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CNTs as composite PCMs in thermal energy storage
Accepted Manuscript Title: Thermal properties and morphologies of MA–SA eutectics/CNTs as composite PCMs in thermal energy storage Author: Yaojie Tang Guruprasad Alva Xiang Huang Di Su Lingkun Liu Guiyin Fang PII: DOI: Reference:
S0378-7788(16)30520-5 http://dx.doi.org/doi:10.1016/j.enbuild.2016.06.031 ENB 6766
To appear in:
ENB
Received date: Revised date: Accepted date:
16-2-2016 29-5-2016 9-6-2016
Please cite this article as: Yaojie Tang, Guruprasad Alva, Xiang Huang, Di Su, Lingkun Liu, Guiyin Fang, Thermal properties and morphologies of MA–SA eutectics/CNTs as composite PCMs in thermal energy storage, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2016.06.031 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.
Thermal properties and morphologies of MA–SA eutectics/CNTs as composite PCMs in thermal energy storage
Yaojie Tang, Guruprasad Alva, Xiang Huang, Di Su, Lingkun Liu, Guiyin Fang* School of Physics, Nanjing University, Nanjing 210093, China *Corresponding author, E-mail address: [email protected]
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Graphical Abstract
Graphical abstract shows the SEM images of the (a) CNTs, (b) CPCM3, (c) CPCM4 and (d) CPCM5.
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Highlights
1. Myristic-stearic acids eutectics/CNTs as composite PCMs were prepared. 2. The minimum mass fraction of the CNTs in the composite PCMs was 9%. 3. Supercooling degrees of the composite PCMs decrease with the increase of the CNTs. 4. Thermal conductivity of the composite PCMs can be improved by adding CNTs.
Abstract: Myristic-stearic acids (MA-SA) binary eutectics with appropriate phase change temperature were prepared based on theoretical calculation. The melting temperature of the MA-SA eutectics and the mass fraction of the myristic acid (MA) in the eutectic mixtures are 42.70 ℃ and 54%, which was measured by Different Scanning Calorimeter (DSC). In order to enhance the thermal conductivity of the MA-SA eutectics, the carbon nanotubes (CNTs) were added into the MA-SA eutectics, and the effects of the CNTs on thermal conductivity of the MA-SA eutectics were investigated experimentally. The minimum mass fraction of the CNTs in the composite PCMs (CPCMs) was determined to be 9% without stratification. The thermal properties and microstructure of the CPCMs were investigated by the DSC and scanning electron microscopy (SEM), respectively. The DSC results showed that the CPCMs exhibited the similar solid-liquid phase change properties as the MA-SA eutectics, and the supercooling degrees of the CPCMs decrease obviously with increase of the CNTs content in the CPCMs. The thermal conductivity Meter (TCM) was used to measure the thermal conductivities of the CPCMs, and the test results indicated that the thermal conductivities of
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the CPCMs increase significantly from 0.173 W/(m• K) to 0.213 W/(m• K), 0.258 W/ (m• K) and 0.283 W/(m• K) as the mass fractions of the CNTs in the CPCMs were 9 wt. %, 12 wt. % and 15 wt. %. The CPCMs with enhanced thermodynamic property will contribute to the energy management and storage in building systems.
Keywords:
Eutectic phase change material; Carbon nanotubes; Thermal properties;
Thermal conductivity enhancement; Thermal energy storage
1. Introduction In recent decades, there is more and more attention on thermal energy storage (TES) because of the global scenario of growing energy consumption and lack of fossil fuel resources. TES is a promising method to improve thermal management, efficiency and energy conservation. For instance, TES is used to redistribute the energy requirement on-peak demand condition (daytime) and off-peak consumption situation (nighttime) [1]. Thermal energy usually being generated through some energy conversion can be stored by sensible heat, reversible thermochemical reactions and latent heat thermal energy storage (LHTES). Among the three energy storage techniques above, the LHTES mainly based on the utilization of phase change materials (PCMs) has gained great impetus on account of the advantages of relatively high energy density and nearly constant operating temperature during phase transition processes [2]. With development of the LHTES, it is widely employed in building systems [3-4], air-conditioning [5–7], electronic devices [8], solar energy storage [9], etc. In general, PCMs that are intended for latent thermal energy storage can be categorized
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into three types, namely, organic phase change materials (OPCMs), inorganic phase change materials (IPCMs) and their eutectic mixtures [10]. Although the IPCMs have an advantage over the OPCMs in energy storage density, easy phase segregation and high supercooling have greatly limited the application of the IPCMs without effective means [11]. The OPCMs, especially fatty acids, taking advantages of good chemical stability, little supercooling, high latent heat, nontoxic and self-nucleation behavior, etc. have been increasingly investigated on their synthesis and thermal properties [12–13]. In order to meet requirements for various phase change temperatures, two or three PCMs can be mixed to form binary or ternary eutectics [14–16]. Li et al. investigated binary fatty acid and the binary fatty acids/diatomite composite PCMs, and the results indicated that their corresponding phase change temperatures coincided well with experimental results [17]. However, there are two main defects in the OPCMs and their eutectics as LHTES materials. One is liquid leakage of the OPCMs while ambient temperature is over the phase change temperature and the other is low thermal conductivity, which restricts their application for thermal energy storage in the charging and discharging processes [12]. Encapsulating and supporting OPCMs is a popular effective method to the defects by using high heat-conducting supporting materials, such as metal foam [18–19], carbon nanospheres [20] and graphite-based materials [21–23]. Wang et al. studied the positive impact of expanded graphite (EG) on thermal conductivity of the OPCMs and the results showed that the conductivity of the compound greatly increased by 443.6% when the mass fraction of the EG was 10% [24]. Adding carbon nanoscale additives into the OPCMs has been investigated for enhancing thermal conductivity. Carbon nanotubes (CNTs) possess
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remarkable heat transfer properties with pretty high thermal conductivity and have a strong immunity for corrosion, decomposition and chemical attack, which make it possible to be incorporated with the OPCMs. The thermal conductivity of single-walled carbon nanotubes (SWCNTs) reach up to 4000 W• (m• K)-1 and that of multi-walled carbon nanotubes (MWCNTs) is approximately 2000 W• (m• K)-1 [25]. Zhang et al. [26] and Cui et al. [27] proved thermal conductivity of the PCMs could be enhanced with the increasing mass fraction of the CNTs respectively. Yu et al. [28] and Alshaer et al. [29] investigated the influence of CNTs on the thermal conductivity of the PCMs and confirmed that the thermal conductivity would be significantly increased as the CNTs loading contents increased. However, CNTs are easily collapsed and gathered, and the methods for CNTs dispersion usually include optimum physical blending, in situ polymerization and chemical functionalization [30]. In addition, the larger mass fraction of the CNTs is, the lower latent heat of the PCMs will be. Therefore, more reasonable proportion of the CNTs in the PCMs is required to investigate in consideration of practical applications [31]. So that the larger latent heat value of the PCMs can be acquired, but also the higher thermal conductivity value of the PCMs can be attained. In order to meet requirements for various phase change temperatures in building applications, two fatty acids can be mixed to form binary eutectics. The phase change temperature of the eutectics is lower than that of two fatty acids. In this work, the eutectic PCMs with a melting point around 43 ℃ can be used as latent heat storage materials for heat pump and solar hot water systems in building applications. The purpose of this work was to prepare the MA-SA binary fatty-acid eutectics with
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suitable phase change temperature and improve the thermal conductivity of the MA-SA eutectics by utilizing the CNTs. The phase change temperature and proportion of the MA-SA eutectics were determined by the experimental values. The CNTs were added into the eutectics with different mass fractions. The thermal properties, surface morphology and thermal conductivities of the CPCMs were investigated by DSC, SEM and TCM. Moreover, a concise formula was presented for predicting the thermal conductivity of CPCMs in this work and the factors for the supercooling decrease of the PCMs were also investigated by adding the CNTs acted as nucleating agents, which is seldom reported in the previous works. The results revealed that the thermal conductivity of the CPCMs can be enhanced and the supercooling of the CPCMs can be eliminated.
2. Experimental 2.1 Materials Stearic acid (C18H36O2, octadecanoic acid, Chemically Pure) and myristic acid (C14H28O2, tetradecane acid, Chemically Pure) were obtained from Sinopharm Chemical Reagent Co., Ltd and Qingdao Chemical technology Co., Ltd, respectively, and used as the PCMs for latent energy storage. Multi-walled carbon nanotubes (MWCNTs, Diameter: 20~30 nm, Length: 10~30 μm, Purity: >95%, Special surface area: >55 m2/g, Density: ~2.1 g/cm3) were provided by Nanjing XFNANO Materials Co., Ltd, and used as a thermal conductivity promoter. 2.2 Preparation of the MA-SA eutectics The mass fraction of the MA in the MA-SA eutectics was determined by taking
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experimental and theoretical values into consideration. A set of different mass fractions of the MA (0%, 25%, 54%, 57%, 63%, 66%, 72%, 90% and 100%) in the mixtures was prepared by water bath at 70 ℃ and the mixtures were stirred with a magnetic stirrer at a rate of 500 rpm for 30min. The thermal properties of different mixtures were measured by DSC, and shown in Table 1, Figs. 1 and 2. As seen in Table 1, Figs. 1 and 2, the melting temperatures of the MA and SA are 55.42 ℃ and 54.27 ℃. The melting temperature of the eutectics is 42.70 ℃ while the mass fraction of the MA in eutectics is 54%. The stearic acid (SA) and myristic acid (MA) are mixed together to form the eutectic mixtures. The atomic percentage ratio of the eutectic mixture is roughly determined by the Schrader equation [32]:
(1)
where TA ,
and
respectively represent the phase change temperature, the
enthalpy change and the molar fraction of the component A. R is the gas constant and T is the phase change temperature of the eutectic mixture. In this work, The melting temperature and latent heat of the SA is 54.27 ℃ and 189.82 kJ• kg-1, while the melting temperature and latent heat of the MA is 55.42 ℃ and 199.73 kJ• kg-1. The calculated phase change temperature of the mixture according to Eq. (1) is shown in Fig. 3. The experimental results were also shown in Fig. 3 for comparison with theoretical results. The mass fraction of the MA in eutectics is 54%, which could be derived from the Fig. 3. Fig. 1 Fig. 2 Fig. 3 8
Table 1
2.3 Preparation of the composite PCMs The MA–SA eutectics with the mass ratio 54/46 was prepared by water bath at 70 ℃ and the mixtures were stirred with a magnetic stirrer at a rate of 500 rpm for 30 min. In order to make the CNTs uniformly distribute in the MA-SA eutectics, the CNTs need to be functionalized with carboxyl [30]. The CNTs with carboxyl were added into the MA-SA eutectics according to the mass ratio of the CNTs listed in Table 2 while the eutectics was heated to 85 ℃ in water bath, and the mixtures were stirred continuously with a magnetic stirrer at a rate of 1000 rpm for 1 h [26]. After the mixtures were cooled down to the room temperature and dried in vacuum oven for 24 h, the composite PCMs were obtained. Fig. 4 shows a photograph of five CPCMs with different mass fractions of the CNTs (3%, 6%, 9%, 12% and 15%). It is seen that the CPCM1 (with 3% CNTs) and CPCM2 (with 6% CNTs) appeared stratification phenomenon obviously. So, the two CPCMs are not suitable as composite PCMs, and were not investigated in this work. However, CPCM3 (with 9% CNTs), CPCM4 (with 12% CNTs) and CPCM5 (with 15% CNTs) display uniformly distribution. Therefore, the CPCM3, CPCM4 and CPCM5 are used as composite PCMs, and thermal properties of the CPCM3, CPCM4 and CPCM5 were studied further. Fig. 4 Table 2
2.4 Characterization of the composite PCMs
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A field emission scanning electron microscope (SEM, ZEISS Ultra 55, Carl Zeiss, Germany) was used to characterize the morphology and microstructure of the composite PCMs. The gold was sputtered on surfaces of the samples to form electric conductor. The thermal properties of the CPCMs were determined by differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin-Elmer) at 5 ℃•min-1 under a constant stream of argon and the temperature range is 0-90 ℃. The accuracy of temperature measurements was ±0.2 ℃ and the enthalpy accuracy was ±5%. A thermal conductivity meter (TC 3020, Xiatech Electronic Technology co., Ltd) was used to measure the thermal conductivities of the CPCMs by using the hot-wire method. In order to determine the accuracy and reliability of the TC 3020, the pyrex glass (Pyrex 7740) and stainless steel (304L) were used as referenced materials to calibrate the accuracy of the thermal conductivity meter. After being calibrated, the accuracy of the thermal conductivity meter (TC 3020) is ±2.0%. The samples were melted in a beaker and the temperature was maintained at 52 ℃. A hot-wire probe was inserted into the samples vertically. When they are in thermal equilibrium, the line heat source should be heated with a constant heat flux. The temperatures of the line heat source and measured sample will rise, which makes the measurement of the thermal conductivity available on analyzing the resulting temperature curve by the software in the TC 3020. After that, the thermal conductivity values of the liquid CPCMs were recorded by TC 3020. Then, the samples were cooled to constant temperature of 32 ℃ while the hot-wire probe still stayed in the samples, and the thermal conductivity values of the solid CPCMs were recorded. The thermal conductivities of the CPCMs were measured repeatedly five times.
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3. Results and discussion 3.1 Microstructure of the CPCMs Fig. 5 shows the SEM photographs of the CNTs, CPCMs with different mass fractions of the CNTs. Fig. 5a presents the morphology of the CNTs while Figs. 5b-d shows that of the CPCM3, CPCM4 and CPCM5. The CNTs are easily agglomerate and difficult to be dispersed in composites because of the nanoscale diameter with high aspect ratio, large surface area and usually being supplied in the form of seriously tangled bundles [33]. Fig. 5a displays that the CNTs formed bundles widely. In this work, a magnetic stirrer was used to disperse the CNTs by physical methods. As shown in Figs. 5b-d, the eutectic PCMs were adsorbed on surfaces of the CNTs, and were filled in interspace among the CNTs. The samples are still melted partially but not broken up, which provides convenience for watching whether the CNTs formed serious tangled bundles or not. The results in the Figs. 5b-d indicate that the degree of aggregation of the CNTs is slight and the interstice of the CPCMs increases, which will retard volume expansion in phase change process. Fig. 5
3.2 Thermal properties of the CPCMs The latent heat, phase change temperature and supercooling degree are three important characteristics for latent heat storage. DSC test was carried out to determine these parameters in this work. In order to determine the reproducibility of the DSC test results, three phase change cycles were performed during DSC measurements. Figs. 6 and 7 illustrate the DSC curves of the melting and solidifying process of the MA-SA eutectics, CPCM3, CPCM4 and
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CPCM5, respectively, and the parameter values are presented in Table 3. The melting and solidifying latent heats of the CPCMs decrease with the increasing mass fraction of the CNTs because only the eutectic PCMs in the CPCMs absorb and release the thermal energy during melting and solidifying processes. As shown in Figs. 6 and 7, the phase change processes of the CPCMs are very similar to those of the MA-SA eutectics. The melting temperatures of the CPCM3, CPCM4 and CPCM5 are 42.60 ℃, 42.59 ℃ and 41.82 ℃, and are lower than that of the eutectics. The relationship between curved interface and equilibrium parameters (Gibbs-Thomson relation) may describe the reason of the melting temperature decrease, which is expressed by the following equation [34]:
Tr Tm
Tm SL s 1 1 ( ) lSL r1 r2
(2)
In Eq. (2), Tr is the melting temperature of the CPCMs and Tm is the melting point of the MA-SA eutectics.
SL
is the difference between the solid/wall interfacial energy and
liquid/wall interfacial energy and is about 10--2 – 10-3 J/m2 [35, 36]. s is the average volume of a single molecule of MA and SA and is about 5×10-28 m3 as the density and molar mass of fatty acids are about 0.8 g/cm3 and 220 g/mol. lSL is the average melting latent heat of a single molecule of MA and SA, and is about 6×10-20 J as the latent heat of MA-SA eutectic is 173.79 J/g. r1 and r2 are the minimum and maximum curvature radius of solid/liquid interface and they are also called principal curvature radius. If the interface is concave, r1 or r2 is less than 0. If the interface is convex, r1 or r2 is greater than 0. During the melting process of the CPCMs, the interface between the crystal and melt of the eutectics is convex. r1 is approximate to the radius of the CNTs (10–15 nm), 1/ r2 can be 12
ignored compared to the 1/ r1 . As a result, the Tr becomes smaller than the Tm. As seen from Table 3, the onset solidifying temperatures of the CPCMs are 0.70 ℃ (CPCM3), 0.83 ℃ (CPCM4) and 0.82 ℃ (CPCM5) higher than that of the MA-SA eutectics. Meanwhile, the supercooling degrees of the CPCMs decrease significantly with the increase of the CNTs content. The solidifying temperature of the CPCMs can be determined by the following equation [37]:
T Tm
Tm Gm Lm
(3)
where T is the solidifying temperature of the CPCMs, Tm is the melting point of the MA-SA eutectics, Lm is the latent heat of the MA-SA eutectics and
Gm
is the nucleating Gibbs free
energy. Tm and Lm are assumed to be constant. Therefore, the smaller
Gm
is, the higher T is.
The CNTs as nucleating agent can reduce the nucleating Gibbs free energy according to the theory of heterogeneous nucleation [38]. Therefore, the T will increase after the addition of CNTs. As shown in Table 3, the supercooling degree decrease of the CPCMs results from two factors. One factor is the decrease of the melting temperature of the CPCMs while the other factor is the increase of the solidifying temperature. It is proved that adding the CNTs is an extremely effective way to reduce the supercooling degree of the CPCMs. Fig. 6 Fig. 7 Table 3
3.3 Thermal conductivity of the CPCMs Thermal conductivity is also an important parameter for the PCMs in its application. The 13
thermal conductivity value measurement was repeated five times. The average thermal conductivity values and errors of the MA-SA eutectics, CPCM3, CPCM4 and CPCM5 are showed in Fig. 8 and Table 4. As seen in the Table 4, the thermal conductivity values of the MA-SA eutectics, CPCM3, CPCM4 and CPCM5 are 0.1726 W/(m• K), 0.2127 W/(m• K), 0.2578 W/(m• K) and 0.2825 W/(m• K) in the solid state (32 ℃), and 0.1711 W/(m• K), 0.1977 W/(m• K), 0.2258 W/(m• K) and 0.2391 W/(m• K) in the liquid state (52 ℃). Compared to the thermal conductivities of the MA-SA eutectics, the thermal conductivities of the CPCMs increase by 23.2%, 49.4% and 63.7% in the solid state, and 15.6%, 32.0% and 39.7% in the liquid state as the mass fractions of the CNTs are 9%, 12% and 15%. Obviously, the thermal conductivities of the solid CPCMs increase greater than those of the liquid CPCMs. It is known that the thermal conductivity of the CPCMs in the solid state is higher than that of the CPCMs in the liquid state due to its crystalline nature. The mechanisms of the thermal conductivity enhancement in solid state and liquid state have been clearly explained by Schiffres et al [39], Harish et al [40] and Angayarkanni et al [41]. The higher thermal conductivity enhancement of the CPCMs in the solid state is possibly due to the formation of continuous networking structure during the phase transition process. When the crystals begin to nucleate forming needle-like structures during solidifying, the CNTs are gradually pushed to the grain boundaries, thereby leading to form a continuous quasi-2D network of bundles which in turn recovers its original form when melted back. There is no significant thermal conductivity improvement of the CPCMs in the liquid state due to the absence of such continuous structures as the heat conduction is heavily limited by high interface resistance
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between the CNTs and the surrounding fluid and contact resistance between the CNTs [40]. There are several theoretical and empirical models to predict the thermal conductivity of two-phase composites [42]. Harish et al [43, 44] analyzed experimental thermal conductivity results based on the modified effective medium theory (EMT) model. The EMT model calculations show that the thermal conductivity enhancement is highly sensitive to the thermal interface resistance and the particle aspect ratio more than the thermal conductivity of particle itself. The experimental thermal conductivity enhancement can be compared with effective medium theory by taking the effect of interfacial thermal transport into account. In order to conveniently determine the thermal conductivity of the CPCMs in practical applications, the geometric mean (GMM) model is applied for calculating thermal conductivity values of the CPCMs in this work. In consideration of the quite high aspect ratio and thermal conductivity of the CNTs, the GMM model is taken into consideration and improved to approach the experimental value. The effective thermal conductivity of the composites is expressed by the following equation:
Km Ke1V Kc2V /3
(4)
In GMM model, Kc= 2000 W/(m• K) [25] is applied in Eq.3. Figs. 9 and 10 display the experimental and calculated thermal conductivity values of the CPCMs. The calculated thermal conductivity values of the GMM model basically corresponds to the experimental values. As seen in Figs. 9 and 10, the experimental thermal conductivities of the MA-SA eutectics and CPCMs in solid state are more close to the GMM model calculated values than those of the MA-SA eutectics and CPCMs in liquid state. This is due to a fact that, in the liquid CPCMs state, the GMM model ignored the boundary layer and heat transfer between
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the CNTs and the eutectics. The GMM model was also applied to predict thermal conductivity values of the palmitic-stearic acid (PA-SA)/CNTs. The calculated thermal conductivity values of the PA-SA/CNTs were compared with the experimental values [26]. Fig.11 presents the calculated thermal conductivity values of the PA-SA/CNTs and the experimental values. As known in Fig.11, the calculated values of the PA-SA/CNTs are in agreement with the experimental values. Therefore, The GMM model can be used to calculate thermal conductivity values of the composite PCMs with the CNTs. Generally, the enhancement of thermal conductivity can only attain 20%–30% when nano-oxides are used as thermal conductivity promoters. The nano-metal may acquire higher the enhancement of thermal conductivity, but the nano-metal can easily result in sedimentation in the CPCMs. In this work, thermal conductivity values of the CPCMs with the CNTs are in agreement with the experimental values [26]. So, the use of the CNTs as the promoters is reasonable, which can improve the thermal conductivity of the CPCMs in practical applications. Table 5 shows the comparison of the thermal conductivities of the prepared CPCMs with those of the CPCMs in the literatures. From Table 5, it is confirmed that the CPCMs obtain well thermal conductivity improvement in this work. Table 4 Fig. 8 Fig. 9 Fig. 10 Fig. 11
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Table 5
4. Conclusions The MA-SA eutectics were prepared in this work. The melting temperature of the MA-SA eutectics and the mass fraction of the MA in the eutectic mixtures are 42.70 ℃ and 54%. Then, a series of the MA-SA/CNTs with different mass fractions of the CNTs (3%, 6%, 9%, 12% and 15%) were successfully synthesized in order to investigate the thermal properties of the CPCMs. The minimum mass fraction of the CNTs in the CPCMs was determined to be 9% without stratification. The suitable ratio between CPCMs should be practically connected to the applications, and it can be determined in the practical applications [26, 31]. The phase change processes of the CPCMs are very similar to those of the MA-SA eutectics, but the latent heats of the CPCMs decrease with the increasing content of the CNTs. The supercooling degrees of the CPCMs are significantly reduced by adding the CNTs. The TCM test results showed that, compared to the thermal conductivities of the MA-SA eutectics, the thermal conductivities of the CPCMs increase by 23.2%, 49.4% and 63.7% in the solid state, and 15.6%, 32.0% and 39.7% in the liquid state as the mass fractions of the CNTs are 9%, 12% and 15%. However, the higher thermal conductivity results in the lower latent heat, the suitable ratio between CPCMs is still required to develop in practical applications [31]. According to the calculated and experimental results, the GMM model can be used to calculate thermal conductivity values of the composite PCMs with the CNTs. Based on all results above, the CNTs not only is the nucleating agent, but also is the thermal conductivity promoter for the CPCMs in building systems.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 51376087) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors also wish to thank the reviewers and editor for kindly giving revising suggestions.
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24
Figure captions Fig.1. DSC curves of the (a) MA, (b)–(g) MA-SA mixtures with different mass fractions of the MA and (h) SA during the melting process. Fig.2. DSC curves of the (a) MA, (b)–(g) MA-SA mixtures with different mass fractions of the MA and (h) SA during the solidifying process. Fig.3. Experimental and theoretical phase change temperature values of the MA-SA mixtures with different mass fractions of the MA. Fig.4. Photographs of the composite PCMs with different mass fractions of the CNTs in solid state. Fig.5. SEM images of the (a) CNTs, (b) CPCM3, (c) CPCM4 and (d) CPCM5. Fig.6. DSC curves of the MA eutectics, CPCM3, CPCM4 and CPCM5 during the melting process. Fig.7. DSC curves of the MA eutectics, CPCM3, CPCM4 and CPCM5 during the solidifying process. Fig.8. Thermal conductivities of the MA-SA eutectics and CPCMs with different mass fractions of the CNTs in solid and liquid states. Fig.9. Calculated and experimental thermal conductivities of the MA-SA eutectics and CPCMs with different mass fractions of the CNTs in solid state. Fig.10. Calculated and experimental thermal conductivities of the MA-SA eutectics and CPCMs with different mass fractions of the CNTs in liquid state.
25
Fig.11. Calculated thermal conductivity values of the PA-SA/CNTs and the experimental values [26].
26
Tables
Tables with Captions
Table 1 DSC data of the MA, PCM1–PCM7 and SA. Samples
Melting Onset
Peak
Solidifying Latent
Onset
Peak
Mass Latent
Temperature Temperature heat(k • Temperature Temperature heat(k • (℃)
(℃)
kg-1)
(℃)
(℃)
kg-1)
ratio of MA (%)
SA
54.27
56.79
189.82
53.06
52.05
178.87
0
PCM1
46.45
49.75
179.11
47.21
46.00
171.96
25
PCM2
42.70
45.08
174.30
42.04
40.81
168.62
54
PCM3
42.81
44.81
173.07
41.63
40.63
167.43
57
PCM4
42.94
45.32
172.60
41.06
39.54
167.28
63
PCM5
43.35
45.40
169.69
40.90
39.54
165.66
66
PCM6
43.85
46.05
170.38
41.15
40.07
162.70
72
PCM7
49.41
52.75
180.99
47.66
46.58
174.05
90
MA
55.42
56.49
199.73
52.67
52.09
198.41
100
27
Table 2 The compositions of the MA–SA eutectics/CNTs composite PCMs. Samples
MA-SA eutectics (g)
Nanotubes (g)
Mass ratio of nanotubes (%)
CPCM1
29.1
0.9
3
CPCM2
28.2
1.8
6
CPCM3
27.3
2.7
9
CPCM4
26.4
3.6
12
CPCM5
25.5
4.5
15
28
Table 3 DSC data of the MA–SA eutectics and CPCM3–CPCM5. Samples
Melting Onset
Peak
Solidifying Latent
Temperature Temperature heat(k • (℃)
(℃)
kg-1)
Onset
Peak
Supercooling Decrease Latent
degree (K)
Temperature Temperature heat(k • (℃)
(℃)
of melting
kg-1)
latent heat (%)
MA-SA
43.13±0.2
45.37±0.2
173.79±8.69 42.02±0.2
40.40±0.2
168.91±8.45
1.11
0
CPCM3
42.60±0.2
45.51±0.2
166.49±8.32 42.32±0.2
41.10±0.2
168.79±8.44
0.28
4.2
CPCM4
42.59±0.2
45.16±0.2
159.53±7.98 42.37±0.2
41.23±0.2
148.44±7.42
0.22
8.2
CPCM5
41.82±0.2
45.24±0.2
148.12±7.41 42.43±0.2
41.22±0.2
138.37±6.92
-0.61
14.8
29
Table 4 Thermal conductivities of the MA–SA eutectics and CPCM3–CPCM5. Samples Thermal conductivity in solid state at 32 ℃ (W• (m• K)-1)
Thermal conductivity in liquid state at 52 ℃ (W• (m• K)-1)
MA–SA
0.1726±0.0012
0.1711±0.0047
CPCM3
0.2127±0.0023
0.1977±0.0027
CPCM4
0.2578±0.0045
0.2258±0.0041
CPCM5
0.2825±0.0032
0.2391±0.0041
Table 5 Comparison of the thermal conductivities of CPCMs from literature with the prepared CPCMs. Samples
The enhancement ratio of
Reference
thermal conductivity (%) Stearic acid+10% CNTs
8.3
[45]
Soy wax +10%CNTs
24.4
[46]
PA-SA+8% CNTs
34.1
[26]
MA-SA +9% CNTs (solid)
49.4
Present study
30
Graphical Abstract
Graphical Abstract
Graphical abstract shows the SEM images of the (a) CNTs, (b) CPCM3, (c) CPCM4 and (d) CPCM5.
1
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7
Fig.8
Fig.9
Fig.10
Fig.11
S0378-7788(16)30520-5 http://dx.doi.org/doi:10.1016/j.enbuild.2016.06.031 ENB 6766
To appear in:
ENB
Received date: Revised date: Accepted date:
16-2-2016 29-5-2016 9-6-2016
Please cite this article as: Yaojie Tang, Guruprasad Alva, Xiang Huang, Di Su, Lingkun Liu, Guiyin Fang, Thermal properties and morphologies of MA–SA eutectics/CNTs as composite PCMs in thermal energy storage, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2016.06.031 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.
Thermal properties and morphologies of MA–SA eutectics/CNTs as composite PCMs in thermal energy storage
Yaojie Tang, Guruprasad Alva, Xiang Huang, Di Su, Lingkun Liu, Guiyin Fang* School of Physics, Nanjing University, Nanjing 210093, China *Corresponding author, E-mail address: [email protected]
1
Graphical Abstract
Graphical abstract shows the SEM images of the (a) CNTs, (b) CPCM3, (c) CPCM4 and (d) CPCM5.
2
Highlights
1. Myristic-stearic acids eutectics/CNTs as composite PCMs were prepared. 2. The minimum mass fraction of the CNTs in the composite PCMs was 9%. 3. Supercooling degrees of the composite PCMs decrease with the increase of the CNTs. 4. Thermal conductivity of the composite PCMs can be improved by adding CNTs.
Abstract: Myristic-stearic acids (MA-SA) binary eutectics with appropriate phase change temperature were prepared based on theoretical calculation. The melting temperature of the MA-SA eutectics and the mass fraction of the myristic acid (MA) in the eutectic mixtures are 42.70 ℃ and 54%, which was measured by Different Scanning Calorimeter (DSC). In order to enhance the thermal conductivity of the MA-SA eutectics, the carbon nanotubes (CNTs) were added into the MA-SA eutectics, and the effects of the CNTs on thermal conductivity of the MA-SA eutectics were investigated experimentally. The minimum mass fraction of the CNTs in the composite PCMs (CPCMs) was determined to be 9% without stratification. The thermal properties and microstructure of the CPCMs were investigated by the DSC and scanning electron microscopy (SEM), respectively. The DSC results showed that the CPCMs exhibited the similar solid-liquid phase change properties as the MA-SA eutectics, and the supercooling degrees of the CPCMs decrease obviously with increase of the CNTs content in the CPCMs. The thermal conductivity Meter (TCM) was used to measure the thermal conductivities of the CPCMs, and the test results indicated that the thermal conductivities of
3
the CPCMs increase significantly from 0.173 W/(m• K) to 0.213 W/(m• K), 0.258 W/ (m• K) and 0.283 W/(m• K) as the mass fractions of the CNTs in the CPCMs were 9 wt. %, 12 wt. % and 15 wt. %. The CPCMs with enhanced thermodynamic property will contribute to the energy management and storage in building systems.
Keywords:
Eutectic phase change material; Carbon nanotubes; Thermal properties;
Thermal conductivity enhancement; Thermal energy storage
1. Introduction In recent decades, there is more and more attention on thermal energy storage (TES) because of the global scenario of growing energy consumption and lack of fossil fuel resources. TES is a promising method to improve thermal management, efficiency and energy conservation. For instance, TES is used to redistribute the energy requirement on-peak demand condition (daytime) and off-peak consumption situation (nighttime) [1]. Thermal energy usually being generated through some energy conversion can be stored by sensible heat, reversible thermochemical reactions and latent heat thermal energy storage (LHTES). Among the three energy storage techniques above, the LHTES mainly based on the utilization of phase change materials (PCMs) has gained great impetus on account of the advantages of relatively high energy density and nearly constant operating temperature during phase transition processes [2]. With development of the LHTES, it is widely employed in building systems [3-4], air-conditioning [5–7], electronic devices [8], solar energy storage [9], etc. In general, PCMs that are intended for latent thermal energy storage can be categorized
4
into three types, namely, organic phase change materials (OPCMs), inorganic phase change materials (IPCMs) and their eutectic mixtures [10]. Although the IPCMs have an advantage over the OPCMs in energy storage density, easy phase segregation and high supercooling have greatly limited the application of the IPCMs without effective means [11]. The OPCMs, especially fatty acids, taking advantages of good chemical stability, little supercooling, high latent heat, nontoxic and self-nucleation behavior, etc. have been increasingly investigated on their synthesis and thermal properties [12–13]. In order to meet requirements for various phase change temperatures, two or three PCMs can be mixed to form binary or ternary eutectics [14–16]. Li et al. investigated binary fatty acid and the binary fatty acids/diatomite composite PCMs, and the results indicated that their corresponding phase change temperatures coincided well with experimental results [17]. However, there are two main defects in the OPCMs and their eutectics as LHTES materials. One is liquid leakage of the OPCMs while ambient temperature is over the phase change temperature and the other is low thermal conductivity, which restricts their application for thermal energy storage in the charging and discharging processes [12]. Encapsulating and supporting OPCMs is a popular effective method to the defects by using high heat-conducting supporting materials, such as metal foam [18–19], carbon nanospheres [20] and graphite-based materials [21–23]. Wang et al. studied the positive impact of expanded graphite (EG) on thermal conductivity of the OPCMs and the results showed that the conductivity of the compound greatly increased by 443.6% when the mass fraction of the EG was 10% [24]. Adding carbon nanoscale additives into the OPCMs has been investigated for enhancing thermal conductivity. Carbon nanotubes (CNTs) possess
5
remarkable heat transfer properties with pretty high thermal conductivity and have a strong immunity for corrosion, decomposition and chemical attack, which make it possible to be incorporated with the OPCMs. The thermal conductivity of single-walled carbon nanotubes (SWCNTs) reach up to 4000 W• (m• K)-1 and that of multi-walled carbon nanotubes (MWCNTs) is approximately 2000 W• (m• K)-1 [25]. Zhang et al. [26] and Cui et al. [27] proved thermal conductivity of the PCMs could be enhanced with the increasing mass fraction of the CNTs respectively. Yu et al. [28] and Alshaer et al. [29] investigated the influence of CNTs on the thermal conductivity of the PCMs and confirmed that the thermal conductivity would be significantly increased as the CNTs loading contents increased. However, CNTs are easily collapsed and gathered, and the methods for CNTs dispersion usually include optimum physical blending, in situ polymerization and chemical functionalization [30]. In addition, the larger mass fraction of the CNTs is, the lower latent heat of the PCMs will be. Therefore, more reasonable proportion of the CNTs in the PCMs is required to investigate in consideration of practical applications [31]. So that the larger latent heat value of the PCMs can be acquired, but also the higher thermal conductivity value of the PCMs can be attained. In order to meet requirements for various phase change temperatures in building applications, two fatty acids can be mixed to form binary eutectics. The phase change temperature of the eutectics is lower than that of two fatty acids. In this work, the eutectic PCMs with a melting point around 43 ℃ can be used as latent heat storage materials for heat pump and solar hot water systems in building applications. The purpose of this work was to prepare the MA-SA binary fatty-acid eutectics with
6
suitable phase change temperature and improve the thermal conductivity of the MA-SA eutectics by utilizing the CNTs. The phase change temperature and proportion of the MA-SA eutectics were determined by the experimental values. The CNTs were added into the eutectics with different mass fractions. The thermal properties, surface morphology and thermal conductivities of the CPCMs were investigated by DSC, SEM and TCM. Moreover, a concise formula was presented for predicting the thermal conductivity of CPCMs in this work and the factors for the supercooling decrease of the PCMs were also investigated by adding the CNTs acted as nucleating agents, which is seldom reported in the previous works. The results revealed that the thermal conductivity of the CPCMs can be enhanced and the supercooling of the CPCMs can be eliminated.
2. Experimental 2.1 Materials Stearic acid (C18H36O2, octadecanoic acid, Chemically Pure) and myristic acid (C14H28O2, tetradecane acid, Chemically Pure) were obtained from Sinopharm Chemical Reagent Co., Ltd and Qingdao Chemical technology Co., Ltd, respectively, and used as the PCMs for latent energy storage. Multi-walled carbon nanotubes (MWCNTs, Diameter: 20~30 nm, Length: 10~30 μm, Purity: >95%, Special surface area: >55 m2/g, Density: ~2.1 g/cm3) were provided by Nanjing XFNANO Materials Co., Ltd, and used as a thermal conductivity promoter. 2.2 Preparation of the MA-SA eutectics The mass fraction of the MA in the MA-SA eutectics was determined by taking
7
experimental and theoretical values into consideration. A set of different mass fractions of the MA (0%, 25%, 54%, 57%, 63%, 66%, 72%, 90% and 100%) in the mixtures was prepared by water bath at 70 ℃ and the mixtures were stirred with a magnetic stirrer at a rate of 500 rpm for 30min. The thermal properties of different mixtures were measured by DSC, and shown in Table 1, Figs. 1 and 2. As seen in Table 1, Figs. 1 and 2, the melting temperatures of the MA and SA are 55.42 ℃ and 54.27 ℃. The melting temperature of the eutectics is 42.70 ℃ while the mass fraction of the MA in eutectics is 54%. The stearic acid (SA) and myristic acid (MA) are mixed together to form the eutectic mixtures. The atomic percentage ratio of the eutectic mixture is roughly determined by the Schrader equation [32]:
(1)
where TA ,
and
respectively represent the phase change temperature, the
enthalpy change and the molar fraction of the component A. R is the gas constant and T is the phase change temperature of the eutectic mixture. In this work, The melting temperature and latent heat of the SA is 54.27 ℃ and 189.82 kJ• kg-1, while the melting temperature and latent heat of the MA is 55.42 ℃ and 199.73 kJ• kg-1. The calculated phase change temperature of the mixture according to Eq. (1) is shown in Fig. 3. The experimental results were also shown in Fig. 3 for comparison with theoretical results. The mass fraction of the MA in eutectics is 54%, which could be derived from the Fig. 3. Fig. 1 Fig. 2 Fig. 3 8
Table 1
2.3 Preparation of the composite PCMs The MA–SA eutectics with the mass ratio 54/46 was prepared by water bath at 70 ℃ and the mixtures were stirred with a magnetic stirrer at a rate of 500 rpm for 30 min. In order to make the CNTs uniformly distribute in the MA-SA eutectics, the CNTs need to be functionalized with carboxyl [30]. The CNTs with carboxyl were added into the MA-SA eutectics according to the mass ratio of the CNTs listed in Table 2 while the eutectics was heated to 85 ℃ in water bath, and the mixtures were stirred continuously with a magnetic stirrer at a rate of 1000 rpm for 1 h [26]. After the mixtures were cooled down to the room temperature and dried in vacuum oven for 24 h, the composite PCMs were obtained. Fig. 4 shows a photograph of five CPCMs with different mass fractions of the CNTs (3%, 6%, 9%, 12% and 15%). It is seen that the CPCM1 (with 3% CNTs) and CPCM2 (with 6% CNTs) appeared stratification phenomenon obviously. So, the two CPCMs are not suitable as composite PCMs, and were not investigated in this work. However, CPCM3 (with 9% CNTs), CPCM4 (with 12% CNTs) and CPCM5 (with 15% CNTs) display uniformly distribution. Therefore, the CPCM3, CPCM4 and CPCM5 are used as composite PCMs, and thermal properties of the CPCM3, CPCM4 and CPCM5 were studied further. Fig. 4 Table 2
2.4 Characterization of the composite PCMs
9
A field emission scanning electron microscope (SEM, ZEISS Ultra 55, Carl Zeiss, Germany) was used to characterize the morphology and microstructure of the composite PCMs. The gold was sputtered on surfaces of the samples to form electric conductor. The thermal properties of the CPCMs were determined by differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin-Elmer) at 5 ℃•min-1 under a constant stream of argon and the temperature range is 0-90 ℃. The accuracy of temperature measurements was ±0.2 ℃ and the enthalpy accuracy was ±5%. A thermal conductivity meter (TC 3020, Xiatech Electronic Technology co., Ltd) was used to measure the thermal conductivities of the CPCMs by using the hot-wire method. In order to determine the accuracy and reliability of the TC 3020, the pyrex glass (Pyrex 7740) and stainless steel (304L) were used as referenced materials to calibrate the accuracy of the thermal conductivity meter. After being calibrated, the accuracy of the thermal conductivity meter (TC 3020) is ±2.0%. The samples were melted in a beaker and the temperature was maintained at 52 ℃. A hot-wire probe was inserted into the samples vertically. When they are in thermal equilibrium, the line heat source should be heated with a constant heat flux. The temperatures of the line heat source and measured sample will rise, which makes the measurement of the thermal conductivity available on analyzing the resulting temperature curve by the software in the TC 3020. After that, the thermal conductivity values of the liquid CPCMs were recorded by TC 3020. Then, the samples were cooled to constant temperature of 32 ℃ while the hot-wire probe still stayed in the samples, and the thermal conductivity values of the solid CPCMs were recorded. The thermal conductivities of the CPCMs were measured repeatedly five times.
10
3. Results and discussion 3.1 Microstructure of the CPCMs Fig. 5 shows the SEM photographs of the CNTs, CPCMs with different mass fractions of the CNTs. Fig. 5a presents the morphology of the CNTs while Figs. 5b-d shows that of the CPCM3, CPCM4 and CPCM5. The CNTs are easily agglomerate and difficult to be dispersed in composites because of the nanoscale diameter with high aspect ratio, large surface area and usually being supplied in the form of seriously tangled bundles [33]. Fig. 5a displays that the CNTs formed bundles widely. In this work, a magnetic stirrer was used to disperse the CNTs by physical methods. As shown in Figs. 5b-d, the eutectic PCMs were adsorbed on surfaces of the CNTs, and were filled in interspace among the CNTs. The samples are still melted partially but not broken up, which provides convenience for watching whether the CNTs formed serious tangled bundles or not. The results in the Figs. 5b-d indicate that the degree of aggregation of the CNTs is slight and the interstice of the CPCMs increases, which will retard volume expansion in phase change process. Fig. 5
3.2 Thermal properties of the CPCMs The latent heat, phase change temperature and supercooling degree are three important characteristics for latent heat storage. DSC test was carried out to determine these parameters in this work. In order to determine the reproducibility of the DSC test results, three phase change cycles were performed during DSC measurements. Figs. 6 and 7 illustrate the DSC curves of the melting and solidifying process of the MA-SA eutectics, CPCM3, CPCM4 and
11
CPCM5, respectively, and the parameter values are presented in Table 3. The melting and solidifying latent heats of the CPCMs decrease with the increasing mass fraction of the CNTs because only the eutectic PCMs in the CPCMs absorb and release the thermal energy during melting and solidifying processes. As shown in Figs. 6 and 7, the phase change processes of the CPCMs are very similar to those of the MA-SA eutectics. The melting temperatures of the CPCM3, CPCM4 and CPCM5 are 42.60 ℃, 42.59 ℃ and 41.82 ℃, and are lower than that of the eutectics. The relationship between curved interface and equilibrium parameters (Gibbs-Thomson relation) may describe the reason of the melting temperature decrease, which is expressed by the following equation [34]:
Tr Tm
Tm SL s 1 1 ( ) lSL r1 r2
(2)
In Eq. (2), Tr is the melting temperature of the CPCMs and Tm is the melting point of the MA-SA eutectics.
SL
is the difference between the solid/wall interfacial energy and
liquid/wall interfacial energy and is about 10--2 – 10-3 J/m2 [35, 36]. s is the average volume of a single molecule of MA and SA and is about 5×10-28 m3 as the density and molar mass of fatty acids are about 0.8 g/cm3 and 220 g/mol. lSL is the average melting latent heat of a single molecule of MA and SA, and is about 6×10-20 J as the latent heat of MA-SA eutectic is 173.79 J/g. r1 and r2 are the minimum and maximum curvature radius of solid/liquid interface and they are also called principal curvature radius. If the interface is concave, r1 or r2 is less than 0. If the interface is convex, r1 or r2 is greater than 0. During the melting process of the CPCMs, the interface between the crystal and melt of the eutectics is convex. r1 is approximate to the radius of the CNTs (10–15 nm), 1/ r2 can be 12
ignored compared to the 1/ r1 . As a result, the Tr becomes smaller than the Tm. As seen from Table 3, the onset solidifying temperatures of the CPCMs are 0.70 ℃ (CPCM3), 0.83 ℃ (CPCM4) and 0.82 ℃ (CPCM5) higher than that of the MA-SA eutectics. Meanwhile, the supercooling degrees of the CPCMs decrease significantly with the increase of the CNTs content. The solidifying temperature of the CPCMs can be determined by the following equation [37]:
T Tm
Tm Gm Lm
(3)
where T is the solidifying temperature of the CPCMs, Tm is the melting point of the MA-SA eutectics, Lm is the latent heat of the MA-SA eutectics and
Gm
is the nucleating Gibbs free
energy. Tm and Lm are assumed to be constant. Therefore, the smaller
Gm
is, the higher T is.
The CNTs as nucleating agent can reduce the nucleating Gibbs free energy according to the theory of heterogeneous nucleation [38]. Therefore, the T will increase after the addition of CNTs. As shown in Table 3, the supercooling degree decrease of the CPCMs results from two factors. One factor is the decrease of the melting temperature of the CPCMs while the other factor is the increase of the solidifying temperature. It is proved that adding the CNTs is an extremely effective way to reduce the supercooling degree of the CPCMs. Fig. 6 Fig. 7 Table 3
3.3 Thermal conductivity of the CPCMs Thermal conductivity is also an important parameter for the PCMs in its application. The 13
thermal conductivity value measurement was repeated five times. The average thermal conductivity values and errors of the MA-SA eutectics, CPCM3, CPCM4 and CPCM5 are showed in Fig. 8 and Table 4. As seen in the Table 4, the thermal conductivity values of the MA-SA eutectics, CPCM3, CPCM4 and CPCM5 are 0.1726 W/(m• K), 0.2127 W/(m• K), 0.2578 W/(m• K) and 0.2825 W/(m• K) in the solid state (32 ℃), and 0.1711 W/(m• K), 0.1977 W/(m• K), 0.2258 W/(m• K) and 0.2391 W/(m• K) in the liquid state (52 ℃). Compared to the thermal conductivities of the MA-SA eutectics, the thermal conductivities of the CPCMs increase by 23.2%, 49.4% and 63.7% in the solid state, and 15.6%, 32.0% and 39.7% in the liquid state as the mass fractions of the CNTs are 9%, 12% and 15%. Obviously, the thermal conductivities of the solid CPCMs increase greater than those of the liquid CPCMs. It is known that the thermal conductivity of the CPCMs in the solid state is higher than that of the CPCMs in the liquid state due to its crystalline nature. The mechanisms of the thermal conductivity enhancement in solid state and liquid state have been clearly explained by Schiffres et al [39], Harish et al [40] and Angayarkanni et al [41]. The higher thermal conductivity enhancement of the CPCMs in the solid state is possibly due to the formation of continuous networking structure during the phase transition process. When the crystals begin to nucleate forming needle-like structures during solidifying, the CNTs are gradually pushed to the grain boundaries, thereby leading to form a continuous quasi-2D network of bundles which in turn recovers its original form when melted back. There is no significant thermal conductivity improvement of the CPCMs in the liquid state due to the absence of such continuous structures as the heat conduction is heavily limited by high interface resistance
14
between the CNTs and the surrounding fluid and contact resistance between the CNTs [40]. There are several theoretical and empirical models to predict the thermal conductivity of two-phase composites [42]. Harish et al [43, 44] analyzed experimental thermal conductivity results based on the modified effective medium theory (EMT) model. The EMT model calculations show that the thermal conductivity enhancement is highly sensitive to the thermal interface resistance and the particle aspect ratio more than the thermal conductivity of particle itself. The experimental thermal conductivity enhancement can be compared with effective medium theory by taking the effect of interfacial thermal transport into account. In order to conveniently determine the thermal conductivity of the CPCMs in practical applications, the geometric mean (GMM) model is applied for calculating thermal conductivity values of the CPCMs in this work. In consideration of the quite high aspect ratio and thermal conductivity of the CNTs, the GMM model is taken into consideration and improved to approach the experimental value. The effective thermal conductivity of the composites is expressed by the following equation:
Km Ke1V Kc2V /3
(4)
In GMM model, Kc= 2000 W/(m• K) [25] is applied in Eq.3. Figs. 9 and 10 display the experimental and calculated thermal conductivity values of the CPCMs. The calculated thermal conductivity values of the GMM model basically corresponds to the experimental values. As seen in Figs. 9 and 10, the experimental thermal conductivities of the MA-SA eutectics and CPCMs in solid state are more close to the GMM model calculated values than those of the MA-SA eutectics and CPCMs in liquid state. This is due to a fact that, in the liquid CPCMs state, the GMM model ignored the boundary layer and heat transfer between
15
the CNTs and the eutectics. The GMM model was also applied to predict thermal conductivity values of the palmitic-stearic acid (PA-SA)/CNTs. The calculated thermal conductivity values of the PA-SA/CNTs were compared with the experimental values [26]. Fig.11 presents the calculated thermal conductivity values of the PA-SA/CNTs and the experimental values. As known in Fig.11, the calculated values of the PA-SA/CNTs are in agreement with the experimental values. Therefore, The GMM model can be used to calculate thermal conductivity values of the composite PCMs with the CNTs. Generally, the enhancement of thermal conductivity can only attain 20%–30% when nano-oxides are used as thermal conductivity promoters. The nano-metal may acquire higher the enhancement of thermal conductivity, but the nano-metal can easily result in sedimentation in the CPCMs. In this work, thermal conductivity values of the CPCMs with the CNTs are in agreement with the experimental values [26]. So, the use of the CNTs as the promoters is reasonable, which can improve the thermal conductivity of the CPCMs in practical applications. Table 5 shows the comparison of the thermal conductivities of the prepared CPCMs with those of the CPCMs in the literatures. From Table 5, it is confirmed that the CPCMs obtain well thermal conductivity improvement in this work. Table 4 Fig. 8 Fig. 9 Fig. 10 Fig. 11
16
Table 5
4. Conclusions The MA-SA eutectics were prepared in this work. The melting temperature of the MA-SA eutectics and the mass fraction of the MA in the eutectic mixtures are 42.70 ℃ and 54%. Then, a series of the MA-SA/CNTs with different mass fractions of the CNTs (3%, 6%, 9%, 12% and 15%) were successfully synthesized in order to investigate the thermal properties of the CPCMs. The minimum mass fraction of the CNTs in the CPCMs was determined to be 9% without stratification. The suitable ratio between CPCMs should be practically connected to the applications, and it can be determined in the practical applications [26, 31]. The phase change processes of the CPCMs are very similar to those of the MA-SA eutectics, but the latent heats of the CPCMs decrease with the increasing content of the CNTs. The supercooling degrees of the CPCMs are significantly reduced by adding the CNTs. The TCM test results showed that, compared to the thermal conductivities of the MA-SA eutectics, the thermal conductivities of the CPCMs increase by 23.2%, 49.4% and 63.7% in the solid state, and 15.6%, 32.0% and 39.7% in the liquid state as the mass fractions of the CNTs are 9%, 12% and 15%. However, the higher thermal conductivity results in the lower latent heat, the suitable ratio between CPCMs is still required to develop in practical applications [31]. According to the calculated and experimental results, the GMM model can be used to calculate thermal conductivity values of the composite PCMs with the CNTs. Based on all results above, the CNTs not only is the nucleating agent, but also is the thermal conductivity promoter for the CPCMs in building systems.
17
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 51376087) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors also wish to thank the reviewers and editor for kindly giving revising suggestions.
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Figure captions Fig.1. DSC curves of the (a) MA, (b)–(g) MA-SA mixtures with different mass fractions of the MA and (h) SA during the melting process. Fig.2. DSC curves of the (a) MA, (b)–(g) MA-SA mixtures with different mass fractions of the MA and (h) SA during the solidifying process. Fig.3. Experimental and theoretical phase change temperature values of the MA-SA mixtures with different mass fractions of the MA. Fig.4. Photographs of the composite PCMs with different mass fractions of the CNTs in solid state. Fig.5. SEM images of the (a) CNTs, (b) CPCM3, (c) CPCM4 and (d) CPCM5. Fig.6. DSC curves of the MA eutectics, CPCM3, CPCM4 and CPCM5 during the melting process. Fig.7. DSC curves of the MA eutectics, CPCM3, CPCM4 and CPCM5 during the solidifying process. Fig.8. Thermal conductivities of the MA-SA eutectics and CPCMs with different mass fractions of the CNTs in solid and liquid states. Fig.9. Calculated and experimental thermal conductivities of the MA-SA eutectics and CPCMs with different mass fractions of the CNTs in solid state. Fig.10. Calculated and experimental thermal conductivities of the MA-SA eutectics and CPCMs with different mass fractions of the CNTs in liquid state.
25
Fig.11. Calculated thermal conductivity values of the PA-SA/CNTs and the experimental values [26].
26
Tables
Tables with Captions
Table 1 DSC data of the MA, PCM1–PCM7 and SA. Samples
Melting Onset
Peak
Solidifying Latent
Onset
Peak
Mass Latent
Temperature Temperature heat(k • Temperature Temperature heat(k • (℃)
(℃)
kg-1)
(℃)
(℃)
kg-1)
ratio of MA (%)
SA
54.27
56.79
189.82
53.06
52.05
178.87
0
PCM1
46.45
49.75
179.11
47.21
46.00
171.96
25
PCM2
42.70
45.08
174.30
42.04
40.81
168.62
54
PCM3
42.81
44.81
173.07
41.63
40.63
167.43
57
PCM4
42.94
45.32
172.60
41.06
39.54
167.28
63
PCM5
43.35
45.40
169.69
40.90
39.54
165.66
66
PCM6
43.85
46.05
170.38
41.15
40.07
162.70
72
PCM7
49.41
52.75
180.99
47.66
46.58
174.05
90
MA
55.42
56.49
199.73
52.67
52.09
198.41
100
27
Table 2 The compositions of the MA–SA eutectics/CNTs composite PCMs. Samples
MA-SA eutectics (g)
Nanotubes (g)
Mass ratio of nanotubes (%)
CPCM1
29.1
0.9
3
CPCM2
28.2
1.8
6
CPCM3
27.3
2.7
9
CPCM4
26.4
3.6
12
CPCM5
25.5
4.5
15
28
Table 3 DSC data of the MA–SA eutectics and CPCM3–CPCM5. Samples
Melting Onset
Peak
Solidifying Latent
Temperature Temperature heat(k • (℃)
(℃)
kg-1)
Onset
Peak
Supercooling Decrease Latent
degree (K)
Temperature Temperature heat(k • (℃)
(℃)
of melting
kg-1)
latent heat (%)
MA-SA
43.13±0.2
45.37±0.2
173.79±8.69 42.02±0.2
40.40±0.2
168.91±8.45
1.11
0
CPCM3
42.60±0.2
45.51±0.2
166.49±8.32 42.32±0.2
41.10±0.2
168.79±8.44
0.28
4.2
CPCM4
42.59±0.2
45.16±0.2
159.53±7.98 42.37±0.2
41.23±0.2
148.44±7.42
0.22
8.2
CPCM5
41.82±0.2
45.24±0.2
148.12±7.41 42.43±0.2
41.22±0.2
138.37±6.92
-0.61
14.8
29
Table 4 Thermal conductivities of the MA–SA eutectics and CPCM3–CPCM5. Samples Thermal conductivity in solid state at 32 ℃ (W• (m• K)-1)
Thermal conductivity in liquid state at 52 ℃ (W• (m• K)-1)
MA–SA
0.1726±0.0012
0.1711±0.0047
CPCM3
0.2127±0.0023
0.1977±0.0027
CPCM4
0.2578±0.0045
0.2258±0.0041
CPCM5
0.2825±0.0032
0.2391±0.0041
Table 5 Comparison of the thermal conductivities of CPCMs from literature with the prepared CPCMs. Samples
The enhancement ratio of
Reference
thermal conductivity (%) Stearic acid+10% CNTs
8.3
[45]
Soy wax +10%CNTs
24.4
[46]
PA-SA+8% CNTs
34.1
[26]
MA-SA +9% CNTs (solid)
49.4
Present study
30
Graphical Abstract
Graphical Abstract
Graphical abstract shows the SEM images of the (a) CNTs, (b) CPCM3, (c) CPCM4 and (d) CPCM5.
1
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7
Fig.8
Fig.9
Fig.10
Fig.11