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Development of a stable inorganic phase change material for thermal energy storage in buildings
Solar Energy Materials & Solar Cells 208 (2020) 110420
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Development of a stable inorganic phase change material for thermal energy storage in buildings Xiaohua Bao a, Haibin Yang a, b, Xiaoxiao Xu a, Tao Xu c, Hongzhi Cui a, *, Waiching Tang d, Guochen Sang e, W.H. Fung b a
College of Civil and Transportation Engineering, Shenzhen University, Shenzhen, 518060, China Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong School of Civil Engineering, Guangzhou University, Guangzhou, 510006, China d School of Architecture and Built Environment, The University of Newcastle, Callaghan, NSW, 2308, Australia e Xi’an University of Technology, School of Civil Engineering and Architecture, Xi’an, 710048, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase change material Industrial grade calcium chloride hexahydrate Flake graphite Super absorbent polymer Supercooling Segregation
Building energy consumption is influenced evidently by solar radiation. To achieve a stable indoor temperature by minimizing the heat fluctuations resulted from solar radiation, latent heat thermal energy storage systems with phase change materials (PCMs) in building envelope have been studied. Though inorganic PCMs have promising potential applications among all kinds of PCMs, the supercooling and segregation are the major issues which have restricted their practical applications. The purpose of this study is to lessen the supercooling degree of an industrial grade PCM (CaCl2⋅6H2O) by using a nucleating agent - flake graphite. A super absorbent polymer (SAP) was also used as a thickener to prevent segregation of CaCl2⋅6H2O during phase transition. The combined method of thermogravimetric analysis/differential thermal analysis (TGA-DTA) was firstly proposed to evaluate the segregation of inorganic PCM innovatively. The results showed that using 0.5 wt% flake graphite not only eliminated the supercooling of CaCl2⋅6H2O, but also improved its thermal conductivity for better thermal per formance. The results of Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) demonstrated that the flake graphite did play an important role on the crystallization of CaCl2⋅6H2O. Numerical simulation demonstrated that using modified CaCl2⋅6H2O within walls of buildings in Hong Kong and Changsha are economically feasible with the payback periods of 18.3 years and 8.4 years respectively.
1. Introduction Globally, the building sector consumed approximately 20.1% of the total energy consumption in 2016 [1]. Building energy consumption is influenced evidently by solar radiation in outdoor environment. Therefore, the building envelope has been regarded as one of the most crucial impacts on the building energy conservation. Latent heat thermal energy storage systems using PCMs in building envelope has been considered as an effective strategy to improve the energy saving or in door thermal comfort of buildings [2–5]. According to the report stated by Xie et al. [6], PCMs-based building envelope can effectively reduce indoor heating and cooling loads caused by variable outdoor sur roundings, ultimately offset the mismatch between availability and supply of heat energy. Besides, PCMs-based building envelopes also could achieve a human comfort level (22 � C–28 � C) owing to PCMs have
an appropriate phase change temperature [7]. Moreover, Zhao et al. [8] and Min et al. [9] have verified that PCMs with excellent chemical and thermal stability could meet the durability requirements of most appli cations. In a word, PCMs are suitable for use in buildings to reduce energy consumption and/or improve indoor thermal comfort. PCM can generally be classified as either organic or inorganic PCMs. Organic PCM is chemically inert and do not undergo phase segregation. However, they are flammable and relatively high costs which limit their large-scale thermal energy storage application. Compared with organic PCMs, inorganic PCMs are nonflammable, higher thermal energy stor age capacity and larger melting enthalpy [10]. Therefore, inorganic PCMs can be the excellent candidate for application in energy storage systems. However, it is well-known that inorganic PCMs usually suffer from serious supercooling and phase segregation [11,12]. Furthermore, the poor thermal conductivity of inorganic PCM also results in low
* Corresponding author. E-mail address: [email protected] (H. Cui). https://doi.org/10.1016/j.solmat.2020.110420 Received 17 May 2019; Received in revised form 8 January 2020; Accepted 15 January 2020 Available online 21 January 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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Table 1 Summary of the nucleating agents for CaCl2⋅6H2O. Study
Type of PCM
Nucleating agents
Supercooling degree (oC)
Mass concentration
Li and Zhou et al. [17] Li and Zhang et al. [18] Xie and Niu et al. [19] Li and Zhou et al. [20] Xu and Dong et al. [21]
CaCl2⋅6H2O CaCl2⋅6H2O–MgCl2⋅6H2O CaCl2⋅6H2O CaCl2⋅6H2O CaCl2⋅6H2O
SrCl2⋅6H2O/oxidized expanded graphite SrCl2⋅6H2O/SrCO3 Nano CsXWO3 Nano Al2O3 Nano SiO2
0.6 2.0 0.6 0.3 0.8
1-3 wt% 1-3 wt% 0.75 wt% 1 wt% 5 wt%
charging and discharging rates and limits its overall energy storage ef ficiency. So, it is necessary to resolve the issues related to supercooling, phase segregation and thermal conductivity of inorganic PCMs. CaCl2⋅6H2O, an inorganic PCM and a non-toxic crystalline hydrated salt, has drawn lots of attention from engineers and researchers around the world, especially during the past decade, owing to its large melting enthalpy (160–209 J/g) [13]. CaCl2⋅6H2O has been proved to be a promising inorganic PCM candidate in building application because of its excellent thermal reliability. Tyagi and Buddhi [14] carried accel erated thermal cycling tests on CaCl2⋅6H2O and found that there was only a minor variation in enthalpy value after 1,000 thermal cycle testing. High supercooling degree causes extra energy to be released, espe cially in the beginning stage of supercooling. As a result, less energy can be available for the subsequent phase transition or crystallization [15]. Hence, supercooling is still the major issue which restricts the applica tion of inorganic PCMs in large scales [16]. To reduce supercooling degree of CaCl2⋅6H2O, several nucleating agents have been used as shown in Table 1. Li and Zhou et al. [17] used SrCl2⋅6H2O and oxidized expanded graphite as nucleating agents to modify CaCl2⋅6H2O. They showed that the combination of SrCl2⋅6H2O and oxidized expanded graphite could reduced the supercooling degree of CaCl2⋅6H2O, but using SrCl2⋅6H2O or oxidized expanded graphite individually is not effective at suppressing supercooling. Li and Zhang et al. [18] proposed that the supercooling degree of an eutectic hydrate salt CaCl2⋅6H2O–MgCl2⋅6H2O was effectively reduced when strontium chloride (SrCl2⋅6H2O) and strontium carbonate (SrCO3) were added as nucleating agents. Nevertheless, the supercooling reported in their study was still as high as 2 � C, and thus more effective and inexpensive nucleating agents have to be developed. Xie et al. [19] found the supercooling of CaCl2⋅6H2O could be reduced to 0.61 � C owing to the addition of 0.75 wt% cesium tungsten bronze (CsXWO3) nanoparticles, but it also caused a slight decrease in the thermal conductivity. Simi larly, Li and Zhou et al. [20] and Xu and Dong et al. [21] respectively incorporated nano Al2O3 and nano SiO2 into CaCl2⋅6H2O, but its supercooling degree still was not eliminated. In comparison of above nucleating agents, Johansen et al. [22] and Dannemand et al. [23] agreed that graphite flake, which has a high thermal conductivity of 256–500 W/mK, as nucleating agent was more effective. In their study, the supercooling degree of inorganic PCM (so dium acetate trihydrate, SAT) was eliminated completely, and its ther mal conductivity could improve by 39%–62% as the addition of 1%–5 wt.% graphite flake. Since there is no related literature on investigating the effect of graphite flake on the supercooling of CaCl2⋅6H2O, the present work is an effort to provide some expanded research on this area. Besides, segregation is related to the occurrence of supercooling because salt hydrates gradually transform to sediment when PCMs start to be deprived of its nucleating capacity as well as crystal water lost [24, 25]. It has been reported that the lower hydrate or anhydrous salt in inorganic salt hydrate would be precipitated as thermal cycling increased, thereby deposited at the bottom [26,27]. Consequently, the substances between the top and bottom layers of liquid CaCl2⋅6H2O would be different. Many measures have been studied to address segregation of CaCl2⋅6H2O during melting process. The most common method is using thickeners to improve segregation resistance of PCM during phase-transition. However, there are limited studies that can
Table 2 Physical properties of the industrial grade CaCl2⋅6H2O. Items of properties
Data
Density (26 C) Phase change temperature
1.475 g/cm3 ~29 � C
�
Fig. 1. XRD spectrum of the industrial grade CaCl2⋅6H2O and a reagent grade CaCl2⋅6H2O.
provide with appropriate methods to examine the effect of thickeners on segregation of inorganic PCMs [28–30]. So, the effectiveness of thick eners cannot be well examined. Hence, it is essential to develop a more suitable technique to evaluate the influence of thickeners on anti-segregation. In this study, super absorbent polymer (SAP) was selected as a thickening agent and its effect on segregation of calcium chloride hexahydrate liquid was investigated. Moreover, flake graphite as a nucleating agent was used to mitigate the supercooling of an industrial grade CaCl2⋅6H2O. Furthermore, the combined method of thermogra vimetric analysis/differential thermal analysis (TGA-DTA) was firstly proposed to assess the influence of thickener on segregation of inorganic PCM. It’s worth noting that the TAG-DTA method can allow a quanti tative segregation analysis of inorganic PCM and the effectiveness of thickeners can be well understood. In this study, a stable inorganic PCM, based on industrial grade CaCl2⋅6H2O without supercooling and without phase segregation was successfully prepared. Besides, the effect of the modified PCM using on buildings was revealed by means of numerical simulation. 2. Experimental and simulation work 2.1. Materials In this study, an industrial grade calcium chloride hexahydrate (CaCl2⋅6H2O, 98% purity) from Chengdu Kelong chemical Corporation 2
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2.2. Sample preparation and characterization 2.2.1. Preparation of modified CaCl2⋅6H2O The procedures for preparing modified CaCl2⋅6H2O are as follows: First, 50g solid CaCl2⋅6H2O was placed inside a glass beaker, and then the beaker was kept in a water-bath at 60 � C until the PCM melted completely. Because the water content in industrial grade CaCl2⋅6H2O was low (see Table 3) which would affect the phase change temperature and enthalpy value of PCM, and inevitably restrict the PCM to function properly. Hence, an proper amount of water was incorporated into the PCM mixture to compensate the water loss. Subsequently, the flake graphite with different dosages (0.2 wt%, 0.5 wt% and 0.8 wt%) mix tures were dispersed in PCM mixture by magnetic stirring with the speed of 500 rpm for 30 min, and followed by ultrasonic dispersion (JY-92-II N, Ningbo Xinzhi Biotech Co., Ltd. 25 kHz, 375 W) for about 30 min, respectively. To mitigate segregation, 25 wt% of SAP (thickener agent) was added and dispersed uniformly in the mixtures by stirring vigor ously under the speed of 1500 rpm for 15 min. Finally, the modified CaCl2⋅6H2O was prepared successfully.
Fig. 2. Microstructure of the flake graphite powder.
2.2.2. The evaluation of supercooling The temperature history method (T-history method) developed by Zhang et al. [31] in 1999 was adopted to dynamically record the tem perature variations of PCMs during heating and cooling processes. The supercooling degree of CaCl2⋅6H2O was calculated by measuring the difference between the highest temperature that occurred during the heating process and the lowest temperature that occurred during the cooling process. For T-history test, thermocouples (Type K, resolution � 0.3 � C) were utilized and placed in the center of the melted PCM. The multichannel data recorder (TP700, Shenzhen TOPRIE Electronics Co., Ltd.) was used for recording the temperature.
Fig. 3. The appearance of super absorbent polymer (SAP).
2.2.3. The evaluation of segregation The combined method of TGA-DTA (NETZSCH STA 409 PC) was proposed to evaluate the segregation of PCM composites. Because segregation would occur during the heating process, testing samples about 5 mL were collected from the bottom and top layers of melted CaCl2⋅6H2O (100 mL) and then stored in a refrigerator below 20� C until testing.
Table 3 Loss on ignition of two kinds of materials. Weight
Weight of solid (g) Weight of water loss (g)
Industrial grade CaCl2⋅6H2O
Reagent grade CaCl2⋅6H2O
Before heating
After heating
Before heating
After heating
10.00 3.97
6.03
10.00 4.98
5.02
2.2.4. Thermo-physical performance of modified CaCl2⋅6H2O The phase change temperature and latent heat value of modified PCM composites were evaluated by the differential scanning calorimetry (DSC, Netzsch DSC 200 F3). The temperature range of measurement process was set from 10� C to 50� C under a heating rate of 2� C/min in a continuous stream of nitrogen flowing at a rate of 50 ml/min.
Limited was used as the inorganic PCM. Table 2 shows the physical properties of CaCl2⋅6H2O, while Fig. 1 shows the X-ray diffraction (XRD) spectrum of the industrial grade PCM. By comparing its peak positions in the spectrum to those of reagent grade PCM, it can be known that there are some impurities in the industrial grade PCM. Although the industrial grade PCM is not as pure as the reagent grade, the industrial grade product is cheaper and more convenient to obtain. Flake graphite powder (carbon purity>98%, 800 mesh) from Tianjin Fuchen chemical Corporation Limited was used as nucleating agents. Fig. 2 shows the micrograph of graphite powder. A commercial grade SAP from Shanghai Funa Corporation Limited (see Fig. 3) was used as an anti-segregation agent for the inorganic PCM. Crystal water is very important to the properties of CaCl2. Table 3 shows the difference in crystal water between the industrial grade CaCl2⋅6H2O before modification and the reagent grade CaCl2⋅6H2O samples, after ignition at high temperature (>300 � C) for 30 min. The reagent CaCl2⋅6H2O (with a water loss of 4.98g) contains more crystal water than that of the industrial grade CaCl2⋅6H2O (with a water loss of 3.97g). Less quantity of water inside the crystallization of CaCl2⋅6H2O may result in different spectrums observed by the XRD, and this in fluences the interior CaCl2 molecule on matching water molecule.
2.2.5. Chemical structure and microstructure of modified CaCl2⋅6H2O The effect of flake graphite on supercooling reduction was analyzed by using Fourier-transform infrared spectroscopy (FT-IR, Nicolet 6700). Environmental Scanning electron microscope (ESEM, FEI Quanta TM 250 FEG) was employed to demonstrate the morphology of micro structure of PCM and modified PCM. In addition, optical microscope was also employed for microscopic analysis. 2.2.6. Thermal conductivity of modified CaCl2⋅6H2O In order to evaluate the thermal conductivity of the CaCl2⋅6H2O and modified CaCl2⋅6H2O, a hot wire method was conducted with a thermal conductivity tester (XIATECH, TC3000). This device uses the transient hot wire method conforming to Chinese standard GB/T 10297-2015 (Test method for thermal conductivity of nonmetal solid materi als—Hot-wire method). The PCM samples were tested at indoor tem peratures of 20 � C and 30 � C for evaluating their thermal conductivity in liquid and solid state, respectively.
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case study areas and the performance of modified PCM in buildings located in these two areas was simulated. Table 4 indicates the typical climatic conditions in Hong Kong and Changsha including the basic parameters such as the mean temperatures of the coldest month and the hottest month. The electricity consumption, parameters of air condi tioning system such as setting temperature and service time were considered and the details are indicated in Table 5. The energy performance of a house model (see Fig. 5) constructed with modified PCM walls was evaluated using a numerical simulation software- Energy plus. Fig. 5(a) shows the appearance and dimension of the house model. The building has a height of 3.0 m and floor area of 20 m2 (4.0 � 5.0 m). The size of window is 1.8 m � 1.5 m and the distance between window’s bottom and the floor is 0.9 m. The size of the door is 1.8 m � 0.9 m. Construction details and key material parameters are shown in Tables 6 and 7, respectively. Fig. 5(b) presented the schematic diagram of PCM wall. The other parameters representing PCMs such as thermal conductivity, phase change temperature and enthalpy were measured from the above said experiments. The accuracy of the simu lation model used in this study has been verified in our previous research [32–34], and thus it is also suitable to analyze the energy consumption of buildings using the modified CaCl2⋅6H2O. The numerical values including the electricity consumption (kW⋅h) and the average indoor air temperature were determined and the effectiveness of modified PCM was revealed. The monthly electricity consumption of nine continuous months (March–November) and the average indoor air temperature in August (hourly) were also presented in this paper.
Fig. 4. Climate regions in China [32]. Table 4 Temperature characteristics in Hong Kong and Changsha. City
Hong Kong Changsha
Climate regions
Hot summer & warm winter (HS&WW) Hot summer & cold winter (HS&CW)
Mean temperature Coldest month
Hottest month
>10 � C
25 � C–29 � C
4 � C–5 � C
>30 � C
Table 6 Construction details.
Table 5 Air conditioning system setting situation. Parameter System Energy type Equipment energy efficiency ratio Temperature Service time
Items
Design parameters
PCM wall (four faces)
200 mm concrete With or without 5 mm modified PCM 12 mm floated coat 100 mm concrete 40 mm concrete 25 mm insulation board 50 mm concrete
Floor Roof
Refrigeration system Central air conditioning control system Electricity 3.28 27 � C March–November, Monday-Saturday, 9.00 am - 18.00 pm.
Table 7 Thermal physics parameters.
2.2.7. Numerical simulation of PCM application Diverse regions in China have their own unique climate character istics as shown in Fig. 4. In the previous study [32], Hong Kong and Changsha were identified the most and less remarkable significant areas with respect to the saving of electricity when applying PCM in buildings. Therefore, in this study, Hong Kong and Changsha were selected as the
Building materials
Thermal conductivity (W/m K)
Specific heat (J/ kg K)
Latent heat (kJ/kg)
Concrete Insulation board Floated coat
1.74 0.085
920 920
0 0
0.93
1050
0
Fig. 5. The construction of simulated model: (a) The appearance of house model; and (b) Schematic diagram of PCM wall. 4
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CaCl2⋅6H2O samples with different dosages of flake graphite are shown in Fig. 7(a). The supercooling degree of samples with 0.2 wt%, 0.5 wt% and 0.8 wt% flake graphite were found to be 1.7 � C, 0.9 � C and 2.6 � C, respectively. It is worth noting that the supercooling degree of CaCl2⋅6H2O added with 0.5 wt% flake graphite decreased by 83% compared with the control samples (see Fig. 7(b)). Hence, the addition of 0.5 wt% flake graphite has a significant effect in reducing super cooling of CaCl2⋅6H2O. This phenomenon can be explained from the chemical structure of flake graphite. Fig. 8 shows the FT-IR spectra of –C flake graphite. The peak located at 1619 cm 1 was caused by the C– stretching mode [35]. The peaks occurred at 1388 cm 1 and 1033 cm 1 were corresponding to the stretching vibrations of C–OH and C–O, respectively [36,37]. These results illustrate that the surface of graphite particles decorated by some of oxygen-containing functional groups. This observation also reveal that the functional groups on flake graphite might have played an important role in crystal attachments which accelerated the crystallization of CaCl2⋅6H2O during the cooling process and thus reducing the supercooling of CaCl2⋅6H2O. Similar observations were made by Liu and Yang [38]. In addition, graphite flake in CaCl2⋅6H2O matrix, can serve as the main sites for attachment and growth of CaCl2⋅6H2O crystals as discussed on Section 3.6. Moreover, the addition of graphite flake would result in the enhancement of
Fig. 6. The cooling curves for two kinds of CaCl2⋅6H2O with T-history method.
3. Results and discussion 3.1. Thermal performance of CaCl2⋅6H2O In this section, the supercooling of pure industrial grade and reagent CaCl2⋅6H2O were evaluated by T-history method under cooling process as shown in Fig. 6. The results show that the phase change temperature of reagent grade CaCl2⋅6H2O was lower than that of industrial grade CaCl2⋅6H2O. Moreover, it is surprising to note that the supercooling degree of reagent grade CaCl2⋅6H2O was 25.7 � C, while it was 5.1� C for the pure industrial grade CaCl2⋅6H2O. The huge difference is because the reagent grade CaCl2⋅6H2O is lack of nucleating agents stimulating the crystallization. According to the results shown in Table 2, the difference in thermal performances between the industrial and reagent grade PCMs is probably attributed to their difference in crystal water volumes. Although the industrial grade CaCl2⋅6H2O is not purified, its thermal performance is better than the pure CaCl2⋅6H2O in terms of reduced supercooling degree. 3.2. Thermal performance of modified CaCl2⋅6H2O by flake graphite
Fig. 8. FT-IR spectra of flake graphite.
The temperature curves of cooling process of the modified
Fig. 7. Supercooling degrees of the different PCMs under cooling process: (a) CaCl2⋅6H2O with different contents of flake graphite; (b) Comparison between CaCl2⋅6H2O with 0.5 wt% flake graphite and control group (pure CaCl2⋅6H2O). 5
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Fig. 9. TGA curves of top and bottom layers of CaCl2⋅6H2O samples: (a) Without any modification; and (b) With 25 wt% SAP.
Fig. 10. DTA curves of top and bottom layers of CaCl2⋅6H2O samples: (a) Without any modification; and (b) With 25 wt% SAP.
viscosity, and thus the crystallization rate of CaCl2⋅6H2O would be accelerated due to the constricted molecular motion region [39]. So, it can be concluded that flake graphite can be worked as nucleating agent to reduce the supercooling of CaCl2⋅6H2O. However, it should be stressed that the supercooling degree increased when more than 0.5 wt% graphite flake was added. As shown in Fig. 7(a), the supercooling degree of CaCl2⋅6H2O with 0.8 wt% graphite flake went up as high as 2.6 � C. According to the previous research [40], it can be explained that the excess graphite would agglomerate, and thus part of the graphite flake lost its nucleating effect.
thickener was 4.08% at the end of thermal dehydration process. How ever, the curves as shown in Fig. 9(b) indicate the effectiveness of thickener as the weight loss between the top and bottom samples was almost the same. At the end of thermal dehydration process, the maximum weight loss of water between the top and bottom layers of CaCl2⋅6H2O containing thickener was only 0.13%. Based on the TGA results, it can be deducted that adding 25 wt% SAP can improve the segregation resistance of the industrial grade CaCl2⋅6H2O significantly. Besides, the effect of thickener on heat flow of top and bottom layers of CaCl2⋅6H2O can be seen in Fig. 10. The point No. 1 represents the complete melting point whereas the point No. 2 refers to the second peak melting point. Table 8 shows the heat flow differences occurred at two melting points between the top and bottom layers of CaCl2⋅6H2O with and without thickener. From Fig. 10(a), it is obvious that the significant difference in heat flow between the top and bottom layers of CaCl2⋅6H2O with thickener occurred at temperatures around 70 � C and 160 � C. This shows segregation of CaCl2⋅6H2O occurred because of different heat flow observed between the top and bottom layers. The structure of CaCl2⋅6H2O molecule might vary during the heating process and lead to difference in substances between the two layers which finally caused
3.3. Segregation behavior of modified CaCl2⋅6H2O by SAP To investigate the effect of thickener on segregation of CaCl2⋅6H2O, TGA and DTA analyses were used to identify any variation in thermal properties between the top and bottom layers of modified CaCl2⋅6H2O during the heating process. Referring to the curves shown in Fig. 9, the TGA results apparently can illustrate the effect of thickener on weight loss of PCM. Also, it can be found from Fig. 9(a) that the percentage of weight loss between the top and bottom layers of CaCl2⋅6H2O without 6
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3.4. Thermo-physical performance of the modified CaCl2⋅6H2O
Table 8 The results of the chosen spots from the DTA curves. Position
Point No.1 (mW/mg) Without SAP
Top layer Bottom layer
0.30 0.38
Based on the above investigations, a new CaCl2⋅6H2O composite can be developed to avoid both the supercooling and segregation issues. It is consisted of industrial grade CaCl2⋅6H2O with 0.5 wt% flake graphite, 25 wt% SAP and 7.5 wt% H2O (used to compensate crystal water loss). The specific amount of water being used could compensate partial water loss inside the industrial grade CaCl2⋅6H2O. However excess water should be avoided as too much water addition may aggravate segrega tion. Fig. 11 shows the cooling process of the newly modified CaCl2⋅6H2O compared to that of the industrial grade CaCl2⋅6H2O. The supercooling of the modified CaCl2⋅6H2O was completely eliminated (see red line in Fig. 11). It is worth reinstating here that the addition of 25 wt% SAP in CaCl2⋅6H2O was effective in eliminating segregation problem. Therefore, the newly modified CaCl2⋅6H2O can be used without the concerns of supercooling and segregation. Fig. 12 demonstrated the DSC results of CaCl2⋅6H2O with or without modification. From Fig. 12(a), it can be seen that, raw CaCl2⋅6H2O so lidified below 1.6 � C, which meant that it suffered a huge super cooling. The melting and freezing temperatures of the new PCM composite were 27.5 � C and 28.7 � C, whereas its corresponding enthalpy values were 142.2 J/g and 158.4 J/g, respectively (see Fig. 12(b)). Be sides, owing to the additives would not provide latent heat during melting process, the latent heat of modified CaCl2⋅6H2O was lower compared to the industrial grade CaCl2⋅6H2O. Because its phase change temperature is within the range of human thermal comfort (22–28 � C) [41], the modified CaCl2⋅6H2O has a great potential for use in buildings to store energy and adjust indoor tem perature. Moreover, the modified CaCl2⋅6H2O is relatively economical and can be applied to many fields, such as solar energy as well as building energy conservation.
Point No.2 (mW/mg)
SAP 0.27 0.28
Without SAP 0.54 0.61
SAP 0.49 0.51
Fig. 11. Cooling curves of the industrial grade CaCl2⋅6H2O and CaCl2⋅6H2O modified by 0.5 wt% flake graphite, 25 wt% SAP and 7.5 wt% H2O.
segregation. Similar to the TGA results, the effectiveness of thickener can be revealed by the DTA results as shown in Fig. 10(b). It is obvious that the DTA curves between the top and bottom samples were almost the same. Based on the TGA and DTA results, it can be concluded that the incor poration of 25 wt% SAP in CaCl2⋅6H2O has a positive influence in ho mogenizing the substance between the top and bottom layers of PCM. Undoubtedly, the thickener can reduce the segregation of CaCl2⋅6H2O and make the substances in CaCl2⋅6H2O system evenly distributed. This results further prove that the combined method of TGA-DTA is capable to characterize the effect of thickening agents on segregation resistance of CaCl2⋅6H2O and can be used to assess different thickeners or additives on segregation resistance of other kinds of inorganic PCM systems.
3.5. The microstructure of modified CaCl2⋅6H2O The microstructure of pure industrial grade CaCl2⋅6H2O and modi fied CaCl2⋅6H2O can be seen in Fig. 13. From Fig. 13(a), it can be found that the surface of pure industrial grade CaCl2⋅6H2O was smooth and flat. However, the surface of modified CaCl2⋅6H2O was generally rough as shown in Fig. 13(b). Therefore, it can be concluded that flake graphite can provide a suitable place for crystal growth of PCM. A stereomicroscope was also used to observe the surface of the modified CaCl2⋅6H2O. The photographs of smooth and irregular surfaces of the modified CaCl2⋅6H2O are shown in Fig. 14(a) and Fig. 14(b), respectively. The flake graphite was uniformly distributed in
Fig. 12. DSC curves of inorganic PCMs: (a) Industrial grade CaCl2⋅6H2O; and (b) CaCl2⋅6H2O modified by 0.5 wt% flake graphite, 25 wt% SAP and 7.5 wt% H2O. 7
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Fig. 13. Microstructure of CaCl2⋅6H2O: (a) Pure industrial grade CaCl2⋅6H2O; and (b) Modified CaCl2⋅6H2O.
Fig. 14. Stereomicroscope photographs of PCM system: (a) Smooth back surface of the sample (1,000X); and (b) Rough top surface of the sample (500X).
Fig. 15. The optical microscopy photos of sample of the modified PCM system (0.5 wt% flake graphiteþ 25 wt% SAP þ 7.5 wt% H2O): (a) Smooth back surface of the sample; and (b) Rough top surface of the sample.
CaCl2⋅6H2O and the crystals grew around the graphite properly. In order to observe the role of SAP in modified CaCl2⋅6H2O, the optical microscopy images of CaCl2⋅6H2O with and without SAP were taken and are shown in Figs. 15 and 16, respectively. The modified CaCl2⋅6H2O with SAP showed a high degree of crystallization compared to the one without SAP. Indeed, the thickeners can influence the solid ification [23]. Moreover, it may stimulate the growth of CaCl2⋅6H2O crystals.
3.6. The thermal conductivity of modified CaCl2⋅6H2O The thermal conductivity of PCM is one of the important parameters for latent heat thermal energy storage applications. The thermal con ductivity of CaCl2⋅6H2O was determined at the testing temperatures of 20 � C and 30 � C, which were corresponding to the solid and liquid states of CaCl2⋅6H2O. The test results of the thermal conductivity of CaCl2⋅6H2O and modified CaCl2⋅6H2O are presented in Table 9. Thermal conductivity coefficient of graphite can reach about 390 W/(m K) [42], 8
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Fig. 16. The optical microscopy photos of sample of the modified CaCl2⋅6H2O without SAP (0.5 wt% flake graphiteþ 7.5 wt% H2O): (a) Smooth surface of the sample bottom; and (b) Rough surface of the sample top. Table 9 Thermal conductivity of CaCl2⋅6H2O under different temperatures. Temperatures
Raw CaCl2⋅6H2O
Modifed CaCl2⋅6H2O
Improvement (%)
20 C 30 � C
0.697 W/m⋅K 0.542 W/m⋅K
0.824 W/m⋅K 0.617 W/m⋅K
18.2% 13.8%
�
Fig. 18. Electricity consumption of PCM model and control model: (a) Hong Kong; and (b) Changsha.
which nearly hundreds times of that of CaCl2⋅6H2O. Therefore, the thermal conductivity of CaCl2⋅6H2O increased by 18.2% and 13.8% with respect to the solid and liquid states, respectively, due to the addition of 0.5 wt% flake graphite. Fig. 17. Average indoor air temperature of PCM model and control model: (a) Hong Kong; and (b) Changsha.
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Table 10 The cost-benefit analysis of application of CaCl2⋅6H2O in Hong Kong and Changsha. City
Hong Kong Changsha
The cost of using CaCl2⋅6H2O
Electricity saving per year
Static payback period (Year)
Total dosage of CaCl2⋅6H2O (kg)
The price of modified CaCl2⋅6H2O (RMB/kg)
Cost (RMB)
Total electricity saving (kW⋅h)
Electricity price (RMB/kW⋅h)
Saving (RMB)
SPP¼Cpcm/Selectricity
395.890
4.420
1749.834
68.670
1.389
95.383
18.345
395.890
2.360
934.300
122.350
0.906
110.849
8.429
Note: 1 USD ¼ 7.15 RMB; 1 USD ¼ 7.84 HKD.
3.7. Numerical analysis of energy consumption for modified CaCl2⋅6H2O in buildings
liquid CaCl2⋅6H2O. The conclusions can be drawn as follows. 1. Although the initial purpose of using graphite was to enhance ther mal conductivity, flake graphite could be used to decrease super cooling of CaCl2⋅6H2O. It is found that the addition of 0.5 wt% flake graphite in CaCl2⋅6H2O appeared to be most effective as the super cooling degree was reduced less than 1 � C. 2. The addition of 25 wt% SAP improved the segregation resistance of CaCl2⋅6H2O. The combined method of TGA-DTA was able to evaluate the effect of thickeners on segregation of CaCl2⋅6H2O. It is found that TGA-DTA method is a suitable, powerful and accurate test method in evaluating the effect of thickeners in CaCl2⋅6H2O. 3. The addition of 0.5 wt% flake graphite, 25 wt% SAP and 7.5 wt% H2O in industrial grade CaCl2⋅6H2O could completely eliminate the supercooling degree of CaCl2⋅6H2O and improve its segregation resistance. The phase change temperature and enthalpy value of modified CaCl2⋅6H2O were 27.9 � C and 147.9 J/g respectively. The proposed modified CaCl2⋅6H2O is relatively economical and is suit able to apply in many fields including solar energy, building energy conservation and so on. 4. With the presence of 0.5 wt% flake graphite, the thermal conduc tivity of CaCl2⋅6H2O in solid and liquid states increased by 13.8% and 18.2%, respectively. 5. From the numerical simulation, it can be known that the room model with PCM walls consumed less electricity compared to the control room, and the reduction became more significant in February to April or October to November. Moreover, the indoor temperature of room model with PCM walls can be maintained within a comfortable range. Furthermore, the payback period of using modified CaCl2⋅6H2O in Hong Kong and Changsha are about 18.3 and 8.4 years, respectively, which are much smaller than the average life span (60 years) for a residential building.
Fig. 17 shows the average indoor temperature of a room model with modified PCM compared with the control model in both Hong Kong and Changsha areas. Red curve presents the control model and the blue curve presents the PCM model (with phase change effect). From the figure, the control model consistently shows higher indoor peak air temperatures compared with the room model containing PCM walls. Specifically, it can be seen from the enlarged images, the indoor tem perature differences between the control and PCM model can reach 0.95 � C and 1.85 � C in Hong Kong and Changsha respectively. This could reveal the effective function of PCM in reducing the indoor temperature. It can also be speculated that the region of hot summer and cold winter (Changsha) is more suitable for the application of phase change material in buildings. At night time, PCM model could still work to relase energy maintaining the indoor temperatures which were higher than those observed in the model without PCM. To quantitatively reveal the effect of energy saving of bulidings with modified CaCl2⋅6H2O, the electricity consumptions were also compared between the control and the PCM model. Fig. 18 demonstrated the caculated results within a period of nine months. The figure also shows the consumed electicity reduction (estimated reduction and reduction percentage compared to the control group) for each month. As for Hong Kong, it can be seen from Fig. 18(a) that the variation of reduction on electricity is different. The reduction rate was below 10% in the months from May to September. In summer (May to September), the electicity consumption was relatively high as air-conditioning is set to maintain the stability of indoor temperature. In other seasons (February to April or October to November), PCM however started to work effectively and resulted in higher electicity reductions. Compared with Hong Kong, the application of PCM in Changsha had a more remarkable effect in the saving of electricity. It can be found that the saving of electricity were over 20% from May to September. Furthermore, the total electricity saving in Changsha could reach to 122.350 kW h per year, while it was only 68.670 kW h in Hong Kong. In order to further assess the feasibility of using modified CaCl2⋅6H2O in building walls, a cost-benefit analysis was also conducted. The static payback period, which has been proved to be an effective method [32], was used in this study. As showed in Table 10, the cost of using modified CaCl2⋅6H2O in the room model was about 1749.8 RMB and 934.3 RMB in Hong Kong and Changsha, whereas the savings of electricity in one year were 95.4 RMB and 110.9 RMB, respectively. Therefore, the payback periods could be calculated as 18.3 years and 8.4 years, respectively. The payback periods of using PCM in Hong Kong and Changsha are much smaller than the average life span of 60 years for a residential building. Therefore, the incorporation of PCM in building wall is economically feasible.
Declaration of competing interest The authors declare no conflict of interest. CRediT authorship contribution statement Xiaohua Bao: Formal analysis. Haibin Yang: Formal analysis. Xiaoxiao Xu: Formal analysis. Tao Xu: Formal analysis. Hongzhi Cui: Formal analysis. Waiching Tang: Writing - review & editing. Guochen Sang: Formal analysis. W.H. Fung: Writing - review & editing. Acknowledgement The work presented in this paper was fully supported by a grant from Natural Science Foundation of China (51925804).
4. Conclusions and recommendations
Appendix A. Supplementary data
In this investigation, flake graphite was used to decrease the super cooling of industrial grade CaCl2⋅6H2O. Besides, super absorbent poly mer was used as a thickener to improve the segregation resistance of
Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110420.
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Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Development of a stable inorganic phase change material for thermal energy storage in buildings Xiaohua Bao a, Haibin Yang a, b, Xiaoxiao Xu a, Tao Xu c, Hongzhi Cui a, *, Waiching Tang d, Guochen Sang e, W.H. Fung b a
College of Civil and Transportation Engineering, Shenzhen University, Shenzhen, 518060, China Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong School of Civil Engineering, Guangzhou University, Guangzhou, 510006, China d School of Architecture and Built Environment, The University of Newcastle, Callaghan, NSW, 2308, Australia e Xi’an University of Technology, School of Civil Engineering and Architecture, Xi’an, 710048, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase change material Industrial grade calcium chloride hexahydrate Flake graphite Super absorbent polymer Supercooling Segregation
Building energy consumption is influenced evidently by solar radiation. To achieve a stable indoor temperature by minimizing the heat fluctuations resulted from solar radiation, latent heat thermal energy storage systems with phase change materials (PCMs) in building envelope have been studied. Though inorganic PCMs have promising potential applications among all kinds of PCMs, the supercooling and segregation are the major issues which have restricted their practical applications. The purpose of this study is to lessen the supercooling degree of an industrial grade PCM (CaCl2⋅6H2O) by using a nucleating agent - flake graphite. A super absorbent polymer (SAP) was also used as a thickener to prevent segregation of CaCl2⋅6H2O during phase transition. The combined method of thermogravimetric analysis/differential thermal analysis (TGA-DTA) was firstly proposed to evaluate the segregation of inorganic PCM innovatively. The results showed that using 0.5 wt% flake graphite not only eliminated the supercooling of CaCl2⋅6H2O, but also improved its thermal conductivity for better thermal per formance. The results of Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) demonstrated that the flake graphite did play an important role on the crystallization of CaCl2⋅6H2O. Numerical simulation demonstrated that using modified CaCl2⋅6H2O within walls of buildings in Hong Kong and Changsha are economically feasible with the payback periods of 18.3 years and 8.4 years respectively.
1. Introduction Globally, the building sector consumed approximately 20.1% of the total energy consumption in 2016 [1]. Building energy consumption is influenced evidently by solar radiation in outdoor environment. Therefore, the building envelope has been regarded as one of the most crucial impacts on the building energy conservation. Latent heat thermal energy storage systems using PCMs in building envelope has been considered as an effective strategy to improve the energy saving or in door thermal comfort of buildings [2–5]. According to the report stated by Xie et al. [6], PCMs-based building envelope can effectively reduce indoor heating and cooling loads caused by variable outdoor sur roundings, ultimately offset the mismatch between availability and supply of heat energy. Besides, PCMs-based building envelopes also could achieve a human comfort level (22 � C–28 � C) owing to PCMs have
an appropriate phase change temperature [7]. Moreover, Zhao et al. [8] and Min et al. [9] have verified that PCMs with excellent chemical and thermal stability could meet the durability requirements of most appli cations. In a word, PCMs are suitable for use in buildings to reduce energy consumption and/or improve indoor thermal comfort. PCM can generally be classified as either organic or inorganic PCMs. Organic PCM is chemically inert and do not undergo phase segregation. However, they are flammable and relatively high costs which limit their large-scale thermal energy storage application. Compared with organic PCMs, inorganic PCMs are nonflammable, higher thermal energy stor age capacity and larger melting enthalpy [10]. Therefore, inorganic PCMs can be the excellent candidate for application in energy storage systems. However, it is well-known that inorganic PCMs usually suffer from serious supercooling and phase segregation [11,12]. Furthermore, the poor thermal conductivity of inorganic PCM also results in low
* Corresponding author. E-mail address: [email protected] (H. Cui). https://doi.org/10.1016/j.solmat.2020.110420 Received 17 May 2019; Received in revised form 8 January 2020; Accepted 15 January 2020 Available online 21 January 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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Table 1 Summary of the nucleating agents for CaCl2⋅6H2O. Study
Type of PCM
Nucleating agents
Supercooling degree (oC)
Mass concentration
Li and Zhou et al. [17] Li and Zhang et al. [18] Xie and Niu et al. [19] Li and Zhou et al. [20] Xu and Dong et al. [21]
CaCl2⋅6H2O CaCl2⋅6H2O–MgCl2⋅6H2O CaCl2⋅6H2O CaCl2⋅6H2O CaCl2⋅6H2O
SrCl2⋅6H2O/oxidized expanded graphite SrCl2⋅6H2O/SrCO3 Nano CsXWO3 Nano Al2O3 Nano SiO2
0.6 2.0 0.6 0.3 0.8
1-3 wt% 1-3 wt% 0.75 wt% 1 wt% 5 wt%
charging and discharging rates and limits its overall energy storage ef ficiency. So, it is necessary to resolve the issues related to supercooling, phase segregation and thermal conductivity of inorganic PCMs. CaCl2⋅6H2O, an inorganic PCM and a non-toxic crystalline hydrated salt, has drawn lots of attention from engineers and researchers around the world, especially during the past decade, owing to its large melting enthalpy (160–209 J/g) [13]. CaCl2⋅6H2O has been proved to be a promising inorganic PCM candidate in building application because of its excellent thermal reliability. Tyagi and Buddhi [14] carried accel erated thermal cycling tests on CaCl2⋅6H2O and found that there was only a minor variation in enthalpy value after 1,000 thermal cycle testing. High supercooling degree causes extra energy to be released, espe cially in the beginning stage of supercooling. As a result, less energy can be available for the subsequent phase transition or crystallization [15]. Hence, supercooling is still the major issue which restricts the applica tion of inorganic PCMs in large scales [16]. To reduce supercooling degree of CaCl2⋅6H2O, several nucleating agents have been used as shown in Table 1. Li and Zhou et al. [17] used SrCl2⋅6H2O and oxidized expanded graphite as nucleating agents to modify CaCl2⋅6H2O. They showed that the combination of SrCl2⋅6H2O and oxidized expanded graphite could reduced the supercooling degree of CaCl2⋅6H2O, but using SrCl2⋅6H2O or oxidized expanded graphite individually is not effective at suppressing supercooling. Li and Zhang et al. [18] proposed that the supercooling degree of an eutectic hydrate salt CaCl2⋅6H2O–MgCl2⋅6H2O was effectively reduced when strontium chloride (SrCl2⋅6H2O) and strontium carbonate (SrCO3) were added as nucleating agents. Nevertheless, the supercooling reported in their study was still as high as 2 � C, and thus more effective and inexpensive nucleating agents have to be developed. Xie et al. [19] found the supercooling of CaCl2⋅6H2O could be reduced to 0.61 � C owing to the addition of 0.75 wt% cesium tungsten bronze (CsXWO3) nanoparticles, but it also caused a slight decrease in the thermal conductivity. Simi larly, Li and Zhou et al. [20] and Xu and Dong et al. [21] respectively incorporated nano Al2O3 and nano SiO2 into CaCl2⋅6H2O, but its supercooling degree still was not eliminated. In comparison of above nucleating agents, Johansen et al. [22] and Dannemand et al. [23] agreed that graphite flake, which has a high thermal conductivity of 256–500 W/mK, as nucleating agent was more effective. In their study, the supercooling degree of inorganic PCM (so dium acetate trihydrate, SAT) was eliminated completely, and its ther mal conductivity could improve by 39%–62% as the addition of 1%–5 wt.% graphite flake. Since there is no related literature on investigating the effect of graphite flake on the supercooling of CaCl2⋅6H2O, the present work is an effort to provide some expanded research on this area. Besides, segregation is related to the occurrence of supercooling because salt hydrates gradually transform to sediment when PCMs start to be deprived of its nucleating capacity as well as crystal water lost [24, 25]. It has been reported that the lower hydrate or anhydrous salt in inorganic salt hydrate would be precipitated as thermal cycling increased, thereby deposited at the bottom [26,27]. Consequently, the substances between the top and bottom layers of liquid CaCl2⋅6H2O would be different. Many measures have been studied to address segregation of CaCl2⋅6H2O during melting process. The most common method is using thickeners to improve segregation resistance of PCM during phase-transition. However, there are limited studies that can
Table 2 Physical properties of the industrial grade CaCl2⋅6H2O. Items of properties
Data
Density (26 C) Phase change temperature
1.475 g/cm3 ~29 � C
�
Fig. 1. XRD spectrum of the industrial grade CaCl2⋅6H2O and a reagent grade CaCl2⋅6H2O.
provide with appropriate methods to examine the effect of thickeners on segregation of inorganic PCMs [28–30]. So, the effectiveness of thick eners cannot be well examined. Hence, it is essential to develop a more suitable technique to evaluate the influence of thickeners on anti-segregation. In this study, super absorbent polymer (SAP) was selected as a thickening agent and its effect on segregation of calcium chloride hexahydrate liquid was investigated. Moreover, flake graphite as a nucleating agent was used to mitigate the supercooling of an industrial grade CaCl2⋅6H2O. Furthermore, the combined method of thermogra vimetric analysis/differential thermal analysis (TGA-DTA) was firstly proposed to assess the influence of thickener on segregation of inorganic PCM. It’s worth noting that the TAG-DTA method can allow a quanti tative segregation analysis of inorganic PCM and the effectiveness of thickeners can be well understood. In this study, a stable inorganic PCM, based on industrial grade CaCl2⋅6H2O without supercooling and without phase segregation was successfully prepared. Besides, the effect of the modified PCM using on buildings was revealed by means of numerical simulation. 2. Experimental and simulation work 2.1. Materials In this study, an industrial grade calcium chloride hexahydrate (CaCl2⋅6H2O, 98% purity) from Chengdu Kelong chemical Corporation 2
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2.2. Sample preparation and characterization 2.2.1. Preparation of modified CaCl2⋅6H2O The procedures for preparing modified CaCl2⋅6H2O are as follows: First, 50g solid CaCl2⋅6H2O was placed inside a glass beaker, and then the beaker was kept in a water-bath at 60 � C until the PCM melted completely. Because the water content in industrial grade CaCl2⋅6H2O was low (see Table 3) which would affect the phase change temperature and enthalpy value of PCM, and inevitably restrict the PCM to function properly. Hence, an proper amount of water was incorporated into the PCM mixture to compensate the water loss. Subsequently, the flake graphite with different dosages (0.2 wt%, 0.5 wt% and 0.8 wt%) mix tures were dispersed in PCM mixture by magnetic stirring with the speed of 500 rpm for 30 min, and followed by ultrasonic dispersion (JY-92-II N, Ningbo Xinzhi Biotech Co., Ltd. 25 kHz, 375 W) for about 30 min, respectively. To mitigate segregation, 25 wt% of SAP (thickener agent) was added and dispersed uniformly in the mixtures by stirring vigor ously under the speed of 1500 rpm for 15 min. Finally, the modified CaCl2⋅6H2O was prepared successfully.
Fig. 2. Microstructure of the flake graphite powder.
2.2.2. The evaluation of supercooling The temperature history method (T-history method) developed by Zhang et al. [31] in 1999 was adopted to dynamically record the tem perature variations of PCMs during heating and cooling processes. The supercooling degree of CaCl2⋅6H2O was calculated by measuring the difference between the highest temperature that occurred during the heating process and the lowest temperature that occurred during the cooling process. For T-history test, thermocouples (Type K, resolution � 0.3 � C) were utilized and placed in the center of the melted PCM. The multichannel data recorder (TP700, Shenzhen TOPRIE Electronics Co., Ltd.) was used for recording the temperature.
Fig. 3. The appearance of super absorbent polymer (SAP).
2.2.3. The evaluation of segregation The combined method of TGA-DTA (NETZSCH STA 409 PC) was proposed to evaluate the segregation of PCM composites. Because segregation would occur during the heating process, testing samples about 5 mL were collected from the bottom and top layers of melted CaCl2⋅6H2O (100 mL) and then stored in a refrigerator below 20� C until testing.
Table 3 Loss on ignition of two kinds of materials. Weight
Weight of solid (g) Weight of water loss (g)
Industrial grade CaCl2⋅6H2O
Reagent grade CaCl2⋅6H2O
Before heating
After heating
Before heating
After heating
10.00 3.97
6.03
10.00 4.98
5.02
2.2.4. Thermo-physical performance of modified CaCl2⋅6H2O The phase change temperature and latent heat value of modified PCM composites were evaluated by the differential scanning calorimetry (DSC, Netzsch DSC 200 F3). The temperature range of measurement process was set from 10� C to 50� C under a heating rate of 2� C/min in a continuous stream of nitrogen flowing at a rate of 50 ml/min.
Limited was used as the inorganic PCM. Table 2 shows the physical properties of CaCl2⋅6H2O, while Fig. 1 shows the X-ray diffraction (XRD) spectrum of the industrial grade PCM. By comparing its peak positions in the spectrum to those of reagent grade PCM, it can be known that there are some impurities in the industrial grade PCM. Although the industrial grade PCM is not as pure as the reagent grade, the industrial grade product is cheaper and more convenient to obtain. Flake graphite powder (carbon purity>98%, 800 mesh) from Tianjin Fuchen chemical Corporation Limited was used as nucleating agents. Fig. 2 shows the micrograph of graphite powder. A commercial grade SAP from Shanghai Funa Corporation Limited (see Fig. 3) was used as an anti-segregation agent for the inorganic PCM. Crystal water is very important to the properties of CaCl2. Table 3 shows the difference in crystal water between the industrial grade CaCl2⋅6H2O before modification and the reagent grade CaCl2⋅6H2O samples, after ignition at high temperature (>300 � C) for 30 min. The reagent CaCl2⋅6H2O (with a water loss of 4.98g) contains more crystal water than that of the industrial grade CaCl2⋅6H2O (with a water loss of 3.97g). Less quantity of water inside the crystallization of CaCl2⋅6H2O may result in different spectrums observed by the XRD, and this in fluences the interior CaCl2 molecule on matching water molecule.
2.2.5. Chemical structure and microstructure of modified CaCl2⋅6H2O The effect of flake graphite on supercooling reduction was analyzed by using Fourier-transform infrared spectroscopy (FT-IR, Nicolet 6700). Environmental Scanning electron microscope (ESEM, FEI Quanta TM 250 FEG) was employed to demonstrate the morphology of micro structure of PCM and modified PCM. In addition, optical microscope was also employed for microscopic analysis. 2.2.6. Thermal conductivity of modified CaCl2⋅6H2O In order to evaluate the thermal conductivity of the CaCl2⋅6H2O and modified CaCl2⋅6H2O, a hot wire method was conducted with a thermal conductivity tester (XIATECH, TC3000). This device uses the transient hot wire method conforming to Chinese standard GB/T 10297-2015 (Test method for thermal conductivity of nonmetal solid materi als—Hot-wire method). The PCM samples were tested at indoor tem peratures of 20 � C and 30 � C for evaluating their thermal conductivity in liquid and solid state, respectively.
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case study areas and the performance of modified PCM in buildings located in these two areas was simulated. Table 4 indicates the typical climatic conditions in Hong Kong and Changsha including the basic parameters such as the mean temperatures of the coldest month and the hottest month. The electricity consumption, parameters of air condi tioning system such as setting temperature and service time were considered and the details are indicated in Table 5. The energy performance of a house model (see Fig. 5) constructed with modified PCM walls was evaluated using a numerical simulation software- Energy plus. Fig. 5(a) shows the appearance and dimension of the house model. The building has a height of 3.0 m and floor area of 20 m2 (4.0 � 5.0 m). The size of window is 1.8 m � 1.5 m and the distance between window’s bottom and the floor is 0.9 m. The size of the door is 1.8 m � 0.9 m. Construction details and key material parameters are shown in Tables 6 and 7, respectively. Fig. 5(b) presented the schematic diagram of PCM wall. The other parameters representing PCMs such as thermal conductivity, phase change temperature and enthalpy were measured from the above said experiments. The accuracy of the simu lation model used in this study has been verified in our previous research [32–34], and thus it is also suitable to analyze the energy consumption of buildings using the modified CaCl2⋅6H2O. The numerical values including the electricity consumption (kW⋅h) and the average indoor air temperature were determined and the effectiveness of modified PCM was revealed. The monthly electricity consumption of nine continuous months (March–November) and the average indoor air temperature in August (hourly) were also presented in this paper.
Fig. 4. Climate regions in China [32]. Table 4 Temperature characteristics in Hong Kong and Changsha. City
Hong Kong Changsha
Climate regions
Hot summer & warm winter (HS&WW) Hot summer & cold winter (HS&CW)
Mean temperature Coldest month
Hottest month
>10 � C
25 � C–29 � C
4 � C–5 � C
>30 � C
Table 6 Construction details.
Table 5 Air conditioning system setting situation. Parameter System Energy type Equipment energy efficiency ratio Temperature Service time
Items
Design parameters
PCM wall (four faces)
200 mm concrete With or without 5 mm modified PCM 12 mm floated coat 100 mm concrete 40 mm concrete 25 mm insulation board 50 mm concrete
Floor Roof
Refrigeration system Central air conditioning control system Electricity 3.28 27 � C March–November, Monday-Saturday, 9.00 am - 18.00 pm.
Table 7 Thermal physics parameters.
2.2.7. Numerical simulation of PCM application Diverse regions in China have their own unique climate character istics as shown in Fig. 4. In the previous study [32], Hong Kong and Changsha were identified the most and less remarkable significant areas with respect to the saving of electricity when applying PCM in buildings. Therefore, in this study, Hong Kong and Changsha were selected as the
Building materials
Thermal conductivity (W/m K)
Specific heat (J/ kg K)
Latent heat (kJ/kg)
Concrete Insulation board Floated coat
1.74 0.085
920 920
0 0
0.93
1050
0
Fig. 5. The construction of simulated model: (a) The appearance of house model; and (b) Schematic diagram of PCM wall. 4
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CaCl2⋅6H2O samples with different dosages of flake graphite are shown in Fig. 7(a). The supercooling degree of samples with 0.2 wt%, 0.5 wt% and 0.8 wt% flake graphite were found to be 1.7 � C, 0.9 � C and 2.6 � C, respectively. It is worth noting that the supercooling degree of CaCl2⋅6H2O added with 0.5 wt% flake graphite decreased by 83% compared with the control samples (see Fig. 7(b)). Hence, the addition of 0.5 wt% flake graphite has a significant effect in reducing super cooling of CaCl2⋅6H2O. This phenomenon can be explained from the chemical structure of flake graphite. Fig. 8 shows the FT-IR spectra of –C flake graphite. The peak located at 1619 cm 1 was caused by the C– stretching mode [35]. The peaks occurred at 1388 cm 1 and 1033 cm 1 were corresponding to the stretching vibrations of C–OH and C–O, respectively [36,37]. These results illustrate that the surface of graphite particles decorated by some of oxygen-containing functional groups. This observation also reveal that the functional groups on flake graphite might have played an important role in crystal attachments which accelerated the crystallization of CaCl2⋅6H2O during the cooling process and thus reducing the supercooling of CaCl2⋅6H2O. Similar observations were made by Liu and Yang [38]. In addition, graphite flake in CaCl2⋅6H2O matrix, can serve as the main sites for attachment and growth of CaCl2⋅6H2O crystals as discussed on Section 3.6. Moreover, the addition of graphite flake would result in the enhancement of
Fig. 6. The cooling curves for two kinds of CaCl2⋅6H2O with T-history method.
3. Results and discussion 3.1. Thermal performance of CaCl2⋅6H2O In this section, the supercooling of pure industrial grade and reagent CaCl2⋅6H2O were evaluated by T-history method under cooling process as shown in Fig. 6. The results show that the phase change temperature of reagent grade CaCl2⋅6H2O was lower than that of industrial grade CaCl2⋅6H2O. Moreover, it is surprising to note that the supercooling degree of reagent grade CaCl2⋅6H2O was 25.7 � C, while it was 5.1� C for the pure industrial grade CaCl2⋅6H2O. The huge difference is because the reagent grade CaCl2⋅6H2O is lack of nucleating agents stimulating the crystallization. According to the results shown in Table 2, the difference in thermal performances between the industrial and reagent grade PCMs is probably attributed to their difference in crystal water volumes. Although the industrial grade CaCl2⋅6H2O is not purified, its thermal performance is better than the pure CaCl2⋅6H2O in terms of reduced supercooling degree. 3.2. Thermal performance of modified CaCl2⋅6H2O by flake graphite
Fig. 8. FT-IR spectra of flake graphite.
The temperature curves of cooling process of the modified
Fig. 7. Supercooling degrees of the different PCMs under cooling process: (a) CaCl2⋅6H2O with different contents of flake graphite; (b) Comparison between CaCl2⋅6H2O with 0.5 wt% flake graphite and control group (pure CaCl2⋅6H2O). 5
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Fig. 9. TGA curves of top and bottom layers of CaCl2⋅6H2O samples: (a) Without any modification; and (b) With 25 wt% SAP.
Fig. 10. DTA curves of top and bottom layers of CaCl2⋅6H2O samples: (a) Without any modification; and (b) With 25 wt% SAP.
viscosity, and thus the crystallization rate of CaCl2⋅6H2O would be accelerated due to the constricted molecular motion region [39]. So, it can be concluded that flake graphite can be worked as nucleating agent to reduce the supercooling of CaCl2⋅6H2O. However, it should be stressed that the supercooling degree increased when more than 0.5 wt% graphite flake was added. As shown in Fig. 7(a), the supercooling degree of CaCl2⋅6H2O with 0.8 wt% graphite flake went up as high as 2.6 � C. According to the previous research [40], it can be explained that the excess graphite would agglomerate, and thus part of the graphite flake lost its nucleating effect.
thickener was 4.08% at the end of thermal dehydration process. How ever, the curves as shown in Fig. 9(b) indicate the effectiveness of thickener as the weight loss between the top and bottom samples was almost the same. At the end of thermal dehydration process, the maximum weight loss of water between the top and bottom layers of CaCl2⋅6H2O containing thickener was only 0.13%. Based on the TGA results, it can be deducted that adding 25 wt% SAP can improve the segregation resistance of the industrial grade CaCl2⋅6H2O significantly. Besides, the effect of thickener on heat flow of top and bottom layers of CaCl2⋅6H2O can be seen in Fig. 10. The point No. 1 represents the complete melting point whereas the point No. 2 refers to the second peak melting point. Table 8 shows the heat flow differences occurred at two melting points between the top and bottom layers of CaCl2⋅6H2O with and without thickener. From Fig. 10(a), it is obvious that the significant difference in heat flow between the top and bottom layers of CaCl2⋅6H2O with thickener occurred at temperatures around 70 � C and 160 � C. This shows segregation of CaCl2⋅6H2O occurred because of different heat flow observed between the top and bottom layers. The structure of CaCl2⋅6H2O molecule might vary during the heating process and lead to difference in substances between the two layers which finally caused
3.3. Segregation behavior of modified CaCl2⋅6H2O by SAP To investigate the effect of thickener on segregation of CaCl2⋅6H2O, TGA and DTA analyses were used to identify any variation in thermal properties between the top and bottom layers of modified CaCl2⋅6H2O during the heating process. Referring to the curves shown in Fig. 9, the TGA results apparently can illustrate the effect of thickener on weight loss of PCM. Also, it can be found from Fig. 9(a) that the percentage of weight loss between the top and bottom layers of CaCl2⋅6H2O without 6
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3.4. Thermo-physical performance of the modified CaCl2⋅6H2O
Table 8 The results of the chosen spots from the DTA curves. Position
Point No.1 (mW/mg) Without SAP
Top layer Bottom layer
0.30 0.38
Based on the above investigations, a new CaCl2⋅6H2O composite can be developed to avoid both the supercooling and segregation issues. It is consisted of industrial grade CaCl2⋅6H2O with 0.5 wt% flake graphite, 25 wt% SAP and 7.5 wt% H2O (used to compensate crystal water loss). The specific amount of water being used could compensate partial water loss inside the industrial grade CaCl2⋅6H2O. However excess water should be avoided as too much water addition may aggravate segrega tion. Fig. 11 shows the cooling process of the newly modified CaCl2⋅6H2O compared to that of the industrial grade CaCl2⋅6H2O. The supercooling of the modified CaCl2⋅6H2O was completely eliminated (see red line in Fig. 11). It is worth reinstating here that the addition of 25 wt% SAP in CaCl2⋅6H2O was effective in eliminating segregation problem. Therefore, the newly modified CaCl2⋅6H2O can be used without the concerns of supercooling and segregation. Fig. 12 demonstrated the DSC results of CaCl2⋅6H2O with or without modification. From Fig. 12(a), it can be seen that, raw CaCl2⋅6H2O so lidified below 1.6 � C, which meant that it suffered a huge super cooling. The melting and freezing temperatures of the new PCM composite were 27.5 � C and 28.7 � C, whereas its corresponding enthalpy values were 142.2 J/g and 158.4 J/g, respectively (see Fig. 12(b)). Be sides, owing to the additives would not provide latent heat during melting process, the latent heat of modified CaCl2⋅6H2O was lower compared to the industrial grade CaCl2⋅6H2O. Because its phase change temperature is within the range of human thermal comfort (22–28 � C) [41], the modified CaCl2⋅6H2O has a great potential for use in buildings to store energy and adjust indoor tem perature. Moreover, the modified CaCl2⋅6H2O is relatively economical and can be applied to many fields, such as solar energy as well as building energy conservation.
Point No.2 (mW/mg)
SAP 0.27 0.28
Without SAP 0.54 0.61
SAP 0.49 0.51
Fig. 11. Cooling curves of the industrial grade CaCl2⋅6H2O and CaCl2⋅6H2O modified by 0.5 wt% flake graphite, 25 wt% SAP and 7.5 wt% H2O.
segregation. Similar to the TGA results, the effectiveness of thickener can be revealed by the DTA results as shown in Fig. 10(b). It is obvious that the DTA curves between the top and bottom samples were almost the same. Based on the TGA and DTA results, it can be concluded that the incor poration of 25 wt% SAP in CaCl2⋅6H2O has a positive influence in ho mogenizing the substance between the top and bottom layers of PCM. Undoubtedly, the thickener can reduce the segregation of CaCl2⋅6H2O and make the substances in CaCl2⋅6H2O system evenly distributed. This results further prove that the combined method of TGA-DTA is capable to characterize the effect of thickening agents on segregation resistance of CaCl2⋅6H2O and can be used to assess different thickeners or additives on segregation resistance of other kinds of inorganic PCM systems.
3.5. The microstructure of modified CaCl2⋅6H2O The microstructure of pure industrial grade CaCl2⋅6H2O and modi fied CaCl2⋅6H2O can be seen in Fig. 13. From Fig. 13(a), it can be found that the surface of pure industrial grade CaCl2⋅6H2O was smooth and flat. However, the surface of modified CaCl2⋅6H2O was generally rough as shown in Fig. 13(b). Therefore, it can be concluded that flake graphite can provide a suitable place for crystal growth of PCM. A stereomicroscope was also used to observe the surface of the modified CaCl2⋅6H2O. The photographs of smooth and irregular surfaces of the modified CaCl2⋅6H2O are shown in Fig. 14(a) and Fig. 14(b), respectively. The flake graphite was uniformly distributed in
Fig. 12. DSC curves of inorganic PCMs: (a) Industrial grade CaCl2⋅6H2O; and (b) CaCl2⋅6H2O modified by 0.5 wt% flake graphite, 25 wt% SAP and 7.5 wt% H2O. 7
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Fig. 13. Microstructure of CaCl2⋅6H2O: (a) Pure industrial grade CaCl2⋅6H2O; and (b) Modified CaCl2⋅6H2O.
Fig. 14. Stereomicroscope photographs of PCM system: (a) Smooth back surface of the sample (1,000X); and (b) Rough top surface of the sample (500X).
Fig. 15. The optical microscopy photos of sample of the modified PCM system (0.5 wt% flake graphiteþ 25 wt% SAP þ 7.5 wt% H2O): (a) Smooth back surface of the sample; and (b) Rough top surface of the sample.
CaCl2⋅6H2O and the crystals grew around the graphite properly. In order to observe the role of SAP in modified CaCl2⋅6H2O, the optical microscopy images of CaCl2⋅6H2O with and without SAP were taken and are shown in Figs. 15 and 16, respectively. The modified CaCl2⋅6H2O with SAP showed a high degree of crystallization compared to the one without SAP. Indeed, the thickeners can influence the solid ification [23]. Moreover, it may stimulate the growth of CaCl2⋅6H2O crystals.
3.6. The thermal conductivity of modified CaCl2⋅6H2O The thermal conductivity of PCM is one of the important parameters for latent heat thermal energy storage applications. The thermal con ductivity of CaCl2⋅6H2O was determined at the testing temperatures of 20 � C and 30 � C, which were corresponding to the solid and liquid states of CaCl2⋅6H2O. The test results of the thermal conductivity of CaCl2⋅6H2O and modified CaCl2⋅6H2O are presented in Table 9. Thermal conductivity coefficient of graphite can reach about 390 W/(m K) [42], 8
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Fig. 16. The optical microscopy photos of sample of the modified CaCl2⋅6H2O without SAP (0.5 wt% flake graphiteþ 7.5 wt% H2O): (a) Smooth surface of the sample bottom; and (b) Rough surface of the sample top. Table 9 Thermal conductivity of CaCl2⋅6H2O under different temperatures. Temperatures
Raw CaCl2⋅6H2O
Modifed CaCl2⋅6H2O
Improvement (%)
20 C 30 � C
0.697 W/m⋅K 0.542 W/m⋅K
0.824 W/m⋅K 0.617 W/m⋅K
18.2% 13.8%
�
Fig. 18. Electricity consumption of PCM model and control model: (a) Hong Kong; and (b) Changsha.
which nearly hundreds times of that of CaCl2⋅6H2O. Therefore, the thermal conductivity of CaCl2⋅6H2O increased by 18.2% and 13.8% with respect to the solid and liquid states, respectively, due to the addition of 0.5 wt% flake graphite. Fig. 17. Average indoor air temperature of PCM model and control model: (a) Hong Kong; and (b) Changsha.
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Table 10 The cost-benefit analysis of application of CaCl2⋅6H2O in Hong Kong and Changsha. City
Hong Kong Changsha
The cost of using CaCl2⋅6H2O
Electricity saving per year
Static payback period (Year)
Total dosage of CaCl2⋅6H2O (kg)
The price of modified CaCl2⋅6H2O (RMB/kg)
Cost (RMB)
Total electricity saving (kW⋅h)
Electricity price (RMB/kW⋅h)
Saving (RMB)
SPP¼Cpcm/Selectricity
395.890
4.420
1749.834
68.670
1.389
95.383
18.345
395.890
2.360
934.300
122.350
0.906
110.849
8.429
Note: 1 USD ¼ 7.15 RMB; 1 USD ¼ 7.84 HKD.
3.7. Numerical analysis of energy consumption for modified CaCl2⋅6H2O in buildings
liquid CaCl2⋅6H2O. The conclusions can be drawn as follows. 1. Although the initial purpose of using graphite was to enhance ther mal conductivity, flake graphite could be used to decrease super cooling of CaCl2⋅6H2O. It is found that the addition of 0.5 wt% flake graphite in CaCl2⋅6H2O appeared to be most effective as the super cooling degree was reduced less than 1 � C. 2. The addition of 25 wt% SAP improved the segregation resistance of CaCl2⋅6H2O. The combined method of TGA-DTA was able to evaluate the effect of thickeners on segregation of CaCl2⋅6H2O. It is found that TGA-DTA method is a suitable, powerful and accurate test method in evaluating the effect of thickeners in CaCl2⋅6H2O. 3. The addition of 0.5 wt% flake graphite, 25 wt% SAP and 7.5 wt% H2O in industrial grade CaCl2⋅6H2O could completely eliminate the supercooling degree of CaCl2⋅6H2O and improve its segregation resistance. The phase change temperature and enthalpy value of modified CaCl2⋅6H2O were 27.9 � C and 147.9 J/g respectively. The proposed modified CaCl2⋅6H2O is relatively economical and is suit able to apply in many fields including solar energy, building energy conservation and so on. 4. With the presence of 0.5 wt% flake graphite, the thermal conduc tivity of CaCl2⋅6H2O in solid and liquid states increased by 13.8% and 18.2%, respectively. 5. From the numerical simulation, it can be known that the room model with PCM walls consumed less electricity compared to the control room, and the reduction became more significant in February to April or October to November. Moreover, the indoor temperature of room model with PCM walls can be maintained within a comfortable range. Furthermore, the payback period of using modified CaCl2⋅6H2O in Hong Kong and Changsha are about 18.3 and 8.4 years, respectively, which are much smaller than the average life span (60 years) for a residential building.
Fig. 17 shows the average indoor temperature of a room model with modified PCM compared with the control model in both Hong Kong and Changsha areas. Red curve presents the control model and the blue curve presents the PCM model (with phase change effect). From the figure, the control model consistently shows higher indoor peak air temperatures compared with the room model containing PCM walls. Specifically, it can be seen from the enlarged images, the indoor tem perature differences between the control and PCM model can reach 0.95 � C and 1.85 � C in Hong Kong and Changsha respectively. This could reveal the effective function of PCM in reducing the indoor temperature. It can also be speculated that the region of hot summer and cold winter (Changsha) is more suitable for the application of phase change material in buildings. At night time, PCM model could still work to relase energy maintaining the indoor temperatures which were higher than those observed in the model without PCM. To quantitatively reveal the effect of energy saving of bulidings with modified CaCl2⋅6H2O, the electricity consumptions were also compared between the control and the PCM model. Fig. 18 demonstrated the caculated results within a period of nine months. The figure also shows the consumed electicity reduction (estimated reduction and reduction percentage compared to the control group) for each month. As for Hong Kong, it can be seen from Fig. 18(a) that the variation of reduction on electricity is different. The reduction rate was below 10% in the months from May to September. In summer (May to September), the electicity consumption was relatively high as air-conditioning is set to maintain the stability of indoor temperature. In other seasons (February to April or October to November), PCM however started to work effectively and resulted in higher electicity reductions. Compared with Hong Kong, the application of PCM in Changsha had a more remarkable effect in the saving of electricity. It can be found that the saving of electricity were over 20% from May to September. Furthermore, the total electricity saving in Changsha could reach to 122.350 kW h per year, while it was only 68.670 kW h in Hong Kong. In order to further assess the feasibility of using modified CaCl2⋅6H2O in building walls, a cost-benefit analysis was also conducted. The static payback period, which has been proved to be an effective method [32], was used in this study. As showed in Table 10, the cost of using modified CaCl2⋅6H2O in the room model was about 1749.8 RMB and 934.3 RMB in Hong Kong and Changsha, whereas the savings of electricity in one year were 95.4 RMB and 110.9 RMB, respectively. Therefore, the payback periods could be calculated as 18.3 years and 8.4 years, respectively. The payback periods of using PCM in Hong Kong and Changsha are much smaller than the average life span of 60 years for a residential building. Therefore, the incorporation of PCM in building wall is economically feasible.
Declaration of competing interest The authors declare no conflict of interest. CRediT authorship contribution statement Xiaohua Bao: Formal analysis. Haibin Yang: Formal analysis. Xiaoxiao Xu: Formal analysis. Tao Xu: Formal analysis. Hongzhi Cui: Formal analysis. Waiching Tang: Writing - review & editing. Guochen Sang: Formal analysis. W.H. Fung: Writing - review & editing. Acknowledgement The work presented in this paper was fully supported by a grant from Natural Science Foundation of China (51925804).
4. Conclusions and recommendations
Appendix A. Supplementary data
In this investigation, flake graphite was used to decrease the super cooling of industrial grade CaCl2⋅6H2O. Besides, super absorbent poly mer was used as a thickener to improve the segregation resistance of
Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110420.
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