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expanded graphite shape-stabilized composite phase change material for electric radiant floor heating
Applied Thermal Engineering 150 (2019) 1177–1185
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Thermal properties enhancement and application of a novel sodium acetate trihydrate-formamide/expanded graphite shape-stabilized composite phase change material for electric radiant floor heating
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Yutang Fang , Yifan Ding, Yufeng Tang, Xianghui Liang, Ce Jin, Shuangfeng Wang, Xuenong Gao, Zhengguo Zhang Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou 510640, China
H I GH L IG H T S
EG-based CPCM using SAT-FA eutectic mixture as the PCM was prepared. • AThenovel CPCM under EG owned good nucleating behavior and thermal reliability. • Thermal conductivity of the CPCM could be significantly enhanced by adding EG. • The electric radiant floor heating system with the CPCM showed great thermal performance. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Eutectic mixture Expanded graphite Composite phase change material Shape stability Electric radiant floor heating
Electric radiant floor heating system (ERFHS) with hydrate salt phase change material (PCM) as thermal storage medium owns the advantages of improving indoor comfort with high energy efficiency and favorable economic applicability. In this paper, based on sodium acetate trihydrate (SAT)-formamide (FA) eutectic mixture as PCM and expanded graphite (EG) as supporting carrier, a novel SAT-FA/EG composite PCM (CPCM) for ERFHS was prepared by physical blending method. The shape stability, thermal properties and thermal reliability of the SAT-FA eutectic mixture under EG were emphatically discussed. The heat storage and release performances of a simulation room established by ERFHS integrated with such CPCM were investigated. Experimental results showed that the SAT-FA/EG composite containing 8% EG displayed high phase change enthalpy (187.6 kJ/kg), suitable phase change temperature (38.54 °C), negligible supercooling degree (0.83 °C) and eminent thermal conductivity (3.11 W/m·K), along with the excellent shape stability and thermal reliability. The simulation showed that the ERFHS with CPCM layer presented smaller indoor operative temperature fluctuation in vertical orientation and longer total thermal comfort time (12.65 h), which greatly exceeds that of the one without CPCM layer (1.836 h). All the superior characters make the obtained SAT-FA/EG composite a promising candidate for ERFHS.
1. Introduction Tremendous energy consumption along with low utilization efficiency is always a main concern in the industrial economy. Particularly, the consumption accounted in building sector has reached definitely around 40% in overall energy consumption and it is expected to increase by 28% in 2035 [1,2], mainly for the extensive installment of electric regulated temperature facilities such as heating ventilating, air conditioning and floor heating system. Such circumstance, as well as the conventional energy crisis, have promoted the technological
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innovation to implement better energy utilization efficiency, thereby reducing consumption. The thermal energy storage (TES), which is obtained by heating, cooling, solidifying, melting, or vaporizing a certain material along with the energy becoming available, has been confirmed to be an environment-friendly energy-saving technology in building [3]. Accordingly, phase change material (PCM), which absorb and release latent heat by experiencing solid-liquid phase transition, has attracted wide interests and extensive researches in TES application, for its high energy storage capacity with a narrow temperature change range, slight volume change during phase transformation and
Corresponding author. E-mail address: [email protected] (Y. Fang).
https://doi.org/10.1016/j.applthermaleng.2019.01.069 Received 1 August 2018; Received in revised form 29 December 2018; Accepted 21 January 2019 Available online 29 January 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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enhancement of thermal conductivity for PCM could remarkably improve the energy efficiency of ERFHS and reduce the cost of insulating materials [21,22]. A simple and effective way is to fabricate a formstable composite by embedding the PCM into porous framework materials which possess high thermal conductivity, such as expanded graphite (EG) [23,24] and boron nitride [25,26], etc. The materials are predicted to provide a mechanical strength and generate capillary force to alleviate the leakage of melted PCM from porous structure, ultimately to obtain a form-stable composite which also owns conveniences on engineering fabrication and customization for special application [27,28]. Expanded graphite (EG) is a loose and porous worm-like graphitic carrier with cheap price. Many studies have prepared the EGbased composite PCM due to its good thermal conductivity, long-term thermal reliability and the function of as heterogeneous nucleating agent to reduce the supercooling degree of salt hydrates [29–32]. Gathering above superiorities it is undisputed that EG was a multifunctional additive for PCM to obtain better practical performance. Herein this paper, the SAT/FA pseudo-binary eutectic mixture selected as base PCM and EG selected as thermal conductivity enhancer, form-stable carrier as well as nucleating promoter were composited by employing a physical mixing method. Firstly, the most favorable adsorption capacity of EG was determined by comparing the melting behavior and supercooling degree of all samples. The following measurements including heat transfer property, morphology and thermal reliability of the obtained SAT-FA/EG CPCM were carried out. Finally, a simulation room established by ERFHS integrated with such CPCM was analyzed by monitoring the indoor air temperature fluctuation and counting the thermal comfort time. The results not only clearly demonstrated the great application potential of the CPCM for ERFHS but also enriched the preparing and applicating method of such similar materials for building.
excellent availability in abundance [4]. Such merits of PCM are particularly favorable for PCM to be applied in building field, while floor and wall could provide enough heat exchange areas which can maximally utilize the thermal properties of PCM. Recent decades, researchers and engineers have focused on the integration of PCM into building elements such as concrete [5], bricks [6] and wallboard [7], to obtain better energy utilization efficiency without compromising the requirements of indoor comfort. Electric radiant floor heating system (ERFHS), a low temperature heating system, has been recognized as an economically efficient preference in residential and commercial building. Such system saves space and produce no noise, meanwhile it can provide uniform temperature distribution which is beneficial for improving thermal comfort degree [8]. Thermal mass material combined with ERFHS is often adapted to storage off-peak electricity source. In such way the peak load could be cut down and transferred to nighttime since electricity price is lower [9]. Conventionally the dense material such as concrete are adapted as thermal mass, but nowadays more researchers are seeking the possibility of using PCM to replace the traditional material. This is because ERFHS integrated with PCM could cause smaller indoor temperature fluctuation and longer thermal comfort time in the energy-saving method. Barrio et al. [10] studied the performance of the ERFHS with neopentyl glycol PCM floor. Results showed the temperature fluctuation at inner and surface was 3 times smaller and the charging time was 2.8 h longer, than those of sand land. Barzin et al. [11] designed a underfloor heating system combined with commercialized PCM wallboards in a price-based operating method. Experimental results showed the total energy saving and electrical cost saving in five days came up to 18.8% and 28.7%, respectively. Various studies have illustrated the significant improvement on energy efficiency and cost reduction of the PCM-combined ERFHS if the suitable PCM was selected. When evaluating the feasibility of a certain PCM applied in building TES, its thermophysical, kinetic and chemical properties should be taken into consideration [12]. Favorable characters of salt hydrates including high volumetric energy storage capacity, high thermal conductivity, nontoxicity and noninflammability, make them attractive PCMs candidates for TES in building. Salt hydrates are commonly modified by adding the nucleating agents and thickening agent to alleviate defects of great supercooling and phase separation which are caused by its coherent poor nucleating ability and incongruent melting behavior respectively [13,14]. But the multiple additives are difficult to control in an accurate way and may reduce the latent heat of salt hydrate. In order to reduce additives some scholars have focused on seizing the binary or multicomponent eutectic mixtures in which the components could undergo phase-change at the same temperature. The eutectic mixtures possess congruent melting behavior and great thermal reliability [15]. Many studies have adapted eutectic mixtures as PCM to obtain better thermal reliability performance such as auric acid-stearic acid eutectic [16] and MgCl2·6H2O-Mg(NO3)2·6H2O eutectic [17]. We are inspired that it is feasible to obtain the superior thermal reliability by preparing a eutectic salt hydrate, meanwhile possessing the suitable thermal properties which are consistent with the requires for ERFHS. In an earlier study, Takahiro M et al. [18] investigated the pseudobinary system of CH3COONa·3H2O-HCONH2 (sodium acetate trihydrate-formamide, SAT-FA). It was found that when the FA mass fraction was 25%, the SAT-FA pseudo-binary eutectic mixture was formed with congruent melting point of 40.5 °C. The desirable melting behavior is precisely required for ERFHS [19]. SAT is a nontoxic and uninflammable inorganic salt and owns a relatively high heat fusion (264 kJ/kg) with moderate price [20], which makes it applicable for generous operation as building materials. Concerning the above merits of eutectic mixture and SAT, it is expected that the SAT-FA eutectic mixture could be a potential PCM candidate for ERFHS. Nevertheless, the thermal conductivity of the eutectic mixture needs to be improved in order to accelerate heat transfer rate during the phase change process, according to the certified conclusion that the moderate
2. Experiment 2.1. Materials Sodium acetate trihydrate (SAT, CH3COONa·3H2O, AR), Formamide (FA, HCONH2, AR) and Disodium phosphate dodecahydrate (DSP, Na2HPO4·12H2O, AR) were purchased from Tianjin Kemious Chemical Reagent Co., Ltd. Expandable graphite powder (50 mesh, expansion rate of 300 mL/g) was obtained from Qingdao Modou Graphite Materials Manufacturing Co., Ltd. 2.2. Preparation of SAT-FA/EG composite 2.2.1. Preparation of EG Graphite powder was expanded using microwave method for its convenience on operation, uniform expansion and high energy efficiency [33]. The powder was firstly dried at 65 °C in vacuum oven for 24 h and then exposed to microwave radiation at 800 W for 30 s. After being cooled to room-temperature the expanded graphite (EG) with high porosity was obtained. The N2 adsorption isotherm of the prepared EG was executed at 77 K adapting an ASAP 2460 porosity analyzer (Micromeritics) to identify its microporous and mesoporous structure. The results of adsorption isotherm and pore size distribution (insert) were shown in Fig. 1. According to the numerical calculation by density functional theory (DFT), the BET surface area of the prepared EG was 53.13 m2/g, of which the mesopore surface area was 34.13 m2/g. The curve of pore size distribution suggested the pore diameter was mainly in the range of 2–50 nm. The great increase of N2 adsorption quantity at the relative pressure range from 0.5 to 1.0 along with a hysteresis behavior of desorption also indicated the abundant mesopore porosity. It was predicted that EG would not only provide strong capillary force to adsorb melted mixture, but also provide generous heterogeneous nucleation sites for salt hydrate to reduce supercooling. 1178
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properties of the EM were measured by DSC (see Fig. 2). Obviously, a single strong peak with the phase change temperature of 40.88 °C and phase change enthalpy of 233.9 kJ/kg was observed in the curve of SAT-FA (c), which was located between the specific peaks of pure FA (a) and SAT (b). It meant the pseudo-binary eutectic mixture with high latent heat was formed. 2.2.3. Preparation of SAT-FA/EG composite A simple blending method was used to prepare SAT-FA/EG composite phase change material (CPCM). Firstly, 20 g SAT-FA solid eutectic mixture placed in glass bottle was completely melted by continuous stirring in water bath of 50 °C. Then the extra EG of 3%, 5%, 8%, 10% was added into the bottles, respectively. Secondly, the mixtures were mechanically stirred in sealed condition for 4 h and then transferred it to a thermostat of 20 °C. The novel solid SAT-FA/EG CPCM finally was obtained. Fig. 1. N2 adsorption isotherm and pore size distribution (insert) of prepared EG.
2.3. Characterization of SAT-FA/EG composite The phase change behaviors of the samples with different mass fraction of EG were investigated using a Q20 differential scanning calorimeter (DSC, TA Instrument Inc., USA) at a heating rate of 5 °C·min−1 under nitrogen atmosphere, with an accuracy within ± 1%. The measurements of thermal conductivities were carried out using a TPS2500 thermal constant analyzer (Hot Disk, Sweden) and transient plane source method within the accuracy of ± 3% and the reproductivity of ± 1%. The experiment set-up and samples arrangement were displayed in Fig. 3. Firstly, the composites were molded into two identical cylindrical blocks (Φ 40 × 10 mm) with the density of 1000 kg/m3 and then a testing sensor (Type 7577) with radius of 2.01 mm was sandwiched between the two samples. Each measurement was repeatedly carried out for 3 times to reduce experimental error. The shape stable behaviors of samples were observed by a DM2500P polarizing microscope (POM, Leica, Germany) and the morphologies were photographed at 20 °C, 40 °C and 60 °C after isothermal heat treatment for 5 min respectively. A S-3700N scanning electron microscope (SEM, HITACHI, Japan) was employed to observe the morphologies of samples with scanning voltage of 15 kv. Considering the practical application of the simulation room established by ERFHS, the melting and crystallization temperatures of samples were set at 45 °C and 30 °C respectively by referring to the initial temperature and stop temperature of the electric heating film. In this condition the heat storage and exothermic processes of samples were investigated. For the melting process, the sealed glass bottle loaded with 20 g solid CPCM was placed in a preset incubator of 45 °C to absorb heat. For the heat release experiment, the above completely melted sample was naturally cooled and crystallized to release heat in a preset incubator of 30 °C. During the entire process a K-type thermocouple with the accuracy of ± 0.1 °C was inserted into the sample center to collect the temperature data, which was logged by an Agilent
Fig. 2. DSC curves of FA (a), SAT (b) and SAT-FA EM (c).
2.2.2. Preparation of SAT-FA eutectic mixture According to our earlier research work [34], SAT-FA eutectic mixture was prepared by melt-blending method. Mixtures of 75% SAT-FA (mass fraction, the same below, 15 g) and 25% FA (5 g) and additional 2% DSP (0.4 g) as nucleating agent were vigorously stirred in a water bath of 60 °C until completely melted. After continuingly stirring for 0.5 h and a successively cooling treatment in a thermostat of 20 °C, the solid SAT-FA eutectic mixture (EM) was finally obtained. The thermal
Fig. 3. The schematic diagram for thermal conductivity measurement. 1179
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Fig. 4. Appearance of simulation room (a), CPCM layer (b), floor construction profile and temperature collection point (c). Table 1 DSC data of SAT-FA/EG CPCM with different EG mass fraction. Mass fraction of EG (%)
Ti (°C)
Tm (°C)
ΔHm (kJ·kg−1)
0 3 5 8 10
38.88 36.36 33.46 29.07 29.98
40.88 40.29 40.68 37.54 37.7
233.9 226.3 215.8 187.6 177
Fig. 6. Thermal conductivities of SAT-FA/EG CPCMs with different EG mass fraction.
cooling treatment of 90 min. Above temperature procedure was the single thermal cycle. After 100, 200, 300 and 400 thermal cycles, the followed DSC measurements and the supercooling experiments were carried out respectively. 2.4. Simulation room test for ERFHS with SAT-FA/EG composite For the purpose of examining the potential application performance of the prepared SAT-FA/EG composite in real working environment, a simulation room established by ERFHS with CPCM as heat-transfer medium was built. As shown in Fig. 4(a), the room was a cubic model with the size of 1.15 × 1.15 × 1.30 m (inner size: 1.10 × 1.10 × 1.10 m). The walls and ceiling of the room were sealed with the adiabatic polystyrene foam board, and its bottom was equipped with multilevel floor which was assembled by insulation layer, electric heating film, PCM layer and wood floor in a bottom-to-up sequence. Although porous EG has a shape stable effect on SAT-FA eutectic mixture, it cannot completely eliminate the leakage of water
Fig. 5. DSC curves (a) and step cooling curves (b) of SAT-FA/EG CPCM with different EG mass fraction.
34970A data acquisition instrument. The thermal reliability of the sample was estimated by accelerated thermal cycling experiment in a WHH-080 constant temperature chamber (Dongguan Weihuang Instrument Co., Ltd., China). The sample was placed in the chamber to be thermostatically heated at 50 °C for 90 min and then the temperature was changed to 15 °C for a
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Fig. 7. Melting (a) and freezing curves (b) of SAT-FA EM and SAT-FA/EG CPCM.
comfort as the standard, the heating time (th) and cooling time (tc) were used to measure the heat storage capacity and heat release ability respectively. The total duration (td) from 20 °C to 26 °C, namely the sum of th and tc, was marked as the parameter to indicate the ability of maintaining thermal comfort. 3. Results and discussion 3.1. Phase change behavior and nucleation promotion under EG Fig. 5(a) showed DSC curves of SAT-FA/EG CPCM with different EG mass fraction and the test results were tabulated in Table 1. With the increase of EG mass fraction, the initial melting temperature (Ti) of the CPCM decreased and the melting peak became smoother along with an acceptable decline of phase change temperature (Tm) and enthalpy (ΔHm). The decrease of ΔHm was caused by the addition of EG which did not undergo phase change. Meanwhile, the porous EG generated capillary force with the molecules of SAT, water and FA, meanwhile weakened the inter-molecular force among eutectic mixture, consequently resulting in a decline of Ti and Tm. When EG mass fraction was 3–5%, the peaks of the samples presented partly incongruent melting, which meant the load of EG was not sufficient to absorb all SAT-FA PCM into its pores. By contrast, the samples with 8–10% EG displayed a desired congruent melting behavior. Salt hydrate PCMs always exhibit inherent supercooling degree (ΔT) which will deteriorate the latent heat and stability. Research indicated that the pure melted SAT would easily be supercooled below 0 °C, and the maximum ΔT was even about 89 °C [35]. As shown from Fig. 5(b), with DSP introduced as nucleating agent, the ΔT of SAT-FA eutectic mixture (EM) has been successfully reduced to 2.15 °C. After composited with EG, an obvious increase of onset phase change temperature (Tonset) along with the smaller of ΔT within 2 °C were observed. The phenomenon implied that EG could be functioned as heterogeneous nucleating agent to provide heterogeneous sites for the crystallization of SAT-FA EM, resulting in a smaller ΔT. When the mass fraction of EG was less than 5%, it was observed that the EM could not be absorbed completely by EG. Considering that the excess EG could presented negative influence on latent heat, it was concluded that the CPCM with 8% EG was most favorable, which still had a sizable latent heat (197.6 kJ/kg) and a neglectable supercooling degree (0.81 °C).
Fig. 8. FT-IR spectrums of SAT-FA EM, EG and SAT-FA/EG CPCM.
vapor from CPCM surface. So further seal was needed to carry out. As shown in Fig. 4(b), the polypropylene rectangular box with the size of 17.6 × 10 × 2.4 cm was adapted to load the CPCM. The weight of CPCM in each box was about 450 g and all the phase change boxes were assembled together to form a CPCM layer. The electric heating film with the size of 1 m × 1 m × 0.25 mm and power of 220 W/m2 was installed beneath CPCM layer. The floor construction profile and temperature collection points of simulation room were shown in Fig. 4(c). At each layer in ERFHS, a T-type thermocouple was adapted to monitor the temperature fluctuation. Meanwhile, the indoor operative temperature was also detected by setting some vertically distributed T-type thermocouples to explore the temperature difference at different height. All the temperature data was collected by a data acquisition instrument. According to the standard of 《Thermal Environmental Conditions for Human Occupancy》 (ANSI/ASHRAE standard 55-2013), the acceptable range of indoor operative temperature is 20 °C to 27 °C in the low air speed (0.1 m/s) and the allowable range of floor temperature is 19 °C to 29 °C. Considering the enclosed testing environment, we set the range of 20–26 °C as thermal comfort interval and 28 °C as upper limit of acceptable floor temperature in our experiment. In order to make a clear comparison, the simulation room with CPCM layer and the one without CPCM layer were examined through same procedure. The outdoor environment temperature was 15 °C. The electric heating system was at continuous work to produce heat, and only to switch it off once the indoor operative temperature reached 26 °C, or the floor surface temperature reached 28 °C. Taking the human body thermal
3.2. Thermal conductivity enhancement under EG The thermal conductivity could be a decisive indicator which greatly influence the energy utilization efficiency in the building TES. As shown in Fig. 6, the thermal conductivities of the CPCMs with EG of 3%, 5%, 8% and 10% were respectively 2.46, 2.54, 3.11 and 3.47 W/ m·K, which were 3.11, 3.21, 3.94 and 4.39 times higher than that of 1181
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Fig. 9. SEM of images EG (a: 1000×, b: 5000×), SAT-FA/EG CPCM (c: 1000×, d: 5000×).
transfer performance under EG. From the melting curves Fig. 7(a), the temperature-rise period of samples could be divided into three stages, including sensible heat stage from room temperature to 40 °C (solidsolid), latent heat stage in the narrow temperature range from 40 °C to 42 °C (solid-liquid) and sensible heat stage from 42 °C to the preset 45 °C (liquid-liquid). In the whole experiment process, it could be seen that the temperature of the sample with EG rose faster than that of the one without EG. The total thermal storage time of the CPCM with 8% EG was about 3200 s, which was far shorter than that of SAT-FA EM (4600 s). Meanwhile, as shown from Fig. 7(b), the heat release time of SAT-FA EM was about 8000 s, while 6000 s for the CPCM. The character was highly consistent with the requirement of fast heat charging and recharging rate in ERFHS, for the purpose to improve energy utilization efficiency. It was also inferred that the SAT-FA with EG will be more sensitive to the temperature variation and present a narrower working temperature range to reduce temperature fluctuation when adapted as energy storage medium in ERFHS. 3.3. Shape stability under EG Fig. 10. Images of the monolithic SAT-FA and SAT-FA/EG CPCM heated at 20 °C (a) and 50 °C (b).
Primarily, the FT-IR spectrum results (Fig. 8) of the prepared EG, SAT-FA eutectic mixture and SAT-FA/EG composite were analyzed to figure out the chemical structure. In the spectrum of EG, the peak at 3500 cm−1 presents the stretching vibration of OH group. Two peaks at 2922 cm−1 and 2854 cm−1 were due to the symmetric and asymmetric stretching vibration of eCH2, while peak at 1429 cm−1 was ascribed to asymmetric bending of CeC in eCH2 group. For spectrum of SAT-FA, it showed peaks at 1693 cm−1 (stretching vibration of C]O in acylamino), 1550 cm−1 (bending vibration of NeH) and 1412 cm−1 (stretching vibration of CeN in acylamino). Noticeably, all absorption
SAT/FA EM (0.79 W/m·K). The obvious growth trend of thermal conductivity indicated that the compact thermal conductivity network was gradually formed with the increase of EG. With favorable load of EG, the faster internal heat transfer rate would be obtained, which could better meet the requirement of improving energy efficiency in ERFHS. The heat storage and release experiments of the SAT-FA/EG with 8% EG and the one without EG were executed to identify the heat 1182
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Fig. 11. POM images of SAT-FA EM (a) and SAT-FA/EG CPCM (b) at different temperatures.
Fig. 12. Supercooling degrees and thermal properties of SAT-FA/EG CPCM after different thermal cycles.
maintain its shape without obvious leakage when heated at 50 °C, while the SAT-FA EM presented obvious liquid outflow on the filter paper after melting. Furthermore, the crystallization behaviors of SAT-FA EM (a) and SAT-FA/EG CPCM (b) at different temperatures photographed by POM were shown in Fig. 11. When placed at 20 °C, the SAT-FA EM presented an obvious white crystallization refraction. When heated at 40 °C which was near its melting temperature, it was in melting process with the crystal refraction becoming weaker and a dark melt behavior observed. Continuing heating to 60 °C, the white crystal patterns disappeared and the liquid PCM converged to be big and flowable droplets, which meant the crystal structure of SAT-FA EM has been changed greatly, resulting in big possibility of leakage. Instead, from the Fig. 11(b), although the strength of white crystal patterns gradually decreased with the increase of temperature, the crystallization
peaks of SAT-FA EM and EG could still be observed in the curve of SATFA/EG with no new peaks found. It was indicated that the newly formed eutectic mixture was physically fabricated into the EG without changing their chemical properties. Fig. 9 showed SEM images of EG (a, b) and the SAT-FA/EG CPCM (c, d). It was seen that EG presented a porous and layered structure which can provide abundant adsorption surface area. Instead, the flatter surface was observed in SAT-FA/EG CPCM, which indicated that the interlayer of EG have been filled by SAT-FA EM and a shape-stable composite was obtained. With the carrier support of EG, the capillary force between the surface of pores and molecules of SAT-FA could also alleviate the leakage of PCM during the melting process. The shape stability of SAT-FA/EG could also be certified by the physical photos (Fig. 10). It was shown that the SAT-FA/EG could still 1183
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3.4. Thermal reliability of SAT-FA/EG CPCM The thermal reliability of SAT-FA/EG CPCM was identified by measuring the thermal properties including phase change temperature, latent heat and supercooling degree after different thermal cycles. As the results shown in Fig. 12, it was calculated that deviations of phase change temperature were less than 5% which were acceptable fluctuation for practical application. Meanwhile, the latent heat of the CPCM was respectively 181.2, 180.5, 177.9 and 169.5 kJ/kg after 100, 200, 300 and 400 thermal cycles with the decrement of 3.41%, 3.78%, 5.17% and 9.65% compared with the original sample (187.6 kJ/kg), which were still available for thermal energy storage. Meanwhile, the supercooling degrees of the CPCM after cycles were all within 1.1 °C, which was neglectable for practical application. These results indicated that the obtained SAT-FA/EG CPCM owned a comparable thermal reliability. 3.5. Simulation room performance established by ERFHS with SAT-FA/EG CPCM
Fig. 13. Vertical temperature difference of ERFHS with CPCM layer and the one without CPCM layer.
For the simulation room test, we firstly made a comparison for indoor operative temperature fluctuations in vertical direction under floor heating for 1 h. As the result shown in Fig. 13, the indoor operative temperature at the height of 40 cm was set as referential value and the temperature differences at different heights were calculated and plotted. At the height range from 10 to 100 cm, the temperature deviations in vertical direction of the two systems were all within 0.2 °C. However, at the height of 5 cm, the temperature difference of the system containing CPCM layer was 3.09 °C, while the one without CPCM layer was 8.71 °C. The data indicated the vertical temperature distribution was more uniform after installing CPCM layer, which was beneficial to improve indoor comfort. Secondly, the synchronously simulative uses of the ERFHS with different layer structures were carried out. The temperature variation at each layer of ERFHS without CPCM layer were plotted in Fig. 14(a). During the heating process, the indoor operative temperature rapidly reached 28.08 °C within 1.139 h, and the thermal comfort time (th) was only 0.422 h, while the floor surface temperature reached 45.11 °C which far exceeded to 28 °C. For the cooling process, only after 2.739 h the indoor operative temperature rapidly dropped to 20 °C, and thermal comfort time (tc) was only 1.414 h, accounting the total td less than 2 h. Above results indicated that the rising and falling rate of indoor operative temperature of the room without CPCM layer were too fast to satisfy the requirement of thermal comfort and energy-saving economy. Instead, in the contrast test shown in Fig. 14(b), the longer thermal storage time (3.878 h) and thermal comfort time (th, 3.364 h) in the heating process of ERFHS containing CPCM layer were observed, both which indicated the better energy storage capacity. Moreover, during the heating process the maximum indoor temperature was maintained below 24 °C which was in the comfortable temperature range. For the cooling process, the heat release time was 9.287 h which was greatly longer than the control group (2.739 h). Overseeing the all procedure the td was almost 13 h, over 6 times longer than that of the one without CPCM layer. With the CPCM layer as heat storage medium, more gradual temperature curve was obtained, which meant the indoor temperature fluctuation has been weakened. The great increase of th, tc as well as td indicated that the heat storage capacity, heat release performance and indoor comfort have been improved with favorable energy utilization efficiency. Above all, it showed great potential for SAT-FA/ EG CPCM used in ERFHS and provided the experience for other PCMs applied in building applications.
Fig. 14. Temperature variation of ERFHS without CPCM layer (a) and the one with CPCM layer (b).
refraction of SAT-FA/EG CPCM was observed all the time, even above 20 °C higher than its melting point. It was suggested that the SAT-FA EM was steadily absorbed into the pores of EG and the shape-stable composite was successfully obtained.
4. Conclusions A novel shape-stabilized composite PCM using SAT-FA pseudobinary eutectic mixture as PCM and EG as supporting carrier, 1184
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heterogeneous nucleating agent as well as thermal conductivity enhancer has been prepared. The most favorable EG mass fraction was determined to be 8% in SAT-FA/EG CPCM. The obtained CPCM possessed a suitable phase change temperature (37.54 °C) for ERFHS, excellent latent heat (187.6 kJ/kg) and thermal conductivity (3.11 W/ m·K), as well as a rather low supercooling degree (0.81 °C). After 400 thermal cycles, the variation of thermal properties was still acceptable for practical application. With the addition of EG, SAT-FA eutectic mixture manifested an excellent form stability, crystallization promoting behavior and the enhancement of heat storage and release performance. The simulation room of ERFHS with SAT-FA/EG CPCM layer presented excellent performance in decreasing indoor temperature fluctuation and improving indoor comfort. It has smaller temperature fluctuation in vertical orientation and longer total thermal comfort time (12.65 h), which was over 6 times longer than that the one without CPCM layer (1.836 h). The favorable thermal properties suggest that the SAT-FA/EG composite PCM owns great potentials to be applied in electric radiant floor heating system.
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Acknowledgement This work was supported by National Natural Science Foundation of China (No. 51536003, No.21471059). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.01.069. References [1] T. Khadiran, M.Z. Hussein, Z. Zainal, R. Rusli, Advanced energy storage materials for building applications and their thermal performance characterization: A review, Renew. Sustain. Energy Rev. 57 (2016) 916–928. [2] A. Bland, M. Khzouz, T. Statheros, E.I. Gkanas, PCMs for residential building applications: a short review focused on disadvantages and proposals for future development, Buildings 7 (2017) 78 (18 pp.)-78 (18 pp.). [3] K. Pielichowska, K. Pielichowski, Phase change materials for thermal energy storage, Prog. Mater. Sci. 65 (2014) 67–123. [4] S. Ramakrishnan, X. Wang, J. Sanjayan, J. Wilson, Heat transfer performance enhancement of paraffin/expanded perlite phase change composites with graphene nano-platelets, in: J. Yan, F. Sun, S.K. Chou, U. Desideri, H. Li, P. Campana, R. Xiong (Eds.), 8th International Conference on Applied Energy, vol. 105, 2017. [5] J. Darkwa, Mathematical evaluation of a buried phase change concrete cooling system for buildings, Appl. Energy 86 (2009) 706–711. [6] L. Fu, Q. Wang, R. Ye, X. Fang, Z. Zhang, A calcium chloride hexahydrate/expanded perlite composite with good heat storage and insulation properties for building energy conservation, Renew. Energy 114 (2017) 733–743. [7] K. Peng, J. Zhang, H. Yang, J. Ouyang, Acid- hybridized expanded perlite as a composite phase-change material in wallboards, RSC Adv. 5 (2015) 66134–66140. [8] Y. Xia, X.S. Zhang, Experimental research on a double-layer radiant floor system with phase change material under heating mode, Appl. Thermal Eng. 96 (2016) 600–606. [9] A.K. Athienitis, T. Chen, Experimental and theoretical investigation of floor heating with thermal storage, ASHRAE Trans. (1993). [10] M. Barrio, J. Font, D.O. Lopez, J. Muntasell, J.L. Tamarit, Floor radiant system with heat storage by a solid-solid phase transition material, Sol. Energy Mater. Sol. Cells 27 (1992) 127–133. [11] R. Barzin, J.J.J. Chen, B.R. Young, M.M. Farid, Application of PCM underfloor heating in combination with PCM wallboards for space heating using price based control system, Appl. Energy 148 (2015) 39–48. [12] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Convers. Manage. 45 (2004) 263–275.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Thermal properties enhancement and application of a novel sodium acetate trihydrate-formamide/expanded graphite shape-stabilized composite phase change material for electric radiant floor heating
T
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Yutang Fang , Yifan Ding, Yufeng Tang, Xianghui Liang, Ce Jin, Shuangfeng Wang, Xuenong Gao, Zhengguo Zhang Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou 510640, China
H I GH L IG H T S
EG-based CPCM using SAT-FA eutectic mixture as the PCM was prepared. • AThenovel CPCM under EG owned good nucleating behavior and thermal reliability. • Thermal conductivity of the CPCM could be significantly enhanced by adding EG. • The electric radiant floor heating system with the CPCM showed great thermal performance. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Eutectic mixture Expanded graphite Composite phase change material Shape stability Electric radiant floor heating
Electric radiant floor heating system (ERFHS) with hydrate salt phase change material (PCM) as thermal storage medium owns the advantages of improving indoor comfort with high energy efficiency and favorable economic applicability. In this paper, based on sodium acetate trihydrate (SAT)-formamide (FA) eutectic mixture as PCM and expanded graphite (EG) as supporting carrier, a novel SAT-FA/EG composite PCM (CPCM) for ERFHS was prepared by physical blending method. The shape stability, thermal properties and thermal reliability of the SAT-FA eutectic mixture under EG were emphatically discussed. The heat storage and release performances of a simulation room established by ERFHS integrated with such CPCM were investigated. Experimental results showed that the SAT-FA/EG composite containing 8% EG displayed high phase change enthalpy (187.6 kJ/kg), suitable phase change temperature (38.54 °C), negligible supercooling degree (0.83 °C) and eminent thermal conductivity (3.11 W/m·K), along with the excellent shape stability and thermal reliability. The simulation showed that the ERFHS with CPCM layer presented smaller indoor operative temperature fluctuation in vertical orientation and longer total thermal comfort time (12.65 h), which greatly exceeds that of the one without CPCM layer (1.836 h). All the superior characters make the obtained SAT-FA/EG composite a promising candidate for ERFHS.
1. Introduction Tremendous energy consumption along with low utilization efficiency is always a main concern in the industrial economy. Particularly, the consumption accounted in building sector has reached definitely around 40% in overall energy consumption and it is expected to increase by 28% in 2035 [1,2], mainly for the extensive installment of electric regulated temperature facilities such as heating ventilating, air conditioning and floor heating system. Such circumstance, as well as the conventional energy crisis, have promoted the technological
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innovation to implement better energy utilization efficiency, thereby reducing consumption. The thermal energy storage (TES), which is obtained by heating, cooling, solidifying, melting, or vaporizing a certain material along with the energy becoming available, has been confirmed to be an environment-friendly energy-saving technology in building [3]. Accordingly, phase change material (PCM), which absorb and release latent heat by experiencing solid-liquid phase transition, has attracted wide interests and extensive researches in TES application, for its high energy storage capacity with a narrow temperature change range, slight volume change during phase transformation and
Corresponding author. E-mail address: [email protected] (Y. Fang).
https://doi.org/10.1016/j.applthermaleng.2019.01.069 Received 1 August 2018; Received in revised form 29 December 2018; Accepted 21 January 2019 Available online 29 January 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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enhancement of thermal conductivity for PCM could remarkably improve the energy efficiency of ERFHS and reduce the cost of insulating materials [21,22]. A simple and effective way is to fabricate a formstable composite by embedding the PCM into porous framework materials which possess high thermal conductivity, such as expanded graphite (EG) [23,24] and boron nitride [25,26], etc. The materials are predicted to provide a mechanical strength and generate capillary force to alleviate the leakage of melted PCM from porous structure, ultimately to obtain a form-stable composite which also owns conveniences on engineering fabrication and customization for special application [27,28]. Expanded graphite (EG) is a loose and porous worm-like graphitic carrier with cheap price. Many studies have prepared the EGbased composite PCM due to its good thermal conductivity, long-term thermal reliability and the function of as heterogeneous nucleating agent to reduce the supercooling degree of salt hydrates [29–32]. Gathering above superiorities it is undisputed that EG was a multifunctional additive for PCM to obtain better practical performance. Herein this paper, the SAT/FA pseudo-binary eutectic mixture selected as base PCM and EG selected as thermal conductivity enhancer, form-stable carrier as well as nucleating promoter were composited by employing a physical mixing method. Firstly, the most favorable adsorption capacity of EG was determined by comparing the melting behavior and supercooling degree of all samples. The following measurements including heat transfer property, morphology and thermal reliability of the obtained SAT-FA/EG CPCM were carried out. Finally, a simulation room established by ERFHS integrated with such CPCM was analyzed by monitoring the indoor air temperature fluctuation and counting the thermal comfort time. The results not only clearly demonstrated the great application potential of the CPCM for ERFHS but also enriched the preparing and applicating method of such similar materials for building.
excellent availability in abundance [4]. Such merits of PCM are particularly favorable for PCM to be applied in building field, while floor and wall could provide enough heat exchange areas which can maximally utilize the thermal properties of PCM. Recent decades, researchers and engineers have focused on the integration of PCM into building elements such as concrete [5], bricks [6] and wallboard [7], to obtain better energy utilization efficiency without compromising the requirements of indoor comfort. Electric radiant floor heating system (ERFHS), a low temperature heating system, has been recognized as an economically efficient preference in residential and commercial building. Such system saves space and produce no noise, meanwhile it can provide uniform temperature distribution which is beneficial for improving thermal comfort degree [8]. Thermal mass material combined with ERFHS is often adapted to storage off-peak electricity source. In such way the peak load could be cut down and transferred to nighttime since electricity price is lower [9]. Conventionally the dense material such as concrete are adapted as thermal mass, but nowadays more researchers are seeking the possibility of using PCM to replace the traditional material. This is because ERFHS integrated with PCM could cause smaller indoor temperature fluctuation and longer thermal comfort time in the energy-saving method. Barrio et al. [10] studied the performance of the ERFHS with neopentyl glycol PCM floor. Results showed the temperature fluctuation at inner and surface was 3 times smaller and the charging time was 2.8 h longer, than those of sand land. Barzin et al. [11] designed a underfloor heating system combined with commercialized PCM wallboards in a price-based operating method. Experimental results showed the total energy saving and electrical cost saving in five days came up to 18.8% and 28.7%, respectively. Various studies have illustrated the significant improvement on energy efficiency and cost reduction of the PCM-combined ERFHS if the suitable PCM was selected. When evaluating the feasibility of a certain PCM applied in building TES, its thermophysical, kinetic and chemical properties should be taken into consideration [12]. Favorable characters of salt hydrates including high volumetric energy storage capacity, high thermal conductivity, nontoxicity and noninflammability, make them attractive PCMs candidates for TES in building. Salt hydrates are commonly modified by adding the nucleating agents and thickening agent to alleviate defects of great supercooling and phase separation which are caused by its coherent poor nucleating ability and incongruent melting behavior respectively [13,14]. But the multiple additives are difficult to control in an accurate way and may reduce the latent heat of salt hydrate. In order to reduce additives some scholars have focused on seizing the binary or multicomponent eutectic mixtures in which the components could undergo phase-change at the same temperature. The eutectic mixtures possess congruent melting behavior and great thermal reliability [15]. Many studies have adapted eutectic mixtures as PCM to obtain better thermal reliability performance such as auric acid-stearic acid eutectic [16] and MgCl2·6H2O-Mg(NO3)2·6H2O eutectic [17]. We are inspired that it is feasible to obtain the superior thermal reliability by preparing a eutectic salt hydrate, meanwhile possessing the suitable thermal properties which are consistent with the requires for ERFHS. In an earlier study, Takahiro M et al. [18] investigated the pseudobinary system of CH3COONa·3H2O-HCONH2 (sodium acetate trihydrate-formamide, SAT-FA). It was found that when the FA mass fraction was 25%, the SAT-FA pseudo-binary eutectic mixture was formed with congruent melting point of 40.5 °C. The desirable melting behavior is precisely required for ERFHS [19]. SAT is a nontoxic and uninflammable inorganic salt and owns a relatively high heat fusion (264 kJ/kg) with moderate price [20], which makes it applicable for generous operation as building materials. Concerning the above merits of eutectic mixture and SAT, it is expected that the SAT-FA eutectic mixture could be a potential PCM candidate for ERFHS. Nevertheless, the thermal conductivity of the eutectic mixture needs to be improved in order to accelerate heat transfer rate during the phase change process, according to the certified conclusion that the moderate
2. Experiment 2.1. Materials Sodium acetate trihydrate (SAT, CH3COONa·3H2O, AR), Formamide (FA, HCONH2, AR) and Disodium phosphate dodecahydrate (DSP, Na2HPO4·12H2O, AR) were purchased from Tianjin Kemious Chemical Reagent Co., Ltd. Expandable graphite powder (50 mesh, expansion rate of 300 mL/g) was obtained from Qingdao Modou Graphite Materials Manufacturing Co., Ltd. 2.2. Preparation of SAT-FA/EG composite 2.2.1. Preparation of EG Graphite powder was expanded using microwave method for its convenience on operation, uniform expansion and high energy efficiency [33]. The powder was firstly dried at 65 °C in vacuum oven for 24 h and then exposed to microwave radiation at 800 W for 30 s. After being cooled to room-temperature the expanded graphite (EG) with high porosity was obtained. The N2 adsorption isotherm of the prepared EG was executed at 77 K adapting an ASAP 2460 porosity analyzer (Micromeritics) to identify its microporous and mesoporous structure. The results of adsorption isotherm and pore size distribution (insert) were shown in Fig. 1. According to the numerical calculation by density functional theory (DFT), the BET surface area of the prepared EG was 53.13 m2/g, of which the mesopore surface area was 34.13 m2/g. The curve of pore size distribution suggested the pore diameter was mainly in the range of 2–50 nm. The great increase of N2 adsorption quantity at the relative pressure range from 0.5 to 1.0 along with a hysteresis behavior of desorption also indicated the abundant mesopore porosity. It was predicted that EG would not only provide strong capillary force to adsorb melted mixture, but also provide generous heterogeneous nucleation sites for salt hydrate to reduce supercooling. 1178
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properties of the EM were measured by DSC (see Fig. 2). Obviously, a single strong peak with the phase change temperature of 40.88 °C and phase change enthalpy of 233.9 kJ/kg was observed in the curve of SAT-FA (c), which was located between the specific peaks of pure FA (a) and SAT (b). It meant the pseudo-binary eutectic mixture with high latent heat was formed. 2.2.3. Preparation of SAT-FA/EG composite A simple blending method was used to prepare SAT-FA/EG composite phase change material (CPCM). Firstly, 20 g SAT-FA solid eutectic mixture placed in glass bottle was completely melted by continuous stirring in water bath of 50 °C. Then the extra EG of 3%, 5%, 8%, 10% was added into the bottles, respectively. Secondly, the mixtures were mechanically stirred in sealed condition for 4 h and then transferred it to a thermostat of 20 °C. The novel solid SAT-FA/EG CPCM finally was obtained. Fig. 1. N2 adsorption isotherm and pore size distribution (insert) of prepared EG.
2.3. Characterization of SAT-FA/EG composite The phase change behaviors of the samples with different mass fraction of EG were investigated using a Q20 differential scanning calorimeter (DSC, TA Instrument Inc., USA) at a heating rate of 5 °C·min−1 under nitrogen atmosphere, with an accuracy within ± 1%. The measurements of thermal conductivities were carried out using a TPS2500 thermal constant analyzer (Hot Disk, Sweden) and transient plane source method within the accuracy of ± 3% and the reproductivity of ± 1%. The experiment set-up and samples arrangement were displayed in Fig. 3. Firstly, the composites were molded into two identical cylindrical blocks (Φ 40 × 10 mm) with the density of 1000 kg/m3 and then a testing sensor (Type 7577) with radius of 2.01 mm was sandwiched between the two samples. Each measurement was repeatedly carried out for 3 times to reduce experimental error. The shape stable behaviors of samples were observed by a DM2500P polarizing microscope (POM, Leica, Germany) and the morphologies were photographed at 20 °C, 40 °C and 60 °C after isothermal heat treatment for 5 min respectively. A S-3700N scanning electron microscope (SEM, HITACHI, Japan) was employed to observe the morphologies of samples with scanning voltage of 15 kv. Considering the practical application of the simulation room established by ERFHS, the melting and crystallization temperatures of samples were set at 45 °C and 30 °C respectively by referring to the initial temperature and stop temperature of the electric heating film. In this condition the heat storage and exothermic processes of samples were investigated. For the melting process, the sealed glass bottle loaded with 20 g solid CPCM was placed in a preset incubator of 45 °C to absorb heat. For the heat release experiment, the above completely melted sample was naturally cooled and crystallized to release heat in a preset incubator of 30 °C. During the entire process a K-type thermocouple with the accuracy of ± 0.1 °C was inserted into the sample center to collect the temperature data, which was logged by an Agilent
Fig. 2. DSC curves of FA (a), SAT (b) and SAT-FA EM (c).
2.2.2. Preparation of SAT-FA eutectic mixture According to our earlier research work [34], SAT-FA eutectic mixture was prepared by melt-blending method. Mixtures of 75% SAT-FA (mass fraction, the same below, 15 g) and 25% FA (5 g) and additional 2% DSP (0.4 g) as nucleating agent were vigorously stirred in a water bath of 60 °C until completely melted. After continuingly stirring for 0.5 h and a successively cooling treatment in a thermostat of 20 °C, the solid SAT-FA eutectic mixture (EM) was finally obtained. The thermal
Fig. 3. The schematic diagram for thermal conductivity measurement. 1179
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Fig. 4. Appearance of simulation room (a), CPCM layer (b), floor construction profile and temperature collection point (c). Table 1 DSC data of SAT-FA/EG CPCM with different EG mass fraction. Mass fraction of EG (%)
Ti (°C)
Tm (°C)
ΔHm (kJ·kg−1)
0 3 5 8 10
38.88 36.36 33.46 29.07 29.98
40.88 40.29 40.68 37.54 37.7
233.9 226.3 215.8 187.6 177
Fig. 6. Thermal conductivities of SAT-FA/EG CPCMs with different EG mass fraction.
cooling treatment of 90 min. Above temperature procedure was the single thermal cycle. After 100, 200, 300 and 400 thermal cycles, the followed DSC measurements and the supercooling experiments were carried out respectively. 2.4. Simulation room test for ERFHS with SAT-FA/EG composite For the purpose of examining the potential application performance of the prepared SAT-FA/EG composite in real working environment, a simulation room established by ERFHS with CPCM as heat-transfer medium was built. As shown in Fig. 4(a), the room was a cubic model with the size of 1.15 × 1.15 × 1.30 m (inner size: 1.10 × 1.10 × 1.10 m). The walls and ceiling of the room were sealed with the adiabatic polystyrene foam board, and its bottom was equipped with multilevel floor which was assembled by insulation layer, electric heating film, PCM layer and wood floor in a bottom-to-up sequence. Although porous EG has a shape stable effect on SAT-FA eutectic mixture, it cannot completely eliminate the leakage of water
Fig. 5. DSC curves (a) and step cooling curves (b) of SAT-FA/EG CPCM with different EG mass fraction.
34970A data acquisition instrument. The thermal reliability of the sample was estimated by accelerated thermal cycling experiment in a WHH-080 constant temperature chamber (Dongguan Weihuang Instrument Co., Ltd., China). The sample was placed in the chamber to be thermostatically heated at 50 °C for 90 min and then the temperature was changed to 15 °C for a
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Fig. 7. Melting (a) and freezing curves (b) of SAT-FA EM and SAT-FA/EG CPCM.
comfort as the standard, the heating time (th) and cooling time (tc) were used to measure the heat storage capacity and heat release ability respectively. The total duration (td) from 20 °C to 26 °C, namely the sum of th and tc, was marked as the parameter to indicate the ability of maintaining thermal comfort. 3. Results and discussion 3.1. Phase change behavior and nucleation promotion under EG Fig. 5(a) showed DSC curves of SAT-FA/EG CPCM with different EG mass fraction and the test results were tabulated in Table 1. With the increase of EG mass fraction, the initial melting temperature (Ti) of the CPCM decreased and the melting peak became smoother along with an acceptable decline of phase change temperature (Tm) and enthalpy (ΔHm). The decrease of ΔHm was caused by the addition of EG which did not undergo phase change. Meanwhile, the porous EG generated capillary force with the molecules of SAT, water and FA, meanwhile weakened the inter-molecular force among eutectic mixture, consequently resulting in a decline of Ti and Tm. When EG mass fraction was 3–5%, the peaks of the samples presented partly incongruent melting, which meant the load of EG was not sufficient to absorb all SAT-FA PCM into its pores. By contrast, the samples with 8–10% EG displayed a desired congruent melting behavior. Salt hydrate PCMs always exhibit inherent supercooling degree (ΔT) which will deteriorate the latent heat and stability. Research indicated that the pure melted SAT would easily be supercooled below 0 °C, and the maximum ΔT was even about 89 °C [35]. As shown from Fig. 5(b), with DSP introduced as nucleating agent, the ΔT of SAT-FA eutectic mixture (EM) has been successfully reduced to 2.15 °C. After composited with EG, an obvious increase of onset phase change temperature (Tonset) along with the smaller of ΔT within 2 °C were observed. The phenomenon implied that EG could be functioned as heterogeneous nucleating agent to provide heterogeneous sites for the crystallization of SAT-FA EM, resulting in a smaller ΔT. When the mass fraction of EG was less than 5%, it was observed that the EM could not be absorbed completely by EG. Considering that the excess EG could presented negative influence on latent heat, it was concluded that the CPCM with 8% EG was most favorable, which still had a sizable latent heat (197.6 kJ/kg) and a neglectable supercooling degree (0.81 °C).
Fig. 8. FT-IR spectrums of SAT-FA EM, EG and SAT-FA/EG CPCM.
vapor from CPCM surface. So further seal was needed to carry out. As shown in Fig. 4(b), the polypropylene rectangular box with the size of 17.6 × 10 × 2.4 cm was adapted to load the CPCM. The weight of CPCM in each box was about 450 g and all the phase change boxes were assembled together to form a CPCM layer. The electric heating film with the size of 1 m × 1 m × 0.25 mm and power of 220 W/m2 was installed beneath CPCM layer. The floor construction profile and temperature collection points of simulation room were shown in Fig. 4(c). At each layer in ERFHS, a T-type thermocouple was adapted to monitor the temperature fluctuation. Meanwhile, the indoor operative temperature was also detected by setting some vertically distributed T-type thermocouples to explore the temperature difference at different height. All the temperature data was collected by a data acquisition instrument. According to the standard of 《Thermal Environmental Conditions for Human Occupancy》 (ANSI/ASHRAE standard 55-2013), the acceptable range of indoor operative temperature is 20 °C to 27 °C in the low air speed (0.1 m/s) and the allowable range of floor temperature is 19 °C to 29 °C. Considering the enclosed testing environment, we set the range of 20–26 °C as thermal comfort interval and 28 °C as upper limit of acceptable floor temperature in our experiment. In order to make a clear comparison, the simulation room with CPCM layer and the one without CPCM layer were examined through same procedure. The outdoor environment temperature was 15 °C. The electric heating system was at continuous work to produce heat, and only to switch it off once the indoor operative temperature reached 26 °C, or the floor surface temperature reached 28 °C. Taking the human body thermal
3.2. Thermal conductivity enhancement under EG The thermal conductivity could be a decisive indicator which greatly influence the energy utilization efficiency in the building TES. As shown in Fig. 6, the thermal conductivities of the CPCMs with EG of 3%, 5%, 8% and 10% were respectively 2.46, 2.54, 3.11 and 3.47 W/ m·K, which were 3.11, 3.21, 3.94 and 4.39 times higher than that of 1181
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Fig. 9. SEM of images EG (a: 1000×, b: 5000×), SAT-FA/EG CPCM (c: 1000×, d: 5000×).
transfer performance under EG. From the melting curves Fig. 7(a), the temperature-rise period of samples could be divided into three stages, including sensible heat stage from room temperature to 40 °C (solidsolid), latent heat stage in the narrow temperature range from 40 °C to 42 °C (solid-liquid) and sensible heat stage from 42 °C to the preset 45 °C (liquid-liquid). In the whole experiment process, it could be seen that the temperature of the sample with EG rose faster than that of the one without EG. The total thermal storage time of the CPCM with 8% EG was about 3200 s, which was far shorter than that of SAT-FA EM (4600 s). Meanwhile, as shown from Fig. 7(b), the heat release time of SAT-FA EM was about 8000 s, while 6000 s for the CPCM. The character was highly consistent with the requirement of fast heat charging and recharging rate in ERFHS, for the purpose to improve energy utilization efficiency. It was also inferred that the SAT-FA with EG will be more sensitive to the temperature variation and present a narrower working temperature range to reduce temperature fluctuation when adapted as energy storage medium in ERFHS. 3.3. Shape stability under EG Fig. 10. Images of the monolithic SAT-FA and SAT-FA/EG CPCM heated at 20 °C (a) and 50 °C (b).
Primarily, the FT-IR spectrum results (Fig. 8) of the prepared EG, SAT-FA eutectic mixture and SAT-FA/EG composite were analyzed to figure out the chemical structure. In the spectrum of EG, the peak at 3500 cm−1 presents the stretching vibration of OH group. Two peaks at 2922 cm−1 and 2854 cm−1 were due to the symmetric and asymmetric stretching vibration of eCH2, while peak at 1429 cm−1 was ascribed to asymmetric bending of CeC in eCH2 group. For spectrum of SAT-FA, it showed peaks at 1693 cm−1 (stretching vibration of C]O in acylamino), 1550 cm−1 (bending vibration of NeH) and 1412 cm−1 (stretching vibration of CeN in acylamino). Noticeably, all absorption
SAT/FA EM (0.79 W/m·K). The obvious growth trend of thermal conductivity indicated that the compact thermal conductivity network was gradually formed with the increase of EG. With favorable load of EG, the faster internal heat transfer rate would be obtained, which could better meet the requirement of improving energy efficiency in ERFHS. The heat storage and release experiments of the SAT-FA/EG with 8% EG and the one without EG were executed to identify the heat 1182
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Fig. 11. POM images of SAT-FA EM (a) and SAT-FA/EG CPCM (b) at different temperatures.
Fig. 12. Supercooling degrees and thermal properties of SAT-FA/EG CPCM after different thermal cycles.
maintain its shape without obvious leakage when heated at 50 °C, while the SAT-FA EM presented obvious liquid outflow on the filter paper after melting. Furthermore, the crystallization behaviors of SAT-FA EM (a) and SAT-FA/EG CPCM (b) at different temperatures photographed by POM were shown in Fig. 11. When placed at 20 °C, the SAT-FA EM presented an obvious white crystallization refraction. When heated at 40 °C which was near its melting temperature, it was in melting process with the crystal refraction becoming weaker and a dark melt behavior observed. Continuing heating to 60 °C, the white crystal patterns disappeared and the liquid PCM converged to be big and flowable droplets, which meant the crystal structure of SAT-FA EM has been changed greatly, resulting in big possibility of leakage. Instead, from the Fig. 11(b), although the strength of white crystal patterns gradually decreased with the increase of temperature, the crystallization
peaks of SAT-FA EM and EG could still be observed in the curve of SATFA/EG with no new peaks found. It was indicated that the newly formed eutectic mixture was physically fabricated into the EG without changing their chemical properties. Fig. 9 showed SEM images of EG (a, b) and the SAT-FA/EG CPCM (c, d). It was seen that EG presented a porous and layered structure which can provide abundant adsorption surface area. Instead, the flatter surface was observed in SAT-FA/EG CPCM, which indicated that the interlayer of EG have been filled by SAT-FA EM and a shape-stable composite was obtained. With the carrier support of EG, the capillary force between the surface of pores and molecules of SAT-FA could also alleviate the leakage of PCM during the melting process. The shape stability of SAT-FA/EG could also be certified by the physical photos (Fig. 10). It was shown that the SAT-FA/EG could still 1183
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3.4. Thermal reliability of SAT-FA/EG CPCM The thermal reliability of SAT-FA/EG CPCM was identified by measuring the thermal properties including phase change temperature, latent heat and supercooling degree after different thermal cycles. As the results shown in Fig. 12, it was calculated that deviations of phase change temperature were less than 5% which were acceptable fluctuation for practical application. Meanwhile, the latent heat of the CPCM was respectively 181.2, 180.5, 177.9 and 169.5 kJ/kg after 100, 200, 300 and 400 thermal cycles with the decrement of 3.41%, 3.78%, 5.17% and 9.65% compared with the original sample (187.6 kJ/kg), which were still available for thermal energy storage. Meanwhile, the supercooling degrees of the CPCM after cycles were all within 1.1 °C, which was neglectable for practical application. These results indicated that the obtained SAT-FA/EG CPCM owned a comparable thermal reliability. 3.5. Simulation room performance established by ERFHS with SAT-FA/EG CPCM
Fig. 13. Vertical temperature difference of ERFHS with CPCM layer and the one without CPCM layer.
For the simulation room test, we firstly made a comparison for indoor operative temperature fluctuations in vertical direction under floor heating for 1 h. As the result shown in Fig. 13, the indoor operative temperature at the height of 40 cm was set as referential value and the temperature differences at different heights were calculated and plotted. At the height range from 10 to 100 cm, the temperature deviations in vertical direction of the two systems were all within 0.2 °C. However, at the height of 5 cm, the temperature difference of the system containing CPCM layer was 3.09 °C, while the one without CPCM layer was 8.71 °C. The data indicated the vertical temperature distribution was more uniform after installing CPCM layer, which was beneficial to improve indoor comfort. Secondly, the synchronously simulative uses of the ERFHS with different layer structures were carried out. The temperature variation at each layer of ERFHS without CPCM layer were plotted in Fig. 14(a). During the heating process, the indoor operative temperature rapidly reached 28.08 °C within 1.139 h, and the thermal comfort time (th) was only 0.422 h, while the floor surface temperature reached 45.11 °C which far exceeded to 28 °C. For the cooling process, only after 2.739 h the indoor operative temperature rapidly dropped to 20 °C, and thermal comfort time (tc) was only 1.414 h, accounting the total td less than 2 h. Above results indicated that the rising and falling rate of indoor operative temperature of the room without CPCM layer were too fast to satisfy the requirement of thermal comfort and energy-saving economy. Instead, in the contrast test shown in Fig. 14(b), the longer thermal storage time (3.878 h) and thermal comfort time (th, 3.364 h) in the heating process of ERFHS containing CPCM layer were observed, both which indicated the better energy storage capacity. Moreover, during the heating process the maximum indoor temperature was maintained below 24 °C which was in the comfortable temperature range. For the cooling process, the heat release time was 9.287 h which was greatly longer than the control group (2.739 h). Overseeing the all procedure the td was almost 13 h, over 6 times longer than that of the one without CPCM layer. With the CPCM layer as heat storage medium, more gradual temperature curve was obtained, which meant the indoor temperature fluctuation has been weakened. The great increase of th, tc as well as td indicated that the heat storage capacity, heat release performance and indoor comfort have been improved with favorable energy utilization efficiency. Above all, it showed great potential for SAT-FA/ EG CPCM used in ERFHS and provided the experience for other PCMs applied in building applications.
Fig. 14. Temperature variation of ERFHS without CPCM layer (a) and the one with CPCM layer (b).
refraction of SAT-FA/EG CPCM was observed all the time, even above 20 °C higher than its melting point. It was suggested that the SAT-FA EM was steadily absorbed into the pores of EG and the shape-stable composite was successfully obtained.
4. Conclusions A novel shape-stabilized composite PCM using SAT-FA pseudobinary eutectic mixture as PCM and EG as supporting carrier, 1184
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heterogeneous nucleating agent as well as thermal conductivity enhancer has been prepared. The most favorable EG mass fraction was determined to be 8% in SAT-FA/EG CPCM. The obtained CPCM possessed a suitable phase change temperature (37.54 °C) for ERFHS, excellent latent heat (187.6 kJ/kg) and thermal conductivity (3.11 W/ m·K), as well as a rather low supercooling degree (0.81 °C). After 400 thermal cycles, the variation of thermal properties was still acceptable for practical application. With the addition of EG, SAT-FA eutectic mixture manifested an excellent form stability, crystallization promoting behavior and the enhancement of heat storage and release performance. The simulation room of ERFHS with SAT-FA/EG CPCM layer presented excellent performance in decreasing indoor temperature fluctuation and improving indoor comfort. It has smaller temperature fluctuation in vertical orientation and longer total thermal comfort time (12.65 h), which was over 6 times longer than that the one without CPCM layer (1.836 h). The favorable thermal properties suggest that the SAT-FA/EG composite PCM owns great potentials to be applied in electric radiant floor heating system.
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