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Enhanced properties of diatomite-based composite phase change materials for thermal energy storage
Renewable Energy 147 (2020) 265e274
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Enhanced properties of diatomite-based composite phase change materials for thermal energy storage Chuanchang Li*, Mengfan Wang, Baoshan Xie, Huan Ma, Jian Chen School of Energy and Power Engineering, Changsha University of Science and Technology, Changsha, 410114, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2019 Received in revised form 14 August 2019 Accepted 1 September 2019 Available online 2 September 2019
Diatomite-based composite phase change materials (PCMs) with enhanced properties were prepared by vacuum impregnation method for thermal energy storage. Diatomite is used as a supporting matrix in composite PCMs due to its natural high porosity and relatively low price. Particularly, diatomite after microwave-acid treatment (Dm) with higher loading capability compared to raw diatomite was used as a supporting matrix to stabilize lauric acid-stearic acid (LA-SA). To improve the thermal conductivity of the composite PCMs, expanded graphite (EG) was introduced as a support matrix together with Dm at a weight ratio of 1:10 of EG to Dm. FTIR and XRD analysis indicated that the crystal structure of diatomite was not affected by the microwave-acid treatment and addition of EG, and no chemical reaction occurred between LA-SA and supports during impregnation. Devised LA-SA/Dm/EG with 72.2% loadage of LA-SA and a phase change temperature of 31.17 C had latent heat value of 117.30 J g1 for melting and 114.50 J g1 for freezing, respectively. The thermal conductivity of LA-SA/Dm/EG was up to 3.2 times that of LA-SA/Dm. The thermal infrared images presented that the transient temperature response of LA-SA/ Dm was enhanced after introducing of 2.5 wt% EG. Therefore, the prepared composite phase change material is a potential candidate for thermal energy storage in the indoor thermal comfort system, especially the building envelope. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Lauric acid-stearic acid Diatomite Phase change materials Thermal energy storage Enhanced properties
1. Introduction For today's society, energy consumption and environmental pollution are two critical issues of sustainable development [1e5]. Buildings take up a vital part of the entire world energy consumption with 39% [6]. With the rapid development of urbanization, building energy consumption increases along with people's pursuit of a comfortable living environment [7e10]. Thermal energy storage (TES) as an effective method for building energy conservation is playing an increasingly important role in energy conservation and emission reduction [11e13]. Therefore, the idea of the minimum energy consumptions in buildings directs the researchers to build up the up-to-date TES composite materials and investigate their potential for increasing of thermal comfort in buildings [14e16]. Latent heat thermal energy storage (LHTES), incorporated composite phase change material (PCM), has been recognized as the most promising method [17,18]. LHTES is able to shift from peak to off-peak periods during the period of energy
* Corresponding author. E-mail addresses: [email protected], [email protected] (C. Li). https://doi.org/10.1016/j.renene.2019.09.001 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
consumption, adjusting the contradiction between energy supply and demand [19e21]. Up to now, the LHTES has been relatively extensively applied in building energy conservation, such as phase change cement board [22], building insulation wall [23e25], and solar space heating and cooling applications in buildings [26]. Binary fatty acid, characterized by high latent heat, varying and adjustable phase transition temperature, low super-cooling, nontoxic and economy, has been widely used in LHTES [24,27,28]. However, it is limited by a number of deficiencies, such as low thermal conductivity and inferior thermal durability, which can cause leakage of PCM in the process of solid-to-liquid phase change [29e31]. Therefore, many types of research have been committed to constructing the form-stable PCMs (fs-PCMs) to overcome these problems [32e36]. The porous minerals have been generally used as supports to stabilize the PCM and form the fs-PCMs, such as kaolinite [33], bentonite [37], palygorskite [38], expanded perlite [39], expanded vermiculite [23], halloysite [40], sepiolite [41], and diatomite [42,43]. Diatomite, a type of natural amorphous siliceous mineral from geological deposits, has already gained the attention of the scholars due to the highly porous structure, excellent absorption capacity,
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chemical stability, and relatively low price [44,45]. For example, Qian et al. [46] skillfully employed the diatomite as support material to stabilize the PEG, LiNO3, and Na2SO4 that represent the low-, middle- and high-temperature storage medium, obtained the corresponding composites with latent heats of the melting 105.70 J g1, 250.7 J g1, and 101.59 J g1, respectively. Meanwhile, Qian et al. [47] proposed that the pores of raw diatomite are commonly blocked by several types of impurities, which will decrease the micro-porosity, filtration efficiency, and commercial applicability of the diatomite. Thus, raw diatomite needs to be purified. Similar studies presented positive and promising results in previous literature [48,49]. Zhang et al. [48] commendably tested the modification approaches including acid treatment, high-speed shear, and ultrasound on the microstructure of diatomite, not only improved the diatomite content but also modified the pore structure of diatomite. Simultaneously, considering the low thermal conductivity of porous minerals, activated carbon (AC) [45], carbon nanotubes (CNTs) [50e53], graphite sheets [54], and expanded graphite (EG) [55,56] were introduced into the fs-PCMs to improve the thermal conductivity of fs-PCMs due to the superior performance of carbon materials [57e60]. Wen et al. [56] used diatomite to absorb CA-LA and added the EG to enhance thermal conductivity, and the thermal conductivity of CA-LA/diatomite contained 10 wt% EG was successfully increased by 113.2%. However, the thermal enthalpy showed decreased due to the less impregnation ratio of PCMs in the composites. Sarı et al. [61] resoundingly enhanced thermal conductivity as well as increasing its incorporation ratio: PEG was successfully confined as 42.8, 44.5 and 51.7 wt% in the novel raw diatomite (RD)/CNTs/PEG composite PCMs (latent heat capacity between 53.8 and 62.9 J g1) including 0.57, 1.70 and 2.50 wt% CNTs; compared to the RD/PEG composite, the thermal conductivities of RD/CNTs/PEG composites were enhanced between 73% and 93% as well as the latent heat capacity of them was increased in the range of 5e31%; the increasing latent heat capacity only attribute to the CNTs. Based on the above research, it is possible to simultaneously rely on diatomite and high thermal conductivity material to synchronously increase latent heat capacity and thermal conductivity. In this study, it is trying to enhance the loading capability of raw diatomite and synchronously increase the thermal conductivity of diatomite-based composite PCMs. A binary fatty acid with suitable phase change temperature was prepared by means of melt blending of lauric acid (LA) and stearic acid (SA) at certainly blending ratio. Acid treatment with hydrochloric acid (8%) and microwave was firstly conducted on raw diatomite to enhance the loading capability. Expanded graphite (EG) was then introduced into the treated diatomite (Dm) to increase thermal conductivity. The devised LA-SA/Dm/EG composite PCM was fabricated through the vacuum impregnation method. Moreover, the study on microstructure, chemical stability, and thermal energy storage performance was carried out to investigate the application potential of composite PCMs. 2. Experimental 2.1. Materials The raw diatomite (325 mesh) were obtained from Jilin Jiapeng Diatomite Co., Ltd. China. The typical chemical composition given by the manufacturer is (mass%): SiO2, >70%, Fe2O3, <0.1%, and the loss on ignition, 10.0%. Lauric acid (LA, C12H24O2) was purchased from Shanghai Chemical Industry Chemical Reagents. Stearic acid (SA, CH3(CH2)16COOH) was supplied by Tianjin Hengxing Chemical Reagent Co., Ltd., China. Expanded graphite (EG) was provided by Qingdao Tengshengda Carbon Co., Ltd. China.
2.2. Preparation The preparation process of composite PCMs is shown in Fig. 1. In this work, microwave heating was chosen in this study instead of the traditional heating method due to the uniform and rapid heating of the microwave technique [62,63]. At first, in order to enhance the loading capability of diatomite for PCM, the raw diatomite (D) was conducted by microwave-acid treatment: 16.0 g D was mixed with an 8% HCl solution (48.0 g) in a 500 ml beaker and the irradiated at 700 W for 5 min in microwave oven and washed with distilled water until chlorine ion was not detected by AgNO3, followed by vacuum suction filtration and drying at 105 C for 15 h, the sample was labeled as Dm. Also experiments to treat raw diatomite at different microwave powers and times have also been carried out and are provided in the Supplementary Information (SI-1) for illustrating that it has better processing effect at 700 W for 5 min. The main chemical compositions of D and Dm are listed in Table 1. As seen in this table, the content of impurities such as MgO, CaO, and P2O3 reduced, only the amount of SiO2 increased, indicating that the impurities attached to the surface of diatomite were removed by microwave-acid treatment. Before preparing the LA-SA composition, the mixing ratio of LA and SA must be determined. LA was mixed with SA in a mass ratio of 7:3 at a temperature above the melting point to form eutectic mixtures [24]. 14.0 g LA and 6.0 g SA were heated in a thermostatic water bath at 90 C and stirred fully for 30 min to obtain the LA-SA. 4.0 g of support material was added to 16.0 g of excess LA-SA in a conical flask, which was connected to the vacuum pump by a backflow prevention device (Fig. S1). After the air pressure of the conical flask was maintained at 0.1 MPa for 5 min, the conical flask was heated in a constant temperature water bath. The process was to be continued for 30 min at 80 C and then air was then allowed to enter the flask again. Moreover, the mixtures were treated in an ultrasonic water bath apparatus (80 C, 8 min) to further accelerate absorption. The composites, thermally filtration at 80 C for 4 h, was cooled and collected to obtain the final product. These composite PCMs prepared using the D and Dm supports were denominated as LA-SA/D and LA-SA/Dm, respectively. To improve the thermal conductivity of the composite PCM, the mixture of EG and Dm (mEG: mDm ¼ 1:10) were uniformly mixed to form a supporting matrix, which was used to stabilize LA-SA by abovementioned vacuum impregnation to obtain LA-SA/Dm/EG, and the content of EG in LA-SA/Dm/EG is 2.5 wt% according to the subsequent TG results. Also, the mixtures of EG and Dm with mEG: mDm ¼ 1:30 and mEG: mDm ¼ 1:1 were used as supporting matrix to prepare the LA-SA/Dm/EG with 1.2 wt% EG and LA-SA/Dm/EG with 4.7 wt% EG for determining of the EG addition ratio, respectively. It indicated that the mEG: mDm ¼ 1:10 and 2.5 wt% EG are the suitable ratio and addition amount, respectively, and the detailed results and analysis were listed in the Supplementary Information (SI-2). 2.3. Characterization Differential scanning calorimetry (DSC, TA Instruments Q2000, temperature accuracy of ±0.1 C, calorimetric precision of ±0.05%) was used to determine the latent heat and phase change temperature of composite PCMs, which was carried out in N2 atmosphere from 0 C to 60 C at a heating/cooling rate of 5 C min1. The loading capability of diatomite-based composites was determined by thermogravimetric-differential scanning calorimetry (TG-DSC) of HENVEN HCT-3 with a balance sensitivity of 0.1 mg and temperature accuracy of ±0.1 C, the samples were heated form 25 Ce800 C at a speed of 10 C min1 under N2 atmosphere. D8 ADVANCE analyzer (Cu-Ka), running at a current of 40 mA, a voltage of 40 kV, scan range from 5 to 80 and step size of 0.02 , was used to
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Fig. 1. Preparation schematic for the LA-SA/Dm composites PCMs.
Table 1 Chemical compositions of diatomite before and after modification (wt.%). Samples
SiO2
Al2O3
Fe2O3
K2O
Na2O
MgO
CaO
P2O3
D Dm
71.28 72.71
16.40 16.31
4.60 4.11
3.30 3.23
2.00 1.94
0.85 0.59
0.64 0.20
0.14 0.07
XRD peaks of LA-SA and supports were contained in the XRD patterns of diatomite-based composites without additional appearance, manifesting that there was no chemical interaction between the LA-SA and the supports during the preparation of the composites. 3.2. Chemical structure
obtain the X-ray diffraction patterns (XRD) of all samples. The chemical structure analysis of all samples was conducted on a Fourier transform infrared spectroscopy (FTIR, IRtracer-100AH) in the range of 4000e400 cm and mixed with KBr to compress into tablets for measurement at room temperature. The morphologies of samples were characterized by using scanning electron microscopy (SEM; Zeiss Sigma 500). In order to further explore the influence of microwave-acid treatment on D, the ASAP 2020 surface area with an accuracy of 2% was used to obtain the nitrogen gas absorption-desorption isotherms of D and Dm. Thermal conductivities of LA-SA and diatomite-based composites were detected by a DRXeIIeRW thermal conductivity test instrument with an accuracy of 3% (Xiangtan Huafeng Instrument Manufacturing Co., Ltd., Hunan, China.). The thermal infrared imager (FLUKE Ti450, noise equivalent temperature difference is 0.05 C) was used to record the temperature of the sample during thermal storage/ release process. In this test, the samples were compressed into round sheets with a thickness of about 1.8 mm and placed in a mold made of foil with good thermal conductivity (the photo of testing samples is presented in Fig. S5). The molds containing LA-SA, LASA/Dm, and LA-SA/Dm/EG were then placed in the thermostatic heating plate with a temperature of 65 ± 1 C.
The FTIR spectra of all samples are shown in Fig. 3. In the spectra of D (Fig. 3a), the bands at 470 cm1, 796 cm1, and 1060 cm1 were assigned to the bending vibration of OeSieO, SiOeH vibration, and -SieOeSi- stretching vibration, respectively. The FTIR spectra of Dm remained unchanged but the characteristic peak signal of Dm was more intense than that of D. It is because the microwave-acid treatment reduced or eliminated other oxides of diatomite relative to SiO2 (Table 1), making the signal of the functional group stronger. In the spectrum of EG, the band at 1632 cm1 was attributed to the stretching vibration of the C]C functional group. For the LA-SA sample, the symmetrical stretching vibration of eCH2 (2920 cm1) and eCH3 (2850 cm1), the deformation vibrations of eCH2 and eCH3 (1470 cm1), the in-plane bend vibration (1302 cm1) and out-of-plane bending vibration (933 cm1) of OeH, the stretching vibration of C]O (1701 cm1), and the winging vibration of eCH2 (721 cm1) were detected. Moreover, it can be clearly seen from Fig. 3b that all the above mentioned characteristic absorption bands of supports and LA-SA were included in the spectra of diatomite-based composite PCMs. There were no significant new peaks observed, indicating that no chemical reactions took place between supporting materials and LA-SA. Therefore, the FTIR spectra of diatomite-based composites were the result of the interaction between LA-SA and supports through physical effects.
3. Results and discussion 3.3. Morphology and microstructures 3.1. Crystallization characteristics Fig. 2 shows the XRD patterns of supports and composite PCMs. In Fig. 2a, the pattern of diatomite had a broad peak in the range of 2q ¼ 20e30 , which were resulted from the typical non-crystalline diffraction of SiO2 [46]. The quartz crystalline phase (2q ¼ 20.8 and 26.6 ) was also found in Fig. 2a. Compared the XRD spectrums of D, Dm, and Dm/EG, it can be clearly seen that there was no significant change in XRD pattern of Dm. It indicates that the crystal structure of diatomite was not affected by the microwave-acid treatment and addition of EG, only the characteristic peak intensity of diatomite in Dm/EG weakened. From Fig. 2b, the reflections at 2q ¼ 21.71 and 23.90 were the characteristic peaks of the LA-SA, which went up with the content of LA-SA in diatomite-based composites. All of the
Fig. 4 displays the SEM images of D, Dm, and corresponding composites, all of which had a cylindrical structure and numerous pores. It can be clearly observed that the pore structure on the surface of the diatomite was exposed after microwave-acid treatment (Fig. 4b) comparing to the untreated sample (Fig. 4a), indicating that the impurities blocking the channel were removed. That is to say, it made Dm easier for PCM to penetrate its storage space than D. After the impregnation, the surfaces of D-matrixes were occupied entirely by LA-SA in the composites (Fig. 4c and d) and became smooth, and the protuberant edge also got even and glossy, indicating that the LA-SA was well fixed in the pores of D and Dm. Furthermore, the morphology of LA-SA/D and LA-SA/Dm still maintained the tubular structure, signifying that the loadage was
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Fig. 2. XRD patterns of (a) supporting matrixes; (b) LA-SA and the composites.
Fig. 3. FTIR spectra of (a) supporting matrixes; (b) LA-SA and diatomite-based composite PCMs.
the maximum by thermal filter removing the attachment. As shown in Fig. 4e, although the Dm particles were relatively independent form EG while the EG was uniformly introduced, some particles scattered inside the EG (Fig. S6a). After the impregnation, the surfaces of Dm/EG were occupied entirely by LA-SA in the LASA/Dm/EG (Fig. 4f & Fig. S6b). For the entire networks of heat storage and heat transfer, the EG as an intermediate medium was linked to the support material making the networks more compactness. 3.4. Thermal stability and loading capability The TG-DSC curves of diatomite-based composites and LA-SA are presented in Fig. 5. There was a single degradation process for the diatomite-based composites attributing to the decomposition of LA-SA, and no apparent decomposition reaction or mass loss was observed from 25 C to 150 C. The thermal stability of the diatomite-based composites was appraised in regards to their degradation temperatures (T5%) while the mass losses are more than 5% (Fig. 5a). The diatomite-based composites decomposed at a high temperature of above 169 C (T5%). For the LA-SA, a low T5% at 150.7 C was observed, manifesting diatomite-based composites had higher thermal stability than LA-SA. The occurrences of 5% weight loss (T5%) were at 169.3 C, 173.6 C, and 178.0 C for LA-SA/ D, LA-SA/Dm, and LA-SA/Dm/EG, which were much higher than their operating temperature in the built environment, indicating that the diatomite-based composites had excellent thermal stability. From the homologous DSC curves (Fig. 5b), an endothermic
peak at around 217 C in the curve of LA-SA indicated the LA-SA disintegrated. The endothermic peaks of LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG were at 220 C, 224 C, and 230 C, respectively. It indicates that the thermostability of LA-SA was improved by supports. Furthermore, the loadage of LA-SA in the diatomite-based composites was ascertained by the TG. The maximum mass loss of LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG at 500 C were 41.3%, 51.7%, and 72.2%, respectively. It is found that as the higher loadage of LA-SA in diatomite-based composites, the measured enthalpies of diatomite-based composites were enhanced (Fig. 5b). According to it, the loading capability value of LA-SA (LC, %) in diatomite-based composites was calculated by:
LC ¼
mLASA mLASA 100% ¼ 100% msupport 1 mLASA
(1)
where mLASA and msupport were the mass of LA-SA (g) and diatomite-based support (g), respectively. Thus, the loading capability value of LA-SA (LC, %) was calculated as 70.36%, 107.04%, and 259.71%, respectively. It suggest that the LC of diatomite was improved by microwave-acid treatment and further enhanced by introducing EG. The loading capability of diatomite was promoted due to the larger specific surface areas and higher porosity (according to subsequent BET results) (see Fig. 7). 3.5. Phase change behavior DSC was used to explore the phase change behavior of pure LA-
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Fig. 4. SEM images of (a) D, (b) Dm and corresponding composites (c) LA-SA/D and (d) LA-SA/Dm; SEM images of (e) Dm/EG and (f) LA-SA/Dm/EG.
Fig. 5. (a) TG and (b) TG-DSC curves of diatomite-based composite PCMs.
SA, LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG. The DSC results are shown in Fig. 6 and Table 2. From Fig. 6, there was only one endothermic peak and one exothermic peak in the heating and cooling phase transition process of LA-SA, indicating LA and SA were well combined to form the eutectic mixture with a steadystate. The melting phase change temperatures of LA-SA, LA-SA/D,
LA-SA/Dm, and LA-SA/Dm/EG were 31.50 C, 31.16 C, 31.16 C, and 31.17 C, respectively. The melting temperature indicates that the prepared fs-PCMs are supposed to be applied in the indoor thermal comfort system on account of the phase change temperature at around 10e40 C, which is suitable for the thermal comfort application in buildings [44]. The melting (DHm) and freezing (DHf)
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Fig. 6. DSC curves of LA-SA and LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG.
latent heat values of LA-SA were measured as 167.0 J g1 and 166.9 J g1, respectively. Through a comparison with LA-SA/D, Dm presented a positive effect on latent heat of their composites (Table 2). LA-SA/D had a melting and freezing latent of 59.60 J g1 and 55.68 J g1, respectively, while LA-SA/Dm was ascertained to be 76.16 J g1 and 72.04 J g1, and LA-SA/Dm/EG was calculated to be 117.30 J g1 and 114.50 J g1, respectively. It is clearly to obtain that the thermal storage capacity was improved by microwave-acid treatment of diatomite and farther enhanced by adding EG. In addition, the measured enthalpies of diatomite-based composites were lower than their corresponding theoretical values (for the LA-SA/D, DHth 68.97 J g1; for the LA-SA/Dm, DHth 86.34 J g1; for the LA-SA/Dm/ EG, DHm 120.57 J g1). As a matter of fact, the latent heat value is also influenced by interactions between PCM and supports, relating to the crystallinity of the PCM in the composites [64]. Crystallization ratio (Fc) of LA-SA was given by:
Fc ¼
DHcomposite 100% DHPCM b
(2)
where DHcomposite and DHPCM were the enthalpies of composite PCMs and LA-SA, respectively; b represented LA-SA mass fraction in composites. The crystallinity of LA-SA in the LA-SA/Dm was 88.21%, which was 2.1% higher than that of LA-SA/D (86.41%). The LA-SA in the LA-SA/Dm/EG has the highest Fc of 97.28%. The effective energy stored per unit mass of the LA-SA (Eef) was used to assess the effectiveness of the LA-SA in different composites (Table 2). The LASA in the LA-SA/Dm had a higher Eef (147.31 J g1) than that of LA-
SA/D (144.30 J g1), and the LA-SA/Dm/EG had the highest Eef of 162.46 J g1. For LA-SA/D and LA-SA/Dm composites, the physical interactions including surface tension forces and capillary forces only occur between LA-SA and diatomite (D and Dm). Compared with the Fc of LA-SA in the LA-SA/Dm, LA-SA in LA-SA/D had a lower Fc. Previous researches have demonstrated that the phase change can hardly occur on the PCM inside the tiny pores due to the confinement imposed by interaction on molecule motion [47]. In order to unveil this question, the pore structures of D and Dm were investigated by nitrogen gas adsorption-desorption isotherms (Fig. S7). The specific surface areas and pore volume of D and Dm were 61.1122 m2 g1 and 0.100872 m3 g1, and 68.9325 m2 g1 and 0.101852 m3 g1, respectively (Table 3). Therefore, the specific surface area and pore volume of substrates were increased by microwave-acid treatment. The pore sizes distributions of D and Dm are shown in Fig. 7. As shown in the inset of Fig. 7a, D and Dm had the semblable status of the pore size distribution for incremental pore volume and the pore volume was close to zero while pore width was greater than 150 nm. The incremental pore volume of 5e50 nm occupied primary, following by 60e120 nm. The pore volume was mildly increased by microwave-acid treatment of diatomite while the pore width less than 7 nm (inset of Fig. 7a). The cumulative pore volume of Dm was enhanced compared to that of D (Fig. 7b). It is found that 90% of the cumulative pore volume of Dm was distributed in pore width less than 60 nm but that of D less than 70 nm. According to the above results, it is drawn that more LA-SA were stabilized in the pore width of 5e60 nm of Dm while that in the pore width of 5e70 nm of D. It indicates that LA-SA in Dm was more stabilization than that in D. This conclusion is consistent with the TG analysis results. Fig. 7c and d further studied the incremental pore area and cumulative pore area of D and Dm. The incremental pore area presented a peak which corresponds mesoporous with a diameter of 2.5e4 nm, and the incremental pore area of Dm was a little larger than that of D (Fig. 7c). It is observed that the cumulative pore area of Dm rose dramatically comparing with D (Fig. 7d). This phenomenon is because of pores in diatomite increased the pore area after microwave-acid treatment. The cumulative pore area provided higher surface area and more rough surface for diatomite which was a benefit to support and stabilize SA-LA. After vacuum impregnation, the LA-SA was sucked into the pore of supporting materials, it can be concluded that there were more free-moving and melted LA-SA in Dm, which was beneficial to the crystallinity of LA-SA. Therefore, the crystallinity of LA-SA in the LA-SA/Dm composite was improved than that in the LA-SA/D. The maximum loadage of LA-SA in LA-SA/Dm was enhanced by 25.2% than that of LA-SA/D but was lower than the increment of latent heats value (27.8%). The latent heats value of composite was significantly affected by the crystallization of LA-SA, so it can be speculated that the residual increasing percentage of latent heats value was resulted from the higher crystallinity of LA-SA in the LA-SA/Dm
Table 2 Thermal properties of pure LA-SA and the diatomite-based composites. Samples Melting Latent heat of Freezing temperature (Tm, melting (DHm, J g-1) temperature (Tf, C) C)
Latent heat of Loadage Theoretic values of freezing (DHf, J g-1) (b, %) DHm (DHth, J g-1)
LA-SA LA-SA/D LA-SA/ Dm LA-SA/ Dm/ EG
Crystallinity of Efficient energy per unit mass SA (Fc, %) of LA-SA (Eef, J g-1)
31.50 31.16 31.16
167.00 59.60 76.16
28.93 29.27 29.16
166.90 55.68 72.04
100 41.3 51.7
e 68.97 86.34
100 86.41 88.21
e 144.30 147.31
31.17
117.30
30.02
114.50
72.2
120.57
97.28
162.46
Note: DHth ¼ DHpure b; Eef ¼ DHpure Fc.
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Table 3 Porous characteristics of the D and the Dm. Samples
BET surface area (m2 g1)
Single point adsorption total pore volume (m3 g1)
D Dm
61.1122 68.9325
0.100872 0.101852
Fig. 7. The pore size distribution by Invalid Model: N2 - DFT Model for (a) incremental pore volume, (b) cumulative pore volume, (c) incremental pore area, and (d) cumulative pore area of the D and Dm.
composite. For LA-SA/Dm/EG, which was prepared by adding a small amount of EG to replace the same mass of Dm, LA-SA interacts with Dm and EG. LA-SA in the LA-SA/Dm/EG had higher crystallinity than that in the LA-SA/Dm, indicating the crystallization condition of LASA in EG was better than that in Dm. It is the reason that the efficient energy per unit mass of LA-SA increases by degrees in LA-SA/D (144.30 J g1), LA-SA/Dm (147.31 J g1), and LA-SA/Dm/EG (162.46 J g1) (Table 2). As discussed above, LA-SA/Dm/EG, having both suitable phase change temperature and excellent heat characteristics, is the optimum candidate for application in thermal energy storage. The longterm thermal cycling stability is also one of the discriminating criteria for PCMs in practical TES applications. A 50 thermal cycles testing was conducted to access the reliability of LA-SA/Dm/EG composite PCMs and shown in Fig. 8. There was no significant change in latent heat values and phase change temperatures during the 50 cycles of release and storage process. The melting and freezing temperatures of the fs-PCM3 after 50 cycles were increased by 0.24 C and 0.26 C, respectively. Its LHTES capacity was decreased by 1.40% and 1.78%, respectively, and tend to be stable. It suggests that the prepared composite PCMs possesses great stability in thermal energy storage.
Fig. 8. DSC curves of LA-SA/D/EG after 50 thermal cycles.
3.6. Thermal conductivity Fig. 9 shows the measured thermal conductivity of pure LA-SA
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energy storage and release ability. In addition, the thermal properties and the thermal conductivities of the LA-SA/Dm/EG composite PCMs were compared with that of similar diatomite-based composite (Table 4) [26,44,51,55,56,61,66]. The LA-SA/Dm/EG composite that was prepared in this study showed obvious advantages over the reported materials, such as considerable latent heat capacities, appropriate melting temperature, and higher thermal conductivity, indicating that LA-SA/Dm/EG composite has the potential for application in the thermal energy storage, especially in the built environment. 3.7. Temperature response behavior
Fig. 9. Thermal conductivities of LA-SA and composites.
and diatomite-based composites. The thermal conductivity values were 0.27 W m1 K1, 0.17 W m1 K1, 0.16 W m1 K1, and 0.51 W m1 K1 for LA-SA, LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG, respectively. In comparison with LA-SA/Dm, the thermal conductivity of LA-SA/Dm/EG was significantly boosted with the enhancing level as high as 214%. Addition of the EG in LA-SA/Dm/EG was the major attributed reason [65]. The mass ratio of Dm/EG to LA-SA/Dm/ EG was 27.8%, indicating that the addition of only 2.5 wt% EG resulted in a significant improvement in the thermal conductivity of LA-SA/Dm/EG. Therefore, the LA-SA/Dm/EG has good thermal
Thermal infrared imager recorded the endothermic and exothermic processes of the samples. The absorption and release process curves of LA-SA and LA-SA/Dm/EG are shown in Fig. 10. The LA-SA/Dm/EG showed obviously faster heat storage and release rates than LA-SA. In the heating process (Fig. 10a), the center temperature (Tc) and average temperature (Ta, the acquisition method is as shown in the Supplementary Information (SI-3)) of the LA-SA/Dm/EG demonstrated a low heat rate at t ¼ 29 s, due to the occurrence of the phase change process, while it took more time for LA-SA to complete the temperature rising process. During cooling, likewise, Tc and Ta of LA-SA/Dm/EG changed smoothly and cooled into a stable state after 33 s (Fig. 10b), but LA-SA still stayed at the temperature descending state. It manifests that LA-SA/Dm/EG had the superior instantaneous heat absorption compared to LA-SA when both samples were placed on the heating platform at the same time. Furthermore, the heating and cooling process of the fs-PCMs
Table 4 Comparison of the thermal properties of the prepared LA-SA/Dm/EG composite with that of some materials in literature. Composite PCMs
Melting point ( C)
Latent heat of melting (J g1)
Freezing point ( C)
Latent heat of freezing (J g1)
Thermal conductivity (W m1 K1)
References
Paraffin/calcined diatomite Myristate/diatomite/5 wt% EG Laurate/diatomite/5 wt% EG Paraffin/diatomite/ MWCNTs PA-CA/diatomite/5 wt% EG PEG/diatomite/3 wt% EG CA-LA/diatomite/10 wt% EG PEG/Diatomite/2.50 wt% CNTs LA-SA/Dm/2.5 wt% EG
33.04 45.86
89.54 96.21
52.43 44.63
89.80 92.46
e 0.22
[44] [26]
39.03 27.12
63.08 89.40
37.85 26.50
61.14 89.91
0.24 1.6e1.7
[51]
26.7 27.83 23.61 7.31
98.3 83.54 75.84 59.25
21.85 30.76 e 8.33
90.03 80.21 e 65.45
0.292 0.41 0.467 0.29
[55] [66] [56] [61]
31.17
117.30
30.02
114.50
0.51
This work
Fig. 10. Temperature response curves of LA-SA and LA-SA/Dm/EG at (a) heating time (b) cooling time.
C. Li et al. / Renewable Energy 147 (2020) 265e274
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Education (18B148); and the Hunan Province 2011 Collaborative Innovation Center of Clean Energy and Smart Grid. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.09.001. References
Fig. 11. Thermal infrared images LA-SA/Dm (left) and LA-SA/Dm/EG (right) at (a) heating time and (b) cooling time.
with and without EG were studied. Temperature rising can be clearly seen in the infrared images represented by the color change from black to purple (Fig. 11a). Within this time frame, 48 s was required for LA-SA/Dm/EG to rise from initial temperature (18 C) to 36 C. At the same time, it is observed that the surface temperature of LA-SA/Dm was lower than that of LA-SA/Dm/EG, and there was still significant changes in a short time (48e56 s). The same status happened during cooling (Fig. 11b), which signified that the transient temperature response of LA-SA/Dm/EG was greater than that of LA-SA/Dm. It can be concluded that LA-SA/Dm/EG possessed a great advantage compared to LA-SA and LA-SA/Dm in transient temperature response. This result is in accordance with the conclusion obtained in the thermal conductivity analysis. 4. Conclusion In this study, diatomite-based composites (LA-SA/D, LA-SA/Dm, LA-SA/Dm/EG) were synthesized by absorbing LA-SA into the diatomite-based support under melting impregnation process. The LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG composites melt at 31.16 C, 31.16 C, and 31.17 C, with the latent heat at melting of 59.60 J g1 for LA-SA/D, 76.16 J g1 for LA-SA/Dm, and 117.30 J g1 for LA-SA/Dm/ EG. The different loading capability of D and Dm for LA-SA were 70.36% and 107.04%, indicating that the diatomite processed the better loading capability after microwave-acid treatment. From SEM and BET, the surfaces and pore structure of Dm were modified, resulting in the higher crystallization of LA-SA in LA-SA/Dm. The transient temperature response properties of composites were in detail investigated by thermal infrared images, indicating it was improved by introducing EG. The LA-SA/Dm/EG can be considered as promising PCM for application in indoor thermal comfort system especially for the energy-efficient building envelope due to its excellent thermal properties including considerable latent heat capacities, appropriate melting temperature, and higher thermal conductivity. Acknowledgments This work was supported by the National Natural Science Foundation of China (51874047, 51504041); the Training Program for Excellent Young Innovators of Changsha (kq1802007); the Fund for University Young Core Instructors of Hunan Province; the Outstanding Youth Project of Hunan Provincial Department of
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Enhanced properties of diatomite-based composite phase change materials for thermal energy storage Chuanchang Li*, Mengfan Wang, Baoshan Xie, Huan Ma, Jian Chen School of Energy and Power Engineering, Changsha University of Science and Technology, Changsha, 410114, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2019 Received in revised form 14 August 2019 Accepted 1 September 2019 Available online 2 September 2019
Diatomite-based composite phase change materials (PCMs) with enhanced properties were prepared by vacuum impregnation method for thermal energy storage. Diatomite is used as a supporting matrix in composite PCMs due to its natural high porosity and relatively low price. Particularly, diatomite after microwave-acid treatment (Dm) with higher loading capability compared to raw diatomite was used as a supporting matrix to stabilize lauric acid-stearic acid (LA-SA). To improve the thermal conductivity of the composite PCMs, expanded graphite (EG) was introduced as a support matrix together with Dm at a weight ratio of 1:10 of EG to Dm. FTIR and XRD analysis indicated that the crystal structure of diatomite was not affected by the microwave-acid treatment and addition of EG, and no chemical reaction occurred between LA-SA and supports during impregnation. Devised LA-SA/Dm/EG with 72.2% loadage of LA-SA and a phase change temperature of 31.17 C had latent heat value of 117.30 J g1 for melting and 114.50 J g1 for freezing, respectively. The thermal conductivity of LA-SA/Dm/EG was up to 3.2 times that of LA-SA/Dm. The thermal infrared images presented that the transient temperature response of LA-SA/ Dm was enhanced after introducing of 2.5 wt% EG. Therefore, the prepared composite phase change material is a potential candidate for thermal energy storage in the indoor thermal comfort system, especially the building envelope. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Lauric acid-stearic acid Diatomite Phase change materials Thermal energy storage Enhanced properties
1. Introduction For today's society, energy consumption and environmental pollution are two critical issues of sustainable development [1e5]. Buildings take up a vital part of the entire world energy consumption with 39% [6]. With the rapid development of urbanization, building energy consumption increases along with people's pursuit of a comfortable living environment [7e10]. Thermal energy storage (TES) as an effective method for building energy conservation is playing an increasingly important role in energy conservation and emission reduction [11e13]. Therefore, the idea of the minimum energy consumptions in buildings directs the researchers to build up the up-to-date TES composite materials and investigate their potential for increasing of thermal comfort in buildings [14e16]. Latent heat thermal energy storage (LHTES), incorporated composite phase change material (PCM), has been recognized as the most promising method [17,18]. LHTES is able to shift from peak to off-peak periods during the period of energy
* Corresponding author. E-mail addresses: [email protected], [email protected] (C. Li). https://doi.org/10.1016/j.renene.2019.09.001 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
consumption, adjusting the contradiction between energy supply and demand [19e21]. Up to now, the LHTES has been relatively extensively applied in building energy conservation, such as phase change cement board [22], building insulation wall [23e25], and solar space heating and cooling applications in buildings [26]. Binary fatty acid, characterized by high latent heat, varying and adjustable phase transition temperature, low super-cooling, nontoxic and economy, has been widely used in LHTES [24,27,28]. However, it is limited by a number of deficiencies, such as low thermal conductivity and inferior thermal durability, which can cause leakage of PCM in the process of solid-to-liquid phase change [29e31]. Therefore, many types of research have been committed to constructing the form-stable PCMs (fs-PCMs) to overcome these problems [32e36]. The porous minerals have been generally used as supports to stabilize the PCM and form the fs-PCMs, such as kaolinite [33], bentonite [37], palygorskite [38], expanded perlite [39], expanded vermiculite [23], halloysite [40], sepiolite [41], and diatomite [42,43]. Diatomite, a type of natural amorphous siliceous mineral from geological deposits, has already gained the attention of the scholars due to the highly porous structure, excellent absorption capacity,
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chemical stability, and relatively low price [44,45]. For example, Qian et al. [46] skillfully employed the diatomite as support material to stabilize the PEG, LiNO3, and Na2SO4 that represent the low-, middle- and high-temperature storage medium, obtained the corresponding composites with latent heats of the melting 105.70 J g1, 250.7 J g1, and 101.59 J g1, respectively. Meanwhile, Qian et al. [47] proposed that the pores of raw diatomite are commonly blocked by several types of impurities, which will decrease the micro-porosity, filtration efficiency, and commercial applicability of the diatomite. Thus, raw diatomite needs to be purified. Similar studies presented positive and promising results in previous literature [48,49]. Zhang et al. [48] commendably tested the modification approaches including acid treatment, high-speed shear, and ultrasound on the microstructure of diatomite, not only improved the diatomite content but also modified the pore structure of diatomite. Simultaneously, considering the low thermal conductivity of porous minerals, activated carbon (AC) [45], carbon nanotubes (CNTs) [50e53], graphite sheets [54], and expanded graphite (EG) [55,56] were introduced into the fs-PCMs to improve the thermal conductivity of fs-PCMs due to the superior performance of carbon materials [57e60]. Wen et al. [56] used diatomite to absorb CA-LA and added the EG to enhance thermal conductivity, and the thermal conductivity of CA-LA/diatomite contained 10 wt% EG was successfully increased by 113.2%. However, the thermal enthalpy showed decreased due to the less impregnation ratio of PCMs in the composites. Sarı et al. [61] resoundingly enhanced thermal conductivity as well as increasing its incorporation ratio: PEG was successfully confined as 42.8, 44.5 and 51.7 wt% in the novel raw diatomite (RD)/CNTs/PEG composite PCMs (latent heat capacity between 53.8 and 62.9 J g1) including 0.57, 1.70 and 2.50 wt% CNTs; compared to the RD/PEG composite, the thermal conductivities of RD/CNTs/PEG composites were enhanced between 73% and 93% as well as the latent heat capacity of them was increased in the range of 5e31%; the increasing latent heat capacity only attribute to the CNTs. Based on the above research, it is possible to simultaneously rely on diatomite and high thermal conductivity material to synchronously increase latent heat capacity and thermal conductivity. In this study, it is trying to enhance the loading capability of raw diatomite and synchronously increase the thermal conductivity of diatomite-based composite PCMs. A binary fatty acid with suitable phase change temperature was prepared by means of melt blending of lauric acid (LA) and stearic acid (SA) at certainly blending ratio. Acid treatment with hydrochloric acid (8%) and microwave was firstly conducted on raw diatomite to enhance the loading capability. Expanded graphite (EG) was then introduced into the treated diatomite (Dm) to increase thermal conductivity. The devised LA-SA/Dm/EG composite PCM was fabricated through the vacuum impregnation method. Moreover, the study on microstructure, chemical stability, and thermal energy storage performance was carried out to investigate the application potential of composite PCMs. 2. Experimental 2.1. Materials The raw diatomite (325 mesh) were obtained from Jilin Jiapeng Diatomite Co., Ltd. China. The typical chemical composition given by the manufacturer is (mass%): SiO2, >70%, Fe2O3, <0.1%, and the loss on ignition, 10.0%. Lauric acid (LA, C12H24O2) was purchased from Shanghai Chemical Industry Chemical Reagents. Stearic acid (SA, CH3(CH2)16COOH) was supplied by Tianjin Hengxing Chemical Reagent Co., Ltd., China. Expanded graphite (EG) was provided by Qingdao Tengshengda Carbon Co., Ltd. China.
2.2. Preparation The preparation process of composite PCMs is shown in Fig. 1. In this work, microwave heating was chosen in this study instead of the traditional heating method due to the uniform and rapid heating of the microwave technique [62,63]. At first, in order to enhance the loading capability of diatomite for PCM, the raw diatomite (D) was conducted by microwave-acid treatment: 16.0 g D was mixed with an 8% HCl solution (48.0 g) in a 500 ml beaker and the irradiated at 700 W for 5 min in microwave oven and washed with distilled water until chlorine ion was not detected by AgNO3, followed by vacuum suction filtration and drying at 105 C for 15 h, the sample was labeled as Dm. Also experiments to treat raw diatomite at different microwave powers and times have also been carried out and are provided in the Supplementary Information (SI-1) for illustrating that it has better processing effect at 700 W for 5 min. The main chemical compositions of D and Dm are listed in Table 1. As seen in this table, the content of impurities such as MgO, CaO, and P2O3 reduced, only the amount of SiO2 increased, indicating that the impurities attached to the surface of diatomite were removed by microwave-acid treatment. Before preparing the LA-SA composition, the mixing ratio of LA and SA must be determined. LA was mixed with SA in a mass ratio of 7:3 at a temperature above the melting point to form eutectic mixtures [24]. 14.0 g LA and 6.0 g SA were heated in a thermostatic water bath at 90 C and stirred fully for 30 min to obtain the LA-SA. 4.0 g of support material was added to 16.0 g of excess LA-SA in a conical flask, which was connected to the vacuum pump by a backflow prevention device (Fig. S1). After the air pressure of the conical flask was maintained at 0.1 MPa for 5 min, the conical flask was heated in a constant temperature water bath. The process was to be continued for 30 min at 80 C and then air was then allowed to enter the flask again. Moreover, the mixtures were treated in an ultrasonic water bath apparatus (80 C, 8 min) to further accelerate absorption. The composites, thermally filtration at 80 C for 4 h, was cooled and collected to obtain the final product. These composite PCMs prepared using the D and Dm supports were denominated as LA-SA/D and LA-SA/Dm, respectively. To improve the thermal conductivity of the composite PCM, the mixture of EG and Dm (mEG: mDm ¼ 1:10) were uniformly mixed to form a supporting matrix, which was used to stabilize LA-SA by abovementioned vacuum impregnation to obtain LA-SA/Dm/EG, and the content of EG in LA-SA/Dm/EG is 2.5 wt% according to the subsequent TG results. Also, the mixtures of EG and Dm with mEG: mDm ¼ 1:30 and mEG: mDm ¼ 1:1 were used as supporting matrix to prepare the LA-SA/Dm/EG with 1.2 wt% EG and LA-SA/Dm/EG with 4.7 wt% EG for determining of the EG addition ratio, respectively. It indicated that the mEG: mDm ¼ 1:10 and 2.5 wt% EG are the suitable ratio and addition amount, respectively, and the detailed results and analysis were listed in the Supplementary Information (SI-2). 2.3. Characterization Differential scanning calorimetry (DSC, TA Instruments Q2000, temperature accuracy of ±0.1 C, calorimetric precision of ±0.05%) was used to determine the latent heat and phase change temperature of composite PCMs, which was carried out in N2 atmosphere from 0 C to 60 C at a heating/cooling rate of 5 C min1. The loading capability of diatomite-based composites was determined by thermogravimetric-differential scanning calorimetry (TG-DSC) of HENVEN HCT-3 with a balance sensitivity of 0.1 mg and temperature accuracy of ±0.1 C, the samples were heated form 25 Ce800 C at a speed of 10 C min1 under N2 atmosphere. D8 ADVANCE analyzer (Cu-Ka), running at a current of 40 mA, a voltage of 40 kV, scan range from 5 to 80 and step size of 0.02 , was used to
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Fig. 1. Preparation schematic for the LA-SA/Dm composites PCMs.
Table 1 Chemical compositions of diatomite before and after modification (wt.%). Samples
SiO2
Al2O3
Fe2O3
K2O
Na2O
MgO
CaO
P2O3
D Dm
71.28 72.71
16.40 16.31
4.60 4.11
3.30 3.23
2.00 1.94
0.85 0.59
0.64 0.20
0.14 0.07
XRD peaks of LA-SA and supports were contained in the XRD patterns of diatomite-based composites without additional appearance, manifesting that there was no chemical interaction between the LA-SA and the supports during the preparation of the composites. 3.2. Chemical structure
obtain the X-ray diffraction patterns (XRD) of all samples. The chemical structure analysis of all samples was conducted on a Fourier transform infrared spectroscopy (FTIR, IRtracer-100AH) in the range of 4000e400 cm and mixed with KBr to compress into tablets for measurement at room temperature. The morphologies of samples were characterized by using scanning electron microscopy (SEM; Zeiss Sigma 500). In order to further explore the influence of microwave-acid treatment on D, the ASAP 2020 surface area with an accuracy of 2% was used to obtain the nitrogen gas absorption-desorption isotherms of D and Dm. Thermal conductivities of LA-SA and diatomite-based composites were detected by a DRXeIIeRW thermal conductivity test instrument with an accuracy of 3% (Xiangtan Huafeng Instrument Manufacturing Co., Ltd., Hunan, China.). The thermal infrared imager (FLUKE Ti450, noise equivalent temperature difference is 0.05 C) was used to record the temperature of the sample during thermal storage/ release process. In this test, the samples were compressed into round sheets with a thickness of about 1.8 mm and placed in a mold made of foil with good thermal conductivity (the photo of testing samples is presented in Fig. S5). The molds containing LA-SA, LASA/Dm, and LA-SA/Dm/EG were then placed in the thermostatic heating plate with a temperature of 65 ± 1 C.
The FTIR spectra of all samples are shown in Fig. 3. In the spectra of D (Fig. 3a), the bands at 470 cm1, 796 cm1, and 1060 cm1 were assigned to the bending vibration of OeSieO, SiOeH vibration, and -SieOeSi- stretching vibration, respectively. The FTIR spectra of Dm remained unchanged but the characteristic peak signal of Dm was more intense than that of D. It is because the microwave-acid treatment reduced or eliminated other oxides of diatomite relative to SiO2 (Table 1), making the signal of the functional group stronger. In the spectrum of EG, the band at 1632 cm1 was attributed to the stretching vibration of the C]C functional group. For the LA-SA sample, the symmetrical stretching vibration of eCH2 (2920 cm1) and eCH3 (2850 cm1), the deformation vibrations of eCH2 and eCH3 (1470 cm1), the in-plane bend vibration (1302 cm1) and out-of-plane bending vibration (933 cm1) of OeH, the stretching vibration of C]O (1701 cm1), and the winging vibration of eCH2 (721 cm1) were detected. Moreover, it can be clearly seen from Fig. 3b that all the above mentioned characteristic absorption bands of supports and LA-SA were included in the spectra of diatomite-based composite PCMs. There were no significant new peaks observed, indicating that no chemical reactions took place between supporting materials and LA-SA. Therefore, the FTIR spectra of diatomite-based composites were the result of the interaction between LA-SA and supports through physical effects.
3. Results and discussion 3.3. Morphology and microstructures 3.1. Crystallization characteristics Fig. 2 shows the XRD patterns of supports and composite PCMs. In Fig. 2a, the pattern of diatomite had a broad peak in the range of 2q ¼ 20e30 , which were resulted from the typical non-crystalline diffraction of SiO2 [46]. The quartz crystalline phase (2q ¼ 20.8 and 26.6 ) was also found in Fig. 2a. Compared the XRD spectrums of D, Dm, and Dm/EG, it can be clearly seen that there was no significant change in XRD pattern of Dm. It indicates that the crystal structure of diatomite was not affected by the microwave-acid treatment and addition of EG, only the characteristic peak intensity of diatomite in Dm/EG weakened. From Fig. 2b, the reflections at 2q ¼ 21.71 and 23.90 were the characteristic peaks of the LA-SA, which went up with the content of LA-SA in diatomite-based composites. All of the
Fig. 4 displays the SEM images of D, Dm, and corresponding composites, all of which had a cylindrical structure and numerous pores. It can be clearly observed that the pore structure on the surface of the diatomite was exposed after microwave-acid treatment (Fig. 4b) comparing to the untreated sample (Fig. 4a), indicating that the impurities blocking the channel were removed. That is to say, it made Dm easier for PCM to penetrate its storage space than D. After the impregnation, the surfaces of D-matrixes were occupied entirely by LA-SA in the composites (Fig. 4c and d) and became smooth, and the protuberant edge also got even and glossy, indicating that the LA-SA was well fixed in the pores of D and Dm. Furthermore, the morphology of LA-SA/D and LA-SA/Dm still maintained the tubular structure, signifying that the loadage was
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Fig. 2. XRD patterns of (a) supporting matrixes; (b) LA-SA and the composites.
Fig. 3. FTIR spectra of (a) supporting matrixes; (b) LA-SA and diatomite-based composite PCMs.
the maximum by thermal filter removing the attachment. As shown in Fig. 4e, although the Dm particles were relatively independent form EG while the EG was uniformly introduced, some particles scattered inside the EG (Fig. S6a). After the impregnation, the surfaces of Dm/EG were occupied entirely by LA-SA in the LASA/Dm/EG (Fig. 4f & Fig. S6b). For the entire networks of heat storage and heat transfer, the EG as an intermediate medium was linked to the support material making the networks more compactness. 3.4. Thermal stability and loading capability The TG-DSC curves of diatomite-based composites and LA-SA are presented in Fig. 5. There was a single degradation process for the diatomite-based composites attributing to the decomposition of LA-SA, and no apparent decomposition reaction or mass loss was observed from 25 C to 150 C. The thermal stability of the diatomite-based composites was appraised in regards to their degradation temperatures (T5%) while the mass losses are more than 5% (Fig. 5a). The diatomite-based composites decomposed at a high temperature of above 169 C (T5%). For the LA-SA, a low T5% at 150.7 C was observed, manifesting diatomite-based composites had higher thermal stability than LA-SA. The occurrences of 5% weight loss (T5%) were at 169.3 C, 173.6 C, and 178.0 C for LA-SA/ D, LA-SA/Dm, and LA-SA/Dm/EG, which were much higher than their operating temperature in the built environment, indicating that the diatomite-based composites had excellent thermal stability. From the homologous DSC curves (Fig. 5b), an endothermic
peak at around 217 C in the curve of LA-SA indicated the LA-SA disintegrated. The endothermic peaks of LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG were at 220 C, 224 C, and 230 C, respectively. It indicates that the thermostability of LA-SA was improved by supports. Furthermore, the loadage of LA-SA in the diatomite-based composites was ascertained by the TG. The maximum mass loss of LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG at 500 C were 41.3%, 51.7%, and 72.2%, respectively. It is found that as the higher loadage of LA-SA in diatomite-based composites, the measured enthalpies of diatomite-based composites were enhanced (Fig. 5b). According to it, the loading capability value of LA-SA (LC, %) in diatomite-based composites was calculated by:
LC ¼
mLASA mLASA 100% ¼ 100% msupport 1 mLASA
(1)
where mLASA and msupport were the mass of LA-SA (g) and diatomite-based support (g), respectively. Thus, the loading capability value of LA-SA (LC, %) was calculated as 70.36%, 107.04%, and 259.71%, respectively. It suggest that the LC of diatomite was improved by microwave-acid treatment and further enhanced by introducing EG. The loading capability of diatomite was promoted due to the larger specific surface areas and higher porosity (according to subsequent BET results) (see Fig. 7). 3.5. Phase change behavior DSC was used to explore the phase change behavior of pure LA-
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Fig. 4. SEM images of (a) D, (b) Dm and corresponding composites (c) LA-SA/D and (d) LA-SA/Dm; SEM images of (e) Dm/EG and (f) LA-SA/Dm/EG.
Fig. 5. (a) TG and (b) TG-DSC curves of diatomite-based composite PCMs.
SA, LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG. The DSC results are shown in Fig. 6 and Table 2. From Fig. 6, there was only one endothermic peak and one exothermic peak in the heating and cooling phase transition process of LA-SA, indicating LA and SA were well combined to form the eutectic mixture with a steadystate. The melting phase change temperatures of LA-SA, LA-SA/D,
LA-SA/Dm, and LA-SA/Dm/EG were 31.50 C, 31.16 C, 31.16 C, and 31.17 C, respectively. The melting temperature indicates that the prepared fs-PCMs are supposed to be applied in the indoor thermal comfort system on account of the phase change temperature at around 10e40 C, which is suitable for the thermal comfort application in buildings [44]. The melting (DHm) and freezing (DHf)
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Fig. 6. DSC curves of LA-SA and LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG.
latent heat values of LA-SA were measured as 167.0 J g1 and 166.9 J g1, respectively. Through a comparison with LA-SA/D, Dm presented a positive effect on latent heat of their composites (Table 2). LA-SA/D had a melting and freezing latent of 59.60 J g1 and 55.68 J g1, respectively, while LA-SA/Dm was ascertained to be 76.16 J g1 and 72.04 J g1, and LA-SA/Dm/EG was calculated to be 117.30 J g1 and 114.50 J g1, respectively. It is clearly to obtain that the thermal storage capacity was improved by microwave-acid treatment of diatomite and farther enhanced by adding EG. In addition, the measured enthalpies of diatomite-based composites were lower than their corresponding theoretical values (for the LA-SA/D, DHth 68.97 J g1; for the LA-SA/Dm, DHth 86.34 J g1; for the LA-SA/Dm/ EG, DHm 120.57 J g1). As a matter of fact, the latent heat value is also influenced by interactions between PCM and supports, relating to the crystallinity of the PCM in the composites [64]. Crystallization ratio (Fc) of LA-SA was given by:
Fc ¼
DHcomposite 100% DHPCM b
(2)
where DHcomposite and DHPCM were the enthalpies of composite PCMs and LA-SA, respectively; b represented LA-SA mass fraction in composites. The crystallinity of LA-SA in the LA-SA/Dm was 88.21%, which was 2.1% higher than that of LA-SA/D (86.41%). The LA-SA in the LA-SA/Dm/EG has the highest Fc of 97.28%. The effective energy stored per unit mass of the LA-SA (Eef) was used to assess the effectiveness of the LA-SA in different composites (Table 2). The LASA in the LA-SA/Dm had a higher Eef (147.31 J g1) than that of LA-
SA/D (144.30 J g1), and the LA-SA/Dm/EG had the highest Eef of 162.46 J g1. For LA-SA/D and LA-SA/Dm composites, the physical interactions including surface tension forces and capillary forces only occur between LA-SA and diatomite (D and Dm). Compared with the Fc of LA-SA in the LA-SA/Dm, LA-SA in LA-SA/D had a lower Fc. Previous researches have demonstrated that the phase change can hardly occur on the PCM inside the tiny pores due to the confinement imposed by interaction on molecule motion [47]. In order to unveil this question, the pore structures of D and Dm were investigated by nitrogen gas adsorption-desorption isotherms (Fig. S7). The specific surface areas and pore volume of D and Dm were 61.1122 m2 g1 and 0.100872 m3 g1, and 68.9325 m2 g1 and 0.101852 m3 g1, respectively (Table 3). Therefore, the specific surface area and pore volume of substrates were increased by microwave-acid treatment. The pore sizes distributions of D and Dm are shown in Fig. 7. As shown in the inset of Fig. 7a, D and Dm had the semblable status of the pore size distribution for incremental pore volume and the pore volume was close to zero while pore width was greater than 150 nm. The incremental pore volume of 5e50 nm occupied primary, following by 60e120 nm. The pore volume was mildly increased by microwave-acid treatment of diatomite while the pore width less than 7 nm (inset of Fig. 7a). The cumulative pore volume of Dm was enhanced compared to that of D (Fig. 7b). It is found that 90% of the cumulative pore volume of Dm was distributed in pore width less than 60 nm but that of D less than 70 nm. According to the above results, it is drawn that more LA-SA were stabilized in the pore width of 5e60 nm of Dm while that in the pore width of 5e70 nm of D. It indicates that LA-SA in Dm was more stabilization than that in D. This conclusion is consistent with the TG analysis results. Fig. 7c and d further studied the incremental pore area and cumulative pore area of D and Dm. The incremental pore area presented a peak which corresponds mesoporous with a diameter of 2.5e4 nm, and the incremental pore area of Dm was a little larger than that of D (Fig. 7c). It is observed that the cumulative pore area of Dm rose dramatically comparing with D (Fig. 7d). This phenomenon is because of pores in diatomite increased the pore area after microwave-acid treatment. The cumulative pore area provided higher surface area and more rough surface for diatomite which was a benefit to support and stabilize SA-LA. After vacuum impregnation, the LA-SA was sucked into the pore of supporting materials, it can be concluded that there were more free-moving and melted LA-SA in Dm, which was beneficial to the crystallinity of LA-SA. Therefore, the crystallinity of LA-SA in the LA-SA/Dm composite was improved than that in the LA-SA/D. The maximum loadage of LA-SA in LA-SA/Dm was enhanced by 25.2% than that of LA-SA/D but was lower than the increment of latent heats value (27.8%). The latent heats value of composite was significantly affected by the crystallization of LA-SA, so it can be speculated that the residual increasing percentage of latent heats value was resulted from the higher crystallinity of LA-SA in the LA-SA/Dm
Table 2 Thermal properties of pure LA-SA and the diatomite-based composites. Samples Melting Latent heat of Freezing temperature (Tm, melting (DHm, J g-1) temperature (Tf, C) C)
Latent heat of Loadage Theoretic values of freezing (DHf, J g-1) (b, %) DHm (DHth, J g-1)
LA-SA LA-SA/D LA-SA/ Dm LA-SA/ Dm/ EG
Crystallinity of Efficient energy per unit mass SA (Fc, %) of LA-SA (Eef, J g-1)
31.50 31.16 31.16
167.00 59.60 76.16
28.93 29.27 29.16
166.90 55.68 72.04
100 41.3 51.7
e 68.97 86.34
100 86.41 88.21
e 144.30 147.31
31.17
117.30
30.02
114.50
72.2
120.57
97.28
162.46
Note: DHth ¼ DHpure b; Eef ¼ DHpure Fc.
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Table 3 Porous characteristics of the D and the Dm. Samples
BET surface area (m2 g1)
Single point adsorption total pore volume (m3 g1)
D Dm
61.1122 68.9325
0.100872 0.101852
Fig. 7. The pore size distribution by Invalid Model: N2 - DFT Model for (a) incremental pore volume, (b) cumulative pore volume, (c) incremental pore area, and (d) cumulative pore area of the D and Dm.
composite. For LA-SA/Dm/EG, which was prepared by adding a small amount of EG to replace the same mass of Dm, LA-SA interacts with Dm and EG. LA-SA in the LA-SA/Dm/EG had higher crystallinity than that in the LA-SA/Dm, indicating the crystallization condition of LASA in EG was better than that in Dm. It is the reason that the efficient energy per unit mass of LA-SA increases by degrees in LA-SA/D (144.30 J g1), LA-SA/Dm (147.31 J g1), and LA-SA/Dm/EG (162.46 J g1) (Table 2). As discussed above, LA-SA/Dm/EG, having both suitable phase change temperature and excellent heat characteristics, is the optimum candidate for application in thermal energy storage. The longterm thermal cycling stability is also one of the discriminating criteria for PCMs in practical TES applications. A 50 thermal cycles testing was conducted to access the reliability of LA-SA/Dm/EG composite PCMs and shown in Fig. 8. There was no significant change in latent heat values and phase change temperatures during the 50 cycles of release and storage process. The melting and freezing temperatures of the fs-PCM3 after 50 cycles were increased by 0.24 C and 0.26 C, respectively. Its LHTES capacity was decreased by 1.40% and 1.78%, respectively, and tend to be stable. It suggests that the prepared composite PCMs possesses great stability in thermal energy storage.
Fig. 8. DSC curves of LA-SA/D/EG after 50 thermal cycles.
3.6. Thermal conductivity Fig. 9 shows the measured thermal conductivity of pure LA-SA
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energy storage and release ability. In addition, the thermal properties and the thermal conductivities of the LA-SA/Dm/EG composite PCMs were compared with that of similar diatomite-based composite (Table 4) [26,44,51,55,56,61,66]. The LA-SA/Dm/EG composite that was prepared in this study showed obvious advantages over the reported materials, such as considerable latent heat capacities, appropriate melting temperature, and higher thermal conductivity, indicating that LA-SA/Dm/EG composite has the potential for application in the thermal energy storage, especially in the built environment. 3.7. Temperature response behavior
Fig. 9. Thermal conductivities of LA-SA and composites.
and diatomite-based composites. The thermal conductivity values were 0.27 W m1 K1, 0.17 W m1 K1, 0.16 W m1 K1, and 0.51 W m1 K1 for LA-SA, LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG, respectively. In comparison with LA-SA/Dm, the thermal conductivity of LA-SA/Dm/EG was significantly boosted with the enhancing level as high as 214%. Addition of the EG in LA-SA/Dm/EG was the major attributed reason [65]. The mass ratio of Dm/EG to LA-SA/Dm/ EG was 27.8%, indicating that the addition of only 2.5 wt% EG resulted in a significant improvement in the thermal conductivity of LA-SA/Dm/EG. Therefore, the LA-SA/Dm/EG has good thermal
Thermal infrared imager recorded the endothermic and exothermic processes of the samples. The absorption and release process curves of LA-SA and LA-SA/Dm/EG are shown in Fig. 10. The LA-SA/Dm/EG showed obviously faster heat storage and release rates than LA-SA. In the heating process (Fig. 10a), the center temperature (Tc) and average temperature (Ta, the acquisition method is as shown in the Supplementary Information (SI-3)) of the LA-SA/Dm/EG demonstrated a low heat rate at t ¼ 29 s, due to the occurrence of the phase change process, while it took more time for LA-SA to complete the temperature rising process. During cooling, likewise, Tc and Ta of LA-SA/Dm/EG changed smoothly and cooled into a stable state after 33 s (Fig. 10b), but LA-SA still stayed at the temperature descending state. It manifests that LA-SA/Dm/EG had the superior instantaneous heat absorption compared to LA-SA when both samples were placed on the heating platform at the same time. Furthermore, the heating and cooling process of the fs-PCMs
Table 4 Comparison of the thermal properties of the prepared LA-SA/Dm/EG composite with that of some materials in literature. Composite PCMs
Melting point ( C)
Latent heat of melting (J g1)
Freezing point ( C)
Latent heat of freezing (J g1)
Thermal conductivity (W m1 K1)
References
Paraffin/calcined diatomite Myristate/diatomite/5 wt% EG Laurate/diatomite/5 wt% EG Paraffin/diatomite/ MWCNTs PA-CA/diatomite/5 wt% EG PEG/diatomite/3 wt% EG CA-LA/diatomite/10 wt% EG PEG/Diatomite/2.50 wt% CNTs LA-SA/Dm/2.5 wt% EG
33.04 45.86
89.54 96.21
52.43 44.63
89.80 92.46
e 0.22
[44] [26]
39.03 27.12
63.08 89.40
37.85 26.50
61.14 89.91
0.24 1.6e1.7
[51]
26.7 27.83 23.61 7.31
98.3 83.54 75.84 59.25
21.85 30.76 e 8.33
90.03 80.21 e 65.45
0.292 0.41 0.467 0.29
[55] [66] [56] [61]
31.17
117.30
30.02
114.50
0.51
This work
Fig. 10. Temperature response curves of LA-SA and LA-SA/Dm/EG at (a) heating time (b) cooling time.
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Education (18B148); and the Hunan Province 2011 Collaborative Innovation Center of Clean Energy and Smart Grid. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.09.001. References
Fig. 11. Thermal infrared images LA-SA/Dm (left) and LA-SA/Dm/EG (right) at (a) heating time and (b) cooling time.
with and without EG were studied. Temperature rising can be clearly seen in the infrared images represented by the color change from black to purple (Fig. 11a). Within this time frame, 48 s was required for LA-SA/Dm/EG to rise from initial temperature (18 C) to 36 C. At the same time, it is observed that the surface temperature of LA-SA/Dm was lower than that of LA-SA/Dm/EG, and there was still significant changes in a short time (48e56 s). The same status happened during cooling (Fig. 11b), which signified that the transient temperature response of LA-SA/Dm/EG was greater than that of LA-SA/Dm. It can be concluded that LA-SA/Dm/EG possessed a great advantage compared to LA-SA and LA-SA/Dm in transient temperature response. This result is in accordance with the conclusion obtained in the thermal conductivity analysis. 4. Conclusion In this study, diatomite-based composites (LA-SA/D, LA-SA/Dm, LA-SA/Dm/EG) were synthesized by absorbing LA-SA into the diatomite-based support under melting impregnation process. The LA-SA/D, LA-SA/Dm, and LA-SA/Dm/EG composites melt at 31.16 C, 31.16 C, and 31.17 C, with the latent heat at melting of 59.60 J g1 for LA-SA/D, 76.16 J g1 for LA-SA/Dm, and 117.30 J g1 for LA-SA/Dm/ EG. The different loading capability of D and Dm for LA-SA were 70.36% and 107.04%, indicating that the diatomite processed the better loading capability after microwave-acid treatment. From SEM and BET, the surfaces and pore structure of Dm were modified, resulting in the higher crystallization of LA-SA in LA-SA/Dm. The transient temperature response properties of composites were in detail investigated by thermal infrared images, indicating it was improved by introducing EG. The LA-SA/Dm/EG can be considered as promising PCM for application in indoor thermal comfort system especially for the energy-efficient building envelope due to its excellent thermal properties including considerable latent heat capacities, appropriate melting temperature, and higher thermal conductivity. Acknowledgments This work was supported by the National Natural Science Foundation of China (51874047, 51504041); the Training Program for Excellent Young Innovators of Changsha (kq1802007); the Fund for University Young Core Instructors of Hunan Province; the Outstanding Youth Project of Hunan Provincial Department of
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