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expanded graphite as a composite phase change thermal energy storage material for tankless solar water heater
Accepted Manuscript Title: Stearic acid/expanded graphite as a composite phase change thermal energy storage material for tankless solar water heater Authors: Chuanchang Li, Bo Zhang, Baoshan Xie, Xinbo Zhao, Jian Chen, Zhongsheng Chen, Yi Long PII: DOI: Reference:
S2210-6707(18)31589-0 https://doi.org/10.1016/j.scs.2018.10.041 SCS 1317
To appear in: Received date: Revised date: Accepted date:
11-8-2018 27-10-2018 27-10-2018
Please cite this article as: Li C, Zhang B, Xie B, Zhao X, Chen J, Chen Z, Long Y, Stearic acid/expanded graphite as a composite phase change thermal energy storage material for tankless solar water heater, Sustainable Cities and Society (2018), https://doi.org/10.1016/j.scs.2018.10.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Stearic acid/expanded graphite as a composite phase change thermal energy storage material for tankless
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solar water heater
Chuanchang Li *a, Bo Zhang a, Baoshan Xie a, Xinbo Zhao a, Jian Chen *a,
a
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Zhongsheng Chen b, Yi Long c
School of Energy and Power Engineering, Changsha University of Science and
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Technology, Changsha410114, China. E-mail: [email protected];
State Key Laboratory Breeding Base of Nuclear Resources and Environment, East
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b
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[email protected]; Fax: +86-0731-85258409; Tel: +86-0731-85258409
c
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China Institute of Technology, Nanchang 330013, China School of Materials Science and Engineering, Nanyang Technological University, 50
Highlights
The stearic acid/expanded graphite composite (SA/EG6) had the melting and
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Nanyang Avenue, 639798, Singapore
freezing heats of 163.5 J g-1 and 167.3 J g-1, respectively;
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The infrared images manifested that the SA/EG6 was definitely good at transient temperature response; The experiment study on a developed TSWH apparatus with composites was carried out, suggesting that the SA/EG6 has a great potential application in TSWH system.
Abstract: Tankless solar water heater (TSWH) integrated phase change materials (PCMs) is a promising field due to its low cost, low heat loss, and compact structure. To obtain a composite PCMs that suitable for tankless solar water heater application, the expanded graphite (EG) in different mass fraction (2 wt.%, 6 wt.%, and 10 wt.%)
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were added to stearic acid (SA) via melting impregnation. The thermophysical
properties including phase change temperature, latent heat, and thermal conductivity
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of the SA/EG composites were characterized by DSC and thermal conductivity tests.
The melting and freezing heats of SA/EG6 composite were 163.5 J g-1 and 167.3 J g-1,
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respectively. The thermal conductivity of SA/EG6 was as high as 9.6 times that of
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pure SA. Meanwhile, the infrared images suggested the addition of EG enhance the
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heat release capability of composites as well as improve the uniformity of heat
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transfer process of SA/EG6. At last, a developed compact solar water heater was used to estimate the thermal performance of as-prepared composites. The experimental
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results shown that the melting time of SA/EG6 was shorten 63.3% compared with that of the SA and the heat releasing rate of SA/EG6 was the highest, suggesting the
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SA/EG6 composites can be applied in TSWH system. Keywords: stearic acid; expanded graphite; composite phase change material;
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tankless solar water heater
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1. Introduction
Nowadays, the energy dissipation and the environmental pollution are two critical
problems of the sustainable society, and how to utilize energy more efficient and cleaner has been paid largely attention (He et al., 2018; Yao et al., 2019; Zhu et al., 2018). One of the solutions is the thermal energy storage (TES) technology, which
stores excess solar energy or waste heat in a storage medium as internal energy for energy applications demanded in time or space (Reddy, Mudgal, & Mallick, 2018). Energy storage equipment with TES has great improvement in energy efficiency and stability. By using different method, TES is categorized into sensible heat storage, latent heat storage, and thermo chemical storage. The latent heat storage, which using
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phase change materials (PCMs) for energy storage or discharge, have received widespread attention due to its higher energy storage density and smaller temperature
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swing compared with other methods (Farid, Khudhair, Razack, & Al-Hallaj, 2004; Nkwetta, & Haghighat, 2014; Zhang, Xiao, & Ma, 2016). Thus, latent heat storage has
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been widely utilized in solar thermal utilization (Stritih et al., 2018), energy-efficiency
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building (Hasan, Basher, & Shdhan, 2018), industrial waste-heat utilization (Miró,
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Gasia, & Cabeza, 2016), and thermal management (Gao et al., 2018; Yuan et al.,
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2017).
Solar water heater (SWH) is one of the most common ways of solar thermal
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utilization and is receiving rapid tendency. Conventional SWH system consists of an array of solar collector panels and an insulated hot water storage tank, which use the
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sun or heat transfer fluid in a collector to heat water directly (Kee, Munusamy, & Ong, 2018). The storage tank size has to be fully large to store adequate heat because of the
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intermittency and uncertainty of solar radiation, which would make the SWH system more complex and expensive. Latent heat storage system integrated with PCMs is
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used in the SWH system to reduce the size of the storage tank and enhance the performance and reliability of thermal solar system by extending its operational time (Nkwetta, & Haghighat, 2014). The use of solar water heater incorporated with PCMs as TES has the potential to store heat energy during off-peak periods and to be re-used during peak periods (Kee, Munusamy, & Ong, 2018). A typical use of latent heat
storage system in solar energy utilization is tankless solar water heater (TSWH) which developed from conventional solar water heater with water tank. TSWH integrated latent heat storage is a compact solar water heater without a conventional water tank and the storing/releasing of thermal energy is achieved by the utilization of PCMs (Luu, Milani, Nomvar, & Abbas, 2017). The PCMs usually
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integrated in solar collector (e.g. evacuated tube (Feliński, & Sekret, 2017), shell, and tube container (Cao, Yuan, Xiang, & Haghighat, 2018) ) absorb heat and store it in the
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form of latent heat for later use. Due to the excellent chemical stability, high storage
density, and suitable phase change temperature of PCMs (Yang et al., 2018; Zeng et
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al., 2018), many researchers have attempted to do some experiment and simulation
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studies about TSWH integrated PCMs. To develop a high-efficiency TSWH integrated
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PCMs, a number of approaches have been adopted. A compact solar collector with an
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absorber plate integrated paraffin wax for heat storage media was designed by Mettawee et al. (Mettawee, & Assassa, 2006) and the experimental results shown that
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the low thermal conductivity of the solid paraffin had a negative effect on the heat transfer performances. Chen. et al. (Chen, Gu, & Peng, 2010) impregnated the paraffin
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into the aluminum foam and evaluated its performance on a solar flat-plate collector, demonstrating that aluminum foam has great effect on the heat transfer and melting
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rate of paraffin. Evacuated tubes, with a lowest heat loss coefficient because of their transparent insulation (vacuum), have been utilizing as a container of PCMs. An
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evacuated tube collector/storage system was developed by Feliński et al. (Feliński, & Sekret, 2016), wherein the paraffin was served as PCMs and heat pipes as heat exchange units. Li et al. (Li, & Zhai, 2017) presented a evacuated tube solar collector/storage
system
designed
for
mid-temperature
application:
the
erythritol/expanded graphite composite PCM (119 °C for melting temperature) was
filled into aluminum pipes and then placed inside evacuated tubes; the experimental results shown that the composite PCMs could effectively storage energy. Moreover, dynamic model of a domestic tankless solar water heater was presented and an annual simulation was carried out for the long-term operational feasibility studies (Luu, Milani, Nomvar, & Abbas, 2017). Considering the optimization of PCMs energy
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storage systems, the leakage and the thermophysical properties (including melting temperature, latent heat, and thermal conductivity) of solid-liquid PCMs employed in
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TSWH systems requires further development (Zhang et al., 2018b). PCMs with high latent heat, high thermal conductivity and suitable phase change temperature for
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household usage are desirable for TSWH system (Luu, Milani, Nomvar, & Abbas,
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2017; Sharif et al., 2015). Meanwhile, the inexpensive and effective methods to
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prepare suitable PCMs for TWSH should be developed as soon as possible (Zhang et
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al., 2018b).
So far, very little information related to the composite phase change materials
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which used in the TWSH system is available, and only researchers choose paraffin wax for experiment study in the field of TWSH. To obtain a high thermal performance
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composite PCMs used for TSWH, a facile and economic efficiency route to synthesize the shape-stabilized PCMs from the expanded graphite and stearic acid
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was presented in this article. Moreover, the experiment study on a developed TSWH apparatus with composite PCMs was carried out to investigate the thermal
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performance of the composite PCMs. 2. Experimental 2.1. Materials Stearic acid (CH3(CH2)16COOH, SA) with a phase change temperature range of 56-69.9 °C was supplied by Tianjin Hengxing Chemical Reagent Co., Ltd., China.
Expanded graphite (EG) with a mean size of 300 mesh was also provided by that company. 2.2. Preparation of the SA/EG composites SA/EG composites were obtained by melting impregnation. EG was added into a 500 ml beaker with a certain amount of SA at room temperature. The mixture was
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stirred by a glass rod to ensure a homogeneous distribution of the composites and then
kept in a thermostatic water bath at 80 °C for about 1 hour until the SA was
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impregnated into EG matrix completely. After that, those composites with certain mass
as well as pure SA were pour in evacuated tubes one by one, and the equidistant
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helical-coil with a theromocouple was installed. In order to inquire the effects of
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different proportion of EG on the properties of composites, three kinds of samples with
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different EG content (2 wt.%, 6 wt.%, and 10 wt.%) marked SA/EG2, SA/EG6, and
2.3. Characterization
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SA/EG10 were prepared via the above method, respectively.
scanning
electron
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X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and microscopy (SEM)
were performed to
investigate the
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microstructure and morphology of the samples. The XRD analysis was carried out via Rigaku D/max-rA analyzer (Cu-Kα) at a step size of 0.02° in the scan range of 5-70°
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(with voltage of 40kV and current of 40 mA). FTIR spectra were obtained by using a Thermo Electron Corporation Nicolet 6700 FTIR spectrometer in the range of
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4000-400 cm-1. The samples powder was mixed with the KBr and the mixtures were pressed into pellets for FTIR measurements. The SEM images of samples were determined by a JEOL JSM-6360LV scanning electron microscopy. The differential scanning calorimetry (DSC; TA Instruments Q2000) analysis of the pure SA and SA/EG composites were carried out in the range of 20-80 °C, at a heating rate of 5 °C
min-1, under nitrogen flow at atmospheric pressure. The testing process was performed as follow. First, calibration of measuring equipment was carried out. Second, the DSC scan was operated by twice to ensure more accurate of the data. In the first scanning, the sample was sealed in an aluminum crucible and heated to 80°C, kept at this temperature for 5 min (made the contact between aluminum crucible and sample more closely, and
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fired the possible impurity in the sample), and cooled to 20°C. In the second scanning, the sample was heated and cooled again, and the second scan was recorded for
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determining the melting and freezing temperature and latent heat. The thermal conductivities of the pure SA and SA/EG composites at room temperature were
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investigated by steady-state heat method (DRX-II-RW Xiangtan Huafeng Instrument
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Manufacturing Co., Ltd., Hunan, China). The samples were pressure formed to wafer
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with a diameter of 12.5 mm and thickness of 1.2 mm, and then transient temperature
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response and heat transfer process uniformity of samples was measured through a thermal infrared imager (FLIR T62101).
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2.4. Evaluation on the thermal performance of proposed composite PCMs A tankless solar water heater (TSWH) was constructed for studying the thermal
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storage/release performance of the pure SA and SA/EG composites. Fig. 1 is the schematic illustration of the TSWH. The apparatus is mainly composed by a solar
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collector with four evacuated tubes, a data logger, and several flowmeters. The PCMs were placed inside evacuated tubes quipped with a equidistant helical-coil (shown in
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Fig. 1) which performs significantly effective among several heat transfer porpoises than straight pipes (Zachár, 2015). During the storage period, the solar energy was collected by absorber of the evacuated tubes then transferred for charging the pure SA or composites. The process of releasing was carried while valve was opened to pass the water through the heat
pipes for carrying off the heats gathered by the SA and composites. The temperature measurements of PCMs inside the evacuated tubes were recorded in 1-second time intervals by the eight-channel data logger. Besides, flowmeters were used to ensure consistent fluid flow rates. 3. Results and Discussions
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3.1. XRD analysis
The XRD patterns of the SA, EG and SA/EG composites with different contents
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of EG were illustrated in Fig. 2. The pattern of the SA displayed the peaks of 21.6°
and 24.0° caused by its regular crystallization. The reflection at 2θ=26.3° was
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characteristic peak of EG. As shown in Fig. 2, the XRD patterns of SA/EG composites
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contained all of the XRD peaks of SA and EG without appearance of new reflections,
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demonstrating that the SA has been successfully impregnated into the EG. There were
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no chemical interactions between the SA and the EG during the preparation of the composites.
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3.2. SEM analysis
Fig. 3 showed the SEM images of EG and SA/EG6. As can be seen in Fig. 3a and
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Fig. 3b, the EG has a worm-like microstructure with various pores, and these porous networks with high active surface increase the specific surface area of graphite (Wu et
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al., 2016). It makes a mass of liquid SA can be easily impregnated into those pores. As shown in Fig. 3c and Fig. 3d, the SA exhibited a homogeneous distribution without
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leaking owing to the capillary and surface tension force of EG. It is observed that the two phase interfaces between the EG matrix and SA combined compactly and there is no obvious phase separation showing a good compatibility of SA and EG. 3.3. FTIR analysis FTIR spectra of SA, EG and SA/EG composites were depicted in Fig. 4. From
the FTIR spectrum of SA, it is observed that the peaks at 2917 cm-1 and 2849 cm-1 represented the symmetrical stretching vibration of its –CH3 and –CH2 group, respectively. The absorption peak at 1701 cm-1 corresponded to the C=O stretching vibration. The absorption bands at 1471 cm-1 and 1295 cm-1 were ascribed to in-plane bending vibration of the -OH functional group in SA (Fang, Li, Chen, & Liu, 2010).
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The peak at 933 cm-1 and 719 cm-1 were assigned to the out-of-plane bending vibration and the in-planes winging vibration of the –OH functional group,
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respectively. The characteristic vibration peaks of the EG at 1631 cm-1 corresponded to the stretching vibration of the C=C functional group. The FTIR spectra of the
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SA/EG composites included the characteristic vibration peaks of SA and EG, and
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there were no significant new peaks observed, indicating that there was no chemical
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interaction between SA and EG. The results were in good consistent with the analysis
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of XRD patterns, and the interactions between SA and EG in the composites were mainly surface tension and capillary effect which had no impact on chemical structure
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of SA and EG (Yang et al., 2014). 3.4. Phase change behavior
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DSC thermal analysis was used to ascertain the phase change properties of pure SA and SA/EG composites. The DSC curves were shown in Fig. 5 and the phase
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change properties were presented in Table 1. The DSC curves of pure SA illustrated the melting and freezing temperature were 52.91 °C and 53.10 °C. Comparing with the
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pure SA, the melting and freezing temperature of SA/EG composites increased a little, resulting from the combine effects of both the interactions (surface tension forces and capillary forces) between melted SA and the pores of EG and the enhancement of thermal conductivity (Cheng et al., 2017; Wang et al., 2012a; Zhang, Tian, & Xiao, 2007). The phase change characteristic of three composite PCMs was similar to that of
SA, and further testified that there was no chemical reaction between pure SA and EG during the melting impregnation. Moreover, the change in melting and freezing temperature of SA in the composite PCMs was less than 1 °C, manifesting the addition of EG has negligible effect on the phase change temperature of SA. Thermal storage capacity is a significant reference index for evaluating the
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thermal storage performance of composite PCMs. The melting (ΔHm) and freezing (ΔHf) latent heats of pure SA were 189.7 J g-1 and 192.4 J g-1, respectively, in
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comparison, 171.9 J g-1 and 173.2 J g-1 for SA/EG2; 163.5 J g-1 and 167.3 J g-1 for SA/EG6; 154.2 J g-1 and 157.1J g-1 for SA/EG10, respectively. These values of SA/EG
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composites were obviously lower than the theoretical values. According to previous
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reports (Li et al., 2019; Li et al., 2017), the physical interactions between SA and
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supporting materials EG had effect to the crystal of SA, and this impact should be
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considered for the decrease of the latent heat of the composites apart from the low proportion of SA. It is believed that melted SA penetrated deeper into the porous
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network or distributed on the inner surfaces of matrix, causing a limitation of melt and crystallization (Atinafu et al., 2018). Thus, the theoretical latent heat value was less
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than that measured.
Crystallization ratio (Fc) (Li et al., 2019; Li, Xie, & Chen, 2017; Wang et al.,
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2012a; Wang et al., 2012b) is given by: Fc
H
composite
H
pure
100 %
(1)
Where ∆Hcomposite and ∆Hpure are the latent heat of the SA/EG composites and pure
SA, and β represents the mass ratio of the SA in the composites. As compared with the Fc of SA in the SA/EG6 (91.7%), that value of SA/EG2 and SA/EG10 was 0.8% higher and 1.4% less than that of SA/EG6, respectively. Previous researches have
demonstrated that there were disordered SA confined in the composites and ordered SA free in the composites, and the confined SA is unable to crystallize and fail to storage thermal energy (Li, Xie, & Chen, 2017; Li et al., 2017; Zhang et al., 2015). In addition, the difference between the Eef of SA in SA/EG2 and that of SA/FG6 was 1.5 J g−1, which was less than that between SA/EG6 and SA/EG10.
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Thermal conductivity is a crucial factor for representing the rate of energy storage
and release ability of PCMs. The SA as an organic PCM has a low thermal conductivity
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of 0.26 W m−1 K−1. As shown in Fig. 6, the thermal conductivity values were 0.75, 2.50, and 3.56 W m−1 K−1 for SA/EG2, SA/EG6, and SA/EG10, which were 2.9, 9.6, and 13.7
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times of that of pure SA, respectively. It indicated that the addition of EG can
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obviously improve the thermal conductivity of SA, this phenomenon can attributed to
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the porous structure and high thermal conductivity of EG (Zhang et al., 2018a). It is
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notable that the improvement of thermal conductivity is remarkable when the mass fraction of EG increase to 6% compared with that of others. Though the SA/EG10 has
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the largest thermal conductivity, the improvement of thermal conductivity is not remarkable compared with the SA/EG6.
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Additionally, the economics of the composite PCMs should be further considered. As a porous carbon mineral, the prices of EG is about 40 USD per kilogram is much
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higher than that of SA (about 0.8 USD per kilogram). So the cost of pure SA, SA/EG2, SA/EG6, and SA/EG10 are about 0.8, 1.6, 3.2, and 4.7 USD per kilogram, respectively.
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The SA/EG6 shows cheaper than the SA/EG10, revealing obvious advantages for TES application. Besides, the reduction of latent heat capability should be regarded as the mass fraction of EG changed from 6% to 10%. In general, the composite with 6wt.% of EG was more suitable for TSWH application compared to SA/EG10 because of larger heat capability, cheaper prices, and considerable thermal conductivity.
3.5. Transient temperature response and heat transfer process uniformity The transient temperature response behavior and the uniformity of heat transfer process were further demonstrated by the infrared thermal images of the pure SA and the SA/EG6 (shown in Fig.7). By pressing wafer and loaded into cylindrical molds, samples were kept away from leakage during heating and cooling. During heating
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phase, the pure SA and the SA/EG6 were placed on a hot platform with a constant
temperature of 65 °C. As shown in Fig.7a, the increase of average temperature of
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SA/EG6 was much faster than that of the pure SA once the samples were placed on the
platform, indicating SA/EG6 had a superior transient temperature response. The
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surface temperature of the SA/EG6 was always higher than that of the pure SA except
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when t＞110s while the samples approached the steady state. The center temperature
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of samples was also recorded. The difference between the center temperature and
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average temperature of the SA/EG6 (0.2 °C≤t≤1.6 °C) was smaller than that of the pure SA (0.4 °C≤t≤2.4 °C)，implying that the former own a higher rate of thermal
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diffusion than the latter. The outstanding transient temperature response and thermal diffusion performance of SA/EG6 can be indicative of its high thermal conductivity
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which enabling composite to absorb energy more rapidly during heating process. The
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infrared images (shown in Fig.7b) had a direct description of temperature rise stage by transform temperature information into color changes. After steady state, the samples were moved to a wood plate to cool down. The
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cooling curves and the infrared images were shown in Fig.7c and Fig.7d, respectively. It was observed that the average temperature of the pure SA and the SA/EG6 were 58.6 °C and 57.0 °C at t = 4 s. After a obviously drop, the temperature of samples approaching freezing temperature at t=22 s and the phase change occurs while the curves demonstrate a low drop rate. As time increased, the samples completed the
phase transition from liquid to solid, and began to releasing sensible heat. By compare with the average temperature as well as center temperature of the pure SA, it should be noted that SA/EG6 owns a superior behavior of temperature releasing. What’s more, the average value of the diffusion between the average and center temperature is 2.08 °C and 1.59 °C for the pure SA and SA/EG6, respectively. Similarly, temperature
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drop can also be directly seen in infrared camera images represented by the color
change from red to blue in Fig.7d. The surface temperature of SA/EG6 approaches to
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the ambient temperature with a high cooling rate and a superior homogeneous distribution of temperature (e.g. ∆tSA/EG6=1.3°C, ∆tSA=2.1°C at t=56 s). It can be
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concluded that the addition of EG enhance the heat release capability of composites as
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well as improve the uniformity of heat transfer process.
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3.6. Evaluation of the thermal performance of proposed composite PCMs
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Thermal storage and release performance of pure SA and SA/EG composites were assessed by the experiment conducted on TSWH. Fig.8 demonstrated the
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melting and releasing curves of four evacuated tube with SA, SA/EG2, SA/EG6 and SA/EG10, respectively. During the heating process shown in Fig. 8a, the melting time
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of pure SA (8150s) was almost 4 times than that of SA/EG10 (2177s), and that was 7375s and 2990s for SA/EG2 and SA/EG6, respectively. By comparing to the pure SA,
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the melting time of SA/EG2, SA/EG6, and SA/EG10 decreased 15.7%, 63.3%, and 72.3%, respectively. It is easy to draw that the reduction of melting time of SA/EG6
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was remarkable compared to others by increasing the mass fraction of EG to 6%. After the water valve was opening, it took 3482s, 1164s, 876s, and 1014s for pure SA, SA/EG2, SA/EG6, and SA/EG10 to reach the temperature of 35 °C. The SA/EG6 presented the best heat releasing performance among those composites, suggesting the storage heat can be transfer to the heating medium more quickly and efficiency. By
comparing the thermal storage and release performances of pure SA and those of SA/EG composites, it is obvious that the thermal storage and release times of SA/EG composites were reduced by the incorporating of EG. In addition, when the mass fraction of EG increase to 6%, the heat storage performance of composite was significantly improved and the heat release capability of it was superior to others.
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4. Conclusion
In this work, SA/EG composites with different mass fractions of EG were
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synthesized by absorbing SA into the EG under melting impregnation process, and the
thermophysical properties of the composites with different fractions of EG were
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discussed. It was found that there was no chemical interactions between SA and EG
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from XRD and FTIR results. The SEM confirmed that the SA was dispersed in the
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porous network of the EG by capillary and surface tension forces. From the DSC curves and thermal conductivity tests, while the mass percentage of the EG up to 6%,
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the thermal conductivity of composite reached to 2.5 W m−1 K−1 which was 9.6 times
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that of pure SA and the melting latent heat is 163.5 J g-1. Furthermore, the infrared images and heating/releasing curves manifested that the SA/EG6 with high thermal
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conductivity was definitely good at transient temperature response, and its uniformity of heat transfer process was improved compared to that of the pure SA. Then, the
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thermal performances of SA/EG composites were evaluated by integrating them into a developed TSWH. According to the experimental result, the addition of the EG can
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effective improve the thermal storage/release rate of the composites. For the composite with 6 wt.% EG, the melting time of it was 2990s which was 63.3% less than that of pure SA, and the heat release time (876s) was the shortest of all. What's more, the effective performance of integrating the SA/EG6 in the TSWH indicating that the composite with 6wt.% EG has a great potential application in TSWH system.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (51504041, 51874047); the Changsha City Fund for Distinguished and Innovative Young Scholars; the Fund for University Young Core Instructors of Hunan Province; the Natural Science Foundation of Hunan Province
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(2016JJ3009); the Key Research and Development Program of Jiangxi Province
(20171BBH80021); and the Hunan Province 2011 Collaborative Innovation Center of
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Clean Energy and Smart Grid.
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recent ten years. Sustainable Cities and Society, 43, 251-264.
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of shape-stabilized phase change materials embedded in building enclosure in
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Figure captions Fig. 1 Schematic illustration of the experimental apparatus. Fig. 2 XRD patterns of the SA, EG, and SA/EG composites. Fig. 3 SEM images of EG (a and b) and the EG/SA6 (c and d). Fig. 4 FTIR spectra of the SA, EG, and SA/EG composites.
Fig. 5 DSC curves of the pure SA and SA/EG composites. Fig. 6 Thermal conductivity of the pure SA and SA/EG composites. Fig. 7 (a) temperature response of pure SA and SA/EG6 during heating; (b) infrared camera images of pure SA and SA/EG6 at different heating times; (c) temperature response of pure SA and SA/EG6 during cooling; and (d) infrared
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camera images of pure SA and SA/EG6 at different cooling times. In the infrared images, the left sample is pure SA and the right sample is SA/EG6.
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Fig. 8 Temperature-time curves of the pure SA and SA/EG composites (a) storage
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process and (b) release process.
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Fig. 1 Schematic illustration of the experimental apparatus.
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Fig. 2 XRD patterns of the SA, EG, and SA/EG composites.
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Fig. 3 SEM images of EG (a and b) and the EG/SA6 (c and d).
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Fig. 4 FTIR spectra of the SA, EG, and SA/EG composites.
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Fig. 5 DSC curves of the pure SA and SA/EG composites.
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Fig. 6 Thermal conductivity of the pure SA and SA/EG composites.
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Fig. 7 (a) temperature response of pure SA and SA/EG6 during heating; (b) infrared
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camera images of pure SA and SA/EG6 at different heating times; (c) temperature response of pure SA and SA/EG6 during cooling; and (d) infrared camera images of
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pure SA and SA/EG6 at different cooling times. In the infrared images, the left sample
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is pure SA and the right sample is SA/EG6.
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Fig. 8 Temperature-time curves of the pure SA and SA/EG composites (a) storage
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process and (b) release process.
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Table captions
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Table 1 Thermal properties of pure SA, SA/FG, and SA/UGS composites.
Loada-ge (β, %)
Efficient
Melting
Freezing
Latent heat of Latent heat
temperature
temperature
melting
(Tm, ºC)
(Tf, ºC)
(∆Hm, J g-1)
Theoretic
of freezing values of ∆Hm (∆Hf, J g-1)
(∆Hth, J g-1)
Crystallinity of SA (Fc, %)
energy per unit mass of SA
100
52.91
53.10
189.7
192.4
-
100
-
SA/EG2
98
53.16
53.91
171.9
173.2
185.9
92.5
175.5
SA/EG6
94
53.51
53.63
163.5
167.3
178.3
91.7
174.0
90
53.28
53.89
154.2
157.1
170.7
90.3
171.4
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SA
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Sample
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Table 1 Phase change properties of pure SA and SA/EG composites
SA/EG10
(Eef, J g-1)
Note: ∆Hth = ∆Hpure × β; Eef = ∆Hpure × Fc
S2210-6707(18)31589-0 https://doi.org/10.1016/j.scs.2018.10.041 SCS 1317
To appear in: Received date: Revised date: Accepted date:
11-8-2018 27-10-2018 27-10-2018
Please cite this article as: Li C, Zhang B, Xie B, Zhao X, Chen J, Chen Z, Long Y, Stearic acid/expanded graphite as a composite phase change thermal energy storage material for tankless solar water heater, Sustainable Cities and Society (2018), https://doi.org/10.1016/j.scs.2018.10.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Stearic acid/expanded graphite as a composite phase change thermal energy storage material for tankless
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solar water heater
Chuanchang Li *a, Bo Zhang a, Baoshan Xie a, Xinbo Zhao a, Jian Chen *a,
a
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Zhongsheng Chen b, Yi Long c
School of Energy and Power Engineering, Changsha University of Science and
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Technology, Changsha410114, China. E-mail: [email protected];
State Key Laboratory Breeding Base of Nuclear Resources and Environment, East
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b
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[email protected]; Fax: +86-0731-85258409; Tel: +86-0731-85258409
c
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China Institute of Technology, Nanchang 330013, China School of Materials Science and Engineering, Nanyang Technological University, 50
Highlights
The stearic acid/expanded graphite composite (SA/EG6) had the melting and
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Nanyang Avenue, 639798, Singapore
freezing heats of 163.5 J g-1 and 167.3 J g-1, respectively;
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The infrared images manifested that the SA/EG6 was definitely good at transient temperature response; The experiment study on a developed TSWH apparatus with composites was carried out, suggesting that the SA/EG6 has a great potential application in TSWH system.
Abstract: Tankless solar water heater (TSWH) integrated phase change materials (PCMs) is a promising field due to its low cost, low heat loss, and compact structure. To obtain a composite PCMs that suitable for tankless solar water heater application, the expanded graphite (EG) in different mass fraction (2 wt.%, 6 wt.%, and 10 wt.%)
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were added to stearic acid (SA) via melting impregnation. The thermophysical
properties including phase change temperature, latent heat, and thermal conductivity
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of the SA/EG composites were characterized by DSC and thermal conductivity tests.
The melting and freezing heats of SA/EG6 composite were 163.5 J g-1 and 167.3 J g-1,
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respectively. The thermal conductivity of SA/EG6 was as high as 9.6 times that of
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pure SA. Meanwhile, the infrared images suggested the addition of EG enhance the
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heat release capability of composites as well as improve the uniformity of heat
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transfer process of SA/EG6. At last, a developed compact solar water heater was used to estimate the thermal performance of as-prepared composites. The experimental
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results shown that the melting time of SA/EG6 was shorten 63.3% compared with that of the SA and the heat releasing rate of SA/EG6 was the highest, suggesting the
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SA/EG6 composites can be applied in TSWH system. Keywords: stearic acid; expanded graphite; composite phase change material;
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tankless solar water heater
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1. Introduction
Nowadays, the energy dissipation and the environmental pollution are two critical
problems of the sustainable society, and how to utilize energy more efficient and cleaner has been paid largely attention (He et al., 2018; Yao et al., 2019; Zhu et al., 2018). One of the solutions is the thermal energy storage (TES) technology, which
stores excess solar energy or waste heat in a storage medium as internal energy for energy applications demanded in time or space (Reddy, Mudgal, & Mallick, 2018). Energy storage equipment with TES has great improvement in energy efficiency and stability. By using different method, TES is categorized into sensible heat storage, latent heat storage, and thermo chemical storage. The latent heat storage, which using
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phase change materials (PCMs) for energy storage or discharge, have received widespread attention due to its higher energy storage density and smaller temperature
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swing compared with other methods (Farid, Khudhair, Razack, & Al-Hallaj, 2004; Nkwetta, & Haghighat, 2014; Zhang, Xiao, & Ma, 2016). Thus, latent heat storage has
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been widely utilized in solar thermal utilization (Stritih et al., 2018), energy-efficiency
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building (Hasan, Basher, & Shdhan, 2018), industrial waste-heat utilization (Miró,
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Gasia, & Cabeza, 2016), and thermal management (Gao et al., 2018; Yuan et al.,
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2017).
Solar water heater (SWH) is one of the most common ways of solar thermal
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utilization and is receiving rapid tendency. Conventional SWH system consists of an array of solar collector panels and an insulated hot water storage tank, which use the
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sun or heat transfer fluid in a collector to heat water directly (Kee, Munusamy, & Ong, 2018). The storage tank size has to be fully large to store adequate heat because of the
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intermittency and uncertainty of solar radiation, which would make the SWH system more complex and expensive. Latent heat storage system integrated with PCMs is
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used in the SWH system to reduce the size of the storage tank and enhance the performance and reliability of thermal solar system by extending its operational time (Nkwetta, & Haghighat, 2014). The use of solar water heater incorporated with PCMs as TES has the potential to store heat energy during off-peak periods and to be re-used during peak periods (Kee, Munusamy, & Ong, 2018). A typical use of latent heat
storage system in solar energy utilization is tankless solar water heater (TSWH) which developed from conventional solar water heater with water tank. TSWH integrated latent heat storage is a compact solar water heater without a conventional water tank and the storing/releasing of thermal energy is achieved by the utilization of PCMs (Luu, Milani, Nomvar, & Abbas, 2017). The PCMs usually
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integrated in solar collector (e.g. evacuated tube (Feliński, & Sekret, 2017), shell, and tube container (Cao, Yuan, Xiang, & Haghighat, 2018) ) absorb heat and store it in the
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form of latent heat for later use. Due to the excellent chemical stability, high storage
density, and suitable phase change temperature of PCMs (Yang et al., 2018; Zeng et
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al., 2018), many researchers have attempted to do some experiment and simulation
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studies about TSWH integrated PCMs. To develop a high-efficiency TSWH integrated
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PCMs, a number of approaches have been adopted. A compact solar collector with an
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absorber plate integrated paraffin wax for heat storage media was designed by Mettawee et al. (Mettawee, & Assassa, 2006) and the experimental results shown that
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the low thermal conductivity of the solid paraffin had a negative effect on the heat transfer performances. Chen. et al. (Chen, Gu, & Peng, 2010) impregnated the paraffin
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into the aluminum foam and evaluated its performance on a solar flat-plate collector, demonstrating that aluminum foam has great effect on the heat transfer and melting
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rate of paraffin. Evacuated tubes, with a lowest heat loss coefficient because of their transparent insulation (vacuum), have been utilizing as a container of PCMs. An
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evacuated tube collector/storage system was developed by Feliński et al. (Feliński, & Sekret, 2016), wherein the paraffin was served as PCMs and heat pipes as heat exchange units. Li et al. (Li, & Zhai, 2017) presented a evacuated tube solar collector/storage
system
designed
for
mid-temperature
application:
the
erythritol/expanded graphite composite PCM (119 °C for melting temperature) was
filled into aluminum pipes and then placed inside evacuated tubes; the experimental results shown that the composite PCMs could effectively storage energy. Moreover, dynamic model of a domestic tankless solar water heater was presented and an annual simulation was carried out for the long-term operational feasibility studies (Luu, Milani, Nomvar, & Abbas, 2017). Considering the optimization of PCMs energy
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storage systems, the leakage and the thermophysical properties (including melting temperature, latent heat, and thermal conductivity) of solid-liquid PCMs employed in
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TSWH systems requires further development (Zhang et al., 2018b). PCMs with high latent heat, high thermal conductivity and suitable phase change temperature for
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household usage are desirable for TSWH system (Luu, Milani, Nomvar, & Abbas,
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2017; Sharif et al., 2015). Meanwhile, the inexpensive and effective methods to
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prepare suitable PCMs for TWSH should be developed as soon as possible (Zhang et
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al., 2018b).
So far, very little information related to the composite phase change materials
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which used in the TWSH system is available, and only researchers choose paraffin wax for experiment study in the field of TWSH. To obtain a high thermal performance
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composite PCMs used for TSWH, a facile and economic efficiency route to synthesize the shape-stabilized PCMs from the expanded graphite and stearic acid
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was presented in this article. Moreover, the experiment study on a developed TSWH apparatus with composite PCMs was carried out to investigate the thermal
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performance of the composite PCMs. 2. Experimental 2.1. Materials Stearic acid (CH3(CH2)16COOH, SA) with a phase change temperature range of 56-69.9 °C was supplied by Tianjin Hengxing Chemical Reagent Co., Ltd., China.
Expanded graphite (EG) with a mean size of 300 mesh was also provided by that company. 2.2. Preparation of the SA/EG composites SA/EG composites were obtained by melting impregnation. EG was added into a 500 ml beaker with a certain amount of SA at room temperature. The mixture was
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stirred by a glass rod to ensure a homogeneous distribution of the composites and then
kept in a thermostatic water bath at 80 °C for about 1 hour until the SA was
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impregnated into EG matrix completely. After that, those composites with certain mass
as well as pure SA were pour in evacuated tubes one by one, and the equidistant
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helical-coil with a theromocouple was installed. In order to inquire the effects of
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different proportion of EG on the properties of composites, three kinds of samples with
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different EG content (2 wt.%, 6 wt.%, and 10 wt.%) marked SA/EG2, SA/EG6, and
2.3. Characterization
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SA/EG10 were prepared via the above method, respectively.
scanning
electron
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X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and microscopy (SEM)
were performed to
investigate the
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microstructure and morphology of the samples. The XRD analysis was carried out via Rigaku D/max-rA analyzer (Cu-Kα) at a step size of 0.02° in the scan range of 5-70°
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(with voltage of 40kV and current of 40 mA). FTIR spectra were obtained by using a Thermo Electron Corporation Nicolet 6700 FTIR spectrometer in the range of
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4000-400 cm-1. The samples powder was mixed with the KBr and the mixtures were pressed into pellets for FTIR measurements. The SEM images of samples were determined by a JEOL JSM-6360LV scanning electron microscopy. The differential scanning calorimetry (DSC; TA Instruments Q2000) analysis of the pure SA and SA/EG composites were carried out in the range of 20-80 °C, at a heating rate of 5 °C
min-1, under nitrogen flow at atmospheric pressure. The testing process was performed as follow. First, calibration of measuring equipment was carried out. Second, the DSC scan was operated by twice to ensure more accurate of the data. In the first scanning, the sample was sealed in an aluminum crucible and heated to 80°C, kept at this temperature for 5 min (made the contact between aluminum crucible and sample more closely, and
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fired the possible impurity in the sample), and cooled to 20°C. In the second scanning, the sample was heated and cooled again, and the second scan was recorded for
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determining the melting and freezing temperature and latent heat. The thermal conductivities of the pure SA and SA/EG composites at room temperature were
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investigated by steady-state heat method (DRX-II-RW Xiangtan Huafeng Instrument
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Manufacturing Co., Ltd., Hunan, China). The samples were pressure formed to wafer
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with a diameter of 12.5 mm and thickness of 1.2 mm, and then transient temperature
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response and heat transfer process uniformity of samples was measured through a thermal infrared imager (FLIR T62101).
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2.4. Evaluation on the thermal performance of proposed composite PCMs A tankless solar water heater (TSWH) was constructed for studying the thermal
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storage/release performance of the pure SA and SA/EG composites. Fig. 1 is the schematic illustration of the TSWH. The apparatus is mainly composed by a solar
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collector with four evacuated tubes, a data logger, and several flowmeters. The PCMs were placed inside evacuated tubes quipped with a equidistant helical-coil (shown in
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Fig. 1) which performs significantly effective among several heat transfer porpoises than straight pipes (Zachár, 2015). During the storage period, the solar energy was collected by absorber of the evacuated tubes then transferred for charging the pure SA or composites. The process of releasing was carried while valve was opened to pass the water through the heat
pipes for carrying off the heats gathered by the SA and composites. The temperature measurements of PCMs inside the evacuated tubes were recorded in 1-second time intervals by the eight-channel data logger. Besides, flowmeters were used to ensure consistent fluid flow rates. 3. Results and Discussions
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3.1. XRD analysis
The XRD patterns of the SA, EG and SA/EG composites with different contents
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of EG were illustrated in Fig. 2. The pattern of the SA displayed the peaks of 21.6°
and 24.0° caused by its regular crystallization. The reflection at 2θ=26.3° was
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characteristic peak of EG. As shown in Fig. 2, the XRD patterns of SA/EG composites
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contained all of the XRD peaks of SA and EG without appearance of new reflections,
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demonstrating that the SA has been successfully impregnated into the EG. There were
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no chemical interactions between the SA and the EG during the preparation of the composites.
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3.2. SEM analysis
Fig. 3 showed the SEM images of EG and SA/EG6. As can be seen in Fig. 3a and
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Fig. 3b, the EG has a worm-like microstructure with various pores, and these porous networks with high active surface increase the specific surface area of graphite (Wu et
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al., 2016). It makes a mass of liquid SA can be easily impregnated into those pores. As shown in Fig. 3c and Fig. 3d, the SA exhibited a homogeneous distribution without
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leaking owing to the capillary and surface tension force of EG. It is observed that the two phase interfaces between the EG matrix and SA combined compactly and there is no obvious phase separation showing a good compatibility of SA and EG. 3.3. FTIR analysis FTIR spectra of SA, EG and SA/EG composites were depicted in Fig. 4. From
the FTIR spectrum of SA, it is observed that the peaks at 2917 cm-1 and 2849 cm-1 represented the symmetrical stretching vibration of its –CH3 and –CH2 group, respectively. The absorption peak at 1701 cm-1 corresponded to the C=O stretching vibration. The absorption bands at 1471 cm-1 and 1295 cm-1 were ascribed to in-plane bending vibration of the -OH functional group in SA (Fang, Li, Chen, & Liu, 2010).
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The peak at 933 cm-1 and 719 cm-1 were assigned to the out-of-plane bending vibration and the in-planes winging vibration of the –OH functional group,
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respectively. The characteristic vibration peaks of the EG at 1631 cm-1 corresponded to the stretching vibration of the C=C functional group. The FTIR spectra of the
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SA/EG composites included the characteristic vibration peaks of SA and EG, and
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there were no significant new peaks observed, indicating that there was no chemical
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interaction between SA and EG. The results were in good consistent with the analysis
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of XRD patterns, and the interactions between SA and EG in the composites were mainly surface tension and capillary effect which had no impact on chemical structure
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of SA and EG (Yang et al., 2014). 3.4. Phase change behavior
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DSC thermal analysis was used to ascertain the phase change properties of pure SA and SA/EG composites. The DSC curves were shown in Fig. 5 and the phase
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change properties were presented in Table 1. The DSC curves of pure SA illustrated the melting and freezing temperature were 52.91 °C and 53.10 °C. Comparing with the
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pure SA, the melting and freezing temperature of SA/EG composites increased a little, resulting from the combine effects of both the interactions (surface tension forces and capillary forces) between melted SA and the pores of EG and the enhancement of thermal conductivity (Cheng et al., 2017; Wang et al., 2012a; Zhang, Tian, & Xiao, 2007). The phase change characteristic of three composite PCMs was similar to that of
SA, and further testified that there was no chemical reaction between pure SA and EG during the melting impregnation. Moreover, the change in melting and freezing temperature of SA in the composite PCMs was less than 1 °C, manifesting the addition of EG has negligible effect on the phase change temperature of SA. Thermal storage capacity is a significant reference index for evaluating the
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thermal storage performance of composite PCMs. The melting (ΔHm) and freezing (ΔHf) latent heats of pure SA were 189.7 J g-1 and 192.4 J g-1, respectively, in
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comparison, 171.9 J g-1 and 173.2 J g-1 for SA/EG2; 163.5 J g-1 and 167.3 J g-1 for SA/EG6; 154.2 J g-1 and 157.1J g-1 for SA/EG10, respectively. These values of SA/EG
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composites were obviously lower than the theoretical values. According to previous
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reports (Li et al., 2019; Li et al., 2017), the physical interactions between SA and
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supporting materials EG had effect to the crystal of SA, and this impact should be
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considered for the decrease of the latent heat of the composites apart from the low proportion of SA. It is believed that melted SA penetrated deeper into the porous
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network or distributed on the inner surfaces of matrix, causing a limitation of melt and crystallization (Atinafu et al., 2018). Thus, the theoretical latent heat value was less
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than that measured.
Crystallization ratio (Fc) (Li et al., 2019; Li, Xie, & Chen, 2017; Wang et al.,
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2012a; Wang et al., 2012b) is given by: Fc
H
composite
H
pure
100 %
(1)
Where ∆Hcomposite and ∆Hpure are the latent heat of the SA/EG composites and pure
SA, and β represents the mass ratio of the SA in the composites. As compared with the Fc of SA in the SA/EG6 (91.7%), that value of SA/EG2 and SA/EG10 was 0.8% higher and 1.4% less than that of SA/EG6, respectively. Previous researches have
demonstrated that there were disordered SA confined in the composites and ordered SA free in the composites, and the confined SA is unable to crystallize and fail to storage thermal energy (Li, Xie, & Chen, 2017; Li et al., 2017; Zhang et al., 2015). In addition, the difference between the Eef of SA in SA/EG2 and that of SA/FG6 was 1.5 J g−1, which was less than that between SA/EG6 and SA/EG10.
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Thermal conductivity is a crucial factor for representing the rate of energy storage
and release ability of PCMs. The SA as an organic PCM has a low thermal conductivity
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of 0.26 W m−1 K−1. As shown in Fig. 6, the thermal conductivity values were 0.75, 2.50, and 3.56 W m−1 K−1 for SA/EG2, SA/EG6, and SA/EG10, which were 2.9, 9.6, and 13.7
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times of that of pure SA, respectively. It indicated that the addition of EG can
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obviously improve the thermal conductivity of SA, this phenomenon can attributed to
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the porous structure and high thermal conductivity of EG (Zhang et al., 2018a). It is
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notable that the improvement of thermal conductivity is remarkable when the mass fraction of EG increase to 6% compared with that of others. Though the SA/EG10 has
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the largest thermal conductivity, the improvement of thermal conductivity is not remarkable compared with the SA/EG6.
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Additionally, the economics of the composite PCMs should be further considered. As a porous carbon mineral, the prices of EG is about 40 USD per kilogram is much
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higher than that of SA (about 0.8 USD per kilogram). So the cost of pure SA, SA/EG2, SA/EG6, and SA/EG10 are about 0.8, 1.6, 3.2, and 4.7 USD per kilogram, respectively.
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The SA/EG6 shows cheaper than the SA/EG10, revealing obvious advantages for TES application. Besides, the reduction of latent heat capability should be regarded as the mass fraction of EG changed from 6% to 10%. In general, the composite with 6wt.% of EG was more suitable for TSWH application compared to SA/EG10 because of larger heat capability, cheaper prices, and considerable thermal conductivity.
3.5. Transient temperature response and heat transfer process uniformity The transient temperature response behavior and the uniformity of heat transfer process were further demonstrated by the infrared thermal images of the pure SA and the SA/EG6 (shown in Fig.7). By pressing wafer and loaded into cylindrical molds, samples were kept away from leakage during heating and cooling. During heating
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phase, the pure SA and the SA/EG6 were placed on a hot platform with a constant
temperature of 65 °C. As shown in Fig.7a, the increase of average temperature of
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SA/EG6 was much faster than that of the pure SA once the samples were placed on the
platform, indicating SA/EG6 had a superior transient temperature response. The
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surface temperature of the SA/EG6 was always higher than that of the pure SA except
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when t＞110s while the samples approached the steady state. The center temperature
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of samples was also recorded. The difference between the center temperature and
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average temperature of the SA/EG6 (0.2 °C≤t≤1.6 °C) was smaller than that of the pure SA (0.4 °C≤t≤2.4 °C)，implying that the former own a higher rate of thermal
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diffusion than the latter. The outstanding transient temperature response and thermal diffusion performance of SA/EG6 can be indicative of its high thermal conductivity
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which enabling composite to absorb energy more rapidly during heating process. The
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infrared images (shown in Fig.7b) had a direct description of temperature rise stage by transform temperature information into color changes. After steady state, the samples were moved to a wood plate to cool down. The
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cooling curves and the infrared images were shown in Fig.7c and Fig.7d, respectively. It was observed that the average temperature of the pure SA and the SA/EG6 were 58.6 °C and 57.0 °C at t = 4 s. After a obviously drop, the temperature of samples approaching freezing temperature at t=22 s and the phase change occurs while the curves demonstrate a low drop rate. As time increased, the samples completed the
phase transition from liquid to solid, and began to releasing sensible heat. By compare with the average temperature as well as center temperature of the pure SA, it should be noted that SA/EG6 owns a superior behavior of temperature releasing. What’s more, the average value of the diffusion between the average and center temperature is 2.08 °C and 1.59 °C for the pure SA and SA/EG6, respectively. Similarly, temperature
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drop can also be directly seen in infrared camera images represented by the color
change from red to blue in Fig.7d. The surface temperature of SA/EG6 approaches to
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the ambient temperature with a high cooling rate and a superior homogeneous distribution of temperature (e.g. ∆tSA/EG6=1.3°C, ∆tSA=2.1°C at t=56 s). It can be
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concluded that the addition of EG enhance the heat release capability of composites as
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well as improve the uniformity of heat transfer process.
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3.6. Evaluation of the thermal performance of proposed composite PCMs
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Thermal storage and release performance of pure SA and SA/EG composites were assessed by the experiment conducted on TSWH. Fig.8 demonstrated the
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melting and releasing curves of four evacuated tube with SA, SA/EG2, SA/EG6 and SA/EG10, respectively. During the heating process shown in Fig. 8a, the melting time
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of pure SA (8150s) was almost 4 times than that of SA/EG10 (2177s), and that was 7375s and 2990s for SA/EG2 and SA/EG6, respectively. By comparing to the pure SA,
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the melting time of SA/EG2, SA/EG6, and SA/EG10 decreased 15.7%, 63.3%, and 72.3%, respectively. It is easy to draw that the reduction of melting time of SA/EG6
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was remarkable compared to others by increasing the mass fraction of EG to 6%. After the water valve was opening, it took 3482s, 1164s, 876s, and 1014s for pure SA, SA/EG2, SA/EG6, and SA/EG10 to reach the temperature of 35 °C. The SA/EG6 presented the best heat releasing performance among those composites, suggesting the storage heat can be transfer to the heating medium more quickly and efficiency. By
comparing the thermal storage and release performances of pure SA and those of SA/EG composites, it is obvious that the thermal storage and release times of SA/EG composites were reduced by the incorporating of EG. In addition, when the mass fraction of EG increase to 6%, the heat storage performance of composite was significantly improved and the heat release capability of it was superior to others.
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4. Conclusion
In this work, SA/EG composites with different mass fractions of EG were
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synthesized by absorbing SA into the EG under melting impregnation process, and the
thermophysical properties of the composites with different fractions of EG were
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discussed. It was found that there was no chemical interactions between SA and EG
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from XRD and FTIR results. The SEM confirmed that the SA was dispersed in the
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porous network of the EG by capillary and surface tension forces. From the DSC curves and thermal conductivity tests, while the mass percentage of the EG up to 6%,
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the thermal conductivity of composite reached to 2.5 W m−1 K−1 which was 9.6 times
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that of pure SA and the melting latent heat is 163.5 J g-1. Furthermore, the infrared images and heating/releasing curves manifested that the SA/EG6 with high thermal
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conductivity was definitely good at transient temperature response, and its uniformity of heat transfer process was improved compared to that of the pure SA. Then, the
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thermal performances of SA/EG composites were evaluated by integrating them into a developed TSWH. According to the experimental result, the addition of the EG can
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effective improve the thermal storage/release rate of the composites. For the composite with 6 wt.% EG, the melting time of it was 2990s which was 63.3% less than that of pure SA, and the heat release time (876s) was the shortest of all. What's more, the effective performance of integrating the SA/EG6 in the TSWH indicating that the composite with 6wt.% EG has a great potential application in TSWH system.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (51504041, 51874047); the Changsha City Fund for Distinguished and Innovative Young Scholars; the Fund for University Young Core Instructors of Hunan Province; the Natural Science Foundation of Hunan Province
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(2016JJ3009); the Key Research and Development Program of Jiangxi Province
(20171BBH80021); and the Hunan Province 2011 Collaborative Innovation Center of
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Clean Energy and Smart Grid.
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Figure captions Fig. 1 Schematic illustration of the experimental apparatus. Fig. 2 XRD patterns of the SA, EG, and SA/EG composites. Fig. 3 SEM images of EG (a and b) and the EG/SA6 (c and d). Fig. 4 FTIR spectra of the SA, EG, and SA/EG composites.
Fig. 5 DSC curves of the pure SA and SA/EG composites. Fig. 6 Thermal conductivity of the pure SA and SA/EG composites. Fig. 7 (a) temperature response of pure SA and SA/EG6 during heating; (b) infrared camera images of pure SA and SA/EG6 at different heating times; (c) temperature response of pure SA and SA/EG6 during cooling; and (d) infrared
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camera images of pure SA and SA/EG6 at different cooling times. In the infrared images, the left sample is pure SA and the right sample is SA/EG6.
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Fig. 8 Temperature-time curves of the pure SA and SA/EG composites (a) storage
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process and (b) release process.
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Fig. 1 Schematic illustration of the experimental apparatus.
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Fig. 2 XRD patterns of the SA, EG, and SA/EG composites.
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Fig. 3 SEM images of EG (a and b) and the EG/SA6 (c and d).
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Fig. 4 FTIR spectra of the SA, EG, and SA/EG composites.
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Fig. 5 DSC curves of the pure SA and SA/EG composites.
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Fig. 6 Thermal conductivity of the pure SA and SA/EG composites.
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Fig. 7 (a) temperature response of pure SA and SA/EG6 during heating; (b) infrared
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camera images of pure SA and SA/EG6 at different heating times; (c) temperature response of pure SA and SA/EG6 during cooling; and (d) infrared camera images of
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pure SA and SA/EG6 at different cooling times. In the infrared images, the left sample
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is pure SA and the right sample is SA/EG6.
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Fig. 8 Temperature-time curves of the pure SA and SA/EG composites (a) storage
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process and (b) release process.
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Table captions
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Table 1 Thermal properties of pure SA, SA/FG, and SA/UGS composites.
Loada-ge (β, %)
Efficient
Melting
Freezing
Latent heat of Latent heat
temperature
temperature
melting
(Tm, ºC)
(Tf, ºC)
(∆Hm, J g-1)
Theoretic
of freezing values of ∆Hm (∆Hf, J g-1)
(∆Hth, J g-1)
Crystallinity of SA (Fc, %)
energy per unit mass of SA
100
52.91
53.10
189.7
192.4
-
100
-
SA/EG2
98
53.16
53.91
171.9
173.2
185.9
92.5
175.5
SA/EG6
94
53.51
53.63
163.5
167.3
178.3
91.7
174.0
90
53.28
53.89
154.2
157.1
170.7
90.3
171.4
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SA
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Sample
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Table 1 Phase change properties of pure SA and SA/EG composites
SA/EG10
(Eef, J g-1)
Note: ∆Hth = ∆Hpure × β; Eef = ∆Hpure × Fc