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Novel composite phase change materials with enhancement of light-thermal conversion, thermal conductivity and thermal storage capacity
Solar Energy 196 (2020) 419–426
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Novel composite phase change materials with enhancement of light-thermal conversion, thermal conductivity and thermal storage capacity
T
Jiasheng Zhanga, Zongming Wanga, Xiangqi Lia,b, , Xiao Wua ⁎
a b
Institute of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China Key Laboratory of Eco-materials Advanced Technology, Fuzhou University, Fuzhou 350180, China
ARTICLE INFO
ABSTRACT
Keywords: Expanded vermiculite Thermal conductivity Light-thermal conversion Form-stable composite PCMs
Composite phase change materials (CPCMs) play a key role in solar energy conversion and storage. However, it is difficult to efficiently utilize solar energy due to their inherent low thermal conductivity, low light absorption and the usual competitive nature between thermal conductivity and latent heat. In this work, novel CPCMs were developed by using TiO2-TiC-C loaded expanded vermiculite (EVT) as a supporting matrix and lauric-myristicstearic eutectic mixture (LA-MA-SA) as a PCM. The EVT was further acidized (EVTa) to enhance its PCM absorbability. Owing to the high thermal conductive and light absorption nature of TiO2-TiC-C nanocomposite, the prepared CPCMs exhibit efficient light-thermal energy conversion. The thermal conductivities of LA-MA-SA/EVT and LA-MA-SA/EVTa were 0.694 and 0.676 W/(m K), respectively. And the PCM content in LA-MA-SA/EVT and LA-MA-SA/EVTa were 64.5 and 70.6 wt%, respectively. Moreover, the feasibility of CPCM being a water heating source was experimentally confirmed. These novel CPCMs have good application prospect in low temperature solar-thermal installations, such as temperature adaptable greenhouse and water heating system.
1. Introduction Latent heat storage (LHS), using phase change materials (PCMs) as storage media, is one of the most important technologies of thermal energy storage. In LHS system, PCMs can store and release a large amount of latent heat thermal energy through their phase transforming process at a basically stable temperature (Sahan et al., 2019; Rabie et al., 2019; Abd Elbar and Hassan et al., 2019). The extensively investigated PCMs include inorganic PCMs (e.g. salt hydrates, metals, alloys, etc.) and organic PCMs (e.g. fatty acids, paraffin and polyethylene glycol) (Deng et al., 2016; Li et al., 2017; Liu et al., 2019; Raj et al., 2019; Wei and Li, 2017; Zou et al., 2019). Owing to the superior thermal properties and easy accessibilities, organic PCMs have attracted much attention for medium temperature thermal energy storage. However, the organic PCMs are prone to leak during melting process. Accordingly, organic PCMs need to be confined in porous supporting materials or encapsulated with suitable shells, forming shape-stabilized composite phase change materials (CPCMs). The shape-stabilization strategy not only solves the problem of liquid PCM leakage, but also enhances the thermal conduction ability of CPCMs due to the increased heat transfer area. Recently, developing bifunctional CPCMs with both photothermal conversion and thermal storage performance for efficient utilization of ⁎
solar energy, is an attractive research topic (Tang et al., 2017; Liu et al., 2018; Wang et al., 2017; Wei et al., 2018). Solar energy is the most abundant renewable and green energy source, but it is intermittent and available only during day time. The critical challenge for solar energy is to enhance its utilization efficiency by developing solar energy storage technique. Solar energy can be utilized in the forms of electrical, chemical and thermal energy, through photovoltaic, photochemical and photothermal conversion processes, respectively (Avinash and Amartya, 2019; Gao et al., 2019). Among them, the photothermal conversion is the most efficient and important pathway to utilize solar energy. The bifunctional CPCMs are able to effectively capture sunlight, directly convert light to thermal energy, and then efficiently store the generated thermal energy as latent heat. Thus, the CPCMs for solar thermal energy storage should have excellent abilities to absorb sunlight (280–2700 nm), as well as high latent thermal storage capacities and thermal conductivities. However, the organic PCMs can hardly absorb visible and near-infrared radiation from sun, which respectively comprises about 45% and 52% of the solar energy (Gao et al., 2019). In order to overcome the intrinsic shortcomings of organic PCMs, some highly thermal conductive materials for absorbing sunlight, including carbon fiber (CF) (Tang et al., 2017), Ti4O7 (Liu et al., 2018), graphene (Wang et al., 2017), carbon aerogels (Wei et al., 2018), were used to develop CPCMs that owns the capacity of light-thermal conversion and
Corresponding author at: Institute of Materials Science and Engineering, Fuzhou University New Campus, Minhou College Town, Fuzhou, Fujian 350180, China. E-mail address: [email protected] (X. Li).
https://doi.org/10.1016/j.solener.2019.12.041 Received 25 June 2019; Received in revised form 20 November 2019; Accepted 15 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Table 1 The CPCMs with solar-thermal conversion and enhanced thermal conductivity. CPCMs
3%CF/PEG/SiO2 3% Ti4O7/PEG/SiO2 3% Fe3O4-graphene/PCM polymer Carbon aerogel/paraffin (87%)
Melting Tm (°C)
ΔHm (J/g)
57.5 59.8 50.8 40.9
142.6 129.8 101.2 110.9
thermal energy storage. The important properties of some reported CPCMs are listed in Table 1, in which the calculated mass fraction (R) of PCM in CPCM is equal to the melting latent heat ratio of CPCM to pure PCM. The results show that the enhanced thermal conductivities of CPCMs were still not high enough to achieve rapid heat transfer during charging/discharging process. Additionally, the PCM content in CPCMs usually decrease with the addition of sunlight absorbers, leading to a limited latent thermal storage capacity in most reported CPCMs (Tang et al., 2017; Liu et al., 2018; Wang et al., 2017). Therefore, the main objective of this study is to develop advanced CPCMs with simultaneously enhanced sunlight absorption, thermal conductivity and thermal storage capacity. Vermiculite is a natural alum inosilicate mineral with layered structure. The thermal-exfoliated vermiculite, referred as expanded vermiculite (EV), is a traditional lightweight porous filler to fabricate construction products, such as concrete blocks and cement mortars. Recently, EV has been widely studied as an excellent supporting material for shape-stabilization of PCMs due to its chemical inertia, flame retardant, high porosity and low cost (Wen et al., 2016; Zhang et al., 2018, 2019; Xie et al., 2019). Most of the studies have focused on the thermal conductivity enhancement of EV-based CPCMs via adding highly thermal conductive materials, such as carbon (Wei et al., 2016; Guan et al., 2015; Zhang et al., 2017; Shah, 2018), metal (Deng et al., 2016), metal oxide (Wei and Li et al., 2017; Li et al., 2016) or metal carbide (Deng et al., 2018). Among these reported EV-based CPCMs, the CPCMs with carbon additive are particularly concerned here as carbon is not only a thermal conductive improver but also a well-known solar absorber (Gao et al., 2019; Zhu et al., 2018). The thermal conductivity of 54 wt% capric-myristic-stearic acid (CA-MA-SA)/EV could reach 0.667 W/(m K) after introducing carbon via in-situ carbonization of cetyl trimethyl ammonium bromide (Wei et al., 2016). Paraffin/EV CPCM with 53.2 wt% paraffin showed an enhanced thermal conductivity (0.452 W/(m K)) when carbon was added via in-situ carbonization of sucrose (Guan et al., 2015). SA/EV composite PCM with 63.12 wt% SA exhibited an enhanced thermal conductivity of 0.52 W/ (m K) by carbonization of starch (Zhang et al., 2017). We believe that two problems exist in these previous studies of EV-based CPCMs with carbon additive. Firstly, both the PCM content and the thermal conductivity of CPCMs need to be further improved. Secondly, the lightthermal conversion, which could be endowed by the carbon additive in CPCMs, was completely ignored. To the best of our knowledge, there are few investigations conducting on the light-thermal conversion of EV-based CPCMs. This paper has two main novelties: one is providing a new way for develop CPCMs with light-thermal conversion capabilities, simultaneously improved thermal conductivities and thermal storage capacities; the other is expanding the application of EV-based CPCMs in solar energy utilization. In this study, by virtue of broad spectrum absorption of carbon and titanium carbide as well as the excellent heat transfer capability of titanium dioxide and titanium carbide, TiO2-TiC-C nanocomposite is insitu loaded in EV to form a new supporting matrix (EVT) for the CPCMs to realize light-thermal conversion and thermal storage. The formed EVT is further acidized (EVTa) to enhance the LA-MA-SA absorption capacity. Both LA-MA-SA/EVT and LA-MA-SA/EVTa CPCMs exhibit efficient conversion of light into thermal energy and improved
R (%)
Thermal conductivity (W/m K)
Ref.
67.3 61.2 54.9 81.8
0.45 0.45 0.296 0.427
Tang et al. (2017) Liu et al. (2018) Wang et al. (2017) Wei et al. (2018)
performances in thermal storage and heat transfer. The feasibility of the CPCM operating as a water heating source was also experimentally investigated. 2. Experiment 2.1. Materials Lauric acid (LA, C12H24O2, AR), myristic acid (MA, C14H28O2, AR), stearic acid (SA, C18H36O2, AR), nitric acid (HNO3, AR), acetic acid (C2H4O2, AR), ethanol (CH3CH2OH, AR), tetrabutyl titanate (C16H36O4Ti, CP), sucrose (C12H22O11, AR) were purchased from Sinopharm Chemical Reagent Company. The EV was purchased from Lingshou county fengfeng mineral products processing factory. 2.2. Preparation of CPCM The preparation of CPCM involves forming a supporting matrix and impregnating the PCM into the supporting matrix. The schematic figure of preparation process of CPCM is given in Fig. 1. 2.2.1. Fabrication of the matrix As shown in Fig. 1, the EVT matrix was fabricated in the following process: 5 g EV was dispersed in a mixed solution of ethanol (12.78 mL), tetrabutyl titanate (1.42 mL) and acetic acid (0.48 mL). The pH value of the mixed solution was adjusted to 1 by slowly adding HNO3 solution (1 mol/L), and then 13 mL of sucrose solution (0.89 mol/L) was added under vigorous stirring. The mixture was taken to a 100 mL Teflon-lined stainless steel autoclave and kept at 190 °C for 24 h. The black product was collected, washed and dried, followed by a calcination at 1000 °C for 2 h under carbon reducing atmosphere. To prepare EVTa matrix with higher porosity, the obtained EVT was treated with HNO3 solution (6 mol/L) via a process descripted in literature (Wei and Li et al., 2017). The ratio of EVT mass to volume of HNO3 solution was 166 g/L. 2.2.2. Adsorption of PCM in the matrix For the low temperature solar thermal systems used in buildings, the EV-based CPCM is expected to own the phase change temperature around 30 °C. Thus, the ternary eutectic mixture of LA-MA-SA instead of a single component was selected as a PCM. According to the Schrader equation (Chung et al., 2015; Ke et al., 2016), the theoretical mass ratio of LA-MA-SA eutectic mixture was calculated to be LA : MA : SA = 60.7 : 29.9 : 9.4. This calculated value was experimentally amended (Chung et al., 2015; Li et al., 2016). Based on the calculated and experimental results, the actual eutectic mass ratio was determined to be LA : MA : SA = 59.5 : 30 : 10.5. Considering the fact that the measured phase change temperature of PCM was influenced by many factors such as chemical compositions, purity of fatty acids, measuring method and the accuracy of thermal analysis instrument, it is understandable that the eutectic mass ratio of LA-MA-SA system in this study is slightly different from those reported in literatures, such as 59.5 : 32.0 : 8.5 (Wei and Li, 2017), 61.02 : 30.06 : 8.92 (Ke et al., 2016) and 60 : 30 : 10 (Zhao et al., 2014). The CPCMs were prepared by impregnating matrix with melting LA420
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Fig. 1. Schematic figure of the preparation process of CPCM.
MA-SA eutectic mixture (Wei and Li et al., 2017). The initial mass ratio of PCM to supporting matrix was 3:1. In order to remove the excess PCM, the prepared CPCM was placed on a filter paper and kept at 60 °C for at least 48 h, until the mass of the composite PCM kept constant.
comparison, the water temperature of blank sample without CPCM slice was also recorded.
2.3. Characterization
3.1. Structure and morphology
The X-ray diffraction (XRD) patterns were obtained from MiniFlex 600. The morphologies of the samples were observed by scanning electron microscopy (SEM) on SUPRA 55. The UV–vis spectra were obtained from UV–visible-near infrared light spectrophotometer (PE Lambda 950). The thermo-gravimetric analysis (TGA) was performed on STA449-F5 in N2 atmosphere with a heating rate of 10 °C/min. Thermal properties of the samples were studied by using differential scanning calorimetry (DSC, NETZSCH 214) with temperature accuracy ± 0.1 °C and enthalpy accuracy ± 1.0%. The heating rate was 5 °C/min within the temperature ranging from 10 °C to 60 °C. The thermal cycling reliability of the CPCMs was investigated through a method reported in literature (Wei and Li et al., 2017). And the thermal cycling number was 500. The thermal conductivity was measured by using transient hotline method on thermal conductivity analyzer (TC3000E) with the measurement accuracy within ± 5%. Two same slices with a diameter of 30 mm were used for the thermal conductivity measurement. A probe was placed into the gap between the two same slices that were piled closely. The light-thermal conversion and heat transfer abilities of the matrix and CPCMs were evaluated by two different simulation experiments. Fig. 2 shows the diagrammatic sketch of the experimental setup. In the first experiment, each specimen consisting of two same slices were piled tightly under a Xe lamp (300 W) to simulate solar irradiation. The distance between light source and up slice was 42 cm. A thermocouple connected with a numerical thermometer (UT323, Unit) was inserted between the two slices to read the temperature at the center point per 1 s. In the second experiment, a small beaker was placed in a big beaker with their mouths upside. Both of the beakers were filled with water (28 mL). A CPCMs slice was put on the mouth of the small beaker with its lower half immersed in water. The thermocouple was inserted into the small beaker and fixed at a location 1.5 cm below the slice, thus the water temperature could be recorded. For
Fig. 3 shows the XRD patterns of the supporting matrices. It can be seen that EV mainly consists of phlogopite and a small amount of quartz. For the XRD pattern of EVT, the intensity of (0 0 2) diffraction (2θ = 8.9°) of phlogopite decreases, and new diffraction peaks corresponding to enstatite, spinel and olivine are observed, suggesting the partial collapse of the layered structure and the occurrence of phase transformation at the calcination of 1000 °C. The weak diffraction peak at 2θ = 25.8° represents the characteristic diffraction of anatase phase, and another weak one at 41.7° is ascribed to titanium carbide. A comparative sample, EVC that without adding tetrabutyl titanate, was synthesized via the same preparation process as EVT. The XRD pattern of EVC shows no diffraction peaks at around 2θ = 25.8° and 41.7°, indicating the absence of anatase and titanium carbide in EVC. In the XRD pattern of the acid treated matrix, EVTa, a new broad diffraction peak occurs at around 26° and the peak at 2θ = 8.9° disappears, indicating the formation of amorphous silica and a “house of cards” delaminated structure of vermiculite (Stawinski et al., 2016; Wegrzyn et al., 2018). It is worthy noted that no diffraction peak of carbon is present in the XRD patterns of EVC, EVT and EVTa, proving that carbon has the amorphous structure. The SEM images of the supporting matrices and the composite PCMs are shown in Fig. 4. It can be seen that the EVT matrix exhibits a typical layered structure of EV, and a large number of mono-dispersed nanospheres with diameters ranging from 25 to 85 nm can be observed at the surface and interlayer space of EV flakes (Fig. 4a). The acid-treated matrix, EVTa, shows fewer nanospheres and delaminated EV with higher porosity (Fig. 4b). The LA-MA-SA largely filled the space of EVT and EVTa matrices, and also fully covered the nanospheres (Fig. 4c-d). Table 2 reveals the result of the energy dispersive X-ray spectroscopy (EDX) analysis for EVTa and EVT. Both of the two matrices contain very high content of carbon, though the content of carbon in EVTa is much lower than that in EVT. The decrease in carbon content for EVTa can be explained by the fact that some carbon particles were washed away
3. Results and discussion
421
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Fig. 2. Diagrammatic sketch of the experimental set-up for light-thermal conversion.
reaction of TiO2 with carbon. But this reaction happened only when the carbon nanosphere and TiO2 particles were in contact. Thus, the TiO2, TiC and C phases coexisted, forming the TiO2-TiC-C nanocompositeloaded EV. During the acid treatment process, the cation ions in the octahedral sheets of EV were leached, resulting in a delaminated structure of EV with increased porosity (Stawinski et al., 2016). Due to its high porosity and high specific surface area of nanocomposite, this new matrix can absorb large amounts of melting PCM to form a CPCM with excellent thermal storage ability. 3.2. Thermal properties The DSC curves of LA-MA-SA, LA-MA-SA/EV, LA-MA-SA/EVT and LA-MA-SA/EVTa are shown in Fig. 5, together with the thermal data listed in Table 3. The melting and freezing temperatures of pure LA-MASA are 31.0 and 27.6 °C, respectively. Compared with pure LA-MA-SA, all of the three CPCMs show lower melting temperatures, as expected in the case of nanoconfined PCM (Mitran et al., 2015; Yang et al., 2012). The melting and freezing latent heats of pure LA-MA-SA are 164.0 and 153.3 J/g, respectively. For the three CPCMs, LA-MA-SA/EVTa shows the highest values of melting and freezing latent heats, up to 115.7 and 106.9 J/g, respectively. The mass fraction (R) of LA-MA-SA in the CPCM can be calculated by the melting latent heat ratio of CPCM to pure LA-MA-SA. The calculated values of R for LA-MA-SA/EV, LA-MASA/EVT and LA-MA-SA/EVTa are 48.6%, 64.5% and 70.6%, respectively, confirming that the thermal storage capacity of EV-based CPCMs can be effectively enhanced through loading TiO2-TiC-C nanocomposite on the supporting matrix. Due to its large specific surface area and high surface activity, the TiO2-TiC-C nanocomposite can absorb PCM molecules easily. Moreover, the acid treatment caused EV to be highly delaminated and excess carbon nanospheres to be removed, resulting in more space available for storing PCM. Thus, plenty of PCM can be hold in the pores of EVT and EVTa by capillary force and surface tension. Consequently, the latent heats increase in the order LA-MA-SA/EV < LA-MA-SA/EVT < LA-MA-SA/EVTa. The thermal conductivities of the supporting matrices and CPCMs are shown in Fig. 6. And the thermal conductivities of EV, EVC, EVT and EVTa are 0.396, 0.440, 0.597 and 0.548 W/(m K), respectively. Obviously, the thermal conductivity of supporting matrix can be enhanced more effectively via loading TiO2-TiC-C nanocomposite rather than amorphous carbon nanomaterial. This is mainly due to the fact that both titanium dioxide and titanium carbide have much higher
Fig. 3. XRD patterns of EV, EVC, EVT and EVTa.
during the acid-treated process. It can be inferred that the nanospheres observed in SEM images are carbon nanospheres. Actually, the nanospheres appeared in the product of hydrothermal treatment (Fig. 1). As we know, hydrothermal carbonization is a common method to fabricate spherical carbon materials. Joula and Farbod (2015) reported a hydrothermal fabrication of monodisperse carbon nanospheres at 170 °C by using only sucrose as reactant, where the diameters of carbon nanospheres could be reduced to 100 nm by annealing in air. Comparatively, the diameter of carbon nanosphere in this work is much smaller than the reported size, which may be ascribed to the pore confinement and blocking effects of expanded vermiculite flakes. Based on the experimental results, the formation mechanism of the supporting matrix is proposed. As shown in Fig. 1, the preparation of EVTa can be considered as a three-step process: hydrothermal treatment, reductive calcination and acid treatment. In the hydrothermal treatment step, titanium hydroxide was derived from hydrolysis of butyl titanate, and carbon nanospheres formed from sucrose through polymerization, aromatization and carbonization processes (Joula and Farbod, 2015), were evenly dispersed in the interlayer space and surface of EV. At the reductive calcination step, titanium hydroxide dehydrated to form titanium dioxide (anatase structure). A very small amount of titanium carbide formed via the carbonthermal reduction 422
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Fig. 4. SEM images of (a) EVT, (b) EVTa, (c) LA-MA-SA/EVT and (d) LA-MA-SA/EVTa. Table 2 The main elemental composition of EVT and EVTa.
Table 3 Thermal properties of LA-MA-SA and the composite PCMs.
Element
EVT (wt%)
EVTa (wt%)
C O Mg Al Si K Fe Ti Total
75.41 8.92 0.66 1.23 12.25 0.09 1.09 0.35 100
24.67 41.14 2.01 2.14 28.59 0.05 0.00 1.40 100
Samples
LA-MA-SA LA-MA-SA/EV LA-MA-SA/EVT LA-MA-SA/EVTa
Melting
Freezing
R (%)
Tm (°C)
ΔHm (J/g)
Tf (°C)
ΔHf (J/g)
31.0 28.9 30.1 30.4
164.0 79.7 105.8 115.7
27.6 27.7 27.9 28.0
153.3 75.9 100.0 106.9
– 48.6 64.5 70.6
heat and PCM mass fraction in CPCM. Thus, the energy storage capacity of various CPCMs can be evaluated according to the maximum PCM content without leakage. The thermal properties of some CPCMs in literatures (Guan et al., 2015; Wei and Li, 2017; Li et al., 2016; Deng et al., 2016, 2018; Zhang et al., 2017; Li et al., 2017, 2020; Wei et al., 2014; Wen et al., 2017) are compared in Table 4. It is obviously that LAMA-SA/EVT and LA-MA-SA/EVTa in this work show higher mass fractions of PCM and thermal conductivities than most reported EVbased and non-EV based CPCMs. Furthermore, compared with the CPCMs capable of solar-thermal conversion in Table 1, the CPCMs in
thermal conductivities than black carbon. Compared with EVT, the acid-treated matrix, EVTa, shows a decreased thermal conductivity mainly owing to its higher porosity as well as lower content of carbon. For the CPCMs, the thermal conductivity values of LA-MA-SA/EV, LAMA-SA/EVT and LA-MA-SA/EVTa are 0.447, 0.694 and 0.676 W/(m K), respectively, which are approximately 83.9%, 185.6% and 178.2% higher than the thermal conductivity of pure PCM (0.243 W/m K). The latent heat of CPCM is equal to the product of pure PCM latent
Fig. 5. DSC heating (a) and cooling (b) curves of LA-MA-SA and the composite PCMs. 423
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Fig. 6. Thermal conductivity of composite PCMs and matrices.
this work still show their superiorities in the enhancement of thermal properties.
Fig. 7. DSC curves of LA-MA-SA/EVTa before and after 500 thermal cycles.
3.3. Thermal reliability LA-MA-SA/EVTa was chosen for thermal cycling test due to the highest content of PCM. Fig. 7 shows the thermal cycling result of LAMA-SA/EVTa. The melting and freezing temperatures almost keep constant after 500 thermal cycles. The latent heats of melting and freezing slightly decreased by 3.4% and 4.4%, respectively, which may not affect the practical application of LA-MA-SA/EVTa. Obviously, LAMA-SA/EVTa shows a good thermal reliability during the thermal energy storage/release processes. TGA was performed in N2 atmosphere to investigate the thermal stability of the composite PCMs. Fig. 8 shows the TGA curves of LA-MASA, LA-MA-SA/EVT and LA-MA-SA/EVTa up to 500 °C. The weights of LA-MA-SA, LA-MA-SA/EVT and LA-MA-SA/EVTa almost keep constant below 160 °C, and a rapid weight loss occurs in the temperature range of 200–300 °C. Similar to the pure PCM, the observed weight losses for CPCMs are caused solely by the decomposition of LA-MA-SA. The total weight losses for LA-MA-SA/EVT and LA-MA-SA/EVTa are 66.6 wt% and 73.7 wt%, respectively. Comparatively, the calculated mass fraction of PCM (R value are shown in Table 3) is 64.5% for LA-MA-SA/EVT and 70.6% for LA-MA-SA/EVTa. As expected in the case of nanoporous matrix-based composite PCM, the PCM content measured by TGA is obviously higher than the R value calculated by melting latent heat. This fact can be well explained by the loss of energy storage capacity due to the nanoconfining effect (Mitran et al., 2015; Yang et al., 2012; Gao et al., 2014).
Fig. 8. TGA curves of LA-MA-SA, LA-MA-SA/EVT and LA-MA-SA/EVTa.
3.4. Light-thermal conversion and its application in water heating The light absorption property of material plays an important role in achieving an efficient light-thermal conversion. Fig. 9 shows the UV–vis absorption spectra of LA-MA-SA, LA-MA-SA/EV, LA-MA-SA/EVT and LA-MA-SA/EVTa. It can be seen that pure PCM hardly absorbs visible light and LA-MA-SA/EV absorbs visible light between 350 nm and 550 nm due to the absorption of light by the EV matrix. LA-MA-SA/EVT and LA-MA-SA/EVTa show strong absorption in the 350–850 nm region due to the outstanding light absorption of TiO2-TiC-C nanocomposite.
Table 4 The thermal properties of some CPCMs in literatures. PCM
Paraffin LA-MA-SA SA PEG + Ag PEG + SiC SA Paraffin LA-SA CA-SA CA-LA LA-MA-SA LA-MA-SA
Supporting matrix
carbon/EV Al2O3/EV TiO2/EV EV EV carbon/EV Montmorillo-nite Diatomite Perlite EV EVT EVTa
Melting Tm (°C)
ΔHm (J/g)
53.1 28.6 65.9 59.97 51.9 67.1 23.1 31.16 29.6 23.60 27.9 28.0
103.0 113.7 146.8 99.1 64.9 134.3 51.8 76.16 82.1 81.34 105.8 115.7
424
R (%)
Thermal conductivity (W/(mK))
Ref.
53.2 70.7 73.7 58.8 32.4 63.1 37.1 45.6 50.0 57.48 64.5 70.6
0.45 0.671 0.58 0.68 0.53 0.52 0.225 0.16 – 0.135 0.694 0.676
Guan et al. (2015) Wei and Li (2017) Li et al. (2016) Deng et al. (2016) Deng et al. (2018) Zhang et al. (2017) Li et al. (2017) Li et al. (2020) Wei et al. (2014) Wen et al. (2017) This work This work
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Fig. 9. UV–vis absorption spectra of LA-MA-SA, LA-MA-SA/EV, LA-MA-SA/EVT and LA-MA-SA/EVTa.
Fig. 11. The temperature vs time curves of water.
When the CPCMs were under a simulated sunlight irradiation and contacted with water, higher temperatures than the surrounding water can be observed, resulting in heat transfer between CPCM and water. Thus, the prepared CPCMs can act as heat sources for water heating. Fig. 11 shows the temperature vs time curves of water. The rising temperature curve can be divided into two sections: one below 37 °C and the other above 37 °C. In the first section, compared with the case without CPCM, the water temperature was slightly lower when CPCM was used. While in the second section, a reverse consequence occurred. These results can be explained as follows: the temperature of the first section is within the melting range of CPCM, and some of the converted thermal energy was stored in CPCMs as latent heat, resulting in less heat transferred to water. In the second section, the melting process was completely finished, so the converted thermal energy could be almost entirely transferred to water. Different from the temperature curve of LA-MA-SA/EVTa in Fig. 10, the water temperature curve in Fig. 11 shows no rising or dropping temperature plateau, owing to the fact that the heat from CPCMs can be quickly conducted to the bulk water with heat loss through conduction. The temperatures at 7500 s were 46.1, 45.6 and 40.5 °C for the water with LA-MA-SA/EVT, LA-MA-SA/EVTa and non CPCM, respectively. These results demonstrate the feasibility of CPCMs being heat sources. These CPCMs can be applied in low temperature solar-thermal installations in buildings, such as temperature adaptable greenhouse and water heating system.
Besides, LA-MA-SA/EVTa exhibits lower absorption than LA-MA-SA/ EVT, which may be related to the carbon loss caused by the acid-treated process. Under a simulated sunlight irradiation, the matrices and CPCMs containing solar absorber will have a higher temperature than those without solar absorber, owing to the light-thermal conversion. Fig. 10 shows the comparison of light-thermal conversion curves between EV and EVTa, LA-MA-SA/EV and LA-MA-SA/EVTa. The digital photos of the slices for the light-thermal conversion experiment (the insets in Fig. 10) shows EVTa and LA-MA-SA/EVTa are black, and EV and LAMA-SA/EV are brown, thus EVTa and LA-MA-SA/EVTa can absorb more light. As seen from Fig. 10(a), the temperatures of EV and EVTa matrices under simulated solar irradiation for 1044 s were 39.3 and 47.8 °C, respectively, indicating that the light-thermal energy conversion can be significantly improved by introducing TiO2-TiC-C nanocomposite on EV. When the simulated light source was turned off, the two matrices reached the same temperature of 25.3 °C at 1705 s. Fig. 10(b) shows that when the irradiation time is 3340 s, the temperatures of LA-MA-SA/EV and LA-MA-SA/EVTa were 47.4 and 52.4 °C, respectively. Each specimen exhibit a slow-rising temperature plateau at around 30 °C and a longer dropping temperature plateau at around 28 °C, owing to the heat storage/release of CPCMs. Furthermore, the dropping temperature plateau of LA-MA-SA/EVTa is gentler than that of LA-MA-SA/EV, demonstrating an enhanced ability of reducing the temperature fluctuation for LA-MA-SA/EVTa. These results reveal that the light energy absorbed by LA-MA-SA/EVTa can quickly be converted in sensible heat and latent heat. The prepared CPCMs both exhibit excellent light-thermal conversion and high heat storage capability.
4. Conclusion In this study, novel CPCMs with superior thermal conductivity, latent heat and light-thermal conversion were successfully prepared
Fig. 10. Light-thermal conversion curves of matrices (a) and CPCMs (b). Inset: digital photos of the experimental slices. 425
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based on TiO2-TiC-C loaded supporting matrix. The thermal conductivity of LA-MA-SA/EVTa (0.676 W/(m K)) and LA-MA-SA/EVT (0.694 W/(m K)) were 51.2% and 55.3% higher than that of LA-MASA/EV (0.447 W/(m K)). The melting latent heats of LA-MA-SA/EVTa and LA-MA-SA/EVT were 115.7 and 105.8 J/g, respectively, which were 45.2% and 32.7% higher than that of LA-MA-SA/EV (79.7 J/g). The TiO2-TiC-C nanocomposite endowed the prepared CPCMs with efficient light-thermal energy conversion. The feasibility of the CPCMs being heat sources was confirmed experimentally. The TGA and thermal cycling tests verified the chemical and thermal reliabilities. Consequently, LA-MA-SA/EVTa and LA-MA-SA/EVT exhibit great application potential in solar energy conversion and storage.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work is financially supported by Natural Science Foundation of Fujian Province (Grant 2018J01755). References Abd Elbar, A.R., Hassan, H., 2019. Experimental investigation on the impact of thermal energy storage on the solar still performance coupled with PV module via new integration. Sol. Energy 184, 584–593. Avinash, K., Amartya, C., 2019. Reassessment of different antireflection coatings for crystalline silicon solar cell in view of their passive radiative cooling properties. Sol. Energy 183, 410–418. Chung, O., Jeong, S.G., Kim, S., 2015. Preparation of energy efficient paraffinic PCMs/ expanded vermiculite and perlite composites for energy saving in buildings. Sol. Energy Mater. Sol. Cells 137, 107–112. Deng, Y., Li, J., Nian, H., 2018. Polyethylene glycol-enwrapped silicon carbide nanowires network/expanded vermiculite composite phase change materials: Form stabilization, thermal energy storage behavior and thermal conductivity enhancement. Sol. Energy Mater. Sol. Cells 174, 283–291. Deng, Y., Li, J., Qian, T., Guan, W., Li, Y., Yin, X., 2016. Thermal conductivity enhancement of polyethylene glycol/expanded vermiculite shape-stabilized composite phase change materials with silver nanowire for thermal energy storage. Chem. Eng. J. 295, 427–435. Gao, C.F., Wang, L.P., Li, Q.F., Wang, C., Nan, Z.D., Lan, X.Z., 2014. Tuning thermal properties of latent heat storage material through confinement in porous media: The case of (1-CnH2n+NH3)2ZnCl4 (n=10 and 12). Sol. Energy Mater. Sol. Cells 128, 221–230. Gao, M., Zhu, L., Peh, C.K., Ho, G.W., 2019. Solar absorber material and system designs for photothermal water vaporization towards clean water and energy production. Energy Environ. Sci. 12, 841–864. Guan, W.M., Li, J.H., Qian, T.T., Wang, X., Deng, Y., 2015. Preparation of paraffin/expanded vermiculite with enhanced thermal conductivity by implanting network carbon in vermiculite layers. Chem. Eng. J. 277, 56–63. Joula, M.H., Farbod, M., 2015. Synthesis of uniform and size-controllable carbon nanospheres by a simple hydrothermal method and fabrication of carbon nanosphere super-hydrophobic surface. Appl. Surf. Sci. 347, 535–540. Ke, H., Pang, Z., Peng, B., Wang, J., Cai, Y., Huang, F., Wei, Q., 2016. Thermal energy storage and retrieval properties of form-stable phase change nanofibrous mats based on ternary fatty acid eutectics/polyacrylonitrile composite by magnetron sputtering of silver. J. Therm. Anal. Calorim. 123, 1293–1307. Li, C.C., Wang, M.F., Xie, B.S., Ma, H., Chen, J., 2020,,. Enhanced properties of diatomitebased composite phase change materials for thermal energy storage. Renew. Energ. 147, 265–274. Li, M., Guo, Q., Nutt, S., 2017. Carbon nanotube/paraffin/montmorillonite composite phase change material for thermal energy storage. Sol. Energy 146, 1–7. Li, X., Wei, H., Lin, X., Xie, X., 2016. Preparation of stearic acid/modified expanded vermiculite composite phase change material with simultaneously enhanced thermal conductivity and latent heat. Sol. Energy Mater. Sol. Cells 155, 9–13. Liu, L.F., Chen, J.Y., Qu, Y., Xu, T., Wu, H.J., Zhou, X.Q., Zhang, H.G., 2019. Preparation
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Novel composite phase change materials with enhancement of light-thermal conversion, thermal conductivity and thermal storage capacity
T
Jiasheng Zhanga, Zongming Wanga, Xiangqi Lia,b, , Xiao Wua ⁎
a b
Institute of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China Key Laboratory of Eco-materials Advanced Technology, Fuzhou University, Fuzhou 350180, China
ARTICLE INFO
ABSTRACT
Keywords: Expanded vermiculite Thermal conductivity Light-thermal conversion Form-stable composite PCMs
Composite phase change materials (CPCMs) play a key role in solar energy conversion and storage. However, it is difficult to efficiently utilize solar energy due to their inherent low thermal conductivity, low light absorption and the usual competitive nature between thermal conductivity and latent heat. In this work, novel CPCMs were developed by using TiO2-TiC-C loaded expanded vermiculite (EVT) as a supporting matrix and lauric-myristicstearic eutectic mixture (LA-MA-SA) as a PCM. The EVT was further acidized (EVTa) to enhance its PCM absorbability. Owing to the high thermal conductive and light absorption nature of TiO2-TiC-C nanocomposite, the prepared CPCMs exhibit efficient light-thermal energy conversion. The thermal conductivities of LA-MA-SA/EVT and LA-MA-SA/EVTa were 0.694 and 0.676 W/(m K), respectively. And the PCM content in LA-MA-SA/EVT and LA-MA-SA/EVTa were 64.5 and 70.6 wt%, respectively. Moreover, the feasibility of CPCM being a water heating source was experimentally confirmed. These novel CPCMs have good application prospect in low temperature solar-thermal installations, such as temperature adaptable greenhouse and water heating system.
1. Introduction Latent heat storage (LHS), using phase change materials (PCMs) as storage media, is one of the most important technologies of thermal energy storage. In LHS system, PCMs can store and release a large amount of latent heat thermal energy through their phase transforming process at a basically stable temperature (Sahan et al., 2019; Rabie et al., 2019; Abd Elbar and Hassan et al., 2019). The extensively investigated PCMs include inorganic PCMs (e.g. salt hydrates, metals, alloys, etc.) and organic PCMs (e.g. fatty acids, paraffin and polyethylene glycol) (Deng et al., 2016; Li et al., 2017; Liu et al., 2019; Raj et al., 2019; Wei and Li, 2017; Zou et al., 2019). Owing to the superior thermal properties and easy accessibilities, organic PCMs have attracted much attention for medium temperature thermal energy storage. However, the organic PCMs are prone to leak during melting process. Accordingly, organic PCMs need to be confined in porous supporting materials or encapsulated with suitable shells, forming shape-stabilized composite phase change materials (CPCMs). The shape-stabilization strategy not only solves the problem of liquid PCM leakage, but also enhances the thermal conduction ability of CPCMs due to the increased heat transfer area. Recently, developing bifunctional CPCMs with both photothermal conversion and thermal storage performance for efficient utilization of ⁎
solar energy, is an attractive research topic (Tang et al., 2017; Liu et al., 2018; Wang et al., 2017; Wei et al., 2018). Solar energy is the most abundant renewable and green energy source, but it is intermittent and available only during day time. The critical challenge for solar energy is to enhance its utilization efficiency by developing solar energy storage technique. Solar energy can be utilized in the forms of electrical, chemical and thermal energy, through photovoltaic, photochemical and photothermal conversion processes, respectively (Avinash and Amartya, 2019; Gao et al., 2019). Among them, the photothermal conversion is the most efficient and important pathway to utilize solar energy. The bifunctional CPCMs are able to effectively capture sunlight, directly convert light to thermal energy, and then efficiently store the generated thermal energy as latent heat. Thus, the CPCMs for solar thermal energy storage should have excellent abilities to absorb sunlight (280–2700 nm), as well as high latent thermal storage capacities and thermal conductivities. However, the organic PCMs can hardly absorb visible and near-infrared radiation from sun, which respectively comprises about 45% and 52% of the solar energy (Gao et al., 2019). In order to overcome the intrinsic shortcomings of organic PCMs, some highly thermal conductive materials for absorbing sunlight, including carbon fiber (CF) (Tang et al., 2017), Ti4O7 (Liu et al., 2018), graphene (Wang et al., 2017), carbon aerogels (Wei et al., 2018), were used to develop CPCMs that owns the capacity of light-thermal conversion and
Corresponding author at: Institute of Materials Science and Engineering, Fuzhou University New Campus, Minhou College Town, Fuzhou, Fujian 350180, China. E-mail address: [email protected] (X. Li).
https://doi.org/10.1016/j.solener.2019.12.041 Received 25 June 2019; Received in revised form 20 November 2019; Accepted 15 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Table 1 The CPCMs with solar-thermal conversion and enhanced thermal conductivity. CPCMs
3%CF/PEG/SiO2 3% Ti4O7/PEG/SiO2 3% Fe3O4-graphene/PCM polymer Carbon aerogel/paraffin (87%)
Melting Tm (°C)
ΔHm (J/g)
57.5 59.8 50.8 40.9
142.6 129.8 101.2 110.9
thermal energy storage. The important properties of some reported CPCMs are listed in Table 1, in which the calculated mass fraction (R) of PCM in CPCM is equal to the melting latent heat ratio of CPCM to pure PCM. The results show that the enhanced thermal conductivities of CPCMs were still not high enough to achieve rapid heat transfer during charging/discharging process. Additionally, the PCM content in CPCMs usually decrease with the addition of sunlight absorbers, leading to a limited latent thermal storage capacity in most reported CPCMs (Tang et al., 2017; Liu et al., 2018; Wang et al., 2017). Therefore, the main objective of this study is to develop advanced CPCMs with simultaneously enhanced sunlight absorption, thermal conductivity and thermal storage capacity. Vermiculite is a natural alum inosilicate mineral with layered structure. The thermal-exfoliated vermiculite, referred as expanded vermiculite (EV), is a traditional lightweight porous filler to fabricate construction products, such as concrete blocks and cement mortars. Recently, EV has been widely studied as an excellent supporting material for shape-stabilization of PCMs due to its chemical inertia, flame retardant, high porosity and low cost (Wen et al., 2016; Zhang et al., 2018, 2019; Xie et al., 2019). Most of the studies have focused on the thermal conductivity enhancement of EV-based CPCMs via adding highly thermal conductive materials, such as carbon (Wei et al., 2016; Guan et al., 2015; Zhang et al., 2017; Shah, 2018), metal (Deng et al., 2016), metal oxide (Wei and Li et al., 2017; Li et al., 2016) or metal carbide (Deng et al., 2018). Among these reported EV-based CPCMs, the CPCMs with carbon additive are particularly concerned here as carbon is not only a thermal conductive improver but also a well-known solar absorber (Gao et al., 2019; Zhu et al., 2018). The thermal conductivity of 54 wt% capric-myristic-stearic acid (CA-MA-SA)/EV could reach 0.667 W/(m K) after introducing carbon via in-situ carbonization of cetyl trimethyl ammonium bromide (Wei et al., 2016). Paraffin/EV CPCM with 53.2 wt% paraffin showed an enhanced thermal conductivity (0.452 W/(m K)) when carbon was added via in-situ carbonization of sucrose (Guan et al., 2015). SA/EV composite PCM with 63.12 wt% SA exhibited an enhanced thermal conductivity of 0.52 W/ (m K) by carbonization of starch (Zhang et al., 2017). We believe that two problems exist in these previous studies of EV-based CPCMs with carbon additive. Firstly, both the PCM content and the thermal conductivity of CPCMs need to be further improved. Secondly, the lightthermal conversion, which could be endowed by the carbon additive in CPCMs, was completely ignored. To the best of our knowledge, there are few investigations conducting on the light-thermal conversion of EV-based CPCMs. This paper has two main novelties: one is providing a new way for develop CPCMs with light-thermal conversion capabilities, simultaneously improved thermal conductivities and thermal storage capacities; the other is expanding the application of EV-based CPCMs in solar energy utilization. In this study, by virtue of broad spectrum absorption of carbon and titanium carbide as well as the excellent heat transfer capability of titanium dioxide and titanium carbide, TiO2-TiC-C nanocomposite is insitu loaded in EV to form a new supporting matrix (EVT) for the CPCMs to realize light-thermal conversion and thermal storage. The formed EVT is further acidized (EVTa) to enhance the LA-MA-SA absorption capacity. Both LA-MA-SA/EVT and LA-MA-SA/EVTa CPCMs exhibit efficient conversion of light into thermal energy and improved
R (%)
Thermal conductivity (W/m K)
Ref.
67.3 61.2 54.9 81.8
0.45 0.45 0.296 0.427
Tang et al. (2017) Liu et al. (2018) Wang et al. (2017) Wei et al. (2018)
performances in thermal storage and heat transfer. The feasibility of the CPCM operating as a water heating source was also experimentally investigated. 2. Experiment 2.1. Materials Lauric acid (LA, C12H24O2, AR), myristic acid (MA, C14H28O2, AR), stearic acid (SA, C18H36O2, AR), nitric acid (HNO3, AR), acetic acid (C2H4O2, AR), ethanol (CH3CH2OH, AR), tetrabutyl titanate (C16H36O4Ti, CP), sucrose (C12H22O11, AR) were purchased from Sinopharm Chemical Reagent Company. The EV was purchased from Lingshou county fengfeng mineral products processing factory. 2.2. Preparation of CPCM The preparation of CPCM involves forming a supporting matrix and impregnating the PCM into the supporting matrix. The schematic figure of preparation process of CPCM is given in Fig. 1. 2.2.1. Fabrication of the matrix As shown in Fig. 1, the EVT matrix was fabricated in the following process: 5 g EV was dispersed in a mixed solution of ethanol (12.78 mL), tetrabutyl titanate (1.42 mL) and acetic acid (0.48 mL). The pH value of the mixed solution was adjusted to 1 by slowly adding HNO3 solution (1 mol/L), and then 13 mL of sucrose solution (0.89 mol/L) was added under vigorous stirring. The mixture was taken to a 100 mL Teflon-lined stainless steel autoclave and kept at 190 °C for 24 h. The black product was collected, washed and dried, followed by a calcination at 1000 °C for 2 h under carbon reducing atmosphere. To prepare EVTa matrix with higher porosity, the obtained EVT was treated with HNO3 solution (6 mol/L) via a process descripted in literature (Wei and Li et al., 2017). The ratio of EVT mass to volume of HNO3 solution was 166 g/L. 2.2.2. Adsorption of PCM in the matrix For the low temperature solar thermal systems used in buildings, the EV-based CPCM is expected to own the phase change temperature around 30 °C. Thus, the ternary eutectic mixture of LA-MA-SA instead of a single component was selected as a PCM. According to the Schrader equation (Chung et al., 2015; Ke et al., 2016), the theoretical mass ratio of LA-MA-SA eutectic mixture was calculated to be LA : MA : SA = 60.7 : 29.9 : 9.4. This calculated value was experimentally amended (Chung et al., 2015; Li et al., 2016). Based on the calculated and experimental results, the actual eutectic mass ratio was determined to be LA : MA : SA = 59.5 : 30 : 10.5. Considering the fact that the measured phase change temperature of PCM was influenced by many factors such as chemical compositions, purity of fatty acids, measuring method and the accuracy of thermal analysis instrument, it is understandable that the eutectic mass ratio of LA-MA-SA system in this study is slightly different from those reported in literatures, such as 59.5 : 32.0 : 8.5 (Wei and Li, 2017), 61.02 : 30.06 : 8.92 (Ke et al., 2016) and 60 : 30 : 10 (Zhao et al., 2014). The CPCMs were prepared by impregnating matrix with melting LA420
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Fig. 1. Schematic figure of the preparation process of CPCM.
MA-SA eutectic mixture (Wei and Li et al., 2017). The initial mass ratio of PCM to supporting matrix was 3:1. In order to remove the excess PCM, the prepared CPCM was placed on a filter paper and kept at 60 °C for at least 48 h, until the mass of the composite PCM kept constant.
comparison, the water temperature of blank sample without CPCM slice was also recorded.
2.3. Characterization
3.1. Structure and morphology
The X-ray diffraction (XRD) patterns were obtained from MiniFlex 600. The morphologies of the samples were observed by scanning electron microscopy (SEM) on SUPRA 55. The UV–vis spectra were obtained from UV–visible-near infrared light spectrophotometer (PE Lambda 950). The thermo-gravimetric analysis (TGA) was performed on STA449-F5 in N2 atmosphere with a heating rate of 10 °C/min. Thermal properties of the samples were studied by using differential scanning calorimetry (DSC, NETZSCH 214) with temperature accuracy ± 0.1 °C and enthalpy accuracy ± 1.0%. The heating rate was 5 °C/min within the temperature ranging from 10 °C to 60 °C. The thermal cycling reliability of the CPCMs was investigated through a method reported in literature (Wei and Li et al., 2017). And the thermal cycling number was 500. The thermal conductivity was measured by using transient hotline method on thermal conductivity analyzer (TC3000E) with the measurement accuracy within ± 5%. Two same slices with a diameter of 30 mm were used for the thermal conductivity measurement. A probe was placed into the gap between the two same slices that were piled closely. The light-thermal conversion and heat transfer abilities of the matrix and CPCMs were evaluated by two different simulation experiments. Fig. 2 shows the diagrammatic sketch of the experimental setup. In the first experiment, each specimen consisting of two same slices were piled tightly under a Xe lamp (300 W) to simulate solar irradiation. The distance between light source and up slice was 42 cm. A thermocouple connected with a numerical thermometer (UT323, Unit) was inserted between the two slices to read the temperature at the center point per 1 s. In the second experiment, a small beaker was placed in a big beaker with their mouths upside. Both of the beakers were filled with water (28 mL). A CPCMs slice was put on the mouth of the small beaker with its lower half immersed in water. The thermocouple was inserted into the small beaker and fixed at a location 1.5 cm below the slice, thus the water temperature could be recorded. For
Fig. 3 shows the XRD patterns of the supporting matrices. It can be seen that EV mainly consists of phlogopite and a small amount of quartz. For the XRD pattern of EVT, the intensity of (0 0 2) diffraction (2θ = 8.9°) of phlogopite decreases, and new diffraction peaks corresponding to enstatite, spinel and olivine are observed, suggesting the partial collapse of the layered structure and the occurrence of phase transformation at the calcination of 1000 °C. The weak diffraction peak at 2θ = 25.8° represents the characteristic diffraction of anatase phase, and another weak one at 41.7° is ascribed to titanium carbide. A comparative sample, EVC that without adding tetrabutyl titanate, was synthesized via the same preparation process as EVT. The XRD pattern of EVC shows no diffraction peaks at around 2θ = 25.8° and 41.7°, indicating the absence of anatase and titanium carbide in EVC. In the XRD pattern of the acid treated matrix, EVTa, a new broad diffraction peak occurs at around 26° and the peak at 2θ = 8.9° disappears, indicating the formation of amorphous silica and a “house of cards” delaminated structure of vermiculite (Stawinski et al., 2016; Wegrzyn et al., 2018). It is worthy noted that no diffraction peak of carbon is present in the XRD patterns of EVC, EVT and EVTa, proving that carbon has the amorphous structure. The SEM images of the supporting matrices and the composite PCMs are shown in Fig. 4. It can be seen that the EVT matrix exhibits a typical layered structure of EV, and a large number of mono-dispersed nanospheres with diameters ranging from 25 to 85 nm can be observed at the surface and interlayer space of EV flakes (Fig. 4a). The acid-treated matrix, EVTa, shows fewer nanospheres and delaminated EV with higher porosity (Fig. 4b). The LA-MA-SA largely filled the space of EVT and EVTa matrices, and also fully covered the nanospheres (Fig. 4c-d). Table 2 reveals the result of the energy dispersive X-ray spectroscopy (EDX) analysis for EVTa and EVT. Both of the two matrices contain very high content of carbon, though the content of carbon in EVTa is much lower than that in EVT. The decrease in carbon content for EVTa can be explained by the fact that some carbon particles were washed away
3. Results and discussion
421
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Fig. 2. Diagrammatic sketch of the experimental set-up for light-thermal conversion.
reaction of TiO2 with carbon. But this reaction happened only when the carbon nanosphere and TiO2 particles were in contact. Thus, the TiO2, TiC and C phases coexisted, forming the TiO2-TiC-C nanocompositeloaded EV. During the acid treatment process, the cation ions in the octahedral sheets of EV were leached, resulting in a delaminated structure of EV with increased porosity (Stawinski et al., 2016). Due to its high porosity and high specific surface area of nanocomposite, this new matrix can absorb large amounts of melting PCM to form a CPCM with excellent thermal storage ability. 3.2. Thermal properties The DSC curves of LA-MA-SA, LA-MA-SA/EV, LA-MA-SA/EVT and LA-MA-SA/EVTa are shown in Fig. 5, together with the thermal data listed in Table 3. The melting and freezing temperatures of pure LA-MASA are 31.0 and 27.6 °C, respectively. Compared with pure LA-MA-SA, all of the three CPCMs show lower melting temperatures, as expected in the case of nanoconfined PCM (Mitran et al., 2015; Yang et al., 2012). The melting and freezing latent heats of pure LA-MA-SA are 164.0 and 153.3 J/g, respectively. For the three CPCMs, LA-MA-SA/EVTa shows the highest values of melting and freezing latent heats, up to 115.7 and 106.9 J/g, respectively. The mass fraction (R) of LA-MA-SA in the CPCM can be calculated by the melting latent heat ratio of CPCM to pure LA-MA-SA. The calculated values of R for LA-MA-SA/EV, LA-MASA/EVT and LA-MA-SA/EVTa are 48.6%, 64.5% and 70.6%, respectively, confirming that the thermal storage capacity of EV-based CPCMs can be effectively enhanced through loading TiO2-TiC-C nanocomposite on the supporting matrix. Due to its large specific surface area and high surface activity, the TiO2-TiC-C nanocomposite can absorb PCM molecules easily. Moreover, the acid treatment caused EV to be highly delaminated and excess carbon nanospheres to be removed, resulting in more space available for storing PCM. Thus, plenty of PCM can be hold in the pores of EVT and EVTa by capillary force and surface tension. Consequently, the latent heats increase in the order LA-MA-SA/EV < LA-MA-SA/EVT < LA-MA-SA/EVTa. The thermal conductivities of the supporting matrices and CPCMs are shown in Fig. 6. And the thermal conductivities of EV, EVC, EVT and EVTa are 0.396, 0.440, 0.597 and 0.548 W/(m K), respectively. Obviously, the thermal conductivity of supporting matrix can be enhanced more effectively via loading TiO2-TiC-C nanocomposite rather than amorphous carbon nanomaterial. This is mainly due to the fact that both titanium dioxide and titanium carbide have much higher
Fig. 3. XRD patterns of EV, EVC, EVT and EVTa.
during the acid-treated process. It can be inferred that the nanospheres observed in SEM images are carbon nanospheres. Actually, the nanospheres appeared in the product of hydrothermal treatment (Fig. 1). As we know, hydrothermal carbonization is a common method to fabricate spherical carbon materials. Joula and Farbod (2015) reported a hydrothermal fabrication of monodisperse carbon nanospheres at 170 °C by using only sucrose as reactant, where the diameters of carbon nanospheres could be reduced to 100 nm by annealing in air. Comparatively, the diameter of carbon nanosphere in this work is much smaller than the reported size, which may be ascribed to the pore confinement and blocking effects of expanded vermiculite flakes. Based on the experimental results, the formation mechanism of the supporting matrix is proposed. As shown in Fig. 1, the preparation of EVTa can be considered as a three-step process: hydrothermal treatment, reductive calcination and acid treatment. In the hydrothermal treatment step, titanium hydroxide was derived from hydrolysis of butyl titanate, and carbon nanospheres formed from sucrose through polymerization, aromatization and carbonization processes (Joula and Farbod, 2015), were evenly dispersed in the interlayer space and surface of EV. At the reductive calcination step, titanium hydroxide dehydrated to form titanium dioxide (anatase structure). A very small amount of titanium carbide formed via the carbonthermal reduction 422
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Fig. 4. SEM images of (a) EVT, (b) EVTa, (c) LA-MA-SA/EVT and (d) LA-MA-SA/EVTa. Table 2 The main elemental composition of EVT and EVTa.
Table 3 Thermal properties of LA-MA-SA and the composite PCMs.
Element
EVT (wt%)
EVTa (wt%)
C O Mg Al Si K Fe Ti Total
75.41 8.92 0.66 1.23 12.25 0.09 1.09 0.35 100
24.67 41.14 2.01 2.14 28.59 0.05 0.00 1.40 100
Samples
LA-MA-SA LA-MA-SA/EV LA-MA-SA/EVT LA-MA-SA/EVTa
Melting
Freezing
R (%)
Tm (°C)
ΔHm (J/g)
Tf (°C)
ΔHf (J/g)
31.0 28.9 30.1 30.4
164.0 79.7 105.8 115.7
27.6 27.7 27.9 28.0
153.3 75.9 100.0 106.9
– 48.6 64.5 70.6
heat and PCM mass fraction in CPCM. Thus, the energy storage capacity of various CPCMs can be evaluated according to the maximum PCM content without leakage. The thermal properties of some CPCMs in literatures (Guan et al., 2015; Wei and Li, 2017; Li et al., 2016; Deng et al., 2016, 2018; Zhang et al., 2017; Li et al., 2017, 2020; Wei et al., 2014; Wen et al., 2017) are compared in Table 4. It is obviously that LAMA-SA/EVT and LA-MA-SA/EVTa in this work show higher mass fractions of PCM and thermal conductivities than most reported EVbased and non-EV based CPCMs. Furthermore, compared with the CPCMs capable of solar-thermal conversion in Table 1, the CPCMs in
thermal conductivities than black carbon. Compared with EVT, the acid-treated matrix, EVTa, shows a decreased thermal conductivity mainly owing to its higher porosity as well as lower content of carbon. For the CPCMs, the thermal conductivity values of LA-MA-SA/EV, LAMA-SA/EVT and LA-MA-SA/EVTa are 0.447, 0.694 and 0.676 W/(m K), respectively, which are approximately 83.9%, 185.6% and 178.2% higher than the thermal conductivity of pure PCM (0.243 W/m K). The latent heat of CPCM is equal to the product of pure PCM latent
Fig. 5. DSC heating (a) and cooling (b) curves of LA-MA-SA and the composite PCMs. 423
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Fig. 6. Thermal conductivity of composite PCMs and matrices.
this work still show their superiorities in the enhancement of thermal properties.
Fig. 7. DSC curves of LA-MA-SA/EVTa before and after 500 thermal cycles.
3.3. Thermal reliability LA-MA-SA/EVTa was chosen for thermal cycling test due to the highest content of PCM. Fig. 7 shows the thermal cycling result of LAMA-SA/EVTa. The melting and freezing temperatures almost keep constant after 500 thermal cycles. The latent heats of melting and freezing slightly decreased by 3.4% and 4.4%, respectively, which may not affect the practical application of LA-MA-SA/EVTa. Obviously, LAMA-SA/EVTa shows a good thermal reliability during the thermal energy storage/release processes. TGA was performed in N2 atmosphere to investigate the thermal stability of the composite PCMs. Fig. 8 shows the TGA curves of LA-MASA, LA-MA-SA/EVT and LA-MA-SA/EVTa up to 500 °C. The weights of LA-MA-SA, LA-MA-SA/EVT and LA-MA-SA/EVTa almost keep constant below 160 °C, and a rapid weight loss occurs in the temperature range of 200–300 °C. Similar to the pure PCM, the observed weight losses for CPCMs are caused solely by the decomposition of LA-MA-SA. The total weight losses for LA-MA-SA/EVT and LA-MA-SA/EVTa are 66.6 wt% and 73.7 wt%, respectively. Comparatively, the calculated mass fraction of PCM (R value are shown in Table 3) is 64.5% for LA-MA-SA/EVT and 70.6% for LA-MA-SA/EVTa. As expected in the case of nanoporous matrix-based composite PCM, the PCM content measured by TGA is obviously higher than the R value calculated by melting latent heat. This fact can be well explained by the loss of energy storage capacity due to the nanoconfining effect (Mitran et al., 2015; Yang et al., 2012; Gao et al., 2014).
Fig. 8. TGA curves of LA-MA-SA, LA-MA-SA/EVT and LA-MA-SA/EVTa.
3.4. Light-thermal conversion and its application in water heating The light absorption property of material plays an important role in achieving an efficient light-thermal conversion. Fig. 9 shows the UV–vis absorption spectra of LA-MA-SA, LA-MA-SA/EV, LA-MA-SA/EVT and LA-MA-SA/EVTa. It can be seen that pure PCM hardly absorbs visible light and LA-MA-SA/EV absorbs visible light between 350 nm and 550 nm due to the absorption of light by the EV matrix. LA-MA-SA/EVT and LA-MA-SA/EVTa show strong absorption in the 350–850 nm region due to the outstanding light absorption of TiO2-TiC-C nanocomposite.
Table 4 The thermal properties of some CPCMs in literatures. PCM
Paraffin LA-MA-SA SA PEG + Ag PEG + SiC SA Paraffin LA-SA CA-SA CA-LA LA-MA-SA LA-MA-SA
Supporting matrix
carbon/EV Al2O3/EV TiO2/EV EV EV carbon/EV Montmorillo-nite Diatomite Perlite EV EVT EVTa
Melting Tm (°C)
ΔHm (J/g)
53.1 28.6 65.9 59.97 51.9 67.1 23.1 31.16 29.6 23.60 27.9 28.0
103.0 113.7 146.8 99.1 64.9 134.3 51.8 76.16 82.1 81.34 105.8 115.7
424
R (%)
Thermal conductivity (W/(mK))
Ref.
53.2 70.7 73.7 58.8 32.4 63.1 37.1 45.6 50.0 57.48 64.5 70.6
0.45 0.671 0.58 0.68 0.53 0.52 0.225 0.16 – 0.135 0.694 0.676
Guan et al. (2015) Wei and Li (2017) Li et al. (2016) Deng et al. (2016) Deng et al. (2018) Zhang et al. (2017) Li et al. (2017) Li et al. (2020) Wei et al. (2014) Wen et al. (2017) This work This work
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Fig. 9. UV–vis absorption spectra of LA-MA-SA, LA-MA-SA/EV, LA-MA-SA/EVT and LA-MA-SA/EVTa.
Fig. 11. The temperature vs time curves of water.
When the CPCMs were under a simulated sunlight irradiation and contacted with water, higher temperatures than the surrounding water can be observed, resulting in heat transfer between CPCM and water. Thus, the prepared CPCMs can act as heat sources for water heating. Fig. 11 shows the temperature vs time curves of water. The rising temperature curve can be divided into two sections: one below 37 °C and the other above 37 °C. In the first section, compared with the case without CPCM, the water temperature was slightly lower when CPCM was used. While in the second section, a reverse consequence occurred. These results can be explained as follows: the temperature of the first section is within the melting range of CPCM, and some of the converted thermal energy was stored in CPCMs as latent heat, resulting in less heat transferred to water. In the second section, the melting process was completely finished, so the converted thermal energy could be almost entirely transferred to water. Different from the temperature curve of LA-MA-SA/EVTa in Fig. 10, the water temperature curve in Fig. 11 shows no rising or dropping temperature plateau, owing to the fact that the heat from CPCMs can be quickly conducted to the bulk water with heat loss through conduction. The temperatures at 7500 s were 46.1, 45.6 and 40.5 °C for the water with LA-MA-SA/EVT, LA-MA-SA/EVTa and non CPCM, respectively. These results demonstrate the feasibility of CPCMs being heat sources. These CPCMs can be applied in low temperature solar-thermal installations in buildings, such as temperature adaptable greenhouse and water heating system.
Besides, LA-MA-SA/EVTa exhibits lower absorption than LA-MA-SA/ EVT, which may be related to the carbon loss caused by the acid-treated process. Under a simulated sunlight irradiation, the matrices and CPCMs containing solar absorber will have a higher temperature than those without solar absorber, owing to the light-thermal conversion. Fig. 10 shows the comparison of light-thermal conversion curves between EV and EVTa, LA-MA-SA/EV and LA-MA-SA/EVTa. The digital photos of the slices for the light-thermal conversion experiment (the insets in Fig. 10) shows EVTa and LA-MA-SA/EVTa are black, and EV and LAMA-SA/EV are brown, thus EVTa and LA-MA-SA/EVTa can absorb more light. As seen from Fig. 10(a), the temperatures of EV and EVTa matrices under simulated solar irradiation for 1044 s were 39.3 and 47.8 °C, respectively, indicating that the light-thermal energy conversion can be significantly improved by introducing TiO2-TiC-C nanocomposite on EV. When the simulated light source was turned off, the two matrices reached the same temperature of 25.3 °C at 1705 s. Fig. 10(b) shows that when the irradiation time is 3340 s, the temperatures of LA-MA-SA/EV and LA-MA-SA/EVTa were 47.4 and 52.4 °C, respectively. Each specimen exhibit a slow-rising temperature plateau at around 30 °C and a longer dropping temperature plateau at around 28 °C, owing to the heat storage/release of CPCMs. Furthermore, the dropping temperature plateau of LA-MA-SA/EVTa is gentler than that of LA-MA-SA/EV, demonstrating an enhanced ability of reducing the temperature fluctuation for LA-MA-SA/EVTa. These results reveal that the light energy absorbed by LA-MA-SA/EVTa can quickly be converted in sensible heat and latent heat. The prepared CPCMs both exhibit excellent light-thermal conversion and high heat storage capability.
4. Conclusion In this study, novel CPCMs with superior thermal conductivity, latent heat and light-thermal conversion were successfully prepared
Fig. 10. Light-thermal conversion curves of matrices (a) and CPCMs (b). Inset: digital photos of the experimental slices. 425
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based on TiO2-TiC-C loaded supporting matrix. The thermal conductivity of LA-MA-SA/EVTa (0.676 W/(m K)) and LA-MA-SA/EVT (0.694 W/(m K)) were 51.2% and 55.3% higher than that of LA-MASA/EV (0.447 W/(m K)). The melting latent heats of LA-MA-SA/EVTa and LA-MA-SA/EVT were 115.7 and 105.8 J/g, respectively, which were 45.2% and 32.7% higher than that of LA-MA-SA/EV (79.7 J/g). The TiO2-TiC-C nanocomposite endowed the prepared CPCMs with efficient light-thermal energy conversion. The feasibility of the CPCMs being heat sources was confirmed experimentally. The TGA and thermal cycling tests verified the chemical and thermal reliabilities. Consequently, LA-MA-SA/EVTa and LA-MA-SA/EVT exhibit great application potential in solar energy conversion and storage.
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