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diatomite composites as novel form-stable phase change materials for thermal energy storage
Energy and Buildings 104 (2015) 244–249
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Preparation and properties of lauric acid/diatomite composites as novel form-stable phase change materials for thermal energy storage Xiaowei Fu a , Zhimeng Liu a , Yao Xiao a , Jiliang Wang b , Jingxin Lei a,∗ a b
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, China
a r t i c l e
i n f o
Article history: Received 2 April 2015 Received in revised form 12 June 2015 Accepted 23 June 2015 Available online 26 June 2015 Keywords: Phase change material Composite Lauric acid Thermal energy storage Diatomite
a b s t r a c t The lauric acid (LA)/diatomite composite phase change materials (PCMs) were prepared using a direct impregnation method, in which LA was chosen as PCM. The chemical structure, chemical compatibility, element composition, crystalline structure, thermal properties and thermal stability of LA/diatomite composite were extensively investigated by Fourier transform infrared (FTIR) spectroscopy, energy dispersive spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry and thermal gravimetric analysis. FTIR and XRD results indicate that there are only physical interactions between LA and diatomite in LA/diatomite composite. The decrease of XRD intensity of composite PCM confirms the reduction of the crystalline size of LA in LA/diatomite composite. The melting and freezing temperatures of composite PCM are 40.9 ◦ C and 38.7 ◦ C respectively. The latent heat of composite PCM is 57.2 J/g. LA/diatomite composite does not decompose when the surrounding temperature is lower than 157 ◦ C, indicating that the composite PCM has a good thermal stability. © 2015 Published by Elsevier B.V.
1. Introduction In the last two decades, the increasing attention has been paid to latent heat energy storage (LHES) based on phase change materials (PCMs) [1–3]. Among the PCMs, the fatty acid such as lauric acid (LA) is of much importance due to the proper phase transition temperature (PTT) for a given application in building field and textile industries [4–6]. They have the ability to absorb and release abundant heat during their phase transition and retain the thermal inertia at their PTT. It is considered that the thermal energy storage problem such as thermal insulation can be solved by LHES based on PCMs [7]. The thermal comfort can be enhanced by reducing the internal temperature fluctuation when the PCMs are utilized in the fields of the buildings and the textile products [8]. The PCMs will be converted into liquid above the corresponding PTT. Therefore, the effective capsulation of PCMs has to be employed to prevent leaking-out once melted [9–11]. Impregnation composite is a facile and robust technology, which has been used in such fields as building materials and textile industries [12]. Due to the lower thermal conductivity of fatty acid [13] compared with inorganic substance, the papers about fatty acid/inorganic substance composite increase in the last
∗ Corresponding author. E-mail address: [email protected] (J. Lei). http://dx.doi.org/10.1016/j.enbuild.2015.06.059 0378-7788/© 2015 Published by Elsevier B.V.
few years [14]. Karaman’s group [10] studied the polyethylene glycol/diatomite composites as form-stable PCMs. The relevant melting temperature, freezing temperature and latent heat were 27.7 ◦ C, 32.2 ◦ C and 87.09 J/g, respectively. Li and coworkers [15] revealed that the paraffin as the PCM was absorbed into the layered bentonite by using solution intercalation method. The melting temperature and latent heat of the paraffin/bentonite composite were 41.7 ◦ C and 39.84 J/g, respectively. Li et al. [16] prepared binary fatty acid/diatomite composites as form-stable PCMs based on the eutectic caprice-lauric acid blends. 57 wt.% of capricelauric acid blends was absorbed into the porous diatomite and the latent heat of binary acid/diatomite composite reached 66.81 J/g. Xu et al. [17] studied the preparation and thermal properties of paraffin/diatomite composites as form-stable PCMs. The maximum latent heat of paraffin/diatomite composites was found as 70.51 J/g without the leakage of melted paraffin. He’s group [18] reported the preparation and properties of shape-stabilized n-alkanes/silica composites as PCMs which were synthesized in a sol–gel process using sodium silicate precursor. From what was mentioned above, diatomite was selected as the supporting material in many papers [19,20] resulting from its light weight, high porosity, high absorption capability, rigidity and inertness [10]. For the chemical composition and physical structure, the diatomite was suitable in lots of fields such as building materials and sound insulator [21]. Therefore, diatomite was considered to be used as the available supporting material of fatty
X. Fu et al. / Energy and Buildings 104 (2015) 244–249
245
Table 1 Chemical composition of the pure diatomite/%. Series
MgO
Al2 O3
SiO2
K2 O
CaO
Fe2 O3
TiO2
Diatomite
6.93
12.36
69.84
3.43
2.80
4.21
0.43
acid/diatomite composites for thermal energy storage [22]. To the best of our knowledge, no studies have been conducted to develop the lauric acid/diatomite composites as form-stable PCMs to potentially improve the thermal comfort of a given field such as building materials due to the proper PTT (41–48 ◦ C). In this study, the LA/diatomite mixtures were prepared as novel form-stable composite PCMs using a direct impregnation method without vacuum treatment. The diatomite as inorganic supporting materials has good compatibility with concrete as important building materials [23]. Given this, the LA/diatomite composite form-stable PCM has a large potential application in building fields by the direct bending of the resulting form-stable PCM and concrete, enhancing the thermal energy storage capability of the building wall [14,24]. The chemical compatibility and impregnation morphology of prepared composite PCMs were investigated by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The thermal properties and thermal stability were also studied by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA), respectively. 2. Experimental 2.1. Materials Lauric acid (LA, 98.5% pure) was purchased from Chengdu Kelong Chemical Reagent Company (Chengdu, China). The diatomite sample was supplied from China’s Pharmaceutical Industry Co., Ltd (Beijing, China). The chemical composition is given in Table 1. The diatomite samples were dried at 80 ◦ C for 8 h to remove existing water. 2.2. Preparation of form-stable composite PCMs The LA/diatomite composite PCMs were prepared using a direct impregnation method without vacuum treatment [25]. Diatomite was added into six 15 ml beakers with 2 g for each beaker, and then LA of different mass was put into the six beakers respectively. A series of weight ratio of LA and diatomite was set as 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8. After LA and diatomite homogenized by stirring, the beakers were sealed with the plastic wrap and put in the oven at 80 ◦ C for 12 h, aiming for saturated adsorption in thermodynamics. Meanwhile, the sample in each beaker was stirred every 4 h during the adsorption process. Finally, the samples were cooled down to 25 ◦ C. The formula and appearance of the form-stable composite PCMs before and after composite are shown in Table 2. 2.3. Characterization The particle size and distribution of diatomite were measured by MASTERIZER 2000 (Malvern Instruments Ltd, England) at 25 ◦ C.
Fig. 1. Particle size and distribution of diatomite.
The testing particle size ranges are from 0.02 to 2000 m. Before the testing process, the diatomite sample was treated with ultrasound for 5 min to obtain a better dispersion in water. The structures of LA, diatomite and LA/diatomite composite were characterized using a Nicolet-560 Fourier transform infrared (FTIR) spectrometer (USA) with a resolution setting of 4 cm−1 . The scanning range was changed from 4000 to 400 cm−1 . The crystalline structures of diatomite, LA and LA/diatomite composite were investigated using the X-ray diffraction (X’Pert pro MPD, Netherlands). The melting and freezing temperatures as well as latent heat of LA and LA/diatomite composite were studied by a differential scanning calorimeter (DSC 204 F1, German) at 10 ◦ C/min under nitrogen atmosphere. Thermal gravimetric analysis was carried out in a TA Instrument SDT-Q600 thermal analyzer (USA) from 25 to 600 ◦ C under a nitrogen atmosphere with a heating rate of 10 ◦ C/min and about 10 mg of each sample. The obtained TGA results were analyzed by a TA universal analysis program. The micrographs of diatomite and LA/diatomite composite PCMs were observed using a JEOL JSM-5900LV SEM (Japan) with an accelerated voltage of 20 kV. The SEM samples were gold-sputtered before the observation.
3. Results and discussion 3.1. Characterization of the diatomite The diatomite was employed as inorganic supporting materials. Considering the impact on absorption, it is indispensable to investigate the particle size and distribution of the pure diatomite before the LA/diatomite composites were prepared. Fig. 1 reveals the particle size and distribution of diatomite and the particle size parameters of diatomite are listed in Table 3. From Fig. 1, the particle size shows the trimodal distribution and the peak particle sizes are 0.58, 34.15 and 441.02 m respectively. The integral area of three peaks is very asymmetric and the 34.15 m is corresponding to the main peak. In Table 3, the D[50] is only 30.85 m and the specific surface of diatomite is 0.836 m2 /g which is helpful to adsorbing the melting LA.
Table 2 Formula and appearances of the form-stable LA/diatomite composites before and after composite. Ratio of LA/diatomite LA (g) Diatomite (g) Appearance before composite Appearance after composite
0.3:1 0.6 2.0 Powder Powder
0.4:1 0.8 2.0 Powder Powder
0.5:1 1.0 2.0 Powder Powder
0.6:1 1.2 2.0 Powder Powder
0.7:1 1.4 2.0 Powder Bulk
0.8:1 1.6 2.0 Powder Bulk
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Table 3 Size parameter of diatomite. Sample
D[10] (m)
D[50] (m)
D[90] (m)
D[4,3]
D[3,2]
S/g (m2 /g)
Diatomite
5.10
30.85
182.05
72.78
7.17
0.836
The D[10], D[50] and D[90] represented the average particle diameter, when the cumulative percentages were up to 10%, 50% and 90%, respectively. D[4,3] and D[3,2] were the volume-average particle diameter and surface area average particle diameter measured by Malvern. S/g means specific surface on weight basis.
Fig. 2. SEM photographs of (a) diatomite and (b) form-stable LA/diatomite composite PCM.
3.2. Preparation of LA/diatomite composite form-stable PCM The LA/diatomite composite form-stable PCMs were prepared using a direct impregnation without vacuum [26]. To obtain the proper adsorption ratio, a series of LA by weight was mixed with the same amount of diatomite. After the saturated adsorption in thermodynamics at 80 ◦ C for 12 h, the appearances of LA/diatomite composite were observed by eyes. The formula and appearances of LA/diatomite composite form-stable PCMs before and after composite are shown in Table 2. The diatomite still keeps granular when the mass ratio of LA and diatomite is 0.3, 0.4, 0.5 and 0.6. However, the diatomite powders obviously become into a block when the mass ratio of LA and diatomite reaches 0.6, indicating that the added amount of LA exceeds the adsorption capacity in the specific condition. Given the critical point of adsorption, the LA/diatomite composite PCM with the ratio of 0.6 (L0.6) was placed on a dry filter paper at 80 ◦ C to remove the LA on the surface of LA/diatomite composite PCM [13], preventing LA from leaking out in a given application. To further investigate the adsorption situation, the SEM was conducted and the SEM photographs of diatomite and form-stable LA/diatomite composite PCM are shown in Fig. 2. In Fig. 2a, many gully-like and tunnel-like veins are found out in the surface of the diatomite powder, revealing the surface inhomogeneity and the existence of lots of area to be filled. Compared with rough surface of diatomite, the surface of LA/diatomite composite form-stable PCM is smooth, bright and locally even in Fig. 2b, indicating that the LA has been adsorbed fully. It can also be observed that the sharp ridge of holes has been covered, taking account of that the existing holes disappeared, which further manifest the successful adsorption of LA into diatomite.
Fig. 3. As seen from Fig. 3a, the pure diatomite possesses the main absorption bands at 3435, 1638, 1429, 1079 and 789 cm−1 [3]. The bands at 3435 and 789 cm−1 are ascribed to the free silanol groups (SiO H) and the band at 1079 cm−1 is characteristic of the siloxane (Si O Si) group [10]. In Fig. 3b, the characteristic peaks at 1928 and 2844 cm−1 are attributed to C H stretching vibration for LA and carbonyl peaks of LA are observed at 1701 and 1480 cm−1 . As shown in Fig. 3c, the characteristic peaks at 3435, 1079 and 788 cm−1 , ascribed to vibration absorption of diatomite, are detected, indicating the existence of diatomite in the composite form-stable PCM. Meanwhile, the characteristic peaks at 2828, 2844, 1701 and 1467 cm−1 , attributed to the stretching vibration of methyl, methylene and carbonyl of LA, are also observed in Fig. 3c, implying the existence of LA in the prepared LA/diatomite composite PCM. No distinct new absorption peaks appear in Fig. 3c, meaning that
3.3. Chemical compatibility and structure of LA/diatomite composite form-stable PCM To study the chemical compatibility of the LA and diatomite, the FTIR analysis was carried out and FTIR spectra of diatomite, LA and LA/diatomite composite form-stable PCM are shown in
Fig. 3. FTIR spectra of (a) diatomite, (b) LA and (c) form-stable LA/diatomite composite PCM.
X. Fu et al. / Energy and Buildings 104 (2015) 244–249
Fig. 4. Energy dispersive spectroscopy graph of diatomite.
there are only physical interactions between LA as organic PCM and diatomite as supporting material. The physical interactions contain capillary and surface tension forces, preventing the melted LA from leaking out [27]. Fig. 4 shows energy dispersive spectroscopy graph of diatomite. As seen from Fig. 4, the contained elements ranked by weight percentage are O, Si, Al, Mg, Fe, K, Ca and Ti. The first content of O element reaches 48.58 wt.% and second content of Si element is up to 32.65 wt.%. Only the content of Ti element is under 2 wt.%. Therefore, the chemical composition of diatomite is complex and SiO2 is the main oxide in diatomite sample. XRD diffractograms of diatomite, LA and LA/diatomite composite form-stable PCM are shown in Fig. 5. Each sample exhibits the complete crystalline structure. In Fig. 5a, the diatomite shows the strong diffraction peaks at 20.8◦ and 26.6◦ , attributed to feather peaks of (100) and (101) respectively. According to Bragg’s law (Eq. ˚ As seen (1)), the interplanar spacing is respectively 4.24 and 3.34 A. in Fig. 5b, the sharp diffraction peaks at 19.2◦ , 19.9◦ , 21.8◦ , 23.5◦ , 24.5◦ and 26.0◦ are caused by the LA due to its regular crystallization. In Fig. 5c, the characteristic XRD peaks of the pure diatomite
Fig. 6. DSC curves of (a, d) pure LA and (b, c) form-stable LA/diatomite composite PCM.
and LA in the LA/diatomite composite form-stable PCM are both presented. In other word, the XRD peaks of composite PCM are the superposition of pure diatomite and LA. This result indicates that the crystal structures of the LA in the LA/diatomite composite formstable PCM are not affected by the direct impregnation treatment of diatomite. However, the intensity of XRD peaks of LA in Fig. 5c is lower than that of pure LA in Fig. 5b, ascribed to the smaller crystalline size [28]. 2d × sin() = n ×
(1)
where d is the interplanar spacing; is the diffraction angle; is the incident wavelength; n is the diffraction series. 3.4. Thermal properties of LA/diatomite composite form-stable PCM It is usually required for an excellent form-stable PCM that the thermal properties should be extensively investigated by DSC analysis. The DSC curves of LA and LA/diatomite composite PCM are shown in Fig. 6. The detailed melting and freezing temperatures and latent heat are presented in Table 4. As seen from Fig. 6, the melting and freezing curves show a single and blunt peak in every curve, indicating the existence of relatively impeccable crystals under the influence of little impurities, consistent with the XRD results in Fig. 5. In Table 4, the melting and freezing temperatures and latent heats of LA were measured as 44.4 ◦ C and 40.9 ◦ C as well as 141.9 J/g and 142.3 J/g respectively. The undercooling of 3.5 ◦ C occurs due to the existence of impurities. The melting temperature of LA/diatomite composite is 40.9 ◦ C lower than that of LA, taking account of the different thermal history in melting curves. The freezing temperatures are consistent in Fig. 5b and c with the same cooling condition. The undercooling of LA/diatomite composite is 3.1 ◦ C in accord with that of pure LA. As seen from Table 4, the melting and freezing latent heats of LA/diatomite composite (L0.6) respectively reach 57.4 and 57.2 J/g. The percentage of LA in LA/diatomite composite form-stable PCMs (L0.6 and L0.5) respectively reaches 40.19 and 29.78 wt.% calculated by Eq. (2). WL =
Fig. 5. XRD diffractograms of (a) diatomite, (b) LA and (c) form-stable LA/diatomite composite PCM.
247
HF HL
(2)
where WL is the weight percentage of LA in LA/diatomite composite form-stable PCM; HF is the latent heat of LA/diatomite composite form-stable PCM, J/g; HL is the latent heat of pure LA, J/g. The LA/diatomite composite form-stable PCM can potentially be utilized as thermal storage material in the exterior wall of
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Table 4 Latent heat storage properties of LA and the LA/diatomite composite with the various LA content. PCM
Melting point (◦ C)
Latent heat of melting (J/g)
Freezing point (◦ C)
Latent heat of freezing (J/g)
LA LA/diatomite composite (L0.6) LA/diatomite composite (L0.5)
44.4 40.9 40.7
141.9 57.4 43.2
40.9 38.7 37.6
−142.3 −57.2 −42.38
Table 5 Thermal energy storage characteristics of different form-stable composite PCMs in literature. Composite PCM
Melting point (◦ C)
Freezing point (◦ C)
Latent heat (J/g)
Refs.
Capric–myristic acid (20 wt.%)/VMT n-Nonadecane (50 wt.%)/cement Paraffin/bentonite Paraffin/porous silica ceramic Capric–myristic acid/VMT/2 wt.%EG Caprice–myristic acid (20 wt.%)/VMT PMMA/heptadecane microcapsule Capric–myristic acid/polyethylene terephthalate nanofibers Paraffin/diatomite Lauric acid (33.3 wt.%)/activated carbon Caprice–myristic acid (66 wt.%)/diatomite Lauric acid/diatomite Lauric acid/diatomite
19.8 31.9 41.7 20.8 19.7 19.8 18.2 24.98 41.11 44.07 16.74 40.9 40.7
17.1 31.8 43.4 – 17.1 17.1 18.4 13.88 – 42.83 – 38.7 37.6
27.0 69.1 39.84 53.6 26.9 27.0 81.5 57.62 70.51 65.14 66.81 57.4 43.2
[29] [30] [15] [31] [29] [29] [32] [31] [3] [28] [16] Present study Present study
buildings with the PTT of about 40 ◦ C and high latent heat of about 45 J/g, absorbing lots of heat from surrounding air during the day when the ambient temperature exceed PTT and releasing large amounts of heat at night when the ambient temperature is below PTT of form-stable PCMs. Therefore, the exterior wall can retain thermal inertia around 40 ◦ C. Table 5 indicates the thermal storage properties of form-stable PCM in literature [3,15,16,28–32]. By comparison, it is obvious that the fatty acid has been widely used to prepare composite form-stable PCMs with the latent heat of 20–80 J/g. However, LA/diatomite composite PCM with proper PTT has not been investigated. 3.5. Thermal stability of form-stable LA/diatomite composite PCM With PCM’s working at different temperatures, it is very necessary to study the thermal stability of form-stable composite PCM. The TGA curves of diatomite, LA and form-stable LA/diatomite composite PCM are shown in Fig. 7. As shown in Fig. 7a, no thermal degradation stages are detected from 25 to 600 ◦ C, revealing the
Fig. 7. TGA curves of (a) pure diatomite, (b) pure LA and (c) form-stable LA/diatomite composite PCM.
excellent thermal stability of diatomite. The weight of LA is nearly invariable when the temperature is lower than 147 ◦ C. It sharply decreases to 0.61 wt.% once the temperature reaches 189 ◦ C. The maximum decomposition rate appears at 176 ◦ C. As can be seen from Fig. 7b, the degradation of LA in LA/diatomite composite formstable PCM starts at 157 ◦ C and ends at 203 ◦ C, indicating that the thermal stability increases after LA is impregnated into the pores of diatomite. Moreover, the TGA curve confirms that the impregnation ratio has reached 30 wt.% lower than 40.19 wt.% in L0.6 mentioned above, due to that the TGA sample (L0.6) was put on a dry filter paper at 80 ◦ C before analysis to remove the LA on the surface of LA/diatomite composite mentioned above. The TGA result (L0.6) is same as that of DSC (L0.5) in Table 4, proving that the maximum impregnation ratio is 30 wt.%, not 40 wt.% observed by eyes. These results reveal that the LA/diatomite composite PCM has good thermal stability. 4. Conclusions LA/diatomite composite form-stable PCM has been prepared using a direct impregnation method. The structure and thermal properties of LA/diatomite composite have been characterized by FTIR, SEM, EDS, XRD, DSC and TGA. The FTIR and XRD results indicate that there are only physical interactions existing between LA and diatomite, and the crystalline size of LA in LA/diatomite composite becomes small due to the limited pores of diatomite. Obtained from DSC results, the melting and freezing temperatures of LA/diatomite composite form-stable PCM are 40.9 ◦ C and 38.7 ◦ C, respectively. The latent heat of LA/diatomite composite reaches 57.2 J/g. The decomposition of LA in LA/diatomite composite occurs from 157 to 203 ◦ C, higher than that of pure LA, revealing that the thermal stability of LA increases after the LA is impregnated into the porous diatomite. There is no degradation of LA in LA/diatomite composite around the PTT. The LA/diatomite composite has a good thermal stability. Based on all results, the LA/diatomite composite form-stable PCM can be used as energy storage material for passive solar space heating or decreasing indoor temperature fluctuating in buildings due to its good thermal properties (i.e. proper PTT of 38–41 ◦ C and high phase enthalpy of 57.2 J/g), thermal stability and good compatibility of the LA/diatomite composite with concrete. The prepared LA/diatomite composite form-stable PCM has far-reaching application potential in building fields.
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Preparation and properties of lauric acid/diatomite composites as novel form-stable phase change materials for thermal energy storage Xiaowei Fu a , Zhimeng Liu a , Yao Xiao a , Jiliang Wang b , Jingxin Lei a,∗ a b
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, China
a r t i c l e
i n f o
Article history: Received 2 April 2015 Received in revised form 12 June 2015 Accepted 23 June 2015 Available online 26 June 2015 Keywords: Phase change material Composite Lauric acid Thermal energy storage Diatomite
a b s t r a c t The lauric acid (LA)/diatomite composite phase change materials (PCMs) were prepared using a direct impregnation method, in which LA was chosen as PCM. The chemical structure, chemical compatibility, element composition, crystalline structure, thermal properties and thermal stability of LA/diatomite composite were extensively investigated by Fourier transform infrared (FTIR) spectroscopy, energy dispersive spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry and thermal gravimetric analysis. FTIR and XRD results indicate that there are only physical interactions between LA and diatomite in LA/diatomite composite. The decrease of XRD intensity of composite PCM confirms the reduction of the crystalline size of LA in LA/diatomite composite. The melting and freezing temperatures of composite PCM are 40.9 ◦ C and 38.7 ◦ C respectively. The latent heat of composite PCM is 57.2 J/g. LA/diatomite composite does not decompose when the surrounding temperature is lower than 157 ◦ C, indicating that the composite PCM has a good thermal stability. © 2015 Published by Elsevier B.V.
1. Introduction In the last two decades, the increasing attention has been paid to latent heat energy storage (LHES) based on phase change materials (PCMs) [1–3]. Among the PCMs, the fatty acid such as lauric acid (LA) is of much importance due to the proper phase transition temperature (PTT) for a given application in building field and textile industries [4–6]. They have the ability to absorb and release abundant heat during their phase transition and retain the thermal inertia at their PTT. It is considered that the thermal energy storage problem such as thermal insulation can be solved by LHES based on PCMs [7]. The thermal comfort can be enhanced by reducing the internal temperature fluctuation when the PCMs are utilized in the fields of the buildings and the textile products [8]. The PCMs will be converted into liquid above the corresponding PTT. Therefore, the effective capsulation of PCMs has to be employed to prevent leaking-out once melted [9–11]. Impregnation composite is a facile and robust technology, which has been used in such fields as building materials and textile industries [12]. Due to the lower thermal conductivity of fatty acid [13] compared with inorganic substance, the papers about fatty acid/inorganic substance composite increase in the last
∗ Corresponding author. E-mail address: [email protected] (J. Lei). http://dx.doi.org/10.1016/j.enbuild.2015.06.059 0378-7788/© 2015 Published by Elsevier B.V.
few years [14]. Karaman’s group [10] studied the polyethylene glycol/diatomite composites as form-stable PCMs. The relevant melting temperature, freezing temperature and latent heat were 27.7 ◦ C, 32.2 ◦ C and 87.09 J/g, respectively. Li and coworkers [15] revealed that the paraffin as the PCM was absorbed into the layered bentonite by using solution intercalation method. The melting temperature and latent heat of the paraffin/bentonite composite were 41.7 ◦ C and 39.84 J/g, respectively. Li et al. [16] prepared binary fatty acid/diatomite composites as form-stable PCMs based on the eutectic caprice-lauric acid blends. 57 wt.% of capricelauric acid blends was absorbed into the porous diatomite and the latent heat of binary acid/diatomite composite reached 66.81 J/g. Xu et al. [17] studied the preparation and thermal properties of paraffin/diatomite composites as form-stable PCMs. The maximum latent heat of paraffin/diatomite composites was found as 70.51 J/g without the leakage of melted paraffin. He’s group [18] reported the preparation and properties of shape-stabilized n-alkanes/silica composites as PCMs which were synthesized in a sol–gel process using sodium silicate precursor. From what was mentioned above, diatomite was selected as the supporting material in many papers [19,20] resulting from its light weight, high porosity, high absorption capability, rigidity and inertness [10]. For the chemical composition and physical structure, the diatomite was suitable in lots of fields such as building materials and sound insulator [21]. Therefore, diatomite was considered to be used as the available supporting material of fatty
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Table 1 Chemical composition of the pure diatomite/%. Series
MgO
Al2 O3
SiO2
K2 O
CaO
Fe2 O3
TiO2
Diatomite
6.93
12.36
69.84
3.43
2.80
4.21
0.43
acid/diatomite composites for thermal energy storage [22]. To the best of our knowledge, no studies have been conducted to develop the lauric acid/diatomite composites as form-stable PCMs to potentially improve the thermal comfort of a given field such as building materials due to the proper PTT (41–48 ◦ C). In this study, the LA/diatomite mixtures were prepared as novel form-stable composite PCMs using a direct impregnation method without vacuum treatment. The diatomite as inorganic supporting materials has good compatibility with concrete as important building materials [23]. Given this, the LA/diatomite composite form-stable PCM has a large potential application in building fields by the direct bending of the resulting form-stable PCM and concrete, enhancing the thermal energy storage capability of the building wall [14,24]. The chemical compatibility and impregnation morphology of prepared composite PCMs were investigated by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The thermal properties and thermal stability were also studied by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA), respectively. 2. Experimental 2.1. Materials Lauric acid (LA, 98.5% pure) was purchased from Chengdu Kelong Chemical Reagent Company (Chengdu, China). The diatomite sample was supplied from China’s Pharmaceutical Industry Co., Ltd (Beijing, China). The chemical composition is given in Table 1. The diatomite samples were dried at 80 ◦ C for 8 h to remove existing water. 2.2. Preparation of form-stable composite PCMs The LA/diatomite composite PCMs were prepared using a direct impregnation method without vacuum treatment [25]. Diatomite was added into six 15 ml beakers with 2 g for each beaker, and then LA of different mass was put into the six beakers respectively. A series of weight ratio of LA and diatomite was set as 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8. After LA and diatomite homogenized by stirring, the beakers were sealed with the plastic wrap and put in the oven at 80 ◦ C for 12 h, aiming for saturated adsorption in thermodynamics. Meanwhile, the sample in each beaker was stirred every 4 h during the adsorption process. Finally, the samples were cooled down to 25 ◦ C. The formula and appearance of the form-stable composite PCMs before and after composite are shown in Table 2. 2.3. Characterization The particle size and distribution of diatomite were measured by MASTERIZER 2000 (Malvern Instruments Ltd, England) at 25 ◦ C.
Fig. 1. Particle size and distribution of diatomite.
The testing particle size ranges are from 0.02 to 2000 m. Before the testing process, the diatomite sample was treated with ultrasound for 5 min to obtain a better dispersion in water. The structures of LA, diatomite and LA/diatomite composite were characterized using a Nicolet-560 Fourier transform infrared (FTIR) spectrometer (USA) with a resolution setting of 4 cm−1 . The scanning range was changed from 4000 to 400 cm−1 . The crystalline structures of diatomite, LA and LA/diatomite composite were investigated using the X-ray diffraction (X’Pert pro MPD, Netherlands). The melting and freezing temperatures as well as latent heat of LA and LA/diatomite composite were studied by a differential scanning calorimeter (DSC 204 F1, German) at 10 ◦ C/min under nitrogen atmosphere. Thermal gravimetric analysis was carried out in a TA Instrument SDT-Q600 thermal analyzer (USA) from 25 to 600 ◦ C under a nitrogen atmosphere with a heating rate of 10 ◦ C/min and about 10 mg of each sample. The obtained TGA results were analyzed by a TA universal analysis program. The micrographs of diatomite and LA/diatomite composite PCMs were observed using a JEOL JSM-5900LV SEM (Japan) with an accelerated voltage of 20 kV. The SEM samples were gold-sputtered before the observation.
3. Results and discussion 3.1. Characterization of the diatomite The diatomite was employed as inorganic supporting materials. Considering the impact on absorption, it is indispensable to investigate the particle size and distribution of the pure diatomite before the LA/diatomite composites were prepared. Fig. 1 reveals the particle size and distribution of diatomite and the particle size parameters of diatomite are listed in Table 3. From Fig. 1, the particle size shows the trimodal distribution and the peak particle sizes are 0.58, 34.15 and 441.02 m respectively. The integral area of three peaks is very asymmetric and the 34.15 m is corresponding to the main peak. In Table 3, the D[50] is only 30.85 m and the specific surface of diatomite is 0.836 m2 /g which is helpful to adsorbing the melting LA.
Table 2 Formula and appearances of the form-stable LA/diatomite composites before and after composite. Ratio of LA/diatomite LA (g) Diatomite (g) Appearance before composite Appearance after composite
0.3:1 0.6 2.0 Powder Powder
0.4:1 0.8 2.0 Powder Powder
0.5:1 1.0 2.0 Powder Powder
0.6:1 1.2 2.0 Powder Powder
0.7:1 1.4 2.0 Powder Bulk
0.8:1 1.6 2.0 Powder Bulk
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Table 3 Size parameter of diatomite. Sample
D[10] (m)
D[50] (m)
D[90] (m)
D[4,3]
D[3,2]
S/g (m2 /g)
Diatomite
5.10
30.85
182.05
72.78
7.17
0.836
The D[10], D[50] and D[90] represented the average particle diameter, when the cumulative percentages were up to 10%, 50% and 90%, respectively. D[4,3] and D[3,2] were the volume-average particle diameter and surface area average particle diameter measured by Malvern. S/g means specific surface on weight basis.
Fig. 2. SEM photographs of (a) diatomite and (b) form-stable LA/diatomite composite PCM.
3.2. Preparation of LA/diatomite composite form-stable PCM The LA/diatomite composite form-stable PCMs were prepared using a direct impregnation without vacuum [26]. To obtain the proper adsorption ratio, a series of LA by weight was mixed with the same amount of diatomite. After the saturated adsorption in thermodynamics at 80 ◦ C for 12 h, the appearances of LA/diatomite composite were observed by eyes. The formula and appearances of LA/diatomite composite form-stable PCMs before and after composite are shown in Table 2. The diatomite still keeps granular when the mass ratio of LA and diatomite is 0.3, 0.4, 0.5 and 0.6. However, the diatomite powders obviously become into a block when the mass ratio of LA and diatomite reaches 0.6, indicating that the added amount of LA exceeds the adsorption capacity in the specific condition. Given the critical point of adsorption, the LA/diatomite composite PCM with the ratio of 0.6 (L0.6) was placed on a dry filter paper at 80 ◦ C to remove the LA on the surface of LA/diatomite composite PCM [13], preventing LA from leaking out in a given application. To further investigate the adsorption situation, the SEM was conducted and the SEM photographs of diatomite and form-stable LA/diatomite composite PCM are shown in Fig. 2. In Fig. 2a, many gully-like and tunnel-like veins are found out in the surface of the diatomite powder, revealing the surface inhomogeneity and the existence of lots of area to be filled. Compared with rough surface of diatomite, the surface of LA/diatomite composite form-stable PCM is smooth, bright and locally even in Fig. 2b, indicating that the LA has been adsorbed fully. It can also be observed that the sharp ridge of holes has been covered, taking account of that the existing holes disappeared, which further manifest the successful adsorption of LA into diatomite.
Fig. 3. As seen from Fig. 3a, the pure diatomite possesses the main absorption bands at 3435, 1638, 1429, 1079 and 789 cm−1 [3]. The bands at 3435 and 789 cm−1 are ascribed to the free silanol groups (SiO H) and the band at 1079 cm−1 is characteristic of the siloxane (Si O Si) group [10]. In Fig. 3b, the characteristic peaks at 1928 and 2844 cm−1 are attributed to C H stretching vibration for LA and carbonyl peaks of LA are observed at 1701 and 1480 cm−1 . As shown in Fig. 3c, the characteristic peaks at 3435, 1079 and 788 cm−1 , ascribed to vibration absorption of diatomite, are detected, indicating the existence of diatomite in the composite form-stable PCM. Meanwhile, the characteristic peaks at 2828, 2844, 1701 and 1467 cm−1 , attributed to the stretching vibration of methyl, methylene and carbonyl of LA, are also observed in Fig. 3c, implying the existence of LA in the prepared LA/diatomite composite PCM. No distinct new absorption peaks appear in Fig. 3c, meaning that
3.3. Chemical compatibility and structure of LA/diatomite composite form-stable PCM To study the chemical compatibility of the LA and diatomite, the FTIR analysis was carried out and FTIR spectra of diatomite, LA and LA/diatomite composite form-stable PCM are shown in
Fig. 3. FTIR spectra of (a) diatomite, (b) LA and (c) form-stable LA/diatomite composite PCM.
X. Fu et al. / Energy and Buildings 104 (2015) 244–249
Fig. 4. Energy dispersive spectroscopy graph of diatomite.
there are only physical interactions between LA as organic PCM and diatomite as supporting material. The physical interactions contain capillary and surface tension forces, preventing the melted LA from leaking out [27]. Fig. 4 shows energy dispersive spectroscopy graph of diatomite. As seen from Fig. 4, the contained elements ranked by weight percentage are O, Si, Al, Mg, Fe, K, Ca and Ti. The first content of O element reaches 48.58 wt.% and second content of Si element is up to 32.65 wt.%. Only the content of Ti element is under 2 wt.%. Therefore, the chemical composition of diatomite is complex and SiO2 is the main oxide in diatomite sample. XRD diffractograms of diatomite, LA and LA/diatomite composite form-stable PCM are shown in Fig. 5. Each sample exhibits the complete crystalline structure. In Fig. 5a, the diatomite shows the strong diffraction peaks at 20.8◦ and 26.6◦ , attributed to feather peaks of (100) and (101) respectively. According to Bragg’s law (Eq. ˚ As seen (1)), the interplanar spacing is respectively 4.24 and 3.34 A. in Fig. 5b, the sharp diffraction peaks at 19.2◦ , 19.9◦ , 21.8◦ , 23.5◦ , 24.5◦ and 26.0◦ are caused by the LA due to its regular crystallization. In Fig. 5c, the characteristic XRD peaks of the pure diatomite
Fig. 6. DSC curves of (a, d) pure LA and (b, c) form-stable LA/diatomite composite PCM.
and LA in the LA/diatomite composite form-stable PCM are both presented. In other word, the XRD peaks of composite PCM are the superposition of pure diatomite and LA. This result indicates that the crystal structures of the LA in the LA/diatomite composite formstable PCM are not affected by the direct impregnation treatment of diatomite. However, the intensity of XRD peaks of LA in Fig. 5c is lower than that of pure LA in Fig. 5b, ascribed to the smaller crystalline size [28]. 2d × sin() = n ×
(1)
where d is the interplanar spacing; is the diffraction angle; is the incident wavelength; n is the diffraction series. 3.4. Thermal properties of LA/diatomite composite form-stable PCM It is usually required for an excellent form-stable PCM that the thermal properties should be extensively investigated by DSC analysis. The DSC curves of LA and LA/diatomite composite PCM are shown in Fig. 6. The detailed melting and freezing temperatures and latent heat are presented in Table 4. As seen from Fig. 6, the melting and freezing curves show a single and blunt peak in every curve, indicating the existence of relatively impeccable crystals under the influence of little impurities, consistent with the XRD results in Fig. 5. In Table 4, the melting and freezing temperatures and latent heats of LA were measured as 44.4 ◦ C and 40.9 ◦ C as well as 141.9 J/g and 142.3 J/g respectively. The undercooling of 3.5 ◦ C occurs due to the existence of impurities. The melting temperature of LA/diatomite composite is 40.9 ◦ C lower than that of LA, taking account of the different thermal history in melting curves. The freezing temperatures are consistent in Fig. 5b and c with the same cooling condition. The undercooling of LA/diatomite composite is 3.1 ◦ C in accord with that of pure LA. As seen from Table 4, the melting and freezing latent heats of LA/diatomite composite (L0.6) respectively reach 57.4 and 57.2 J/g. The percentage of LA in LA/diatomite composite form-stable PCMs (L0.6 and L0.5) respectively reaches 40.19 and 29.78 wt.% calculated by Eq. (2). WL =
Fig. 5. XRD diffractograms of (a) diatomite, (b) LA and (c) form-stable LA/diatomite composite PCM.
247
HF HL
(2)
where WL is the weight percentage of LA in LA/diatomite composite form-stable PCM; HF is the latent heat of LA/diatomite composite form-stable PCM, J/g; HL is the latent heat of pure LA, J/g. The LA/diatomite composite form-stable PCM can potentially be utilized as thermal storage material in the exterior wall of
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Table 4 Latent heat storage properties of LA and the LA/diatomite composite with the various LA content. PCM
Melting point (◦ C)
Latent heat of melting (J/g)
Freezing point (◦ C)
Latent heat of freezing (J/g)
LA LA/diatomite composite (L0.6) LA/diatomite composite (L0.5)
44.4 40.9 40.7
141.9 57.4 43.2
40.9 38.7 37.6
−142.3 −57.2 −42.38
Table 5 Thermal energy storage characteristics of different form-stable composite PCMs in literature. Composite PCM
Melting point (◦ C)
Freezing point (◦ C)
Latent heat (J/g)
Refs.
Capric–myristic acid (20 wt.%)/VMT n-Nonadecane (50 wt.%)/cement Paraffin/bentonite Paraffin/porous silica ceramic Capric–myristic acid/VMT/2 wt.%EG Caprice–myristic acid (20 wt.%)/VMT PMMA/heptadecane microcapsule Capric–myristic acid/polyethylene terephthalate nanofibers Paraffin/diatomite Lauric acid (33.3 wt.%)/activated carbon Caprice–myristic acid (66 wt.%)/diatomite Lauric acid/diatomite Lauric acid/diatomite
19.8 31.9 41.7 20.8 19.7 19.8 18.2 24.98 41.11 44.07 16.74 40.9 40.7
17.1 31.8 43.4 – 17.1 17.1 18.4 13.88 – 42.83 – 38.7 37.6
27.0 69.1 39.84 53.6 26.9 27.0 81.5 57.62 70.51 65.14 66.81 57.4 43.2
[29] [30] [15] [31] [29] [29] [32] [31] [3] [28] [16] Present study Present study
buildings with the PTT of about 40 ◦ C and high latent heat of about 45 J/g, absorbing lots of heat from surrounding air during the day when the ambient temperature exceed PTT and releasing large amounts of heat at night when the ambient temperature is below PTT of form-stable PCMs. Therefore, the exterior wall can retain thermal inertia around 40 ◦ C. Table 5 indicates the thermal storage properties of form-stable PCM in literature [3,15,16,28–32]. By comparison, it is obvious that the fatty acid has been widely used to prepare composite form-stable PCMs with the latent heat of 20–80 J/g. However, LA/diatomite composite PCM with proper PTT has not been investigated. 3.5. Thermal stability of form-stable LA/diatomite composite PCM With PCM’s working at different temperatures, it is very necessary to study the thermal stability of form-stable composite PCM. The TGA curves of diatomite, LA and form-stable LA/diatomite composite PCM are shown in Fig. 7. As shown in Fig. 7a, no thermal degradation stages are detected from 25 to 600 ◦ C, revealing the
Fig. 7. TGA curves of (a) pure diatomite, (b) pure LA and (c) form-stable LA/diatomite composite PCM.
excellent thermal stability of diatomite. The weight of LA is nearly invariable when the temperature is lower than 147 ◦ C. It sharply decreases to 0.61 wt.% once the temperature reaches 189 ◦ C. The maximum decomposition rate appears at 176 ◦ C. As can be seen from Fig. 7b, the degradation of LA in LA/diatomite composite formstable PCM starts at 157 ◦ C and ends at 203 ◦ C, indicating that the thermal stability increases after LA is impregnated into the pores of diatomite. Moreover, the TGA curve confirms that the impregnation ratio has reached 30 wt.% lower than 40.19 wt.% in L0.6 mentioned above, due to that the TGA sample (L0.6) was put on a dry filter paper at 80 ◦ C before analysis to remove the LA on the surface of LA/diatomite composite mentioned above. The TGA result (L0.6) is same as that of DSC (L0.5) in Table 4, proving that the maximum impregnation ratio is 30 wt.%, not 40 wt.% observed by eyes. These results reveal that the LA/diatomite composite PCM has good thermal stability. 4. Conclusions LA/diatomite composite form-stable PCM has been prepared using a direct impregnation method. The structure and thermal properties of LA/diatomite composite have been characterized by FTIR, SEM, EDS, XRD, DSC and TGA. The FTIR and XRD results indicate that there are only physical interactions existing between LA and diatomite, and the crystalline size of LA in LA/diatomite composite becomes small due to the limited pores of diatomite. Obtained from DSC results, the melting and freezing temperatures of LA/diatomite composite form-stable PCM are 40.9 ◦ C and 38.7 ◦ C, respectively. The latent heat of LA/diatomite composite reaches 57.2 J/g. The decomposition of LA in LA/diatomite composite occurs from 157 to 203 ◦ C, higher than that of pure LA, revealing that the thermal stability of LA increases after the LA is impregnated into the porous diatomite. There is no degradation of LA in LA/diatomite composite around the PTT. The LA/diatomite composite has a good thermal stability. Based on all results, the LA/diatomite composite form-stable PCM can be used as energy storage material for passive solar space heating or decreasing indoor temperature fluctuating in buildings due to its good thermal properties (i.e. proper PTT of 38–41 ◦ C and high phase enthalpy of 57.2 J/g), thermal stability and good compatibility of the LA/diatomite composite with concrete. The prepared LA/diatomite composite form-stable PCM has far-reaching application potential in building fields.
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