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A novel phase-change cement composite for thermal energy storage: Fabrication, thermal and mechanical properties
Applied Energy 170 (2016) 130–139
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
A novel phase-change cement composite for thermal energy storage: Fabrication, thermal and mechanical properties He Zhang a, Feng Xing b, Hong-Zhi Cui b, Da-Zhu Chen a,⇑, Xing Ouyang a,⇑, Su-Zhen Xu a, Jia-Xin Wang c, Yi-Tian Huang a, Jian-Dong Zuo a, Jiao-Ning Tang a a b c
Shenzhen Key Laboratory of Special Functional Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil Engineering, Shenzhen University, Shenzhen 518060, China Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel flaky graphite-doped phase-
change microcapsule (FGD-MPCM) was prepared. FGD-MPCM has substantial latent heat storage capacity (135.8 J/g). FGD-MPCMs/cement composite is capable of reducing indoor temperature fluctuation. Compressive strength of cement composite with 30% FGD-MPCMs can reach to 14.2 MPa. Time (min)
0
5.0
11.2
17.0
23.7
32.8
Raw cement
Composite
a r t i c l e
i n f o
Article history: Received 8 November 2015 Received in revised form 16 February 2016 Accepted 17 February 2016
Keywords: Microencapsulated phase-change materials Cement composite Energy storage Thermal stability
a b s t r a c t Facing upon the increasingly severe energy crisis, one of the key issues for reducing the building energy consumption is to pursue high-performance thermal energy storage technologies based on phase-change materials. In this study, a novel cement composite incorporated with flaky graphite-doped microencapsulated phase-change materials (FGD-MPCMs) was developed. Various techniques, such as field emission-scanning electron microscopy (FE-SEM), optical microscopy (OM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to analyse the composite structure and thermal performances. The results indicate that the spherical microcapsules are well dispersed in the cement matrix. When combined within the cement, the thermal stability of the microcapsules was highly improved, and the inclusion of greater amounts of FGD-MPCMs further increased the latent heat of the composite. The mechanical properties of the cement composites were affected with the increase of FGD-MPCMs dosage and the porosity of the composites. In spite of this, the compressive strength and flexural strength of the cement composite with 30% FGD-MPCM could still reach to as high as 14.2 MPa and 4.1 MPa, respectively. Results from the infrared thermography and the model room test suggested that the composite filled with FGDMPCMs is capable of reducing indoor temperature fluctuation and exhibits good potential for application in buildings to enhance energy savings and thermal comfort. Ó 2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding authors. Tel.: +86 755 2653 5133 (D.Z. Chen), +86 755 2653 1629 (X. Ouyang). E-mail addresses: [email protected] (D.Z. Chen), [email protected] (X. Ouyang). http://dx.doi.org/10.1016/j.apenergy.2016.02.091 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
H. Zhang et al. / Applied Energy 170 (2016) 130–139
1. Introduction With industrial development and population growth, the consumption of resources is increasing, especially that of fossil fuels. Concomitantly, millions of tons of CO2 are emitted into the atmosphere every year because of combustion, which is widely believed to be the main cause of global warming [1]. Among the environmental burdens caused by the excessive use of fossil fuels, 30% of the energy consumption and 1/3 of the greenhouse gas released worldwide are attributed to the building sector [2]. Efforts to increase energy efficiency in the building sector have been intensified to meet heating and cooling requirements [3,4]. Phase-change materials (PCMs) [5] can store or release large quantities of latent heat via a nearly isothermal phase-change process; therefore, combining construction materials and PCMs is believed to be an efficient way to increase the heat energy storage efficiency of construction elements [5,6]. For instance, Paksoy et al. [7] reported that 7% of cooling energy and 28% of heating energy was conserved by combining 3.5 kg of PCMs with insulation panels in a test cabin. However, bulk PCMs are not easy to handle in practical applications because of leakage, erosion and super-cooling problems. To solve these problematic limitations, the materials are generally encapsulated with a polymer or inorganic shell before use [8]. Borreguero et al. [9] and Zhang et al. [10] incorporated microencapsulated PCMs (MPCMs) into gypsum boards and found that the thermal regulation ability of the boards was enhanced as the amount of microcapsules was increased. Similar results were observed in MPCMs-filled, cement-based composites [11–16] and geopolymer mortar [17]. An effective envelopment of PCMs is important to prevent the leakage of core materials, increase the heat-transfer area and control the volume changes during the phase changes [18]. Many polymers and copolymers, such as polystyrene (PS) [19,20], polyurea (PU) [21–23], poly(methyl methacrylate) (PMMA) [24–26] and melamine–formaldehyde (MF) resin [27,28], are commonly used as shells in MPCMs. However, the poor heat conduction of the pure organic polymer shell limits the response velocity of the microcapsules to the environmental temperature change. More recently, there are growing interests in utilizing polymer–inorganic hybrid shells to improve the thermal conductivity and mechanical performance of phase-change microcapsules. Chang et al. [29] synthesized microencapsulated n-octadecane with a PMMA-silica hybrid shell via in-situ polymerization and investigated the effect of coupling agents, ageing time and temperature on the thermal properties of the microcapsules. By mixing Cu (II) methacrylate with the oil phase during the fabrication process, Wu et al. [30] obtained PMMA-SiO2 @ paraffin microcapsules with copper chelation as the ion probe. Yin et al. [31,32] prepared MPCMs with MF resin/SiO2 and a poly(styrene-co-divinylbenzene)/SiO2 shell through Pickering emulsion polymerization and explored the effects of introducing inorganic particles in the MPCMs shell on the mechanical strength, thermal reliability and anti-osmosis performance, which were significantly improved. Moreover, using the same in-situ polymerization method, Chen et al. [33] incorporated nano-Al2O3 particles into the MPCMs shell and observed an obvious enhancement of the thermal stability of the microcapsules. Graphite is one of excellent thermal conductivity materials. Thermal conductivity coefficient of graphite can reach about 390 W/ (m K), which nearly 1000 times of that of polymer. Due to graphite shape likes a platelet, therefore, less research has addressed flaky graphite-doped, microencapsulated PCMs (FGD-MPCMs) and their applications in the field of building materials as the thermal energy storage carriers. Most researches on enhancement of the thermal stability of the microcapsules only focused on the formation of microcapsules and their thermophysical or mechanical properties, in this work, a
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novel cement composite with good thermal energy storage capacity was developed by incorporating self-manufactured, flaky graphite-doped, phase-change paraffin microcapsules via interfacial polycondensation [34]. The structure, phase-change properties, and thermal stabilities of both the microcapsules and the FGD-MPCMs-filled cement composites were well examined. For building thermal energy storage application, both efficiency of the thermal energy storage and the mechanical properties of the cement composite with FGD-MPCMs were evaluated. 2. Experimental 2.1. Materials Paraffin wax (RT28, Rubitherm Technologies Gmbh, Berlin, Germany), which was mainly composed of n-octadecane, was used as the core material of the FGD-MPCMs. 3-Isocyanatomethyl-3,5,5-tri methylcyclohexyl isocyanate (IPDI, Bayer, Taiwan) and diethylene triamine (DETA, Dow Chemical, USA) with purity exceeding 99.5% were used to form the microcapsule shell. Styrene–maleic anhydride copolymer (SMA, ScriptÒ 520, USA) was used as the emulsifier. Flaky graphite powder (800 meshes, Qingdao Tianyuan Graphite Co., Ltd., China) was used as the inorganic filler for the microcapsules. Portland cement powder (PII 42.5R, Guangdong Zhujiang Cement Factory, China) was adopted as the matrix for fabricating the raw cement or cement composite board. Polyacrylic acid (Guangdong Longhu Science and Technology Co., Ltd., China) was used as the water-reducing agent. 2.2. Fabrication 2.2.1. Synthesis of FGD-MPCMs The microencapsulation of paraffin wax doped with flaky graphite powder was achieved through interfacial polycondensation. The fabrication procedure, including the mixing of core materials, emulsification, polymerization, washing and drying, is schematically illustrated in Fig. 1. Paraffin wax, IPDI and flaky graphite were mixed at 50 °C for 30 min to produce the core material with a weight proportion of 30:20:1. Then, the mixture was poured into a 3-ton reaction kettle filled with 1.3 wt% SMA aqueous solution and agitated vigorously at 60 °C for 1 h to generate a stable oil/ water (O/W) emulsion. Subsequently, stoichiometric DETA was added dropwise into the above emulsion to initiate the formation of the polyurea shell. After continuous stirring for three hours, the resultant microcapsules were centrifugally filtered, washed with hot water and dried at room temperature for one week. 2.2.2. Preparation of smart cement composite containing FGD-MPCMs The cement composites were manufactured by mixing Portland cement, FGD-MPCMs and water at room temperature according to the proportions listed in Table 1. For all specimens, the mass ratio of the cement powder and water was 5:2. Compared to the amount of cement powder, the filling contents of FGD-MPCMs in the composites were 10%, 20%, 25% and 30%, and the corresponding composites were named Cement10, Cement20, Cement25 and Cement30, respectively. The fabrication process is illustrated in Fig. 2. The FGD-MPCMs were immersed in water in a 500-ml beaker for 24 h to ensure that they were completely wetted. Then, the mixture was transferred into a metal stirring pot with the cement powder and stirred vigorously for 5 min using a Hobart mixer (Hobart Corporation, Troy, OH, USA). During this step, the remaining water was calculated, and a very small amount of waterreducing agent was added. The resulting slurry was poured into the moulds and kept for one day to form board-like or cylindrical cement samples. Finally, the above samples were inserted into a curing chamber at constant temperature and humidity (Dongxing
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IPDI Paraffin
Graphite
Water
SMA
Oil phase
DETA
Emulsifying
Mixing
SMA solution
Washing & Drying
Stirring
O/W emulsion
Interfacial polymerization
FGD-MPCMs
Fig. 1. Schematic synthesis of the FGD-MPCMs. Table 1 Descriptions of the raw cement and cement composites filled with FGD-MPCMs. Samples
Water (g)
Cement powders (g)
FGD-MPCMs (g)
Water reducing agent (g)
Raw cement Cement10 Cement20 Cement25 Cement30
500 500 500 500 500
1250 1250 1250 1250 1250
0 125 250 312 375
0 0 2.8 3.7 4.8
Building Material Co., Ltd., China) and placed into water for fast curing at 50 °C for three days.
atmosphere. The temperature scanning range was from 10 to 50 °C.
2.3. Characterization
2.3.3. Thermogravimetric analysis (TGA) TGA of the FGD-MPCMs and cement composites were performed under a nitrogen atmosphere using a TA Q50 thermal gravimetric analyser. For each test, the sample was first heated from 30 to 120 °C at a rate of 20 °C/min and was kept at 120 °C for 20 min to remove the adsorbed water. The temperature was then quickly decreased to 30 °C and increased to 500 °C at a heating rate of 10 °C/min.
2.3.1. Structures of the FGD-MPCMs and cement composites The morphologies of the FGD-MPCMs and their dispersion in the cement composites were examined using a Hitachi Su-70 field emission-scanning electron microscope (FE-SEM) at an accelerating voltage of 1.0 kV. The samples of FGD-MPCMs and the fractured surfaces of the composites were sprayed with gold prior to observation. A Mshot MP41 optical microscope (OM) equipped with a MC 50 digital imaging system (Guangzhou Micro-shot Technology Co., Ltd., China) was used to observe the microstructures of the microcapsules. The crystalline phases of the FGD-MPCMs were analysed by an X-ray diffractometer (XRD, Bruker D8) with Cu Ka radiation (k = 0.154 nm) with operating conditions of 40 kV and 200 mA. The XRD pattern was recorded in the 2h range from 10° to 80° with a scanning rate of 3°/min.
2.3.4. Infrared thermography To determine the thermo-regulating performance of the cement composite, infrared thermography was conducted with an infrared thermal imager (FLIR SC655). A round composite disk containing 20% FGD-MPCMs with a diameter of 25 mm and a thickness of 3 mm was prepared in a cylindrical mould. A raw cement disk without FGD-MPCMs was also fabricated as a reference. Both samples were cooled to 3 °C in advance and placed onto a hot stage maintained at a constant temperature of 60 °C. To prevent the sample’s temperature from rising too quickly, a cylindrical PTFE disk (U 25 mm) with a thickness of 5 mm was placed between the hot plate and the test sample. The microscope lens was kept
2.3.2. Differential scanning calorimetry (DSC) The phase-change properties of the FGD-MPCMs and cement composites were analysed using a TA Q200 differential scanning calorimeter at a heating/cooling rate of 5 °C/min in a nitrogen
FGD-MPCMs
Cement powder
Water Water
Water reducing agent
Transferring
Wetting of FGD-MPCMs
Molding & curing
Compounding
Cement composite with FGD-MPCMs
Fig. 2. Schematic fabrication of the FGD-MPCMs-filled cement composite.
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Cement composite wall
or
Raw cement wall Model room without test wall
Model room with composite wall
Thermocouple Data logger
Heat channel
Computer recording system
Heat source Test setup with model room
Installation of model room
Fig. 3. Schematic diagram of the setup used to evaluate the thermal regulation capacity of the FGD-MPCMs-filled cement composite.
at 25 cm from the upper surface of the sample during the test. The infrared thermal images of both the composite and control were taken at different times from 3 to 40 °C and analysed using FLIR Researcher IR software. 2.3.5. Evaluation of the energy storage capacity of the cement composite The energy storage capacity of the cement composite was evaluated using a self-made setup [15] equipped with a small test room, a thermal channel, a heat source and a temperaturerecording system, as shown in Fig. 3. The cubical test room is composed of six walls: one testing cement panel with/without FGD-MPCMs and five other walls made of polystyrene foam to provide a heat-insulating environment. The room is connected to a 300mm-long cylindrical heat channel made of polyvinyl chloride with reflective paper to create a uniform and stable temperature field by a connecting wooden board (500 500 15 mm3) with an opening of 200 200 15 mm3. The cement composite board with 20% FGD-MPCMs and dimensions of 200 200 40 mm3 was inserted into the test room opening as a wall. Then, after installing two contacts of a K-type thermocouple (resolution ± 0.3 °C), the small room was placed into a wooden box with the test wall close to the heat channel. The thermal-electric couple contact attached at the outer surface of the cement composite wall was used to measure the wall temperature, and the one inserted into the centre of the test room was used to examine the ‘‘indoor” temperature. A 500-W lamp was used to supplement the heat source and placed at 200 mm from the heat channel. In this test, the test wall was heated for 1 h and then cooled naturally for 40 min. For comparison, a raw cement board with the same size was measured using the same experimental procedure to serve as a control.
2.3.6. Porosity measurement Mercury intrusion porosimetry (MIP) has proven to be a powerful technique to characterize porous materials with pore sizes ranging in several orders of magnitude [35]. In this study, MIP tests of the cement composites were conducted using an AutoPore IV 9500 (Micrometrics) porosimeter capable of producing up to 33 103 psia.
2.3.7. Mechanical properties of cement composites The mechanical properties of the cement composite with different dosage of FGD-MPCMs were evaluated by studying its compressive strength and flexural strength at the age of three days in this research. The compressive strength (40-mm cube) and flexural strength (40 mm 40 mm 160 mm prism) of the hardened cement paste was determined at the age of 3 days in accordance with GB/T 17671-1999 (method of testing cements-determination of strength) [Method of Testing Cements-Determination of Strength (ISO); GB/T 17671-1999; Chinese National Standard: Beijing, China, 1999]. The loading rates for compressive and flexural strength were 2400 ± 200 N/s and 50 ± 10 N/s. The test specimens with a size of 40 mm 40 mm 160 mm were fast cured for 3 days in a curing room at a temperature of 50 ± 1 °C and a relative humidity of 99%. The loading rate for the flexural and compressive strength were 50 ± 10 N/s and 2400 ± 200 N/s, respectively. Before doing the compressive strength test, the dimensions of the sample were measured, and the mass of the sample was recorded using an electronic balance with a ±0.1 g error. The density of each sample was then calculated, which represented the average of three samples.
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(b)
(a)
(c)
(d)
200 μm Fig. 4. FE-SEM photos of (a) FGD-MPCMs and (b) their cross-sections; (c) OM image and (d) XRD pattern of FGD-MPCMs.
3. Results and discussion
3.3. Phase-change properties
3.1. Microstructure of FGD-MPCMs
Fig. 6 displays the DSC results of phase-change behaviours of the FGD-MPCMs and cement composites containing different amounts of FGD-MPCMs. The melting and crystallization parameters of the FGD-MPCMs and cement composites and the pure PCMs are listed in Table 2. In the tested temperature range, no thermal effect was found for the raw cement, suggesting that the endo-/ exo-thermal peaks for the cement composites could be attributed to the FGD-MPCMs. The whole melting process of the composites falls in the temperature range of approximately 18–33 °C. The melting points (Tm) for the cement composites containing 20%, 25% and 30% microcapsules were 26.76, 27.87 and 28.52 °C, respectively, which are the comfortable temperatures for human body. The solidification of PCMs incorporated in the cement composite occurred within the temperature range of approximately 28–10 °C, and compared to the freezing curve of FGD-MPCMs, a visible left-shift of the crystallization peaks was observed for the cement composite filled with FGD-MPCMs. As confirmed by the XRD pattern above (Fig. 4d), the two crystallization forms of a- and b-crystals of n-octadecane were also noticeable in the cooling DSC curves as two corresponding exothermic peaks [23]. The formation of a-crystals resulted from the heterogeneously nucleated liquid-rotator transition for which the microcapsules and graphite particles acted as nucleating agents. The b-crystals were formed via the homogeneous nucleation mechanism of liquid transition. The latent heats of fusion and solidification were 248.80 and 249.90 J/g for pure PCMs and 135.80 and 137.60 J/g for FGDMPCMs, respectively. According to the equation below [38], the encapsulation efficiency was 54.82%. Table 2 shows that the latent heats of phase change for the composite increased monotonously as the FGD-MPCMs content increased. In the case of cement composite containing 30% FGD-MPCMs, the values for melting (DHm) and freezing (DHf) processes reached to 34.02 and 33.85 J/g, respectively. Therefore, the incorporation of FGD-MPCMs
The morphology and microstructure of the FGD-MPCMs are shown in Fig. 4a–d. The microcapsules have a spherical profile with many surface wrinkles attributed to the reduced volume of the paraffin wax during the phase change from the melting state to the crystal state [36]. The shell thickness of the microcapsules is approximately 7.5 lm (Fig. 4b). In the optical microscope micrographs (Fig. 4c), the black graphite platelets are well dispersed in the microcapsules, as shown by the different contrast between the graphite and shell polymer (PU). The existence of flaky graphite was further confirmed by the XRD patterns of the FGD-MPCMs (Fig. 4d). The strong peak at 2h = 26.48° was assigned to the characteristic diffraction peak of the (0 0 2) plane of graphite. The reflection of the (0 0 4) plane of graphite was observed at 2h of 54.60° [37]. Moreover, the characteristic peaks for the phasechange paraffin, which mainly consisted of n-octadecane, also appeared in the pattern. The peaks at 2h = 24.71° and 44.43° correspond to the diffractions of the (1 1 1) plane of the a-crystal phase and the (2 0 5) plane of the b-crystal phase, respectively [23]. 3.2. Microstructure of FGD-MPCMs-filled cement composites Fig. 5a–c presents the FE-SEM photos of cement composites containing 10%, 20% and 30% FGD-MPCMs. The majority of the microcapsules were in good condition and were uniformly distributed in the cement matrix. Concave nests and some fractured microcapsules resulted from microcapsule pull-out during the fabrication of the cross-sectional specimen were observed. Fig. 5d shows good interfacial bonding between microcapsules and the cement matrix. The wrinkles formed on the microcapsule surface provide substantial points for cement attachment and fixation, favouring the formation of an embedded interfacial bonding mode.
H. Zhang et al. / Applied Energy 170 (2016) 130–139
(a)
135
(b) FGD-MPCMs
Cement
(c)
(d)
Fig. 5. FE-SEM photos of cement composites containing various amounts of FGD-MPCMs: (a) 10%, (b) 20%, (c) 30% and (d) magnified image of a microcapsule buried in the cement matrix.
improved the thermal energy storage efficiency of the cementbased building materials.
E¼
DHm;FGD-MPCMs þ DHf ;FGD-MPCMs 100% DHm;PCM þ DHf ;PCM
ð1Þ
3.4. Thermal stability Thermal degradation curves of the FGD-MPCMs and cement composites containing various amounts of FGD-MPCMs are presented in Fig. 7, and the characteristic temperatures are listed in Table 3. Fig. 7 shows clear two-step weight loss processes for both FGD-MPCMs and their filled cement composites over the heating range of 30–500 °C. In the case of the raw cement material, only a weak decreasing trend of weight loss was observed during the whole heating process because of the free-water evaporation, the bound water and the decomposition of ettringite and portlandite [39]. Hence, the rapid decrease in the weight loss of the cement composites was ascribed to FGD-MPCMs degradation. When greater amounts of FGD-MPCMs were used, the weight loss of the composite material was also greater. However, increasing the FGD-MPCMs content did not change the characteristic temperatures of the cement composites. The first decomposition stage of microcapsules is caused by the evaporation and decomposition of PCMs in the microcapsules [23], and for pure FGD-MPCMs, the starting decomposition temperature (T1,o), the end temperature (T1,e) and the temperature of the maximum degradation rate (T1,m) are 272.05, 283.31 and 278.18 °C, respectively. In contrast, the corresponding temperatures of T1,o, T1,e and T1,m (listed in Table 3) were found with delays of 46.56, 56.08 and 53.05 °C for the composite with 10% FGD-MPCMs; 48.52, 58.35 and 54.37 °C for the composite with 20% FGD-MPCMs; and 46.34, 53.48 and 52.17 °C for the composite with 30% FGD-MPCMs. The decomposition of the shell materials of the microcapsules occurs during the second stage of rapid degradation, when the composites show a similar delay in the characteristic decomposition
temperatures compared to the pure microcapsules. For instance, in the case of the composite with 10% FGD-MPCMs, the delayed temperatures of T2,o, T2,e and T2,m were 81.93, 68.48 and 75.59 °C, respectively, whereas for the composite with 30% FGD-MPCMs, they were 83.56, 67.68 and 75.34 °C, respectively. These results indicate that when incorporated in the composite, the thermal stability of the microcapsules was enhanced by the surrounding inorganic matrix, which is believed to favour improving the durability of the smart cement materials able to store energy when exposed to severe environmental conditions (i.e., high temperature). 3.5. Infrared thermal imaging analysis The surface temperature distribution on the cement disks with/ without FGD-MPCMs at various times was measured by infrared thermography and is shown in Fig. 8. The increase in the temperature of the cement composite was slower than that of the raw cement disk. The temperature differences between the raw cement and the composite at various times were calculated and are illustrated in Fig. 9. When heated for 17 min, both the temperature at the test point (Sp1) and the average surface temperature showed maximum differences (i.e., 5.0 °C at Sp1 and 4.9 °C for the average value), whereas before and after this time, the differences showed a decreasing trend in the opposite direction. During the heating process, the temperature change on the surface of the disk without FGD-MPCMs depends only on the sensible heat of the cement, whereas for the cement composite containing FGD-MPCMs, the latent heat of the PCMs provides much higher storage density [5] and thus shows a less marked heating effect than the raw cement without FGD-MPCMs [38]. Furthermore, when the cement composite was heated for 17 min, the encapsulated PCMs within the composite reached their melting point, and the cement composite exhibits maximum heat absorption during the melting process. These results indicate that the incorporation of FGD-MPCMs allows the cement composite to delay the temperature change.
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Fig. 7. TGA curves of FGD-MPCMs and the cement composites containing various amounts of FGD-MPCMs. The inner plot is a magnification of the first degradation step of the cement composites.
Table 3 Thermal stabilities of the raw cement, FGD-MPCMs and cement composites containing various amounts of FGD-MPCMs. Step 1
Raw cement Cement10 Cement20 Cement25 Cement30 FGD-MPCMs
Fig. 6. DSC thermograms of FGD-MPCMs and the cement composites containing various amounts of FGD-MPCMs during (a) melting and (b) freezing.
Table 2 Thermal properties of the raw cement, FGD-MPCMs and cement composites containing various amounts of FGD-MPCMs. Melting process
Raw cement Cement20 Cement25 Cement30 FGD-MPCMs Pure PCMs
Freezing process
Tm (°C)
DHm (J/g)
Tf,a (°C)
Tf,b (°C)
DHf (J/g)
– 26.76 27.87 28.52 30.58 28.32
– 14.57 22.35 34.02 135.80 248.80
– 21.85 21.45 21.02 21.49 –
– 18.87 19.44 18.76 16.20 24.43
– 12.71 23.25 33.85 137.60 249.90
3.6. Evaluation of the thermal energy storage capacity in a model room To further evaluate the thermo-regulating capacity, the composite board was used as a wall in a self-made model room, as shown in Fig. 3. The temperature variations at the room centre and the walls with or without FGD-MPCMs during the test period, which consisted of 60-min heating and subsequent 40-min cooling, are plotted in Fig. 10. When exposed to the heat source, a visible delay in the temperature response was observed for the room with the FGD-MPCMs-filled wall compared to the control with raw cement wall. In the initial heating stage (i.e., from the start to 17 min for the test wall and 8.5 min for the room centre), the temperature was less than 18 °C, and the PCM phase change did not occur. Because of the lower thermal conductivity of microcapsules compared to the cement matrix, the temperatures of the room with the composite wall were slightly lower than those of the control room. As the process continued, the involved PCMs
Step 2
T1,o (°C)
T1,e (°C)
T1,m (°C)
T2,o (°C)
T2,e (°C)
T2,m (°C)
147.20 318.61 320.57 319.35 318.39 272.05
229.49 339.39 341.66 338.59 336.79 283.31
183.07 331.23 332.55 331.23 330.35 278.18
432.66 415.50 413.00 414.80 417.13 333.57
480.29 438.54 447.41 436.32 437.74 370.06
458.43 427.37 427.24 425.28 427.12 351.78
Note: T1,o and T1,e were determined from the cross-point of two tangent lines at related bending locations during the first rapid decomposition. T1,m was determined as the temperature corresponding to the peak on the derivative curve of the decomposition. Similar methods were adopted to obtain T2,o, T2,e and T2,m.
(paraffin) began to melt and absorb the environmental heat with a larger latent heat mode [5], and the temperature differences between the room with the composite wall and the control room increased. When heated for 40 min, the temperature differences were 7.54 °C at the wall and 3.60 °C at the room centre. Twenty minutes later, the differences were maximized: compared with the control room, the temperatures of the room with the composite wall decreased by 13.10 °C at the wall and by 6.22 °C at the room centre. Correspondingly, the temperature curves of the room with the composite wall were right shifted and showed a wide, downward peak resulting from the phase change of the core materials. Similar temperature regulation capacity of the FGD-MPCMs-filled board was found in the cooling process. Within the tested time range of this study, the temperatures of the room containing the composite wall decreased more slowly than those of the control room. When cooled naturally for 20 min, the temperatures of the room with the composite wall remained 5.88 °C and 2.32 °C lower at the wall and room centre, respectively, than those of the control room. These results indicate that the FGD-MPCMs reduced the indoor temperature fluctuations and thus exhibited good application potential in buildings to achieve energy storage and thermal comfort. 3.7. Porosity, density and mechanical properties of the cement composites Porosities and densities of the cement composites with different dosage of FGD-MPCMs are listed in Table 4. From Table 4, it
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Time (min)
0
5.0
11.2
17.0
23.7
32.8
Raw cement
Composite
Fig. 8. Infrared thermography images of the cement disks with and without FGD-MPCMs heated for different times. The maximum, minimum and average temperatures and the temperatures determined at the marked point (Sp1) are presented under each thermal image.
Fig. 9. Temperature differences between the raw cement disk and the FGDMPCMs-filled cement disk as a function of time.
can be known that the density of cement composite decreased with the increase of FGD-MPCMs dosage. The reason for this is that the density of PCM is lower than that of cement paste. The decrease of density of the cement composite in percentages are approximately proportional to the FGD-MPCMs dosages in the cement composites. In regard to the porosity of cement composite, with FGD-MPCMs dosage increase, it increased from 3.67% to 8.49%. Compared with the FGD-MPCMs dosage increase, the percentage increase in the porosity of cement composite is dramatical. The reason for this phenomenon maybe the increasing FGD-MPCMs in the cement composites would lead to a greater volume of interfacial transition zone (ITZ) between FGD-MPCM and the matrix. Compared with the matrix of cement paste, the ITZ has a high porosity [40,41]. The results of the compressive strength and flexural strength of hardened cement composites with different mass percentages of FGD-MPCMs are presented in Fig. 11. As shown in Fig. 11, the compressive strength and flexural strength of cement composites decreased with the increase in the mass percentage of FGD-MPCMs. The decreases in compressive strength and flexural strength of the cement composites are due to the presence of weaker MPCMs in compression and increasing porosities of the samples. When the mass percentages of FGD-MPCMs are 10%, 20%, 25%, and 30%, the reductions in compressive strength with respect to control cement material are 28.2%, 41.1%, 47.8%, and 58.4%, respectively. However, despite the negative effect of FGDMPCMs on the mechanical strength of the cement composites observed in this study, the compressive strength and flexural strength of cement composite with 30% FGD-MPCMs can reach to
Fig. 10. Comparison of the thermal regulation performance between the raw cement and the composite using a model room. Temperature variations (a) at the test wall and (b) at the room centre.
as high as 14.2 MPa and 4.1 MPa, respectively, according to BS EN 998-2:2002 (Specification for mortar for masonry – Part 2: Masonry mortar), which satisfies the requirements for building application. In further study of the cement composite with FGDMPCMs, the mechanical strength and bonding between FGDMPCMs and the matrix can be enhanced by adding superfine mineral admixture and/or increasing the surface roughness of FGDMPCMs.
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Table 4 Porosities and densities of the cement composites. FGDMPCMs percentage (%)
Density of cement composite (kg/ m3)
Density decrease percentage
Porosity of cement composite (%)
Porosity increase percentage
0 10 20 25 30
2210.4 2007.3 1804.9 1703.5 1601.7
0.0 9.19 18.3 22.9 27.5
3.67 4.91 6.95 7.85 8.49
0.0 33.8 89.4 113.9 131.3
Compressive strength (MPa)
40
(a)
35 30 25 20 15 10 5
Acknowledgements
0 0
5
10
15
20
25
30
Content of FGD-MPCMs (%) 7
(b)
The authors would like to acknowledge the support of the National Natural Science Foundation of China (51173109, 51308345) and the Guangdong Province Natural Science Foundation (2014A030313561). References
6
Flexural strength (MPa)
(3) A novel cement composite with good thermal energy storage ability was developed by using FGD-MPCMs. The FGDMPCMs were well distributed in the matrix, and good interfacial bonding was achieved. (4) Integrating the board with 20% FGD-MPCMs into the wall of the room model, the maximum temperatures of the cement composite walls and the maximum indoor temperatures of the test rooms were decreased obviously and the differences were as high as 13.1 °C and 6.22 °C, respectively. The infrared thermography test results also confirmed that the cement composite with FGD-MPCMs can lower the temperature effectively. These results indicated that the FGDMPCMs-filled cement composite has great potential applications in buildings to achieve high thermal energy-saving capacity. (5) The compressive and flexural strength of the cement composites both exhibited a decreasing tendency with an increase in the dosage of FGD-MPCMs. Cement composite with 30% FGD-MPCMs reduced the flexural strength by 34.9% and the compressive strength by 58.4%. This phenomenon can be attributed to the presence of weaker MPCMs relative to hard cement in compression and increasing porosity of the cement composites.
5 4 3 2 1 0 0
5
10
15
20
25
30
Content of FGD-MPCMs (%) Fig. 11. The compressive strength (a) and flexural strength (b) of raw cement and cement composites with different mass percentages of FGD-MPCMs.
4. Conclusions From the experimental investigation, the following conclusions can be drawn: (1) An innovative microencapsulated phase change material (FGD-MPCM) with polymer–inorganic hybrid shell, was prepared in this study. Flaky graphite was served as an inorganic hybrid material with high thermal conductivity. (2) The SEM, DSC and TGA test results showed that the FGDMPCMs possess a high quality encapsulation, about 55% encapsulation efficiency and good thermal reliability. The FGD-MPCMs have a melting temperature of 30.58 °C and latent heat of 135.8 J/g.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
A novel phase-change cement composite for thermal energy storage: Fabrication, thermal and mechanical properties He Zhang a, Feng Xing b, Hong-Zhi Cui b, Da-Zhu Chen a,⇑, Xing Ouyang a,⇑, Su-Zhen Xu a, Jia-Xin Wang c, Yi-Tian Huang a, Jian-Dong Zuo a, Jiao-Ning Tang a a b c
Shenzhen Key Laboratory of Special Functional Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil Engineering, Shenzhen University, Shenzhen 518060, China Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel flaky graphite-doped phase-
change microcapsule (FGD-MPCM) was prepared. FGD-MPCM has substantial latent heat storage capacity (135.8 J/g). FGD-MPCMs/cement composite is capable of reducing indoor temperature fluctuation. Compressive strength of cement composite with 30% FGD-MPCMs can reach to 14.2 MPa. Time (min)
0
5.0
11.2
17.0
23.7
32.8
Raw cement
Composite
a r t i c l e
i n f o
Article history: Received 8 November 2015 Received in revised form 16 February 2016 Accepted 17 February 2016
Keywords: Microencapsulated phase-change materials Cement composite Energy storage Thermal stability
a b s t r a c t Facing upon the increasingly severe energy crisis, one of the key issues for reducing the building energy consumption is to pursue high-performance thermal energy storage technologies based on phase-change materials. In this study, a novel cement composite incorporated with flaky graphite-doped microencapsulated phase-change materials (FGD-MPCMs) was developed. Various techniques, such as field emission-scanning electron microscopy (FE-SEM), optical microscopy (OM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to analyse the composite structure and thermal performances. The results indicate that the spherical microcapsules are well dispersed in the cement matrix. When combined within the cement, the thermal stability of the microcapsules was highly improved, and the inclusion of greater amounts of FGD-MPCMs further increased the latent heat of the composite. The mechanical properties of the cement composites were affected with the increase of FGD-MPCMs dosage and the porosity of the composites. In spite of this, the compressive strength and flexural strength of the cement composite with 30% FGD-MPCM could still reach to as high as 14.2 MPa and 4.1 MPa, respectively. Results from the infrared thermography and the model room test suggested that the composite filled with FGDMPCMs is capable of reducing indoor temperature fluctuation and exhibits good potential for application in buildings to enhance energy savings and thermal comfort. Ó 2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding authors. Tel.: +86 755 2653 5133 (D.Z. Chen), +86 755 2653 1629 (X. Ouyang). E-mail addresses: [email protected] (D.Z. Chen), [email protected] (X. Ouyang). http://dx.doi.org/10.1016/j.apenergy.2016.02.091 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
H. Zhang et al. / Applied Energy 170 (2016) 130–139
1. Introduction With industrial development and population growth, the consumption of resources is increasing, especially that of fossil fuels. Concomitantly, millions of tons of CO2 are emitted into the atmosphere every year because of combustion, which is widely believed to be the main cause of global warming [1]. Among the environmental burdens caused by the excessive use of fossil fuels, 30% of the energy consumption and 1/3 of the greenhouse gas released worldwide are attributed to the building sector [2]. Efforts to increase energy efficiency in the building sector have been intensified to meet heating and cooling requirements [3,4]. Phase-change materials (PCMs) [5] can store or release large quantities of latent heat via a nearly isothermal phase-change process; therefore, combining construction materials and PCMs is believed to be an efficient way to increase the heat energy storage efficiency of construction elements [5,6]. For instance, Paksoy et al. [7] reported that 7% of cooling energy and 28% of heating energy was conserved by combining 3.5 kg of PCMs with insulation panels in a test cabin. However, bulk PCMs are not easy to handle in practical applications because of leakage, erosion and super-cooling problems. To solve these problematic limitations, the materials are generally encapsulated with a polymer or inorganic shell before use [8]. Borreguero et al. [9] and Zhang et al. [10] incorporated microencapsulated PCMs (MPCMs) into gypsum boards and found that the thermal regulation ability of the boards was enhanced as the amount of microcapsules was increased. Similar results were observed in MPCMs-filled, cement-based composites [11–16] and geopolymer mortar [17]. An effective envelopment of PCMs is important to prevent the leakage of core materials, increase the heat-transfer area and control the volume changes during the phase changes [18]. Many polymers and copolymers, such as polystyrene (PS) [19,20], polyurea (PU) [21–23], poly(methyl methacrylate) (PMMA) [24–26] and melamine–formaldehyde (MF) resin [27,28], are commonly used as shells in MPCMs. However, the poor heat conduction of the pure organic polymer shell limits the response velocity of the microcapsules to the environmental temperature change. More recently, there are growing interests in utilizing polymer–inorganic hybrid shells to improve the thermal conductivity and mechanical performance of phase-change microcapsules. Chang et al. [29] synthesized microencapsulated n-octadecane with a PMMA-silica hybrid shell via in-situ polymerization and investigated the effect of coupling agents, ageing time and temperature on the thermal properties of the microcapsules. By mixing Cu (II) methacrylate with the oil phase during the fabrication process, Wu et al. [30] obtained PMMA-SiO2 @ paraffin microcapsules with copper chelation as the ion probe. Yin et al. [31,32] prepared MPCMs with MF resin/SiO2 and a poly(styrene-co-divinylbenzene)/SiO2 shell through Pickering emulsion polymerization and explored the effects of introducing inorganic particles in the MPCMs shell on the mechanical strength, thermal reliability and anti-osmosis performance, which were significantly improved. Moreover, using the same in-situ polymerization method, Chen et al. [33] incorporated nano-Al2O3 particles into the MPCMs shell and observed an obvious enhancement of the thermal stability of the microcapsules. Graphite is one of excellent thermal conductivity materials. Thermal conductivity coefficient of graphite can reach about 390 W/ (m K), which nearly 1000 times of that of polymer. Due to graphite shape likes a platelet, therefore, less research has addressed flaky graphite-doped, microencapsulated PCMs (FGD-MPCMs) and their applications in the field of building materials as the thermal energy storage carriers. Most researches on enhancement of the thermal stability of the microcapsules only focused on the formation of microcapsules and their thermophysical or mechanical properties, in this work, a
131
novel cement composite with good thermal energy storage capacity was developed by incorporating self-manufactured, flaky graphite-doped, phase-change paraffin microcapsules via interfacial polycondensation [34]. The structure, phase-change properties, and thermal stabilities of both the microcapsules and the FGD-MPCMs-filled cement composites were well examined. For building thermal energy storage application, both efficiency of the thermal energy storage and the mechanical properties of the cement composite with FGD-MPCMs were evaluated. 2. Experimental 2.1. Materials Paraffin wax (RT28, Rubitherm Technologies Gmbh, Berlin, Germany), which was mainly composed of n-octadecane, was used as the core material of the FGD-MPCMs. 3-Isocyanatomethyl-3,5,5-tri methylcyclohexyl isocyanate (IPDI, Bayer, Taiwan) and diethylene triamine (DETA, Dow Chemical, USA) with purity exceeding 99.5% were used to form the microcapsule shell. Styrene–maleic anhydride copolymer (SMA, ScriptÒ 520, USA) was used as the emulsifier. Flaky graphite powder (800 meshes, Qingdao Tianyuan Graphite Co., Ltd., China) was used as the inorganic filler for the microcapsules. Portland cement powder (PII 42.5R, Guangdong Zhujiang Cement Factory, China) was adopted as the matrix for fabricating the raw cement or cement composite board. Polyacrylic acid (Guangdong Longhu Science and Technology Co., Ltd., China) was used as the water-reducing agent. 2.2. Fabrication 2.2.1. Synthesis of FGD-MPCMs The microencapsulation of paraffin wax doped with flaky graphite powder was achieved through interfacial polycondensation. The fabrication procedure, including the mixing of core materials, emulsification, polymerization, washing and drying, is schematically illustrated in Fig. 1. Paraffin wax, IPDI and flaky graphite were mixed at 50 °C for 30 min to produce the core material with a weight proportion of 30:20:1. Then, the mixture was poured into a 3-ton reaction kettle filled with 1.3 wt% SMA aqueous solution and agitated vigorously at 60 °C for 1 h to generate a stable oil/ water (O/W) emulsion. Subsequently, stoichiometric DETA was added dropwise into the above emulsion to initiate the formation of the polyurea shell. After continuous stirring for three hours, the resultant microcapsules were centrifugally filtered, washed with hot water and dried at room temperature for one week. 2.2.2. Preparation of smart cement composite containing FGD-MPCMs The cement composites were manufactured by mixing Portland cement, FGD-MPCMs and water at room temperature according to the proportions listed in Table 1. For all specimens, the mass ratio of the cement powder and water was 5:2. Compared to the amount of cement powder, the filling contents of FGD-MPCMs in the composites were 10%, 20%, 25% and 30%, and the corresponding composites were named Cement10, Cement20, Cement25 and Cement30, respectively. The fabrication process is illustrated in Fig. 2. The FGD-MPCMs were immersed in water in a 500-ml beaker for 24 h to ensure that they were completely wetted. Then, the mixture was transferred into a metal stirring pot with the cement powder and stirred vigorously for 5 min using a Hobart mixer (Hobart Corporation, Troy, OH, USA). During this step, the remaining water was calculated, and a very small amount of waterreducing agent was added. The resulting slurry was poured into the moulds and kept for one day to form board-like or cylindrical cement samples. Finally, the above samples were inserted into a curing chamber at constant temperature and humidity (Dongxing
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IPDI Paraffin
Graphite
Water
SMA
Oil phase
DETA
Emulsifying
Mixing
SMA solution
Washing & Drying
Stirring
O/W emulsion
Interfacial polymerization
FGD-MPCMs
Fig. 1. Schematic synthesis of the FGD-MPCMs. Table 1 Descriptions of the raw cement and cement composites filled with FGD-MPCMs. Samples
Water (g)
Cement powders (g)
FGD-MPCMs (g)
Water reducing agent (g)
Raw cement Cement10 Cement20 Cement25 Cement30
500 500 500 500 500
1250 1250 1250 1250 1250
0 125 250 312 375
0 0 2.8 3.7 4.8
Building Material Co., Ltd., China) and placed into water for fast curing at 50 °C for three days.
atmosphere. The temperature scanning range was from 10 to 50 °C.
2.3. Characterization
2.3.3. Thermogravimetric analysis (TGA) TGA of the FGD-MPCMs and cement composites were performed under a nitrogen atmosphere using a TA Q50 thermal gravimetric analyser. For each test, the sample was first heated from 30 to 120 °C at a rate of 20 °C/min and was kept at 120 °C for 20 min to remove the adsorbed water. The temperature was then quickly decreased to 30 °C and increased to 500 °C at a heating rate of 10 °C/min.
2.3.1. Structures of the FGD-MPCMs and cement composites The morphologies of the FGD-MPCMs and their dispersion in the cement composites were examined using a Hitachi Su-70 field emission-scanning electron microscope (FE-SEM) at an accelerating voltage of 1.0 kV. The samples of FGD-MPCMs and the fractured surfaces of the composites were sprayed with gold prior to observation. A Mshot MP41 optical microscope (OM) equipped with a MC 50 digital imaging system (Guangzhou Micro-shot Technology Co., Ltd., China) was used to observe the microstructures of the microcapsules. The crystalline phases of the FGD-MPCMs were analysed by an X-ray diffractometer (XRD, Bruker D8) with Cu Ka radiation (k = 0.154 nm) with operating conditions of 40 kV and 200 mA. The XRD pattern was recorded in the 2h range from 10° to 80° with a scanning rate of 3°/min.
2.3.4. Infrared thermography To determine the thermo-regulating performance of the cement composite, infrared thermography was conducted with an infrared thermal imager (FLIR SC655). A round composite disk containing 20% FGD-MPCMs with a diameter of 25 mm and a thickness of 3 mm was prepared in a cylindrical mould. A raw cement disk without FGD-MPCMs was also fabricated as a reference. Both samples were cooled to 3 °C in advance and placed onto a hot stage maintained at a constant temperature of 60 °C. To prevent the sample’s temperature from rising too quickly, a cylindrical PTFE disk (U 25 mm) with a thickness of 5 mm was placed between the hot plate and the test sample. The microscope lens was kept
2.3.2. Differential scanning calorimetry (DSC) The phase-change properties of the FGD-MPCMs and cement composites were analysed using a TA Q200 differential scanning calorimeter at a heating/cooling rate of 5 °C/min in a nitrogen
FGD-MPCMs
Cement powder
Water Water
Water reducing agent
Transferring
Wetting of FGD-MPCMs
Molding & curing
Compounding
Cement composite with FGD-MPCMs
Fig. 2. Schematic fabrication of the FGD-MPCMs-filled cement composite.
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Cement composite wall
or
Raw cement wall Model room without test wall
Model room with composite wall
Thermocouple Data logger
Heat channel
Computer recording system
Heat source Test setup with model room
Installation of model room
Fig. 3. Schematic diagram of the setup used to evaluate the thermal regulation capacity of the FGD-MPCMs-filled cement composite.
at 25 cm from the upper surface of the sample during the test. The infrared thermal images of both the composite and control were taken at different times from 3 to 40 °C and analysed using FLIR Researcher IR software. 2.3.5. Evaluation of the energy storage capacity of the cement composite The energy storage capacity of the cement composite was evaluated using a self-made setup [15] equipped with a small test room, a thermal channel, a heat source and a temperaturerecording system, as shown in Fig. 3. The cubical test room is composed of six walls: one testing cement panel with/without FGD-MPCMs and five other walls made of polystyrene foam to provide a heat-insulating environment. The room is connected to a 300mm-long cylindrical heat channel made of polyvinyl chloride with reflective paper to create a uniform and stable temperature field by a connecting wooden board (500 500 15 mm3) with an opening of 200 200 15 mm3. The cement composite board with 20% FGD-MPCMs and dimensions of 200 200 40 mm3 was inserted into the test room opening as a wall. Then, after installing two contacts of a K-type thermocouple (resolution ± 0.3 °C), the small room was placed into a wooden box with the test wall close to the heat channel. The thermal-electric couple contact attached at the outer surface of the cement composite wall was used to measure the wall temperature, and the one inserted into the centre of the test room was used to examine the ‘‘indoor” temperature. A 500-W lamp was used to supplement the heat source and placed at 200 mm from the heat channel. In this test, the test wall was heated for 1 h and then cooled naturally for 40 min. For comparison, a raw cement board with the same size was measured using the same experimental procedure to serve as a control.
2.3.6. Porosity measurement Mercury intrusion porosimetry (MIP) has proven to be a powerful technique to characterize porous materials with pore sizes ranging in several orders of magnitude [35]. In this study, MIP tests of the cement composites were conducted using an AutoPore IV 9500 (Micrometrics) porosimeter capable of producing up to 33 103 psia.
2.3.7. Mechanical properties of cement composites The mechanical properties of the cement composite with different dosage of FGD-MPCMs were evaluated by studying its compressive strength and flexural strength at the age of three days in this research. The compressive strength (40-mm cube) and flexural strength (40 mm 40 mm 160 mm prism) of the hardened cement paste was determined at the age of 3 days in accordance with GB/T 17671-1999 (method of testing cements-determination of strength) [Method of Testing Cements-Determination of Strength (ISO); GB/T 17671-1999; Chinese National Standard: Beijing, China, 1999]. The loading rates for compressive and flexural strength were 2400 ± 200 N/s and 50 ± 10 N/s. The test specimens with a size of 40 mm 40 mm 160 mm were fast cured for 3 days in a curing room at a temperature of 50 ± 1 °C and a relative humidity of 99%. The loading rate for the flexural and compressive strength were 50 ± 10 N/s and 2400 ± 200 N/s, respectively. Before doing the compressive strength test, the dimensions of the sample were measured, and the mass of the sample was recorded using an electronic balance with a ±0.1 g error. The density of each sample was then calculated, which represented the average of three samples.
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(b)
(a)
(c)
(d)
200 μm Fig. 4. FE-SEM photos of (a) FGD-MPCMs and (b) their cross-sections; (c) OM image and (d) XRD pattern of FGD-MPCMs.
3. Results and discussion
3.3. Phase-change properties
3.1. Microstructure of FGD-MPCMs
Fig. 6 displays the DSC results of phase-change behaviours of the FGD-MPCMs and cement composites containing different amounts of FGD-MPCMs. The melting and crystallization parameters of the FGD-MPCMs and cement composites and the pure PCMs are listed in Table 2. In the tested temperature range, no thermal effect was found for the raw cement, suggesting that the endo-/ exo-thermal peaks for the cement composites could be attributed to the FGD-MPCMs. The whole melting process of the composites falls in the temperature range of approximately 18–33 °C. The melting points (Tm) for the cement composites containing 20%, 25% and 30% microcapsules were 26.76, 27.87 and 28.52 °C, respectively, which are the comfortable temperatures for human body. The solidification of PCMs incorporated in the cement composite occurred within the temperature range of approximately 28–10 °C, and compared to the freezing curve of FGD-MPCMs, a visible left-shift of the crystallization peaks was observed for the cement composite filled with FGD-MPCMs. As confirmed by the XRD pattern above (Fig. 4d), the two crystallization forms of a- and b-crystals of n-octadecane were also noticeable in the cooling DSC curves as two corresponding exothermic peaks [23]. The formation of a-crystals resulted from the heterogeneously nucleated liquid-rotator transition for which the microcapsules and graphite particles acted as nucleating agents. The b-crystals were formed via the homogeneous nucleation mechanism of liquid transition. The latent heats of fusion and solidification were 248.80 and 249.90 J/g for pure PCMs and 135.80 and 137.60 J/g for FGDMPCMs, respectively. According to the equation below [38], the encapsulation efficiency was 54.82%. Table 2 shows that the latent heats of phase change for the composite increased monotonously as the FGD-MPCMs content increased. In the case of cement composite containing 30% FGD-MPCMs, the values for melting (DHm) and freezing (DHf) processes reached to 34.02 and 33.85 J/g, respectively. Therefore, the incorporation of FGD-MPCMs
The morphology and microstructure of the FGD-MPCMs are shown in Fig. 4a–d. The microcapsules have a spherical profile with many surface wrinkles attributed to the reduced volume of the paraffin wax during the phase change from the melting state to the crystal state [36]. The shell thickness of the microcapsules is approximately 7.5 lm (Fig. 4b). In the optical microscope micrographs (Fig. 4c), the black graphite platelets are well dispersed in the microcapsules, as shown by the different contrast between the graphite and shell polymer (PU). The existence of flaky graphite was further confirmed by the XRD patterns of the FGD-MPCMs (Fig. 4d). The strong peak at 2h = 26.48° was assigned to the characteristic diffraction peak of the (0 0 2) plane of graphite. The reflection of the (0 0 4) plane of graphite was observed at 2h of 54.60° [37]. Moreover, the characteristic peaks for the phasechange paraffin, which mainly consisted of n-octadecane, also appeared in the pattern. The peaks at 2h = 24.71° and 44.43° correspond to the diffractions of the (1 1 1) plane of the a-crystal phase and the (2 0 5) plane of the b-crystal phase, respectively [23]. 3.2. Microstructure of FGD-MPCMs-filled cement composites Fig. 5a–c presents the FE-SEM photos of cement composites containing 10%, 20% and 30% FGD-MPCMs. The majority of the microcapsules were in good condition and were uniformly distributed in the cement matrix. Concave nests and some fractured microcapsules resulted from microcapsule pull-out during the fabrication of the cross-sectional specimen were observed. Fig. 5d shows good interfacial bonding between microcapsules and the cement matrix. The wrinkles formed on the microcapsule surface provide substantial points for cement attachment and fixation, favouring the formation of an embedded interfacial bonding mode.
H. Zhang et al. / Applied Energy 170 (2016) 130–139
(a)
135
(b) FGD-MPCMs
Cement
(c)
(d)
Fig. 5. FE-SEM photos of cement composites containing various amounts of FGD-MPCMs: (a) 10%, (b) 20%, (c) 30% and (d) magnified image of a microcapsule buried in the cement matrix.
improved the thermal energy storage efficiency of the cementbased building materials.
E¼
DHm;FGD-MPCMs þ DHf ;FGD-MPCMs 100% DHm;PCM þ DHf ;PCM
ð1Þ
3.4. Thermal stability Thermal degradation curves of the FGD-MPCMs and cement composites containing various amounts of FGD-MPCMs are presented in Fig. 7, and the characteristic temperatures are listed in Table 3. Fig. 7 shows clear two-step weight loss processes for both FGD-MPCMs and their filled cement composites over the heating range of 30–500 °C. In the case of the raw cement material, only a weak decreasing trend of weight loss was observed during the whole heating process because of the free-water evaporation, the bound water and the decomposition of ettringite and portlandite [39]. Hence, the rapid decrease in the weight loss of the cement composites was ascribed to FGD-MPCMs degradation. When greater amounts of FGD-MPCMs were used, the weight loss of the composite material was also greater. However, increasing the FGD-MPCMs content did not change the characteristic temperatures of the cement composites. The first decomposition stage of microcapsules is caused by the evaporation and decomposition of PCMs in the microcapsules [23], and for pure FGD-MPCMs, the starting decomposition temperature (T1,o), the end temperature (T1,e) and the temperature of the maximum degradation rate (T1,m) are 272.05, 283.31 and 278.18 °C, respectively. In contrast, the corresponding temperatures of T1,o, T1,e and T1,m (listed in Table 3) were found with delays of 46.56, 56.08 and 53.05 °C for the composite with 10% FGD-MPCMs; 48.52, 58.35 and 54.37 °C for the composite with 20% FGD-MPCMs; and 46.34, 53.48 and 52.17 °C for the composite with 30% FGD-MPCMs. The decomposition of the shell materials of the microcapsules occurs during the second stage of rapid degradation, when the composites show a similar delay in the characteristic decomposition
temperatures compared to the pure microcapsules. For instance, in the case of the composite with 10% FGD-MPCMs, the delayed temperatures of T2,o, T2,e and T2,m were 81.93, 68.48 and 75.59 °C, respectively, whereas for the composite with 30% FGD-MPCMs, they were 83.56, 67.68 and 75.34 °C, respectively. These results indicate that when incorporated in the composite, the thermal stability of the microcapsules was enhanced by the surrounding inorganic matrix, which is believed to favour improving the durability of the smart cement materials able to store energy when exposed to severe environmental conditions (i.e., high temperature). 3.5. Infrared thermal imaging analysis The surface temperature distribution on the cement disks with/ without FGD-MPCMs at various times was measured by infrared thermography and is shown in Fig. 8. The increase in the temperature of the cement composite was slower than that of the raw cement disk. The temperature differences between the raw cement and the composite at various times were calculated and are illustrated in Fig. 9. When heated for 17 min, both the temperature at the test point (Sp1) and the average surface temperature showed maximum differences (i.e., 5.0 °C at Sp1 and 4.9 °C for the average value), whereas before and after this time, the differences showed a decreasing trend in the opposite direction. During the heating process, the temperature change on the surface of the disk without FGD-MPCMs depends only on the sensible heat of the cement, whereas for the cement composite containing FGD-MPCMs, the latent heat of the PCMs provides much higher storage density [5] and thus shows a less marked heating effect than the raw cement without FGD-MPCMs [38]. Furthermore, when the cement composite was heated for 17 min, the encapsulated PCMs within the composite reached their melting point, and the cement composite exhibits maximum heat absorption during the melting process. These results indicate that the incorporation of FGD-MPCMs allows the cement composite to delay the temperature change.
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Fig. 7. TGA curves of FGD-MPCMs and the cement composites containing various amounts of FGD-MPCMs. The inner plot is a magnification of the first degradation step of the cement composites.
Table 3 Thermal stabilities of the raw cement, FGD-MPCMs and cement composites containing various amounts of FGD-MPCMs. Step 1
Raw cement Cement10 Cement20 Cement25 Cement30 FGD-MPCMs
Fig. 6. DSC thermograms of FGD-MPCMs and the cement composites containing various amounts of FGD-MPCMs during (a) melting and (b) freezing.
Table 2 Thermal properties of the raw cement, FGD-MPCMs and cement composites containing various amounts of FGD-MPCMs. Melting process
Raw cement Cement20 Cement25 Cement30 FGD-MPCMs Pure PCMs
Freezing process
Tm (°C)
DHm (J/g)
Tf,a (°C)
Tf,b (°C)
DHf (J/g)
– 26.76 27.87 28.52 30.58 28.32
– 14.57 22.35 34.02 135.80 248.80
– 21.85 21.45 21.02 21.49 –
– 18.87 19.44 18.76 16.20 24.43
– 12.71 23.25 33.85 137.60 249.90
3.6. Evaluation of the thermal energy storage capacity in a model room To further evaluate the thermo-regulating capacity, the composite board was used as a wall in a self-made model room, as shown in Fig. 3. The temperature variations at the room centre and the walls with or without FGD-MPCMs during the test period, which consisted of 60-min heating and subsequent 40-min cooling, are plotted in Fig. 10. When exposed to the heat source, a visible delay in the temperature response was observed for the room with the FGD-MPCMs-filled wall compared to the control with raw cement wall. In the initial heating stage (i.e., from the start to 17 min for the test wall and 8.5 min for the room centre), the temperature was less than 18 °C, and the PCM phase change did not occur. Because of the lower thermal conductivity of microcapsules compared to the cement matrix, the temperatures of the room with the composite wall were slightly lower than those of the control room. As the process continued, the involved PCMs
Step 2
T1,o (°C)
T1,e (°C)
T1,m (°C)
T2,o (°C)
T2,e (°C)
T2,m (°C)
147.20 318.61 320.57 319.35 318.39 272.05
229.49 339.39 341.66 338.59 336.79 283.31
183.07 331.23 332.55 331.23 330.35 278.18
432.66 415.50 413.00 414.80 417.13 333.57
480.29 438.54 447.41 436.32 437.74 370.06
458.43 427.37 427.24 425.28 427.12 351.78
Note: T1,o and T1,e were determined from the cross-point of two tangent lines at related bending locations during the first rapid decomposition. T1,m was determined as the temperature corresponding to the peak on the derivative curve of the decomposition. Similar methods were adopted to obtain T2,o, T2,e and T2,m.
(paraffin) began to melt and absorb the environmental heat with a larger latent heat mode [5], and the temperature differences between the room with the composite wall and the control room increased. When heated for 40 min, the temperature differences were 7.54 °C at the wall and 3.60 °C at the room centre. Twenty minutes later, the differences were maximized: compared with the control room, the temperatures of the room with the composite wall decreased by 13.10 °C at the wall and by 6.22 °C at the room centre. Correspondingly, the temperature curves of the room with the composite wall were right shifted and showed a wide, downward peak resulting from the phase change of the core materials. Similar temperature regulation capacity of the FGD-MPCMs-filled board was found in the cooling process. Within the tested time range of this study, the temperatures of the room containing the composite wall decreased more slowly than those of the control room. When cooled naturally for 20 min, the temperatures of the room with the composite wall remained 5.88 °C and 2.32 °C lower at the wall and room centre, respectively, than those of the control room. These results indicate that the FGD-MPCMs reduced the indoor temperature fluctuations and thus exhibited good application potential in buildings to achieve energy storage and thermal comfort. 3.7. Porosity, density and mechanical properties of the cement composites Porosities and densities of the cement composites with different dosage of FGD-MPCMs are listed in Table 4. From Table 4, it
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Time (min)
0
5.0
11.2
17.0
23.7
32.8
Raw cement
Composite
Fig. 8. Infrared thermography images of the cement disks with and without FGD-MPCMs heated for different times. The maximum, minimum and average temperatures and the temperatures determined at the marked point (Sp1) are presented under each thermal image.
Fig. 9. Temperature differences between the raw cement disk and the FGDMPCMs-filled cement disk as a function of time.
can be known that the density of cement composite decreased with the increase of FGD-MPCMs dosage. The reason for this is that the density of PCM is lower than that of cement paste. The decrease of density of the cement composite in percentages are approximately proportional to the FGD-MPCMs dosages in the cement composites. In regard to the porosity of cement composite, with FGD-MPCMs dosage increase, it increased from 3.67% to 8.49%. Compared with the FGD-MPCMs dosage increase, the percentage increase in the porosity of cement composite is dramatical. The reason for this phenomenon maybe the increasing FGD-MPCMs in the cement composites would lead to a greater volume of interfacial transition zone (ITZ) between FGD-MPCM and the matrix. Compared with the matrix of cement paste, the ITZ has a high porosity [40,41]. The results of the compressive strength and flexural strength of hardened cement composites with different mass percentages of FGD-MPCMs are presented in Fig. 11. As shown in Fig. 11, the compressive strength and flexural strength of cement composites decreased with the increase in the mass percentage of FGD-MPCMs. The decreases in compressive strength and flexural strength of the cement composites are due to the presence of weaker MPCMs in compression and increasing porosities of the samples. When the mass percentages of FGD-MPCMs are 10%, 20%, 25%, and 30%, the reductions in compressive strength with respect to control cement material are 28.2%, 41.1%, 47.8%, and 58.4%, respectively. However, despite the negative effect of FGDMPCMs on the mechanical strength of the cement composites observed in this study, the compressive strength and flexural strength of cement composite with 30% FGD-MPCMs can reach to
Fig. 10. Comparison of the thermal regulation performance between the raw cement and the composite using a model room. Temperature variations (a) at the test wall and (b) at the room centre.
as high as 14.2 MPa and 4.1 MPa, respectively, according to BS EN 998-2:2002 (Specification for mortar for masonry – Part 2: Masonry mortar), which satisfies the requirements for building application. In further study of the cement composite with FGDMPCMs, the mechanical strength and bonding between FGDMPCMs and the matrix can be enhanced by adding superfine mineral admixture and/or increasing the surface roughness of FGDMPCMs.
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Table 4 Porosities and densities of the cement composites. FGDMPCMs percentage (%)
Density of cement composite (kg/ m3)
Density decrease percentage
Porosity of cement composite (%)
Porosity increase percentage
0 10 20 25 30
2210.4 2007.3 1804.9 1703.5 1601.7
0.0 9.19 18.3 22.9 27.5
3.67 4.91 6.95 7.85 8.49
0.0 33.8 89.4 113.9 131.3
Compressive strength (MPa)
40
(a)
35 30 25 20 15 10 5
Acknowledgements
0 0
5
10
15
20
25
30
Content of FGD-MPCMs (%) 7
(b)
The authors would like to acknowledge the support of the National Natural Science Foundation of China (51173109, 51308345) and the Guangdong Province Natural Science Foundation (2014A030313561). References
6
Flexural strength (MPa)
(3) A novel cement composite with good thermal energy storage ability was developed by using FGD-MPCMs. The FGDMPCMs were well distributed in the matrix, and good interfacial bonding was achieved. (4) Integrating the board with 20% FGD-MPCMs into the wall of the room model, the maximum temperatures of the cement composite walls and the maximum indoor temperatures of the test rooms were decreased obviously and the differences were as high as 13.1 °C and 6.22 °C, respectively. The infrared thermography test results also confirmed that the cement composite with FGD-MPCMs can lower the temperature effectively. These results indicated that the FGDMPCMs-filled cement composite has great potential applications in buildings to achieve high thermal energy-saving capacity. (5) The compressive and flexural strength of the cement composites both exhibited a decreasing tendency with an increase in the dosage of FGD-MPCMs. Cement composite with 30% FGD-MPCMs reduced the flexural strength by 34.9% and the compressive strength by 58.4%. This phenomenon can be attributed to the presence of weaker MPCMs relative to hard cement in compression and increasing porosity of the cement composites.
5 4 3 2 1 0 0
5
10
15
20
25
30
Content of FGD-MPCMs (%) Fig. 11. The compressive strength (a) and flexural strength (b) of raw cement and cement composites with different mass percentages of FGD-MPCMs.
4. Conclusions From the experimental investigation, the following conclusions can be drawn: (1) An innovative microencapsulated phase change material (FGD-MPCM) with polymer–inorganic hybrid shell, was prepared in this study. Flaky graphite was served as an inorganic hybrid material with high thermal conductivity. (2) The SEM, DSC and TGA test results showed that the FGDMPCMs possess a high quality encapsulation, about 55% encapsulation efficiency and good thermal reliability. The FGD-MPCMs have a melting temperature of 30.58 °C and latent heat of 135.8 J/g.
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