig. 3b shows the XRD pattern of LA and the composite
3.2. Analysis of the as-prepared form-stable composite PCMs XRD patterns of the raw and modified SEP were given in Fig. 3a, all the samples had the typical crystalline diffraction reflections of SEP at 18
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action. When LA was in the melting state during the SEM testing process, it was not leaked from the pores and surface of HSEP (Fig. 4d). So the maximum appropriate loading of LA in the form-stable composite PCMs (HS-LA4) could be 60%. Fig. 5 shows the TEM images and corresponding energy spectra of HSEP and HS-LA4. HSEP was nanotube structure, and the diameter of the fiber was about 18 nm (Fig. 5a). After impregnation, the pores and surface of HSEP were fully filled with LA by capillary force, and the diameter of fiber with LA increased up to 28 nm (Fig. 5b), which was in accordance with the SEM images. The elemental compositions of carbon in HSEP and HS-LA4 were confirmed to be 12.88 and 82.48 atomic percentage, respectively (Fig. 5c & d). These results obviously demonstrated the high percentage of LA in HS-LA4 form-stable composite PCMs.
Table 3 Thermal characteristics of LA and the as-prepared composite PCMs. LA mass ratio (wt%) Samples LA PCM HS-LA1 HS-LA2 HS-LA3 HS-LA4 S-LA
100 30 40 50 60 40
Melting process
Freezing process
Hm (J/g)
Tm (°C)
Hf (J/g)
Hf (°C)
225.4 58.4 81.5 106.7 125.2 82.6
43.2 42.4 42.5 42.5 42.5 42.5
194 54.3 74.6 94.1 113.9 75.4
41.1 41.5 41.5 41.4 41.3 41.4
PCMs. The reflections at 19.5° and 24.0° were assigned to LA crystals. All the intense and sharp diffraction reflections of LA appeared in the as-prepared composite PCMs, indicating that the crystal structure of LA was not destroyed during the preparation process. FTIR spectra of raw and modified SEP are shown in Fig. 3b. The bands at 796 cm− 1 and 1095 cm− 1 corresponded to the SieOeSi symmetric and asymmetric stretching vibrations, respectively. The band at 466 cm− 1 was due to the SieOeSi bending vibration. The band at 1420 cm− 1 was attributed to the [CO3]2 − internal stretching vibration. As the reaction between calcite and acid solution proceeded, the band disappeared in HSEP. The FTIR spectra of LA and the composites are shown in Fig. 3d. All of the bands of LA appeared in the spectrum of the composites, such as eCH2 asymmetric stretching vibration (2926 cm− 1), eCH2 symmetric stretching vibration (2841 cm− 1), and C]O vibration (1696 cm− 1). All of the HSEP bands were also observed in the spectrum, and no significant new bands were found. These results confirmed no chemical interaction between HSEP and LA. LA with different mixing ratio (30%, 40%, 50%, and 60%) was greatly impregnated in the support of HSEP (Fig. 4). The porous HSEP prevented the leakage of LA by the surface tension force and capillary
3.3. Thermal properties of the form-stable composite PCMs Thermal properties of all the composites, including melting/freezing temperature and latent heats, are summarized in Table 3. Raw SEP with maximum mass ratio of LA was prepared, and the composite PCM was denoted as S-LA. The maximum loading of LA in the HS-LA4 composites could be 60 wt%, and that of raw SEP was 40 wt%, which was increased by 50% (Table 3). The latent heats increased with the increase in the mass ratio of LA, because it is LA but not the supporting HSEP that undergoes the phase change process. So HS-LA4 can be unquestionably accounted as the promising thermal energy storage material. As the DSC curves shown in Fig. 6a, LA and the form-stable composites have similar melting and freezing processes. The optimal HS-LA4 had the melting latent heat of 125.2 J/g at 42.5 °C and the freezing latent heat of 113.9 J/g at 41.3 °C. Theoretical enthalpy of the composites could be determined by Eq. (1) (Qian et al., 2013):
Htheo = ηHLA
(1)
where Htheo is the theoretical enthalpy of the composites; η denotes the
Fig. 6. LA and composite PCMs: (a) DSC curves, (b) comparison of theoretical and actual enthalpies, (c) the relation between LA mass fraction and HHS-LA/HLA and (d) phase change temperatures.
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Table 4 Thermal property comparison between the as-prepares composite PCM in this work and the previously reported composite PCMs. PCM
Paraffin blends Stearic acid Lauric acid Paraffin PEG1000 Galactitol myristate Galactitol myristate Capic/myristic aicd Paraffin Capic/myristic aicd Lauric acid
Support
Opal Kaolin Kaolinite Diatomite Diatomite Diatomite Vermiculite Vermiculite Vermiculite Perlite Sepiolite
Max. ratio (%)
39 39 48 61 50 52 55 20 53 55 60
Melting process
Freezing process
Hm (J/g)
Tm (°C)
Hf (J/g)
Hf (°C)
49.1 66.3 72.5 89.5 87.1 83.8 92.2 27.5 101.1 85.4 125.2
32.8 53.3 43.7 33.0 27.7 44.9 45.9 19.8 48.9 21.7 42.5
48.4 65.6 70.9 89.8 82.2 81.1 92.5 31.4 103.0 89.8 113.9
47.3 52.7 39.3 52.4 32.2 44.3 44.6 17.1 53.0 20.7 41.3
Thermal conductivity (W/mK)
Ref.
– – 0.10 – 0.32 0.19 0.14 0.07 0.45 0.05 0.59
Sun et al., 2013a, b Liu and Yang, 2014 Song et al., 2014b Sun et al., 2013a, b Karaman et al., 2011 Sarı and Biçer, 2012 Sarı and Biçer, 2012 Karaipekli and Sarı, 2009 Guan et al., 2015 Karaipekli and Sarı, 2008 This work
Fig. 8. Thermal conductivity and melting/freezing process curves of LA and HS-LA4.
extents of supercooling could be defined as the difference between freezing and melting temperature. Compared with pure LA, the extent of supercooling in the composite PCMs was reduced by 0.9–1.2 °C (Fig. 6d), indicating that the extents of supercooling could be reduced by impregnating into HSEP support. Because the porous HSEP in the composite PCMs could offer the heat transmission route in LA and then increased the phase change rate (Parameshwaran et al., 2013). According to the thermal property comparison between HS-LA4 and the composite PCMs previously prepared in related references (Table 4), the as-prepared HS-LA4 had suitable melting temperature of 42.5 °C and freezing temperature of 41.3 °C. The form-stable composites demonstrated a relatively higher melting enthalpy of 125.2 J/g and freezing enthalpy of 113.9 J/g than that of other composites. In addition, the as-prepared materials had the larger loading of PCM (60%) than most of the composites. Therefore, the form-stable composites could be the promising thermal energy storage material for application in the building field.
Fig. 7. (a) TGA curves and (b) corresponding DTG thermograms of LA, S-LA and HS-LA4.
mass fraction of LA in the composites; HLA is the enthalpy of pristine. The theoretical enthalpies of the composites were slightly higher than the actual values (Fig. 6b). The reason could be that mesopores restricted the crystal orientation, the arrangement of LA and the steric drag effect of nano (Feng et al., 2011). And the melting enthalpy was always higher than the freezing enthalpy, it could be the increase of mass loss when the composites were heated during melting tests by DSC. The relations between LA mass ratio and HHS-LA/HLA of the composite PCMs are shown in Fig. 6c. The loading of LA was linearly correlated with the latent heat of the composites to certain extent. The
3.4. Thermal stability of the form-stable composite PCMs Thermal stability is an important parameter for thermal energy storage application. The weight loss of S-LA and HS-LA4 were 40.3% and 60.4% (Fig. 7a), which was close to the corresponding designed values (40% and 60%) and was in line with the DSC measurement. Moreover, the weight loss profile of HS-LA4 was almost overlapped with that of LA in the programmed temperature scope of 30–150 °C, suggesting that the form-stable composites had excellent thermal stability before 150 °C. LA exhibited the typical one-step degradation at 243 °C (Fig. 7b), indicating that pure LA experienced a simple 20
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Fig. 9. HS-LA4 after 200 thermal cycling: (a) SEM image, (b) XRD patterns, (c) FTIR spectra and (d) DSC curves.
influenced during thermal cycles. Therefore, the as-prepared formstable composite PCM had stable chemical structure after 200 thermal cycling tests. DSC measurement was performed to investigate the thermal stability (Fig. 9d). The melting temperature of HS-LA4 was changed by 0.1 °C after 200 thermal cycling tests. The changes in melting temperature were not of crucial magnitude for the application. The latent heat of HS-LA4 was changed by 3%. The decrease of latent heat was in a reasonable level, and it was still in a comparatively high level. The latent heats of melting and freezing of HS-LA4/LA composites after thermal cycling showed general advantages over the current PCMs such as paraffin/diatomite composite (Sun et al., 2013a, b), galactitol myristate/vermiculite composite (Sarı and Biçer, 2012), and capricmyristic aicd/expanded perlite composite (Karaipekli and Sarı, 2008). Therefore, the form-stable composite PCM had thermal reliability and could be used as a thermal energy storage material in the buildings.
evaporation. The sharp weight loss of HS-LA4 at 224 °C was attributed to the decomposition of organic ingredient of LA chains during the degradation process. HS-LA4 had the residue of 39.6 wt% at 600 °C, showing that the form-stable composite PCM was homogeneous. 3.5. Thermal conductivity of the form-stable composite PCMs Thermal conductivity is a significant indicator for the applicability of heat storage and release. The thermal conductivity of HS-LA4 was 0.59 W/(m·k), which was 1.8 time of that of pure LA (0.33 W/(m·k)). Compared with the other composite PCMs (Table 4), the as-prepared HS-LA4 composite PCM showed the relatively higher thermal conductivity. The improvement in thermal conductivity was verified by comparing the melting and freezing processes of HS-LA4 and that of the LA. The melting/freezing process was performed in the temperature scope of 20–40 °C. The melting and freezing periods of LA were 27 min and 24 min, and that of HS-LA4 were 14 min and 16 min, respectively (Fig. 8). The decreased time could be the result of the improvement of heat conduction speed during the thermal storage and release process, which indicated that the form-stable composite PCM had the high heat conduction speed.
4. Conclusion A series of methods were used to enhance the adsorption capacity of raw SEP. The modification effects including calcination, alkali leaching and hydrochloric acid treatment on SEP were studied. The maximum loading of LA in HS-LA4 composites could be 60 wt%, which was 50% higher than that of raw SEP. After acid treatment, the pores could be dredged and enlarged, thus leading to a larger pore size, higher BET area and change of the overall morphology. LA was well impregnated into the porous structure of HSEP, which had better compatibility, reduced supercooling extent, thermal stability and reliability. The formstable composite PCM had suitable melting/freezing temperature (42.5 °C/41.3 °C) and large latent heat of melting and freezing (125.2 J/g and 113.9 J/g), and exhibited a thermal conductivity of 0.59 W/(m·k) higher than that of pristine LA. Therefore, the form-stable composite PCM could be the promising thermal energy storage material
3.6. Thermal reliability of the form-stable composite PCM The form-stable composite PCM must be chemically and thermally stable. The fibrous structure of HSEP was entirely covered by LA and nearly no empty pores was observed (Fig. 9a). Moreover, no seepage of melted LA was found after the thermal cycling. The diffraction peaks nearly had no change before and after thermal cycles (Fig. 9b), indicating that the crystal structure of LA in HS-LA4 had not been affected. The frequency values and shapes of all the peaks had not been changed (Fig. 9c). The results indicated that no chemical structure was 21
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for application in the building field. Acknowledgements This work was supported by the National Science Fund for distinguished Young Scholars (51225403), the National “Ten Thousand Talents Program” in China (2016-37), the Strategic Priority Research Program of Central South University (ZLXD2017005), the Hunan Provincial Science and Technology Project (2016RS2004, 2015TP1006), the Innovation Driven Plan of Central South University (2016CX015), the ShengHua Scholar Project of CSU (2016) and the Fundamental Research Funds for the Central Universities of Central South University (2017zzts432). References Chen, Z., Su, D., Qin, M., Fang, G., 2015. Preparation and characteristics of composite phase change material (CPCM) with SiO2 and diatomite as endothermal-hydroscopic material. Energy Build. 86, 1–6. Ding, W., Yan, H., Ouyang, J., Long, H., 2016a. Modified wollastonite sequestrating CO2 and exploratory application of the carbonation products. RSC Adv. 6, 78090–78099. Ding, W., Ouyang, J., Yang, H., 2016b. Synthesis and characterization of nesquehonite (MgCO3·3H2O) powders from natural talc. Powder Technol. 292, 169–175. Feng, L., Zhao, W., Zheng, J., Frisco, S., Song, P., Li, X., 2011. The shape-stabilized phase change materials composed of polyethylene glycol and various mesoporous matrices (AC, SBA-15 and MCM-41). Sol. Energy Mater. Sol. Cells 95, 3550–3556. Fu, L., Yang, H., Hu, Y., Wu, D., Navrotsky, A., 2017a. Tailoring mesoporous γ-Al2O3 properties by transition metal doping: a combined experimental and computational study. Chem. Mater. 29, 1338–1349. Fu, L., Yang, H., Tang, A., Hu, Y., 2017b. Engineering a tubular mesoporous silica nanocontainer with well-preserved clay shell from natural halloysite. Nano Res. http://dx.doi.org/10.1007/s12274-017-1482-x. Guan, W., Li, J., Qian, T., Wang, X., Deng, Y., 2015. Preparation of paraffin/expanded vermiculite with enhanced thermal conductivity by implanting network carbon in vermiculite layers. Chem. Eng. J. 277, 56–63. He, X., Wang, J., Shu, Z., Tang, A., Yang, H., 2016. Y2O3 functionalized natural palygorskite as an adsorbent for methyl blue removal. RSC Adv. 6, 41765–41771. Hou, K., Wen, X., Yan, P., Tang, A., Yang, H., 2017. Tin oxide-carbon coated sepiolite nanofibers with enhanced lithium-ion storage property. Nanoscale Res. Lett. 215, 1–10. Jin, J., Ouyang, J., Yang, H., 2017. Pd nanoparticles and MOFs synergistically hybridized halloysite nanotubes for hydrogen storage. Nanoscale Res. Lett. 240, 1–9. Karaipekli, A., Sarı, A., 2008. Capric–myristic acid/expanded perlite composite as formstable phase change material for latent heat thermal energy storage. Renew. Energy 33, 2599–2605. Karaipekli, A., Sarı, A., 2009. Capric–myristic acid/vermiculite composite as form-stable phase change material for thermal energy storage. Sol. Energy 83, 323–332. Karaipekli, A., Sarι, A., 2016. Development and thermal performance of pumice/organic PCM/gypsum composite plasters for thermal energy storage in buildings. Sol. Energy Mater. Sol. Cells 149, 19–28. Karaman, S., Karaipekli, A., Sarı, A., Biçer, A., 2011. Polyethylene glycol (PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage. Sol. Energy Mater. Sol. Cells 95, 1647–1653. Konuklu, Y., Ersoy, O., 2016. Preparation and characterization of sepiolite-based phase change material nanocomposites for thermal energy storage. Appl. Therm. Eng. 107, 575–582. Kuznik, F., David, D., Johannes, K., Roux, J., 2011. A review on phase change materials integrated in building walls. Renew. Sust. Energ. Rev. 15, 379–391. Li, M., Wu, Z., 2012. A review of intercalation composite phase change material: preparation, structure and properties. Renew. Sust. Energ. Rev. 16, 2094–2101. Li, X., Yang, Q., Ouyang, J., Yang, H., Chang, S., 2016. Chitosan modified halloysite nanotubes as emerging porous microspheres for drug carrier. Appl. Clay Sci. 126, 306–312. Liu, S., Yang, H., 2014. Stearic acid hybridizing coal-series kaolin composite phase change material for thermal energy storage. Appl. Clay Sci. 101, 277–281. Liu, S., Yang, H., 2015. Composite of coal-series kaolinite and Capric–Lauric acid as formstable phase-change material. Energy Technol. 3, 77–83. Liu, S., Yang, H., 2016. Porous ceramic stabilized phase change materials for thermal energy storage. RSC Adv. 6, 48033–48042. Liu, D., Yuan, P., Tan, D., Liu, H., Wang, T., Fan, M., Zhu, J., He, H., 2012. Facile preparation of hierarchically porous carbon using diatomite as both template and catalyst and methylene blue adsorption of carbon products. J. Colloid Interface Sci. 388, 176–184. Liu, S., Yan, Z., Fu, L., Yang, H., 2017. Hierarchical nano-activated silica nanosheets for thermal energy storage. Sol. Energy Mater. Sol. Cells 167, 140–149. Lv, P., Liu, C., Rao, Z., 2017. Review on clay mineral-based form-stable phase change materials: preparation, characterization and applications. Renew. Sust. Energ. Rev. 68, 707–726. Ma, Y., Zhang, G., 2016. Sepiolite nanofiber-supported platinum nanoparticle catalysts toward the catalytic oxidation of formaldehyde at ambient temperature: efficient and stable performance and mechanism. Chem. Eng. J. 288, 70–78. Memon, S., Lo, T., Shi, X., Barbhuiya, S., Cui, H., 2013. Preparation, characterization and
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Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Lauric acid/modified sepiolite composite as a form-stable phase change material for thermal energy storage
MARK
Qiang Shena, Jing Ouyanga,b, Yi Zhanga,b, Huaming Yanga,b,c,⁎ a b c
Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Hunan Key Lab for Mineral Materials and Application, Central South University, Changsha 410083, China State Key Lab of Powder Metallurgy, Central South University, Changsha 410083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Sepiolite Composite phase change materials Lauric acid Modification Thermal energy storage
A series of novel composite phase change materials (PCMs) were prepared by impregnating lauric acid (LA) into the chemically modified sepiolite (SEP) via a vacuum impregnation method. Modification strategy was developed to improve the adsorption capacity of SEP, and the effects of thermal and chemical modification on the physical and chemical properties of SEP were investigated. The loading of LA inside the acid treated SEP could reach up to 60 wt%, which was 50% higher than that of pristine SEP. The corresponding latent heats of the composite PCMs exhibited 125.2 J/g at the melting temperatures of 42.5 °C and 113.9 J/g at the freezing temperatures of 41.3 °C, respectively. The increased latent heat could be attributed to the better microstructure of the modified SEP. The thermal conductivity (0.59 W/(m·k)) of the composite PCMs was higher than that of LA. The composite PCMs presented chemical and thermal reliability after 200 thermal cycling tests. The form-stable composite PCMs could be the promising candidate material for thermal energy storage.
1. Introduction Developments in renewable and sustainable energy have been of prime significance after the oil crisis in the 1970s. Thermal energy storage (TES) has proved to be a low-cost and promising technique for energy saving efficiency improvement (Ding et al., 2016a,b; Li and Wu, 2012; Niu et al., 2016a,b), which also in turn mitigates the energy consumption. For the latent heat of TES, phase change material (PCM) has become an attractive option due to its repeatable utilization property, constant heat source temperature, high heat recovery, and high energy storage density (Rathod and Banerjee, 2013; Shu et al., 2017; Yan et al., 2017). PCM also has been widely used in many fields such as insulation clothing, building energy conservation, air condition systems, solar energy storage, and waste heat recovery (Hou et al., 2017; Kuznik et al., 2011; Peng et al., 2016a,b). In recent years, many PCMs have been widely researched for application in energy conservation buildings (Jin et al., 2017; Peng et al., 2017a, 2017b; Sharma et al., 2013). Building materials including PCMs allow the TES, thus achieving spatial and temporal transfer of energy (Pielichowska and Pielichowski, 2014). During the development of building materials including PCMs, the leakage of PCMs and the thermal transfer between the ambient environment and PCMs are the main difficulties (Chen et al., 2015; He et al., 2016; Ouyang et al.,
⁎
2016). So the form-stable PCMs have been greatly investigated in these years. The form-stable PCM could be prepared via impregnating PCM into the supports like bentonite, expanded perlite, diatomite, vermiculite, and clay mineral (Fu et al., 2017a,b; Karaipekli and Sarι, 2016; Liu and Yang, 2015; Lv et al., 2017; Memon et al., 2013; Sarι et al., 2014; Song et al., 2014a). Despite the innovations made in the previous investigations, most of researchers have continued to use paraffin as PCM in energy storage systems (Shen et al., 2016; Tang et al., 2015). Other PCMs suitable for application in low-temperature heat storage systems (40–75 °C) require further research before practical use. Among the phase transformation matrix for PCMs, lauric acid (LA) possesses a melting point in the desired operating temperature range of application in building field (melting/freezing at 41–45 °C), and exhibits little supercooling during the freezing process and large latent heat with respect to thermal cycling for thermal energy storage in the long term. Additionally, LA has reliable chemical stability with small volume changes during phase transition. So, LA has been considered as a desirable candidate for thermal energy storage systems (Liu and Yang, 2016; Liu et al., 2017; Wen et al., 2016). On the other hand, porous supports have been used in the building field in recent years. Sepiolite (SEP) is considered to be special mineral with its fibrous structure (Konuklu and Ersoy, 2016; Ma and Zhang, 2016; Yang et al., 2016). This unique crystal structure is
Corresponding author at: Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (H. Yang).
http://dx.doi.org/10.1016/j.clay.2017.05.035 Received 14 April 2017; Received in revised form 21 May 2017; Accepted 24 May 2017 0169-1317/ © 2017 Published by Elsevier B.V.
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Fig. 1. SEM images of (a) SEP, (b) CSEP, (c) NSEP and (d) HSEP. TEM images of (e) SEP and (f) HSEP.
China. Before the composite PCMs preparation, SEP was thermally treated at 400 °C for 2 h to obtain CSEP. SEP was immersed in sufficient amount of 10 wt% sodium solution at 30 °C for 2 h to obtain NSEP. SEP was acid-leached in 10 wt% HCl solution at 30 °C for 2 h to form HSEP.
responsible for its important characteristic related to adsorption active centres, dehydration, porosite, and surface area. SEP has good physical properties like non-toxicity, porous structure and fire resistance. And it has great chemical compatibility with organic PCM (Li et al., 2016; Ying and Zhang, 2016). However, SEP has its shortcoming that the pores of natural clay mineral are commonly blocked by impurities. Hence, raw SEP needs to be modified before commercial utilization. Recently, clay minerals could be functionalized upon proper modification, which increased the compatibility with matrix (Zhang et al., 2016a,b,c, 2017; Zhou et al., 2016). Currently, modified SEP supported PCMs for thermal energy storage have been little documented. In this paper, SEP was first treated by calcination, alkali leaching and hydrochloric acid treatment. Then, LA was absorbed in raw and modified SEP by the vacuum impregnation method. Finally, the characterization and properties of the composites were determined. The results indicated that the form-stable composite will be a potential candidate for thermal storage application.
2.2. Preparation of the form-stable composite PCMs The composite PCMs were prepared by vacuum impregnation method. Firstly, LA was placed in a conical flask with HSEP, the conical flask was heated at 80 °C and the vacuum was evacuated to − 0.1 MPa for 40 min. Then, the vacuum pump and air entry was closed to force LA to penetrate into HSEP, with ultrasonic heating at 80 °C for 5 min. To remove excess LA by thermal filtration, the composite PCMs were maintained at 80 °C for 48 h as the mass did not decrease longer. After cooling, the mixture was ground to obtain the composite PCMs. LA with different mass ratio (30%, 40%, 50%, and 60%) is dispersed in HSEP, and the composites were denoted as HS-LA1, HS-LA2, HS-LA3 and HSLA4, respectively.
2. Experimental 2.3. Characterization 2.1. Materials The SEM images of the samples were obtained using a JEOL JSM6360LV SEM. The TEM images were collected with a JEOL JEM-2100F TEM. FTIR (Nicolet 5700) spectra of the samples were investigated in the range of 450–4000 cm− 1. The XRD (RigakuD/max 2550) experi-
Lauric acid (LA, C12H24O2) was analytically pure and supplied by Tianjin Kemiou Chemical Reagent Co., Ltd., China. Natural sepiolite (SEP) was collected from mineral deposit located in Hunan Province in 15
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Table 1 Chemical compositions of SEP before and after modification (wt%). Samples
SiO2
CaO
MgO
Al2O3
Fe2O3
P2O3
TiO2
K2O
Na2O
F
SEP GSEP NSEP HSEP
51.28 51.45 50.58 72.62
27.27 26.20 25.80 0.07
16.56 17.44 17.06 21.23
2.87 2.87 2.77 3.83
0.96 0.91 0.94 1.23
0.17 0.14 0.15 0.01
0.13 0.15 0.10 0.15
0.13 0.14 0.14 0.21
0.10 0.13 0.28 0.09
0.41 0.43 0.49 0.51
Fig. 2. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of SEP, CSEP, NSEP and HSEP.
under N2 flow. TGA (STA8000) was investigated at a heating rate of 10 °C/min up to 600 °C in N2 atmosphere. Thermal conductivities (TC 3000) of the samples were measured by hot-wire method, and 200 accelerated thermal cycles were performed in a thermostatic chamber
ments were performed on the samples at a scan rate of 3°/min in the range of 5–80°. N2 adsorption-desorption isothermals (ASAP 2020) were performed at 77 K. DSC analysis (DSC21400A-0211-L) was investigated in the range of 30–80 °C at a heating rate of 5 °C/min
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originally clogged pores were opened. It is a good phenomenon for the porous SEP as more LA can be loaded. The changes of acid treatment indicated that metallic oxides were the main impurity which plugged up the pore. XRF was applied to confirm the observation. The main chemical compositions of raw SEP and modified SEP were listed in Table 1, SEP was mainly composed of 51.28 wt% SiO2, 27.27 wt% CaO and 16.56 wt% MgO. After the treatment of calcination and alkali leaching, it had no obvious change of the contents of ingredients. However, after the purification of acid treatment, the contents of SiO2 (72.62 wt%) and MgO (21.23 wt%) significantly increased and CaO (0.07 wt%) nearly disappeared, indicating that the hydrochloric acid treatment is an efficient way to remove the impurity which clogs the pore. Fig. 2 illustrates the N2 adsorption-desorption isotherms of raw and modified SEP, the porous textures were summarized in Table 2. The N2 adsorption-desorption isotherms of raw and modified SEP were similar (Fig. 2a), indicating that the microstructure of SEP was maintained after modification treatment. The N2 adsorption-desorption isotherms of raw and modified SEP were classified as H3 hysteresis loops. The hysteresis was related to the capillary condensation of the filling and emptying of mesopores, showing that many mesopores existed in the supports (Yu et al., 2015). In addition, the N2 adsorption quantity increased sharply, indicating the existence of micropores (Liu et al., 2012). This feature demonstrated the coexistence of mesopore and micropore structures, which was in accord with the SEM and TEM results. The specific surface area and pore volume had no obvious change after the calcination and alkali leaching treatment (Table 2). However, three parameters were improved obviously in HSEP as the micropore size was reduced and the mesopore and macropore size increased, which resulted in the increase of the surface area. The pore size distributions of HSEP showed a narrow range of 2–20 nm and the pore diameter shifted to larger diameter (Fig. 2b), which is desirable for
Table 2 Textural properties of raw SEP and modified SEP. Properties
SEP
CSEP
NSEP
HSEP
Specific surface area (m2/g) Pore volume (cc/g) Mean pore diameter (nm)
51.58 0.12 9.46
52.99 0.14 12.69
53.92 0.14 13.82
76.08 0.29 15.53
with the temperature controller. 3. Results and discussion 3.1. Characterization of modified sepiolite Sepiolite (SEP) powder is mainly composed of fibrous structure, some pores can be seen. However, the fiber bundles are agglutinated as a bed and many pores on the surface of SEP were blocked by impurities (Fig. 1a). No obvious improvement was found after calcination treatment (Fig. 1b) and the fibrous structure changed a little after alkali leaching (Fig. 1c). But after acid treatment, the fiber bundles became much more loose while the morphology of the fibrous structure was basically retained. In addition, there were fewer impurities and much more pores on the surface of SEP (Fig. 1d). TEM was applied to confirm the observations from SEM. The thick fiber bundles of raw SEP stick together (Fig. 1e), which was in accord with the SEM observation. After treatment with hydrochloric acid, the fiber bundles became more dispersed than raw SEP (Fig. 1f). Because the acid treatment produced a very strong discontinuity in both octahedral and tetrahedral sheets, causing SEP to transform phyllosilicate-like structure into an inosilicate-like structure (Yebra et al., 2003). A great number of pores can be obviously seen in Fig. 1f. Due to the reaction between metallic oxides and hydrochloric acid, pore edges were enlarged gradually and
Fig. 3. (a, b) XRD patterns and (c, d) FTIR spectra of raw and modified SEP, LA and the composite PCMs.
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Fig. 4. SEM images of (a) HS-LA1, (b) HS-LA2, (c) HS-LA3 and (d) HS-LA4 composite PCMs.
Fig. 5. TEM images of (a) HSEP and (b) HS-LA4, and (c, d) corresponding EDS analysis, respectively.
its role as a PCM carrier.
7.2° and 26.5°, indicating the modification treatment didn't change the crystalline structure of SEP. A large number of calcite crystalline phases was also found in SEP, CSEP and NSEP, but had not appeared in HSEP, which indicated that the impurity was removed after acid treatment, which was well agreement with the microstructure observation and XRF analysis. Fig. 3b shows the XRD pattern of LA and the composite
3.2. Analysis of the as-prepared form-stable composite PCMs XRD patterns of the raw and modified SEP were given in Fig. 3a, all the samples had the typical crystalline diffraction reflections of SEP at 18
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action. When LA was in the melting state during the SEM testing process, it was not leaked from the pores and surface of HSEP (Fig. 4d). So the maximum appropriate loading of LA in the form-stable composite PCMs (HS-LA4) could be 60%. Fig. 5 shows the TEM images and corresponding energy spectra of HSEP and HS-LA4. HSEP was nanotube structure, and the diameter of the fiber was about 18 nm (Fig. 5a). After impregnation, the pores and surface of HSEP were fully filled with LA by capillary force, and the diameter of fiber with LA increased up to 28 nm (Fig. 5b), which was in accordance with the SEM images. The elemental compositions of carbon in HSEP and HS-LA4 were confirmed to be 12.88 and 82.48 atomic percentage, respectively (Fig. 5c & d). These results obviously demonstrated the high percentage of LA in HS-LA4 form-stable composite PCMs.
Table 3 Thermal characteristics of LA and the as-prepared composite PCMs. LA mass ratio (wt%) Samples LA PCM HS-LA1 HS-LA2 HS-LA3 HS-LA4 S-LA
100 30 40 50 60 40
Melting process
Freezing process
Hm (J/g)
Tm (°C)
Hf (J/g)
Hf (°C)
225.4 58.4 81.5 106.7 125.2 82.6
43.2 42.4 42.5 42.5 42.5 42.5
194 54.3 74.6 94.1 113.9 75.4
41.1 41.5 41.5 41.4 41.3 41.4
PCMs. The reflections at 19.5° and 24.0° were assigned to LA crystals. All the intense and sharp diffraction reflections of LA appeared in the as-prepared composite PCMs, indicating that the crystal structure of LA was not destroyed during the preparation process. FTIR spectra of raw and modified SEP are shown in Fig. 3b. The bands at 796 cm− 1 and 1095 cm− 1 corresponded to the SieOeSi symmetric and asymmetric stretching vibrations, respectively. The band at 466 cm− 1 was due to the SieOeSi bending vibration. The band at 1420 cm− 1 was attributed to the [CO3]2 − internal stretching vibration. As the reaction between calcite and acid solution proceeded, the band disappeared in HSEP. The FTIR spectra of LA and the composites are shown in Fig. 3d. All of the bands of LA appeared in the spectrum of the composites, such as eCH2 asymmetric stretching vibration (2926 cm− 1), eCH2 symmetric stretching vibration (2841 cm− 1), and C]O vibration (1696 cm− 1). All of the HSEP bands were also observed in the spectrum, and no significant new bands were found. These results confirmed no chemical interaction between HSEP and LA. LA with different mixing ratio (30%, 40%, 50%, and 60%) was greatly impregnated in the support of HSEP (Fig. 4). The porous HSEP prevented the leakage of LA by the surface tension force and capillary
3.3. Thermal properties of the form-stable composite PCMs Thermal properties of all the composites, including melting/freezing temperature and latent heats, are summarized in Table 3. Raw SEP with maximum mass ratio of LA was prepared, and the composite PCM was denoted as S-LA. The maximum loading of LA in the HS-LA4 composites could be 60 wt%, and that of raw SEP was 40 wt%, which was increased by 50% (Table 3). The latent heats increased with the increase in the mass ratio of LA, because it is LA but not the supporting HSEP that undergoes the phase change process. So HS-LA4 can be unquestionably accounted as the promising thermal energy storage material. As the DSC curves shown in Fig. 6a, LA and the form-stable composites have similar melting and freezing processes. The optimal HS-LA4 had the melting latent heat of 125.2 J/g at 42.5 °C and the freezing latent heat of 113.9 J/g at 41.3 °C. Theoretical enthalpy of the composites could be determined by Eq. (1) (Qian et al., 2013):
Htheo = ηHLA
(1)
where Htheo is the theoretical enthalpy of the composites; η denotes the
Fig. 6. LA and composite PCMs: (a) DSC curves, (b) comparison of theoretical and actual enthalpies, (c) the relation between LA mass fraction and HHS-LA/HLA and (d) phase change temperatures.
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Table 4 Thermal property comparison between the as-prepares composite PCM in this work and the previously reported composite PCMs. PCM
Paraffin blends Stearic acid Lauric acid Paraffin PEG1000 Galactitol myristate Galactitol myristate Capic/myristic aicd Paraffin Capic/myristic aicd Lauric acid
Support
Opal Kaolin Kaolinite Diatomite Diatomite Diatomite Vermiculite Vermiculite Vermiculite Perlite Sepiolite
Max. ratio (%)
39 39 48 61 50 52 55 20 53 55 60
Melting process
Freezing process
Hm (J/g)
Tm (°C)
Hf (J/g)
Hf (°C)
49.1 66.3 72.5 89.5 87.1 83.8 92.2 27.5 101.1 85.4 125.2
32.8 53.3 43.7 33.0 27.7 44.9 45.9 19.8 48.9 21.7 42.5
48.4 65.6 70.9 89.8 82.2 81.1 92.5 31.4 103.0 89.8 113.9
47.3 52.7 39.3 52.4 32.2 44.3 44.6 17.1 53.0 20.7 41.3
Thermal conductivity (W/mK)
Ref.
– – 0.10 – 0.32 0.19 0.14 0.07 0.45 0.05 0.59
Sun et al., 2013a, b Liu and Yang, 2014 Song et al., 2014b Sun et al., 2013a, b Karaman et al., 2011 Sarı and Biçer, 2012 Sarı and Biçer, 2012 Karaipekli and Sarı, 2009 Guan et al., 2015 Karaipekli and Sarı, 2008 This work
Fig. 8. Thermal conductivity and melting/freezing process curves of LA and HS-LA4.
extents of supercooling could be defined as the difference between freezing and melting temperature. Compared with pure LA, the extent of supercooling in the composite PCMs was reduced by 0.9–1.2 °C (Fig. 6d), indicating that the extents of supercooling could be reduced by impregnating into HSEP support. Because the porous HSEP in the composite PCMs could offer the heat transmission route in LA and then increased the phase change rate (Parameshwaran et al., 2013). According to the thermal property comparison between HS-LA4 and the composite PCMs previously prepared in related references (Table 4), the as-prepared HS-LA4 had suitable melting temperature of 42.5 °C and freezing temperature of 41.3 °C. The form-stable composites demonstrated a relatively higher melting enthalpy of 125.2 J/g and freezing enthalpy of 113.9 J/g than that of other composites. In addition, the as-prepared materials had the larger loading of PCM (60%) than most of the composites. Therefore, the form-stable composites could be the promising thermal energy storage material for application in the building field.
Fig. 7. (a) TGA curves and (b) corresponding DTG thermograms of LA, S-LA and HS-LA4.
mass fraction of LA in the composites; HLA is the enthalpy of pristine. The theoretical enthalpies of the composites were slightly higher than the actual values (Fig. 6b). The reason could be that mesopores restricted the crystal orientation, the arrangement of LA and the steric drag effect of nano (Feng et al., 2011). And the melting enthalpy was always higher than the freezing enthalpy, it could be the increase of mass loss when the composites were heated during melting tests by DSC. The relations between LA mass ratio and HHS-LA/HLA of the composite PCMs are shown in Fig. 6c. The loading of LA was linearly correlated with the latent heat of the composites to certain extent. The
3.4. Thermal stability of the form-stable composite PCMs Thermal stability is an important parameter for thermal energy storage application. The weight loss of S-LA and HS-LA4 were 40.3% and 60.4% (Fig. 7a), which was close to the corresponding designed values (40% and 60%) and was in line with the DSC measurement. Moreover, the weight loss profile of HS-LA4 was almost overlapped with that of LA in the programmed temperature scope of 30–150 °C, suggesting that the form-stable composites had excellent thermal stability before 150 °C. LA exhibited the typical one-step degradation at 243 °C (Fig. 7b), indicating that pure LA experienced a simple 20
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Fig. 9. HS-LA4 after 200 thermal cycling: (a) SEM image, (b) XRD patterns, (c) FTIR spectra and (d) DSC curves.
influenced during thermal cycles. Therefore, the as-prepared formstable composite PCM had stable chemical structure after 200 thermal cycling tests. DSC measurement was performed to investigate the thermal stability (Fig. 9d). The melting temperature of HS-LA4 was changed by 0.1 °C after 200 thermal cycling tests. The changes in melting temperature were not of crucial magnitude for the application. The latent heat of HS-LA4 was changed by 3%. The decrease of latent heat was in a reasonable level, and it was still in a comparatively high level. The latent heats of melting and freezing of HS-LA4/LA composites after thermal cycling showed general advantages over the current PCMs such as paraffin/diatomite composite (Sun et al., 2013a, b), galactitol myristate/vermiculite composite (Sarı and Biçer, 2012), and capricmyristic aicd/expanded perlite composite (Karaipekli and Sarı, 2008). Therefore, the form-stable composite PCM had thermal reliability and could be used as a thermal energy storage material in the buildings.
evaporation. The sharp weight loss of HS-LA4 at 224 °C was attributed to the decomposition of organic ingredient of LA chains during the degradation process. HS-LA4 had the residue of 39.6 wt% at 600 °C, showing that the form-stable composite PCM was homogeneous. 3.5. Thermal conductivity of the form-stable composite PCMs Thermal conductivity is a significant indicator for the applicability of heat storage and release. The thermal conductivity of HS-LA4 was 0.59 W/(m·k), which was 1.8 time of that of pure LA (0.33 W/(m·k)). Compared with the other composite PCMs (Table 4), the as-prepared HS-LA4 composite PCM showed the relatively higher thermal conductivity. The improvement in thermal conductivity was verified by comparing the melting and freezing processes of HS-LA4 and that of the LA. The melting/freezing process was performed in the temperature scope of 20–40 °C. The melting and freezing periods of LA were 27 min and 24 min, and that of HS-LA4 were 14 min and 16 min, respectively (Fig. 8). The decreased time could be the result of the improvement of heat conduction speed during the thermal storage and release process, which indicated that the form-stable composite PCM had the high heat conduction speed.
4. Conclusion A series of methods were used to enhance the adsorption capacity of raw SEP. The modification effects including calcination, alkali leaching and hydrochloric acid treatment on SEP were studied. The maximum loading of LA in HS-LA4 composites could be 60 wt%, which was 50% higher than that of raw SEP. After acid treatment, the pores could be dredged and enlarged, thus leading to a larger pore size, higher BET area and change of the overall morphology. LA was well impregnated into the porous structure of HSEP, which had better compatibility, reduced supercooling extent, thermal stability and reliability. The formstable composite PCM had suitable melting/freezing temperature (42.5 °C/41.3 °C) and large latent heat of melting and freezing (125.2 J/g and 113.9 J/g), and exhibited a thermal conductivity of 0.59 W/(m·k) higher than that of pristine LA. Therefore, the form-stable composite PCM could be the promising thermal energy storage material
3.6. Thermal reliability of the form-stable composite PCM The form-stable composite PCM must be chemically and thermally stable. The fibrous structure of HSEP was entirely covered by LA and nearly no empty pores was observed (Fig. 9a). Moreover, no seepage of melted LA was found after the thermal cycling. The diffraction peaks nearly had no change before and after thermal cycles (Fig. 9b), indicating that the crystal structure of LA in HS-LA4 had not been affected. The frequency values and shapes of all the peaks had not been changed (Fig. 9c). The results indicated that no chemical structure was 21
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