expanded graphite composite phase change solar thermal absorption material

expanded graphite composite phase change solar thermal absorption material

Solar Energy Materials and Solar Cells 200 (2019) 110038 Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal ho...

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Solar Energy Materials and Solar Cells 200 (2019) 110038

Contents lists available at ScienceDirect

Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat

A foamed cement blocks with paraffin/expanded graphite composite phase change solar thermal absorption material

T

Lifang Liua, Jiayu Chenb, Yue Qua, Tao Xua,c,*, Huijun Wua, Gongsheng Huangb, Xiaoqing Zhoua, Lixiu Yanga a

Academy of Building Energy Efficiency, School of Civil Engineering, Guangzhou University, Guangzhou, 510006, China Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Ave, Kowloon, Hong Kong c Linköping University - Guangzhou University Research Center on Urban Sustainable Development, Guangzhou University, Guangzhou, 510006, China b

ARTICLE INFO

ABSTRACT

Keywords: Phase change thermal storage material Paraffin Expanded graphite Foamed cement

When solar radiation reaches its peak value during the midday, buildings' external walls absorb the highest amount of heats and result in dramatic spikes in cooling load and energy consumption. To flatten the energy peak, promote energy efficiency, and preserve thermal comfort, this study developed a novel foamed thermal insulation cement block with extraordinary heat storage capacity. To prepare the proposed foamed cement block, a paraffin/expanded graphite composite phase change heat storage material with a phase transition temperature of 41.9 °C and an enthalpy value of 207.8 J/g was used to load paraffin into expanded graphite in a water bath at 44 °C. After mixing the paraffin/expanded graphite composite phase change material (PCM) with cement, foams were added, stirred and poured into the mold to make a novel foaming cement thermal insulation block. Due to its porous structure, the foamed cement has lightweight as well as high heat preservation capacity. To investigate the characteristics of the proposed material, this investigated its physical structure, thermal conductivity, and heat energy storage performance when the material mass fractions of PCM are 10%, 15%, 20%, 25% and 30%, and regular (pure) cement blocks. The experimental results show that there is no chemical reaction during material preparation. In addition, composite materials’ PCM mass fraction and their thermal storage capacity increase simultaneously. The comprehensive analysis shows that foamed cement blocks with 30% PCM contents have the best thermal energy storage performance and can maintain the lowest average indoor temperature. Therefore, the proposed foamed cement blocks can be applied to building outer surfaces and sandwiched middle enclosures to develop energy efficiency insulation walls.

1. Introduction and background Reducing energy consumption and carbon emission have become the targets of sustainable development all over the world. Building industry as the major energy consumer is regarded as an inevitable target to be improved. Traditionally, thermal insulation is one of the most efficient methods to improve building energy efficiency. Thermal insulations for buildings aim to reduce heat losses in the building interior space and heat gains from the outside environment. Efficient insulations can effectively reduce thermal load and energy consumption of the air-conditioning systems in a building [1]. Therefore, developing better thermal insulation materials for buildings is regarded as one of the most effective methods to promote building energy efficiency [2]. In recent years, researchers found the traditional organic insulation materials significant constraints, such as cracking [3], poor durability,

*

flammability, and toxic gases during combustion [4]. It is inevitable to develop alternative materials that can overcome these problems while maintaining a high thermal insulation capacity [5]. With the advances in the ecological energy conservation and resource utilization technologies, foamed cement, a new type of green building material, is expected to replace organic thermal insulation material in the building industry [4]. Foamed cement is a lightweight porous material introducing bubbles generated by chemical foaming agents in cement slurry [6]. Foamed cement is a typical building insulation material has been widely implemented for years. It has excellent thermal insulation properties, lightweight, high adhesive force, good durability, excellent A-grade combustibility, and low carbon emission during production [7]. As the purpose of insulation in the building is to trap heat into specific areas, many researchers used energy storage technology to clip

Corresponding author. Academy of Building Energy Efficiency, School of Civil Engineering, Guangzhou University, Guangzhou, 510006, China. E-mail address: [email protected] (T. Xu).

https://doi.org/10.1016/j.solmat.2019.110038 Received 18 February 2019; Received in revised form 25 June 2019; Accepted 30 June 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.

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the peak and fill the valley to stabilize energy consumption and avoid waste. The phase change material (PCM), as a typical energy storage material, has been recognized in the field of building energy conservation [8]. Researchers proposed to produce phase change thermal storage foamed cement blocks though adding heat storage thermal energy storage PCM into foamed cement blocks. PCM has a large energy storage density, so its temperature remains constant within a certain temperature range during storage and release of energy. Such character makes the phase change material not only can store heat but also regulate temperature [9,10]. PCM is often combined with other building materials (such as gypsum board, wallboard and concrete) to enhance their energy storage capacity to reduce electrical system energy consumption and reduce adjustment frequency of air conditioning systems. By doing so, the balance between indoor temperature and ambient temperature, and indoor thermal comfort level also can be improved [11,12]. According to the phase change mode, PCMs can be divided into four categories: solid-solid phase change, solid-liquid phase change, solidgas phase change, and liquid-gas phase change [13]. Among them, the solid-liquid PCM is a relatively mature PCM at present. It has large latent heat and stable performance, but the solid-liquid phase change material cannot be directly used due to the liquid leakage and low thermal conductivity [14,15]. Inorganic hydrated salt and organic phase change materials (OPCMs) have large latent heat, suitable melting point, and low price, and it becomes the most widely used PCM at present. However, when the temperature is low and being overheated, OPCMs tends to precipitate out, which have corrosion effect on building elements [16]. In recent years, Paraffin is recognized as one of the most promising PCMs due to its large latent heat, low cost and its stable, nontoxic and noncorrosive properties [17]. However, if paraffin is directly added to the building materials, phase separation and leakage are likely to occur [9,18]. Therefore, the paraffin wax needs to be qualitatively treated before implementation. Some researchers developed paraffin microcapsule mortar for wall structure [19,20], it takes a complicated process and capital investment to produce. Expanded graphite (EG) is a potential porous material for heat conduction enhancement in organic OPCMs. Compared with metal foams, it is light weighted, inexpensive and resistant to corrosion. EG shows adsorption capacity to paraffin, which can shape liquid paraffin and solve the problem of leakage and difficulties in packaging [21]. Chen et al. proposed such paraffin/expanded graphite composite phase change material with a mass ratio of EG of 20% (completely absorbed) to cool down the room temperature [22]. Based on previous studies, this study proposes to apply paraffin/expanded graphite composite PCM (phase transition temperature of 44 °C) in the foamed cement. This not only can improve the foamed cement blocks’ thermal energy storage performance, but also mitigate the temperature fluctuation in the room. For example, the surface temperature of many solar-powered walls or roofs can reach as high as 55.5 °C [23], the composite PCM can absorb large amount of heat without increasing temperature dramatically. This feature can effectively hinder heat transfer, as the temperature difference between indoor space and building envelop is smaller. In addition, since the solar radiation gradually decrease after noon, the peak temperature can be flattened by delaying temperature raining, therefore, the room temperature can be stabilized. To identify the optimal balance between the thermal energy storage and conductivity of PCM, this study carried out detailed analysis to investigate the effects of adding different fractions of paraffin/expanded graphite composite PCM on the properties of foamed cement blocks. The foamed cement microstructure, physical structure, thermal energy storage capacity, and thermal conductivity were compared among samples. The research developed a better understanding of the application of PCM in foamed cement and provided references for future applications of such new material.

2. Experiment description 2.1. Raw materials Paraffin (melting point Tm = 44 °C) was provided by Hangzhou Ruhr New Material Technology Co. Ltd. Expandable graphite (expansion ratio: 300 ml/g, average particle size: 300 μm) was supplied by the Qingdao Graphite Co. Ltd. Concentrated high efficiency cement foaming agent (QW-100) was produced by Zhengzhou Pengyi Chemical Building Materials Co. Ltd. Silicate 425 cement was produced by Haimen Conch Cement Co. Ltd. Insulation cotton was provided by Dongguan Baiyue Industrial Co., Ltd. Insulation board (thickness = 20 mm) was supplied by Guangdong Huamei Glass Cotton Products Sales Co., Ltd. 2.2. Equipment To prepare the material samples, following equipment was used, including a microwave oven (EG720FFI-NS, Midea, China), an electronic balance (WT5002 N, Changzhou wantai balance instrument Co. Ltd.), a concrete accelerated curing box (HJ-84, Tianjin Luda Construction Equipment Co. Ltd.), a foaming machine (Yh-fp50, Guangzhou Yongsheng Construction Engineering Co. Ltd.), an electrothermostatic blast oven (DGX-9243, Shanghai Foma Experimental Equipment Co. Ltd.), a scanning electron microscope (JSM-7001F, JEOL, Japan), an X-ray diffraction (PW3040/60, PANalytical, Netherlands), a differential scanning calorimeter (DSC2910, Ta Znstrument Inc company, USA), a thermostatic heating plate (DB - 2B, Lichen Instruments) with Agilent34972A data acquisition system(HP Inc.,USA), and a microscope(SMZ168-TL, MOTIC CHINA GROUP CO., LTD.). 2.3. Preparation of the composite PCM and foamed cement 2.3.1. Configuration of the production system A paraffin/expanded graphite composite PCM was produced by absorbing the liquid paraffin into the expanded graphite in this study. First, the expandable graphite powder was dried in a vacuum drying oven at 65 °C for 16 h. Put 2–3 g dried expandable graphite into a ceramic crucible. Then the crucible was placed in a 700 W microwave oven and heat-treated for 40 seconds to obtain expanded graphite. Then, the paraffin was poured into a 400 mL glass beaker and placed in a constant temperature water bath at 70 °C for 10 min. The expanded graphite was added to the beaker (the mass ratio of expanded graphite to paraffin is 2:8) while stirring and blending the mixture for 1 h. Fig. 1 shows the schematic diagram of the composite process. Finally, a composite phase change thermal storage material of paraffin/expanded graphite could be obtained.

Fig. 1. Schematic diagram of the composite process. 2

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Fig. 2. Schematic diagram of the experiment system.

range of 0.005–500 W m−1 K−1 with an error of ± 3%. The test requires accurate power supply and correct precision resistor selection to handle computer generated signals. A probe of the hot plate was placed between two identical samples to detect their resistance. The recorded resistance is used to assess the quantitative correlation between temperature and time, as shown in the following equation:

2.3.2. Preparation of phase change thermal storage foamed cement blocks Take the same cement into five beakers, add the paraffin/expanded graphite composite PCM (with the cement mass ratio of 10%, 15%, 20%, 25%, 30%) respectively and mix them well. Add distilled water into to the beakers (with a cement mass ratio of 75%) and stir evenly (When the mass fraction of the paraffin/expanded graphite composite PCM is greater than 30%, the liquid becomes thick and cannot be well mixed). At this time, add air bubbles with a density of 50 kg/m3 into the beakers and stirred evenly for about 10 seconds. Pour the uniform slurry into a 40*40*40 mm3 mold and keep it at the room temperature for 24 h. After demoulding, place the module in a cement curing box at 40 °C for 3 days and then in a drying oven at 60 °C for 1.5 days until the weight of the foamed cement blocks remains constant. In the production process of pure cement block, only distilled water (with the cement mass ratio of 75%, no composite PCM, no bubbles) is allowed to be added into the cement. Then the phase change heat storage foamed cement blocks and pure cement blocks (0% PCM) are finally prepared.

R (t ) = R 0 {1 +

[ Ti + Tave ( )]}

(1)

where t is time; is a dimensionless constant; R 0 is the resistance before the sensor is heated (t = 0 ); is the temperature coefficient of resistance (TRC); Ti is the thin layer of insulating material covering the Hot Disk sensor (nickel) temperature difference; Tave is the temperature rise of the sample in contact with the probe. The test for each sample was repeated at least three times and the results were averaged. Each test trial waited at least 30 min after the previous test finished to ensure that the heat was completely dissipated. This also avoids significant convection and vibration in the internal temperature distribution. Finally, the thermal conductivity of pure cement block and foamed cement blocks with different mass fractions of PCM were compared and analyzed.

2.4. Characterization of the materials The scanning electron microscope (SEM) was used to observe the microstructure of expanded graphite, paraffin/expanded graphite composite phased material, and phase-change thermal storage foam cement. Before the scanning, the surface of the sample was first subjected to a gold spray treatment for 30 min to enhance their surface conductivity. The D/max-3A X-ray diffractometer (XRD) was used to analyze the diffraction patterns of phase change thermal storage foamed cement materials. The source of the diffraction is a copper target (CuKα), the tube pressure is 40 kV, and the tube flow is 40 mA. The scanning speed is 8°/min, the scanning pattern is theta/2theta, and scanning mode is continuous. The DSC2910 differential scanning calorimeter was used to investigate the thermophysical properties of paraffin, paraffin/expanded graphite (mass fraction 20%:80%) composite phase change materials, and phase change thermal storage foamed cement. The measurement targets include phase transition temperature, peak temperature, and latent heat of phase change. The variation range of the temperature was controlled within 15 °C–65 °C, the heating rate was studied at 1 °C/min, 3 °C/min and 5 °C/min respectively, and the testing environment used the nitrogen for the operation of melting and solidifying. The PCM latent heat was calculated by integrating the area under the DSC curve peak.

2.6. Testing device of thermal energy storage performance The thermal energy storage test bench consists of a thermostatic heating plate, an Agilent 34972A Data Collector, and a desktop computer. The schematic diagram in Fig. 2 shows the experiment system. First, the thermal insulation cotton was wrapped around the energy storage foamed cement blocks to reduce the measurement error. The upper surface of the samples was covered with blue insulation board with a thickness of 1.5 cm to simulate the insulation material of the wall. Then K-type thermocouple was struck to the center of the lower surface of the cement block (heating platform contact surface a to simulate outdoor temperature), the upper surface of the cement block (insulation board contact surface b to simulate wall sandwich temperature), and the upper surface of insulation board (point c to simulate indoor temperature). the position of the measuring points is shown in Fig. 2. Finally, the wrapped pure cement block (no composite PCM, no bubbles) and the foamed cement blocks (with composite PCM content of 10%, 15%, 20%, 25%, 30%) were placed on the 65 °C heating plate at the same time. Record the temperature changes of the test point until the temperature became constant and stop the experiment. After processing the data, the endothermic curve can be obtained. The temperature was automatically recorded by Agilent 34972A Data Collector with a data interval of 10 seconds.

2.5. Thermal conductivity test The thermal conductivity of the composite PCM was measured by a Hot-Disk Thermal Constant Analyzer (Hot Disk Inc., Sweden) in the 3

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Fig. 3. SEM images of the expanded graphite and paraffin/expanded graphite composite PCM: (a) EG( × 60); (b)EG( × 2000); (c)Composite PCM( × 60); (d)Composite PCM( × 2000).

pores are filled with paraffin/expanded graphite composite PCM. The surface color is darker. Fig. 5 (c) shows the surface of a foamed cement block with 30% mass fraction of composite PCM. Compared with Fig. 5 (b), the number of pores was significantly reduced, as the composite PCM filled in pores. Fig. 6 shows the appearance of the pure foamed cement block magnified 2000 times under the microscope. In the figure, the mixture of the composite phase change material and the cement can be clearly seen.

3. Results and discussion 3.1. SEM analysis 3.1.1. SEM analysis of paraffin/expanded graphite composite PCM Fig. 3 shows the SEM image of the expanded graphite and paraffin/ expanded graphite composite PCM. Fig. 3(a) is an enlarged view of expanded graphite, showing a worm-like structure. Fig. 3(b) is an enlarged view of expanded graphite with an irregular honeycomb grid structure, which contains a large amount of crack-like and network-like micro holes. Fig. 3(c) shows the expanded graphite adsorbed paraffin and still maintains its original porous worm-like morphology. Fig. 3(d) shows the expanded graphite that adsorbed paraffin and still maintains its original porous worm-like morphology. In these figures, paraffin was evenly adsorbed in the pores of expanded graphite and no block appeared on the surface of expanded graphite.

3.2. XRD analysis Fig. 7 shows the XRD patterns of the paraffin, expanded graphite, paraffin/expanded graphite composite PCM, and the phase change thermal storage foamed cement. From Fig. 7 (a), it can be seen that the expanded graphite has one strong peak, the paraffin has three strong peaks, and the paraffin/expanded graphite composite phase change material has four strong peaks. The strong peaks of pure paraffin and expanded graphite appear in the corresponding 2θ in the composite PCM respectively. No other strong peaks appeared in the composite PCM. These results indicate that the process of material production is a physical process with no chemical reaction. Similarly, in Fig. 7 (b), the strong peaks in the composite phase change foaming cement also appear in the corresponding 2θ position in the composite PCM and no other strong peaks appeared, this also suggested no chemical reaction has occurred. Therefore, paraffin/expanded graphite composite PCM and phase change thermal storage foamed cement can maintain its phase change thermal storage properties and chemical properties of paraffin.

3.1.2. Appearance and SEM analysis of pure cement block and foamed cement blocks Fig. 4 shows the appearance characteristics of pure cement block and phase change thermal storage foamed cement blocks (with composite PCM content of 10%, 15%, 20%, 25% and 30%). The surface of pure cement block (no PCM, no pores) is relatively flat and has the lightest color. The surface of phase change thermal storage foamed cement blocks have a large number of pores and composite PCM components. The surface color gradually deepens with the increase of the mass fraction of composite PCM. With the mass fraction increases, the pores of the foamed cement blocks are reduced, since the composite PCM was adsorbed in the pores and occupied the space within pores. Fig. 5 shows the appearance of the pure cement block and the phase change heat storage foamed cement block (the mass fraction of composite PCM is 20%) after magnifying 30 times under the microscope. Fig. 5 (a) shows a pure cement block with a smooth surface, it has no black composite PCM, no pores, and a light gray color. Fig. 5 (b) shows a phase change heat storage foamed cement block (the mass fraction of composite PCM is 20%). It has a large number of pores and some of the

3.3. DSC analysis In this experiment, the effects of three different heating rates of 5 °C/min, 3 °C/min and 1 °C/min on the DSC results were studied. When the heating rate was 1 °C/min, the lower rate leads to a decrease in the monitoring sensitivity of the thermal effect, and the peak values of 4

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Fig. 4. Appearance images of the pure block and phase change thermal storage foamed cement blocks.

Fig. 5. The appearance of the pure cement block and the phase change heat storage foamed cement block: (a) Pure cement ( × 30); (b) Composite PCM with 20% mass fraction ( × 30); (c) composite PCM with 30% mass fraction ( × 30).

samples with lower PCM mass fractions such as 10% and 15% are not easily displayed. When the heating rate is increased to 5 °C/min, the heating rate is faster. Although the peak shape is larger, the reaction lags, the temperature gradient in the sample increases, and the peak

separation ability decreases. Therefore, the DSC result with a heating rate of 3 °C/min is more reasonable, the peak separation effect is better, the peak shape is complete, and the test result is closer to the true value, which satisfies the requirement that the sensitivity of the sensor is

Fig. 6. SEM images of the phase change thermal storage foamed cement: (a) Pure cement ( × 2000); (b) Foaming cement ( × 2000). 5

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Fig. 7. (a) XRD pattern of paraffin, expanded graphite, paraffin/expanded graphite composite PCM, and phase change thermal storage foamed cement; (b) XRD magnification of paraffin/expanded graphite composite PCM and phase change thermal storage foamed cement.

sufficient and does not affect the measurement efficiency. The following is the DSC test result measured at a heating rate of 3 °C/min. Fig. 8 (a) shows the DSC curve of the paraffin and paraffin/expanded graphite (mass ratio 80%:20%) composite PCM. There are two phase change peaks show up when the temperature rises from 15 to 65 °C. One peak is a solid-solid phase change peak around 40 °C and the other is a solid-liquid phase change peak at about 45 °C. The enthalpy values of the endothermic and exothermic processes of paraffin are

similar, 260.5J/g and 261.5J/g, respectively. Similarly, the DSC curve of paraffin/expanded graphite composite PCM also has two peaks when the temperature increase from 15 to 65 °C. The enthalpy of endothermic and exothermic heat of the composite PCM are reduced to 207.7J/g and 208.0J/g, respectively. This can be explained by the reduction of mass ratios of paraffin. The theoretical value of the latent heat (Hthe ) can be calculated with the following equation [10]:

6

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composite PCM and phase change thermal storage foamed cement (with the mass ratio of 10%, 15%, 20%, 25%, and 30%). If the mass of the phase change heat thermal foamed cement block is composed of the mass of the cement and the mass of the composite PCM, W is 9.09%, 13.04%, 16.67%, 20%, and 23.08%). which can be obtained from:

W=

MPCM MC + MPCM

(3)

W is the mass fraction of the composite PCM in the whole phase change thermal storage foamed cement block; MC is the mass of the cement; MPCM is the mass of the composite PCM. Table 1 summaries the material features at different mass fractions. is the composite PCM mass fraction. Ts/ Te and Ts / Te are the melting and solidification temperature during the heat absorption and exothermic process, respectively. H1 and H2 are the absorption and exothermic values of the melting enthalpy, respectively. H is the theoretical enthalpy values of the phase change thermal storage foamed cement block (corresponding to W 9.09%, 13.04%, 16.67%, 20%, 23.08%, respectively), which can be calculated by: H = W·207.7J/g

(4)

where 207.7J/g is the actual enthalpy of paraffin/expanded graphite composite PCM. As can be seen from the figure, with the increase of the mass ratio of composite PCM, the enthalpy of the phase change thermal storage foamed cement also increases. However, the actual enthalpy is smaller than its theoretical value. This can be explained by the reaction of water and cement during the preparation of foamed cement blocks. Although there is no free water after the foamed cement block is dried, during the production process, water and cement react and produce crystal water and hydroxide. The chemical formula is

3CaO·SiO2 + nH2 O= x CaO·SiO2 ·y H2 O+ (3

(5)

x )Ca(OH)2

Therefore, the solution formula for W should be:

W=

%) Hexp,

3.4. Thermal conductivity

(2)

=0

(6)

Ma is the quality of crystal water in the phase change heat storage foamed cement block. Therefore, the actual value of W should be smaller than that was calculated. Even so, the actual measured enthalpy H1 is still smaller than the theoretical enthalpy H . In addition, there may be material loss during the preparation process, which may result in a lower actual measurement threshold.

Fig. 8. (a) DSC curve of paraffin and composite PCM. (b) DSC curve of phase change thermal storage foamed cement.

Hthe = (1

MPCM MC + MPCM + Ma

The thermal conductivity of building materials is one of the key parameters that directly affect the energy performance of buildings. In this work, the thermal conductivity of pure cement blocks and foamed cement blocks with different mass fractions of PCM were measured using a hot-disk analyzer. The result were depicted in Fig. 9. Fig. 9 shows the thermal conductivity of the foamed cement block increased with the increase of the mass fraction of the composite PCM. As shown in the black polyline, the thermal conductivity of samples

where Hexp, = 0 is the experimental value of latent heat when = 0 . In this experiment, % is the content of carrier expanded graphite. Calculation results show that the experimental values of latent heat consistent with the theoretical values of latent heat with a relative error less than 10%. Fig. 8 (b) shows the DSC curves of paraffin/expanded graphite

Table 1 The tested phase change temperature and latent heat of the phase change thermal storage foamed cement with different mass fractions of composite PCM. w (wt%)

Ts (°C)

Te (°C)

H1(J/g)

Ts (°C)

Te (°C)

H2 (J/g)

H (J/g)

paraffin/expanded graphite composite PCM 10% 15% 20% 25% 30%

41.7 37.0 37.5 39.5 40.4 41.8

47.4 42.1 43.2 44.2 44.6 45.3

207.7 8.512 16.43 20.40 27.04 34.87

40.3 40.7 40.5 43.1 42.8 42.6

41.9 32.1 30.9 29.7 32.0 38.5

−208.0 −7.833 −15.98 −21.64 −26.18 −34.47

208.4 18.88 27.08 34.62 41.54 47.94

7

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the composite PCM and the sample with 30 wt% had the highest heating rate. The pure cement block has a similar heating rate as the blocks that have a mass fraction between 20% and 25%. When the mass fraction is higher than 25%, the composite PCM blocks have higher heating rate than the pure cement blocks. In Region B, the composite PCM starts phase changing at about 40 °C. As shown in Fig. 10(a), when the temperature reaches 42.5 °C, the phasing changing the time for 30% composite PCM block is 2990 s and 690 s longer than the pure cement block (2300 s). Among all sample, the block with 30% composite PCM has the highest thermal energy storage performance and feasible in implementing in outer walls to remove indoor temperature fluctuations. In Region C, after the phase change complete, the surface temperature of the foamed cement block continues to rise. The surface temperature of the pure cement block is the lowest, suggesting better thermal insulation when the surrounding temperature is high. In summary, the foamed cement block with composite PCM can delay the temperature raising but can reach a higher temperature than conventional cement blocks in hot climate regions. In addition, it can be seen from Fig. 10 (a) that the maximum upper surface temperature difference between pure cement block and the foamed cement block with 30% composite PCM is 1.45 °C. If the environment temperature is lower than the upper limit of the Region B, adding composite PCM can effectively delay heat transfer and scrape energy peaks at noon. Fig. 10 (b) shows the surface temperature development of a multi-layer insulation board over time. The experiment process simulates the room temperature variation due to the heat transfer (outdoor heat → foamed cement block/pure cement block → insulation board → indoor). The results are similar to Fig. 10 (a) that the foamed cement block with 30% composite PCM show high thermal energy storage capacity and can effectively delay the heat transfer and flatten the peak heating load. All the above measurements were performed at least three times to verify the results [23].

Fig. 9. Thermal conductivity of foamed cement blocks with different mass fractions of PCM.

with mass fraction of 10%,15%,20%,25% and 30% are 0.48 W (m K)−1, 0.49 W (m K)−1,0.53 W (m K)−1,0.57 W (m K)−1 and 0.60 W (m K)−1, respectively. The quantality of the paraffin/expanded graphite composite PCM components is positively associated with the materials’ thermal conductivity because expanded graphite has excellent thermal conductivity. The thermal conductivity of pure cement block is 0.55 W·(m·K)−1, between the thermal conductivity of the mass fraction of 20% and 25% of the foamed cement block. This is because the apparent density of pure cement blocks is much higher than that of foamed cement blocks. 3.5. Thermal insulation capacity of the phase change thermal storage foam cement

3.6. Thermal performance

Fig. 10 shows the thermal energy storage performance and thermal conductivity of the foamed cement blocks with different mass fractions of paraffin/expanded graphite composite PCM. Fig. 10 (a) illustrates the temperature development of the upper surface (point b) of foamed cement blocks and pure cement blocks over time. It can be clearly seen that in Region A, the heating rate of the foamed cement block increased with the increase of the mass fraction of

According to the heat transfer process of the building envelope structure, the thermal characteristics of the building envelope structure can usually be evaluated by thermal resistance R, thermal effusivity S and of thermal inertia index D. The thermal resistance R reflects the blocking ability of the enclosing structure to the heat flow. The larger the thermal resistance, the

Fig. 10. (a) The temperature of the upper surface (point b) of foamed cement blocks and pure cement blocks over time; (b) Temperature of the upper surface (point a) of the insulation board over time. 8

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Table 2 Comparison of thermal performance between pure cement block and phase foamed cement blocks with different mass fractions of paraffin/expanded graphite composite PCM. Material

T hickness δ/mm

D ensity ρ/kg·m³

T hermal conductivity λ/W·(m·K)−1

Specific heat capacity c/kJ·(kg·K)−1

Thermal effusivity S/W·(m2·K)−1

Thermal inertia index D

Pure block 10% 15% 20% 25% 30%

40 40 40 40 40 40

1500 574 587 620 635 663

0.55 0.48 0.49 0.53 0.57 0.60

0.82 0.81 0.90 1.05 1.98 2.78

13.26 7.66 8.22 9.47 13.67 16.95

0.96 0.64 0.67 0.71 0.96 1.13

smaller the heat flux density passed. When the thickness of the wall material layer is constant, its thermal resistance is inversely proportional to the thermal conductivity. For a multi-layer wall material layer, the calculation expression is as shown in equation (5):

Ri = R1 + R2 +…+ Rn =

i

thermal energy storage performance of the foamed cement blocks. (2) The preparation and absorption process is a physical process with no chemical reactions. Thus, the phase change thermal storage foamed cement can maintain the thermal storage properties and chemical properties of paraffin. (3) The thermal conductivity of the pure cement block is similar to the foamed cement block with 20%–25% composite PCM. 30% is an ideal mass ratio for the foamed cement blocks with composite PCM and has the highest thermal inertness index. They can effectively delay the temperature raising and reduce temperature fluctuation but subject to a higher temperature after phase change. Therefore, the proposed materials are suitable for the climate region with peak temperature lower than 42.5 °C.

(7)

i

The thermal effusivity S reflects the sensitivity of the material to the fluctuating heat reaction. Under the same heat wave action, the material with larger thermal effusivity has smaller surface temperature fluctuation and better thermal stability. Formula (6) is the thermal effusivity S of the wall material layer with a period of 1 d (24 h), whose value depends on the thermal conductivity and the specific heat capacity of the material, and also varies depending on the period of the heat flow fluctuation. For a multi-layer wall material layer, the thermal effusivity S of the wall is obtained by weighting the thermal effusivity of each layer of material.

S=

2

c

Therefore, the proposed foamed cement blocks with paraffin/expanded graphite composite PCM can effectively improve building energy efficiency when applied in building envelops, such as walls, roofs, and floors.

(8)

T

Acknowledgements

The thermal inertia index D indicates the ability of the enclosure to resist periodic temperature fluctuations. The larger the D value, the faster the temperature wave decays, and the better the thermal stability of the enclosure. The thermal inertia index D is also the sum of the thermal inertia indices of the materials of each layer, and the calculation formula is as follows:

D = D1 + D2 + ... + Dn =

Ri Si

This work was supported by Guangzhou Science and Technology Program (201704030137), the Research Project of Guangdong Province (2017A050506058), and the Major Research Project of Guangdong Provincial Department of Education (No. 2016KZDXM035). Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solmat.2019.110038.

Table 2 shows the comparison of the thermal performance of the pure cement block with the phase change foam cement blocks studied in this paper. It can be seen from Table 2 that for two kinds of wall materials with a thickness δ of 40 mm, the thermal conductivity of the 30% composite PCM foam cement block is 0.60, which is higher than the thermal conductivity of pure cement block 0.55. But in terms of heat storage performance, the specific heat capacity of the 30% composite PCM foam cement block is about 3.4 times that of the pure cement block. Therefore, the thermal inertia index D of the material is also calculated by the formula to be 1.13, which is larger than the pure cement block of 0.96, reflecting that the material has good heat storage capacity.

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4. Conclusion This study introduces the phase change heat storage foamed cement blocks that prepared from with paraffin, expanded graphite, and Portland cement. The physical structure, thermal conductivity and thermal energy storage performance of foamed cement were detailly studied through experiments. The results can be summarized as the following conclusions: (1) No paraffin leakage was observed when adding the paraffin/expanded graphite (mass ratio is 80%:20%) composite PCM into the foamed cement. Adding paraffin can effectively improve the 9

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