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expanded graphite composite phase change material for thermal energy storage
Composites: Part A 87 (2016) 138–145
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A capric–palmitic–stearic acid ternary eutectic mixture/expanded graphite composite phase change material for thermal energy storage Hua Zhang a, Xuenong Gao a,⇑, Caixing Chen a, Tao Xu b,⇑, Yutang Fang a, Zhengguo Zhang a a Key Laboratory of Enhanced Heat Transfer and Energy Conservation, The Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 25 January 2016 Received in revised form 30 March 2016 Accepted 23 April 2016 Available online 23 April 2016 Keywords: Phase change material CA–PA–SA ternary eutectic mixture Expanded graphite Low-temperature thermal energy storage
a b s t r a c t This paper demonstrated a capric acid–palmitic acid–stearic acid ternary eutectic mixture/expanded graphite (CA–PA–SA/EG) composite phase change material (PCM) for low-temperature heat storage. The CA– PA–SA ternary eutectic mixture with a mass ratio of CA:PA:SA = 79.3:14.7:6.0 was prepared firstly, and its mass ratio in the CA–PA–SA/EG composite can reach as high as 90%. The melting and freezing temperatures of CA–PA–SA/EG composite were 21.33 °C and 19.01 °C, and the corresponding latent heat were 131.7 kJ kg1 and 127.2 kJ kg1. The CA–PA–SA/EG composite powders can be formed into round blocks by dry pressing easily, with much higher thermal conductivity than CA–PA–SA. Thermal performance test showed that the increasing thermal conductivity of CA–PA–SA could obviously decrease the melting/cooling time. Thermal property characterizations after 500 heating/cooling cycles test indicated that CA–PA–SA/EG composite PCM had excellent thermal reliability. Based on all these results, CA–PA–SA/ EG composite PCM is a promising material for low-temperature thermal energy storage applications. Ó 2016 Published by Elsevier Ltd.
1. Introduction Phase change materials (PCMs) refer to materials that can store or release heat energy during their phase transition process at a nearly constant temperature. Numerous efforts have been made to explore PCMs for realizing the control of environmental temperature and matching the energy supply and demand in time and space due to their high energy storage density. PCMs have a heat storage capacity, which is about 5–14 times higher than the conventional (sensible) thermal storage materials such as water, rock and masonry. A large number of PCMs are known to melt with a heat of fusion in any required range [1]. Therefore, PCMs have been explored in many applications such as building energy conservation, solar energy storage, low temperature refrigeration and indoor temperature controlling. Among the numerous investigated PCMs, fatty acids have been widely employed by virtue of their following desirable features, high heat capacity, good thermal and chemical stability, small volume change, self-nucleating behavior, non-toxicity, noncorrosiveness, low vapor pressure, negligible supercooling and cost-effectiveness [2]. However, the individual fatty acids are
⇑ Corresponding authors. E-mail address: [email protected] (X. Gao). http://dx.doi.org/10.1016/j.compositesa.2016.04.024 1359-835X/Ó 2016 Published by Elsevier Ltd.
restricted in some practical applications for their fixed phase change temperatures. Fortunately, the phase transition temperatures can be tailored into a suitable range by preparing binary or ternary eutectic mixtures of fatty acids [3]. Capric acid (CA), lauric acid (LA), myristic acid (MA) and stearic acid (SA) were selected to prepare binary fatty acid eutectic (CA–LA, CA–MA, CA–SA and LA–MA) by Wang and Meng [4], and phase change temperatures of the composites CA–LA/PMMA, CA–MA/PMMA, CA–SA/PMMA and LA–MA/PMMA (50/50 wt.%) were adjusted to a suitable range for building energy conservation. Sarı [5] prepared three binary eutectic mixtures including LA–MA, LA–PA and MA–SA with a melting temperature of 34.2 °C, 35.2 °C and 44.1 °C, which are lower than the individual fatty acids. Cai et al. [6] prepared five binary fatty acid eutectics LA–MA, LA–PA, MA–PA, MA–SA and PA–SA through Schrader equation, whose melting temperatures ranged from 33.27 °C to 53.69 °C, and freezing temperatures ranged from 33.71 °C to 53.45 °C. Then, an innovative type of electrospun binary fatty acid eutectics/PET ultrafine composite fibers for latent heat storage in the field of building energy conservation were successfully fabricated as form-stable PCMs. Nonetheless, fatty acids and their eutectic mixtures have a notable drawback of low thermal conductivity, which seriously limits the heat transfer rates during phase transition. In order to overcome the problem, the combination of PCMs with porous
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matrix materials to prepare form-stable PCMs has been proven to be an effective way [7]. Furthermore, the leakage during solid– liquid phase transition processes can be prevented due to the capillary and surface tension forces of the porous matrix materials [8], The form-stable PCMs containing fatty acids and/or their eutectics can be prepared through various methods such as physical adsorption [9], sol–gel [10,11], microencapsulation [12] and electrospinning [13]. Among the investigated methods, directly incorporating PCMs into porous materials is a simple, convenient, low cost and effective technique. Since previous reported research, expanded graphite (EG) has been verified to be the ideal porous matrix material owing to its advantages of high thermal conductivity, large pore volume, low density and good compatibility [14]. The recent studies are mainly focused on fatty acids and/or their binary eutectics, investigations on ternary fatty acid eutectic mixtures as PCMs have rarely been reported. Zhang et al. [15] prepared LA–MA–PA/EG composite PCM, Liu et al. [16] prepared LA–MA–SA/ EG composite PCM and Yang et al. [17] prepared MA–PA–SA/EG composite PCM with the phase change temperature of 30.94 °C, 29.05 °C and 41.64 °C. To the best of our knowledge, the investigation of CA–PA–SA ternary eutectic mixture as PCM has not been reported. Moreover, compared with medium-temperature and high-temperature applications have been well developed, lowtemperature thermal applications such as building energy conservation, low temperature refrigeration and indoor temperature controlling have dropped behind but show great potential in the future, especially in heat storage systems on the use of PCMs with the phase change temperatures below 25 °C [18]. Hence, this research is aimed at preparing a form-stable PCM for lowtemperature latent heat storage applications. Ternary eutectic mixture of CA–PA–SA with a melting temperature of about 20 °C was prepared firstly. Then, the supporting material EG was prepared through microwave method [7]. Finally, CA–PA–SA were absorbed into EG to fabricate a form-stable CA–PA–SA/EG composite PCM. The microstructure, thermal properties and stability of CA–PA–SA/EG composite PCM were characterized by Scanning electron microscopy (SEM), X-ray diffractometer (XRD), Fourier transfer infrared (FT-IR), Differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). The thermal cycling test was conducted to determine the thermal reliability of CA–PA–SA/ EG composite. Afterward, the influences of EG on thermal conductivity and thermal energy storage/retrieval rates of CA–PA–SA/EG composite PCM were also investigated. 2. Experimental 2.1. Materials Capric acid (CA, CH3(CH2)8COOH, 98%, Chemical Pure), palmitic acid (PA, CH3(CH2)14COOH, 98%, Chemical Pure), stearic acid (SA, CH3(CH2)16COOH, 98%, Analytical Reagent) were supplied by Shanghai Lingfeng Chemical Reagents Co. (Shanghai, China). Expandable graphite (mesh 32, expansion ration: 300 ml g1) was purchased from Qingdao Graphite Co. Ltd., China. The chemicals were used as received without further purification. 2.2. Preparation of CA–PA–SA ternary eutectic mixture Based on the lowest eutectic point theory, fatty acids can be blended together with an eutectic ratio to achieve the eutectic temperature, which is lower than those of individual fatty acids. The eutectic ratios of fatty acid mixture were calculated by the Schrader equation [3,15,17].
T ¼ 1=ð1=T i R ln X i =DHi Þ
ð1Þ
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where Ti and DHi are the phase change temperature and latent heat of the ith fatty acid, T is the phase change temperature of the eutectic mixture, Xi is the content of the ith substance in the eutectic mixture, and R is ideal gas constant. Through the above equation, the mass ratio of CA–PA binary eutectic mixture was calculated first and was acted as a kind of ‘‘single” fatty acid, and then the mass ratio of single fatty acid (i.e., CA–PA) and SA was calculated again. Finally, the mass ratios for CA–PA–SA ternary eutectic mixture were determined at 79.3:14.7:6.0 and the calculated melting point of the mixture was 25.71 °C. The CA–PA–SA ternary eutectic mixture was prepared through melt-blending. Three fatty acids (CA, PA and SA) with the calculated eutectic mass ratio were first mixed in a sealed beaker, then placed into an oven at 70 °C for 2 h until totally melted. Subsequently, the molten mixture was stirred in a magnetic stirrer for 2 h at 300 r min1 to ensure the homogeneity of the mixture, and then slowly cooled down to ambient temperature. 2.3. Preparation of CA–PA–SA/EG composite PCM The CA–PA–SA/EG composite PCMs containing 85 wt.%, 90 wt.%, and 95 wt.% CA–PA–SA were prepared by absorbing different amounts of liquid CA–PA–SA into the pores of EG. To determine the maximum mass fraction of CA–PA–SA that can be absorbed in the composite PCM, the leakage test was carried out as follows. Three CA–PA–SA/EG composite PCM samples containing 85 wt.%, 90 wt.%, and 95 wt.% CA–PA–SA were placed on three pieces of filter paper, and then inserted into an oven at a temperature of 40 °C (above the melting point of CA–PA–SA) for 8 h to make sure the phase change of CA–PA–SA from solid to liquid takes place. After cooling to ambient temperature, the samples were removed from the corresponding filter paper. Finally, a careful examination was given to each piece of filter paper to find out whether there were any traces of liquid CA–PA–SA left on it. Furthermore, the weights of the samples were compared before and after the thermal treatment to further judge whether the leakage happened. 2.4. Fabrication of compressed CA–PA–SA/EG composite PCM The CA–PA–SA/EG composite PCM powders were formed into several round blocks by dry pressing with a cylindrical mold (4 cm inside diameter and 1 cm height) under the pressure of 100 kg cm2. Actual packing densities were calculated by the actual masses to volumes of the formed blocks, which were 435.7, 526.7, 615.4, 710.4 and 743.3 kg m3, respectively. Hot disk thermal constant analyzer (TPS2500, Hot Disk Inc., Sweden) was applied to measure the thermal conductivities of the prepared round blocks with different packing densities, with a type 5501 probe acting as both heat source and sensor. A transient plane source method, in which two samples (round blocks) of the test materials with a similar thickness were required to be placed in contact with the probe and heated at constant power for a setting scanning time, was selected for these measurements. The type of the probe, heating power and scanning time for these tests were chosen based upon the diameter, thickness and range of thermal conductivity of each sample. 2.5. Characterization The phase change temperatures and latent heat of CA, PA, SA, CA–PA–SA and CA–PA–SA/EG composite PCM were analyzed by DSC (DSC Q20, TA Instruments, USA) under nitrogen atmosphere at a flow rate of 50 mL min1. The heating/cooling rate was set at 5 °C min1. Crystalline phases of EG, CA–PA–SA and CA–PA–SA/ EG composite PCM were characterized by an X-ray diffractometer
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3. Results and discussion 3.1. Thermal properties of individual fatty acids and the prepared CA– PA–SA eutectic mixture The DSC curves of CA, PA, SA and CA–PA–SA are displayed in Fig. 1, and their thermal properties are summarized in Table 1. From the DSC curve of CA–PA–SA, only one solid–liquid phase change peak can be observed, which indicating a successful preparation of CA–PA–SA ternary eutectic mixture. The results in Table 1 indicate the phase transition temperatures of CA–PA–SA are much lower than those of individual fatty acids, while latent heat of the eutectic mixture still remains high, thus making CA–PA–SA eutectic mixture promising in potential applications such as building heating/cooling, indoor temperature controlling, and thermoregulating fibers and textiles. Fig. 1. DSC curves of CA, PA, SA and CA–PA–SA ternary eutectic mixture. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(XRD, D8-ADVANCE, Bruker, German) using Cu Ka radiation (k = 1.5406 Å). Fourier transformation infrared (FT-IR) spectra of CA–PA–SA, EG and CA–PA–SA/EG composite PCM were measured at ambient temperature on a spectrometer (Tensor 27, Bruke, Germany). The FT-IR spectra were recorded on a KBr pellet at the frequency range of 4000 –400 cm1. The morphology and microstructure of EG and CA–PA–SA/EG composite PCM were observed by using a scanning electron microscope (SEM, ZEISS Merlin, Germany). The thermal stability of CA–PA–SA and CA– PA–SA/EG composite PCM were investigated by the thermogravimetric analysis (TGA) using a thermal analyzer (Q600 SDT, TA, URT100). The measurements were conducted by heating the samples from ambient temperature to 600 °C at a heating rate of 10 °C min1 under nitrogen atmosphere with a flow rate of 100 mL min1. The thermal reliability of CA–PA–SA/EG composite was studied by placing the sample sealed in an aluminum box into a high-low temperature chamber to experience 100 and 500 heating–cooling cycles, followed by the characterizations by DSC measured under the same conditions as measured before the cycling test. The thermal performances of CA–PA–SA and CA–PA–SA/EG composite PCM during thermal energy storage and retrieval process were also investigated in order to evaluate the effects of EG on heat transfer behavior. Firstly, 20 g samples was put in a 100 ml test tube. Subsequently, a K-type thermocouple was inserted into the center of the test tube and tightly surrounded with the samples. The test tube was then placed in a water bath at a constant temperature of 35 °C for being heated. When the temperature reached balance, the bottle was transferred to another water bath at a constant temperature of 10 °C immediately, where the samples performed process of heat retrieval. After the heat retrieval was finished, the bottle was rapidly subjected to a constant temperature of 35 °C, where the samples performed process of heat storage. The temperature variations (at the time interval of 2 s) were automatically recorded by a data logger (Agilent34970A, USA) with the accuracy of ±0.1 °C.
3.2. Maximum absorption ratio of CA–PA–SA ternary eutectic mixture in the EG Application of a PCM is mainly decided by its phase change temperature and latent heat. And usually the higher latent heat under a proper phase change temperature, the better the PCM performs. It is obvious that the higher the mass ratio of CA–PA– SA in composite the larger latent heat the obtained form-stable composite PCM exhibits. While the content of EG has little influence on the phase change temperature, but greatly affects thermal conductivity, latent heat and solid–liquid leakage phenomenon of composite PCM. Therefore, determining the optimal mass ratio of CA–PA–SA in the composite PCM is of great importance. The photographs of CA–PA–SA/EG composite PCM before (a) and after (b) the thermal treatment are illustrated in Fig. 2. By comparing Fig. 2(a) and (b), it can be found that leakage happened to samples containing 95 wt.% CA–PA–SA, not to those containing 85 wt.% and 90 wt.% CA–PA–SA. Hence, to effectively prevent PCM leakage from EG, the maximum mass percentage of CA–PA– SA in CA–PA–SA/EG composite PCM is about 90%, which is also verified by mass analysis as shown in Table 2. In addition, maximum mass percentages around 90% of fatty acids or their eutectic mixtures in EG-based composites have also been reported [15–17,19]. For convenience, the composite sample containing 90 wt.% CA–PA–SA is in the following called CA–PA–SA/EG composite PCM. 3.3. Chemical characterizations and microstructure of CA–PA–SA/EG composite PCM Fig. 3 shows the XRD patterns of CA–PA–SA, EG and CA–PA–SA/ EG composite PCM. It can be seen that EG has one strong diffraction peak centered at 26.42° with a lattice spacing of 3.3579 Å, and CA–PA–SA has three strong diffraction peaks located at 8.01°, 21.49° and 23.60° with lattice spacing of 7.6938, 4.1294 and 3.77673 Å, respectively. The CA–PA–SA/EG composite exhibits four diffraction peaks, which are just the combination of the peaks of EG and CA–PA–SA. No new diffraction peak is found in the XRD
Table 1 Thermal properties of CA, PA, SA, CA–PA–SA ternary eutectic mixture and CA–PA–SA/EG composite PCM. Fatty acids
Melting temperature (°C)
Melting latent heat (kJ kg1)
Freezing temperature (°C)
Freezing latent heat (kJ kg1)
CA PA SA CA–PA–SA CA–PA–SA/EG
30.98 62.04 68.74 18.9 21.33
162.7 209.9 214.7 147.2 131.7
29.94 60.86 66.73 16.73 19.01
163.2 213.3 216.9 142.3 127.2
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Fig. 2. Photographs of CA–PA–SA/EG composite PCM before (a) and after (b) the thermal treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 Weight of CA–PA–SA/EG composite PCM before and after the thermal treatment. Mass ratio (CA– PA–SA/EG)
Before thermal treatment (g)
After thermal treatment (g)
Percentage (%)
85:15 90:10 95:05
0.2061 0.2125 0.2144
0.2029 0.2081 0.1923
98.4 97.9 89.7
Fig. 4. FT-IR spectra of CA–PA–SA, EG and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. XRD patterns of EG, CA–PA–SA and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
pattern of the composite, indicating that CA–PA–SA/EG composite PCM is just the physical integration of CA–PA–SA with EG. Fig. 4 illustrates the FT-IR spectra of CA–PA–SA, EG and CA–PA– SA/EG composite. In the CA–PA–SA FT-IR spectrum, the peaks at 2929 cm1 and 2857 cm1 represent the stretching vibration of ACH3 and ACH2 group. The absorption band of OAH stretching vibration is in the interval of 3000–2750 cm1 which usually overlaps with the absorption band of aliphatic CAH stretching vibration. The peak at 1710 cm1 is the characteristic absorption peak
for the stretching vibration of C@O. The peak at 1462 cm1 is the ACH2 bending peak, 1280 cm1 represents CAH and CAC bending, 940 cm1 and 722 cm1 responds to rocking vibration and bending, which are all characteristics for aliphatic chain of CA–PA–SA. For CA–PA–SA/EG composite PCM, the characteristic peaks at 2929, 2857, 1710, 1462, 1280, 940 and 722 cm1 are also existed with no distinct new absorption peak appears. These results prove again that CA–PA–SA/EG composite PCM is just the combination of CA–PA–SA and EG without any chemical interaction between the two components. Fig. 5 displays the SEM images of EG (a) and CA–PA–SA/EG composite PCM (b). The SEM image of EG in Fig. 5(a) shows that EG has porous structure. Such porous structure increases its specific surface area, thus making molten CA–PA–SA adsorbed easily to a high value of 90 wt.%. As shown in Fig. 5(b), the SEM image of CA–PA– SA/EG composite PCM indicates that CA–PA–SA distributes uniformly in the porous structure of EG. Based on the above results, it can be concluded that the capillary and surface tension between
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Fig. 5. SEM images of EG (a) and CA–PA–SA/EG composite PCM (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. DSC curves of CA–PA–SA and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
CA–PA–SA and the porous structure of EG can prevent molten CA– PA–SA from leaking [8]. 3.4. Thermal properties of CA–PA–SA/EG composite PCM Fig. 6 illustrates DSC curves of CA–PA–SA and CA–PA–SA/EG composite PCM. And their thermal properties are listed in Table 1. The temperatures of melting and freezing of CA–PA–SA/EG composite are 21.33 °C and 19.01 °C as shown in Table 1. It can be seen that the phase transition temperatures (both melting and freezing temperature) of CA–PA–SA/EG composite are slightly higher than CA–PA–SA. The similar results have also been reported in literatures [17,20,21]. It can be found in Table 1, CA–PA–SA/EG composite PCM possesses a melting latent heat as high as 131.7 kJ kg1
Fig. 8. Relationship between the packing density and thermal conductivity of the CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and a freezing latent heat about 127.2 kJ kg1. Theoretically, the latent heat of composite PCMs can be calculated by multiplying latent heat of PCMs and mass ratio of PCMs in composite PCMs. The calculated melting and freezing latent heat of CA–PA–SA/EG composite PCM are 133.29 kJ kg1 and 127.44 kJ kg1, both slightly higher than the experiment results. This result may be attributed to the prevention of supercooling in the CA–PA–SA/EG composite. 3.5. Thermal conductivity and thermal storage/retrieval behavior of CA–PA–SA/EG composite PCM Thermal conductivity is an important parameter of PCM because it affects the heat transfer rates. In this paper, the cylindri-
Fig. 7. Photographs of the round blocks fabricated from the CA–PA–SA powders by dry pressing (a), together with that of the CA–PA–SA/EG composite PCM samples, whose packing densities were calculated to be 435.7, 526.7, 615.4, 710.4 and 743.3 kg m3 (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Schematic of the setup for thermal performance measurement.
Fig. 10. Melting temperature curves (a) and freezing temperature curves (b) of CA–PA–SA and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
cal compressed CA–PA–SA and CA–PA–SA/EG composite PCMs were formed by dry pressing. All the round blocks have smooth surface without any cracks, indicating that CA–PA–SA and CA– PA–SA/EG composite PCM have good formability, as shown in Fig. 7. The thermal conductivity of CA–PA–SA samples (as shown in Fig. 7(a)) with the density of 850.9 kg m3 is measured to be 0.3407 W (m K)1. Fig. 8 shows the thermal conductivity of CA–
PA–SA/EG composite PCM round blocks with different packing densities. As the packing density of the blocks is increased from 435.7, 526.7, 615.4 and 710.4 to 743.3 kg m3, their thermal conductivity accordingly increases from 1.125, 2.410, 3.677 and 4.844 to 5.225 W (m K)1. The relationship between the packing density (x) and the thermal conductivity (y) of the CA–PA–SA/EG blocks can be fitted into a linear equation: y = 4.641 + 0.0133x, indicating an obvious increase in the thermal conductivity with
Fig. 11. Mass loss curves of EG, CA–PA–SA and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. DSC curves of CA–PA–SA and CA–PA–SA/EG composite PCM before and after thermal cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 3 Phase change temperatures and latent heat of the CA–PA–SA/EG composite PCM before and after thermal cycles. Samples
Melting temperatures (°C)
Melting latent heat (kJ kg1)
Freezing temperatures (°C)
Freezing latent heat (kJ kg1)
CA–PA–SA/EG CA–PA–SA/EG after 100 cycles CA–PA–SA/EG after 500 cycles
21.33 21.24 22.22
131.7 129.0 124.3
19.01 19.88 19.97
127.2 125.6 123.3
the packing density. This phenomenon may be resulted from the reduction of void space within the composite PCM and extension of contact surface area for the composite particles [7]. The linear equation between the thermal conductivity and the packing density of EG-based PCM composite has been demonstrated in previous works [8,22,23]. The addition of EG into CA–PA–SA increased the thermal conductivity is verified by comparing the melting/freezing times of CA–PA–SA with those of CA–PA–SA/EG composite PCM. Fig. 9 displays the experimental setup. The temperature–time curves of CA– PA–SA and CA–PA–SA/EG composite PCM for melting/freezing process are shown in Fig. 10(a) and (b). As seen in Fig. 10(a), it takes 3400 s by heating CA–PA–SA from 10 °C to 35 °C and for CA–PA– SA/EG composite PCM only 300 s. It can be found from Fig. 10(b), when temperature decreases from 35 °C to 10 °C, it takes 6710 s for CA–PA–SA and just 1022 s for CA–PA–SA/EG composite PCM. These results illustrate that the melting/freezing times of CA–PA– SA/EG composite PCM are significantly shorter than CA–PA–SA as a result of the increase in the thermal conductivity. In other words, the thermal energy storage/retrieval rates as well as the heat transfer rates of CA–PA–SA/EG composite PCM are largely increased due to the increased thermal conductivity. 3.6. Thermal stability and reliability of CA–PA–SA/EG composite PCM Fig. 11 illustrates the mass loss curves of EG, CA–PA–SA and CA– PA–SA/EG composite PCM. There is a single degradation process from 30 °C to 600 °C for CA–PA–SA and CA–PA–SA/EG composite because of the decomposition of fatty acids. The EG decomposes rarely in the temperature range, and the initial decomposition temperatures of CA–PA–SA and CA–PA–SA/EG composite are around 108.8 °C and 106.9 °C. Moreover, no decomposition is observed for both CA–PA–SA and CA–PA–SA/EG composite within 100 °C, showing high thermal stability in low-temperature applications. Furthermore, the weight-loss ratio of the CA–PA–SA/EG composite PCM is about 90%, which is equivalent to the mass ratio of CA–PA–SA in the composite. For composite PCMs, good thermal reliabilities over a number of thermal cycles are necessary. In this paper, a thermal cycle experiment was conducted to test the thermal reliabilities of CA–PA–SA/ EG composite PCM. The DSC curves and thermal properties of CA– PA–SA/EG composite before and after thermal cycles are showed in Fig. 12 and Table 3. As it can be seen in Fig. 12, the DSC curves of CA–PA–SA/EG composite PCM after experiencing 100 and 500 heating/cooling cycles are very close to that of the composite PCM before experiencing the test. From Table 3, it can be found that there are no obvious difference in the melting and freezing temperatures, while just a slight reduction in the melting and freezing latent heat from 131.7 kJ kg1 to 129.0 kJ kg1 and 124.3 kJ kg1, and 127.2 kJ kg1 to 125.6 kJ kg1 and 123.3 kJ kg1, respectively. All the results indicate that CA–PA–SA/EG composite PCM has a good thermal reliability. 4. Conclusion In this work, through combining CA–PA–SA as PCM and EG as matrix material, a novel form-stable CA–PA–SA/EG composite
PCM was fabricated. The maximum mass percentage of CA–PA– SA was determined to be 90%. The CA–PA–SA/EG composite PCM powders can be compressed into round blocks by dry pressing. The SEM results showed that CA–PA–SA uniformly distributed into the porous structure of EG and the XRD and FT-IR results indicated that there were no chemical interactions but physical adsorption action between CA–PA–SA and EG. The DSC results revealed that the melting temperature and latent heat of CA–PA–SA/EG composite PCM were 21.33 °C and 131.7 kJ kg1. TG analysis revealed that the prepared CA–PA–SA/EG composite PCM has a high thermal stability. Good thermal reliability of CA–PA–SA/EG composite PCM was found by thermal cycling test. The thermal conductivities of all CA–PA–SA/EG composite PCM round blocks were much higher than CA–PA–SA and linearly increased with the packing density. The addition of EG into CA–PA–SA resulted in a remarkable improvement in the thermal conductivity, which was verified by comparing the melting/freezing times of CA–PA–SA with those of the CA–PA–SA/EG composite PCM. These excellent properties make the CA–PA–SA/EG composite PCM promising in lowtemperature latent heat storage applications such as building energy conservation, solar energy storage, low temperature refrigeration and indoor temperature controlling.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. U1507201) and Guangdong Natural Science Foundation (2014A030312009).
References [1] Hyun DC, Levinson NS, Jeong U, Xia Y. Emerging applications of phase-change materials (PCMs): teaching an old dog new tricks. Angew Chem Int Ed 2014;53 (15):3780–95. [2] Yuan Y, Zhang N, Tao W, Cao X, He Y. Fatty acids as phase change materials: a review. Renew Sustain Energy Rev 2014;29:482–98. [3] Yanping Y, Wenquan T, Xiaoling C, Li B. Theoretic prediction of melting temperature and latent heat for a fatty acid eutectic mixture. J Chem Eng Data 2011;56(6):2889–91. [4] Wang L, Meng D. Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change material for thermal energy storage. Appl Energy 2010;87(8):2660–5. [5] Sarı A. Eutectic mixtures of some fatty acids for low temperature solar heating applications: thermal properties and thermal reliability. Appl Therm Eng 2005;25(14–15):2100–7. [6] Cai Y, Ke H, Lin L, Fei X, Wei Q, Song L, et al. Preparation, morphology and thermal properties of electrospun fatty acid eutectics/polyethylene terephthalate form-stable phase change ultrafine composite fibers for thermal energy storage. Energy Convers Manage 2012;64:245–55. [7] Huang Z, Gao X, Xu T, Fang Y, Zhang Z. Thermal property measurement and heat storage analysis of LiNO3/KCl – expanded graphite composite phase change material. Appl Energy 2014;115:265–71. [8] Wang S, Qin P, Fang X, Zhang Z, Wang S, Liu X. A novel sebacic acid/expanded graphite composite phase change material for solar thermal mediumtemperature applications. Sol Energy 2014;99:283–90. [9] Sarı A, Karaipekli A, Alkan C. Preparation, characterization and thermal properties of lauric acid/expanded perlite as novel form-stable composite phase change material. Chem Eng J 2009;155(3):899–904. [10] Towata A, Hwang H, Yasuoka M, Sando M, Niihara K. Preparation of polycrystalline YAG/alumina composite fibers and YAG fiber by sol–gel method. Compos A: Appl Sci Manuf 2001;32(8):1127–31. [11] Fang G, Li H, Chen Z, Liu X. Preparation and properties of palmitic acid/SiO2 composites with flame retardant as thermal energy storage materials. Sol Energy Mater Sol Cells 2011;95(7):1875–81.
H. Zhang et al. / Composites: Part A 87 (2016) 138–145 [12] Wang Z, Zhang Z, Jia L, Yang L. Paraffin and paraffin/aluminum foam composite phase change material heat storage experimental study based on thermal management of Li-ion battery. Appl Therm Eng 2015;78:428–36. [13] Cai Y, Gao C, Zhang T, Zhang Z, Wei Q, Du J, et al. Influences of expanded graphite on structural morphology and thermal performance of composite phase change materials consisting of fatty acid eutectics and electrospun PA6 nanofibrous mats. Renew Energy 2013;57:163–70. [14] Zhang Z, Zhang N, Peng J, Fang X, Gao X, Fang Y. Preparation and thermal energy storage properties of paraffin/expanded graphite composite phase change material. Appl Energy 2012;91(1):426–31. [15] Zhang N, Yuan Y, Wang X, Cao X, Yang X, Hu S. Preparation and characterization of lauric–myristic–palmitic acid ternary eutectic mixtures/expanded graphite composite phase change material for thermal energy storage. Chem Eng J 2013;231:214–9. [16] Liu C, Yuan Y, Zhang N, Cao X, Yang X. A novel PCM of lauric–myristic–stearic acid/expanded graphite composite for thermal energy storage. Mater Lett 2014;120:43–6. [17] Yang X, Yuan Y, Zhang N, Cao X, Liu C. Preparation and properties of myristic– palmitic–stearic acid/expanded graphite composites as phase change materials for energy storage. Sol Energy 2014;99:259–66.
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[18] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sustain Energy Rev 2009;13(2):318–45. [19] Zhang J, Guan X, Song X, Hou H, Yang Z, Zhu J. Preparation and properties of gypsum based energy storage materials with capric acid–palmitic acid/expanded perlite composite PCM. Energy Buildings 2015;92:155–60. [20] Li M, Wu Z, Kao H. Study on preparation and thermal properties of binary fatty acid/diatomite shape-stabilized phase change materials. Sol Energy Mater Sol Cells 2011;95(8):2412–6. [21] Karaipekli A, Sarı A. Preparation, thermal properties and thermal reliability of eutectic mixtures of fatty acids/expanded vermiculite as novel form-stable composites for energy storage. J Ind Eng Chem 2010;16(5):767–73. [22] Zhang Q, Wang H, Ling Z, Fang X, Zhang Z. RT100/expand graphite composite phase change material with excellent structure stability, photo-thermal performance and good thermal reliability. Sol Energy Mater Sol Cells 2015;140:158–66. [23] Ling Z, Chen J, Xu T, Fang X, Gao X, Zhang Z. Thermal conductivity of an organic phase change material/expanded graphite composite across the phase change temperature range and a novel thermal conductivity model. Energy Convers Manage 2015;102:202–8.
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
A capric–palmitic–stearic acid ternary eutectic mixture/expanded graphite composite phase change material for thermal energy storage Hua Zhang a, Xuenong Gao a,⇑, Caixing Chen a, Tao Xu b,⇑, Yutang Fang a, Zhengguo Zhang a a Key Laboratory of Enhanced Heat Transfer and Energy Conservation, The Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 25 January 2016 Received in revised form 30 March 2016 Accepted 23 April 2016 Available online 23 April 2016 Keywords: Phase change material CA–PA–SA ternary eutectic mixture Expanded graphite Low-temperature thermal energy storage
a b s t r a c t This paper demonstrated a capric acid–palmitic acid–stearic acid ternary eutectic mixture/expanded graphite (CA–PA–SA/EG) composite phase change material (PCM) for low-temperature heat storage. The CA– PA–SA ternary eutectic mixture with a mass ratio of CA:PA:SA = 79.3:14.7:6.0 was prepared firstly, and its mass ratio in the CA–PA–SA/EG composite can reach as high as 90%. The melting and freezing temperatures of CA–PA–SA/EG composite were 21.33 °C and 19.01 °C, and the corresponding latent heat were 131.7 kJ kg1 and 127.2 kJ kg1. The CA–PA–SA/EG composite powders can be formed into round blocks by dry pressing easily, with much higher thermal conductivity than CA–PA–SA. Thermal performance test showed that the increasing thermal conductivity of CA–PA–SA could obviously decrease the melting/cooling time. Thermal property characterizations after 500 heating/cooling cycles test indicated that CA–PA–SA/EG composite PCM had excellent thermal reliability. Based on all these results, CA–PA–SA/ EG composite PCM is a promising material for low-temperature thermal energy storage applications. Ó 2016 Published by Elsevier Ltd.
1. Introduction Phase change materials (PCMs) refer to materials that can store or release heat energy during their phase transition process at a nearly constant temperature. Numerous efforts have been made to explore PCMs for realizing the control of environmental temperature and matching the energy supply and demand in time and space due to their high energy storage density. PCMs have a heat storage capacity, which is about 5–14 times higher than the conventional (sensible) thermal storage materials such as water, rock and masonry. A large number of PCMs are known to melt with a heat of fusion in any required range [1]. Therefore, PCMs have been explored in many applications such as building energy conservation, solar energy storage, low temperature refrigeration and indoor temperature controlling. Among the numerous investigated PCMs, fatty acids have been widely employed by virtue of their following desirable features, high heat capacity, good thermal and chemical stability, small volume change, self-nucleating behavior, non-toxicity, noncorrosiveness, low vapor pressure, negligible supercooling and cost-effectiveness [2]. However, the individual fatty acids are
⇑ Corresponding authors. E-mail address: [email protected] (X. Gao). http://dx.doi.org/10.1016/j.compositesa.2016.04.024 1359-835X/Ó 2016 Published by Elsevier Ltd.
restricted in some practical applications for their fixed phase change temperatures. Fortunately, the phase transition temperatures can be tailored into a suitable range by preparing binary or ternary eutectic mixtures of fatty acids [3]. Capric acid (CA), lauric acid (LA), myristic acid (MA) and stearic acid (SA) were selected to prepare binary fatty acid eutectic (CA–LA, CA–MA, CA–SA and LA–MA) by Wang and Meng [4], and phase change temperatures of the composites CA–LA/PMMA, CA–MA/PMMA, CA–SA/PMMA and LA–MA/PMMA (50/50 wt.%) were adjusted to a suitable range for building energy conservation. Sarı [5] prepared three binary eutectic mixtures including LA–MA, LA–PA and MA–SA with a melting temperature of 34.2 °C, 35.2 °C and 44.1 °C, which are lower than the individual fatty acids. Cai et al. [6] prepared five binary fatty acid eutectics LA–MA, LA–PA, MA–PA, MA–SA and PA–SA through Schrader equation, whose melting temperatures ranged from 33.27 °C to 53.69 °C, and freezing temperatures ranged from 33.71 °C to 53.45 °C. Then, an innovative type of electrospun binary fatty acid eutectics/PET ultrafine composite fibers for latent heat storage in the field of building energy conservation were successfully fabricated as form-stable PCMs. Nonetheless, fatty acids and their eutectic mixtures have a notable drawback of low thermal conductivity, which seriously limits the heat transfer rates during phase transition. In order to overcome the problem, the combination of PCMs with porous
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matrix materials to prepare form-stable PCMs has been proven to be an effective way [7]. Furthermore, the leakage during solid– liquid phase transition processes can be prevented due to the capillary and surface tension forces of the porous matrix materials [8], The form-stable PCMs containing fatty acids and/or their eutectics can be prepared through various methods such as physical adsorption [9], sol–gel [10,11], microencapsulation [12] and electrospinning [13]. Among the investigated methods, directly incorporating PCMs into porous materials is a simple, convenient, low cost and effective technique. Since previous reported research, expanded graphite (EG) has been verified to be the ideal porous matrix material owing to its advantages of high thermal conductivity, large pore volume, low density and good compatibility [14]. The recent studies are mainly focused on fatty acids and/or their binary eutectics, investigations on ternary fatty acid eutectic mixtures as PCMs have rarely been reported. Zhang et al. [15] prepared LA–MA–PA/EG composite PCM, Liu et al. [16] prepared LA–MA–SA/ EG composite PCM and Yang et al. [17] prepared MA–PA–SA/EG composite PCM with the phase change temperature of 30.94 °C, 29.05 °C and 41.64 °C. To the best of our knowledge, the investigation of CA–PA–SA ternary eutectic mixture as PCM has not been reported. Moreover, compared with medium-temperature and high-temperature applications have been well developed, lowtemperature thermal applications such as building energy conservation, low temperature refrigeration and indoor temperature controlling have dropped behind but show great potential in the future, especially in heat storage systems on the use of PCMs with the phase change temperatures below 25 °C [18]. Hence, this research is aimed at preparing a form-stable PCM for lowtemperature latent heat storage applications. Ternary eutectic mixture of CA–PA–SA with a melting temperature of about 20 °C was prepared firstly. Then, the supporting material EG was prepared through microwave method [7]. Finally, CA–PA–SA were absorbed into EG to fabricate a form-stable CA–PA–SA/EG composite PCM. The microstructure, thermal properties and stability of CA–PA–SA/EG composite PCM were characterized by Scanning electron microscopy (SEM), X-ray diffractometer (XRD), Fourier transfer infrared (FT-IR), Differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). The thermal cycling test was conducted to determine the thermal reliability of CA–PA–SA/ EG composite. Afterward, the influences of EG on thermal conductivity and thermal energy storage/retrieval rates of CA–PA–SA/EG composite PCM were also investigated. 2. Experimental 2.1. Materials Capric acid (CA, CH3(CH2)8COOH, 98%, Chemical Pure), palmitic acid (PA, CH3(CH2)14COOH, 98%, Chemical Pure), stearic acid (SA, CH3(CH2)16COOH, 98%, Analytical Reagent) were supplied by Shanghai Lingfeng Chemical Reagents Co. (Shanghai, China). Expandable graphite (mesh 32, expansion ration: 300 ml g1) was purchased from Qingdao Graphite Co. Ltd., China. The chemicals were used as received without further purification. 2.2. Preparation of CA–PA–SA ternary eutectic mixture Based on the lowest eutectic point theory, fatty acids can be blended together with an eutectic ratio to achieve the eutectic temperature, which is lower than those of individual fatty acids. The eutectic ratios of fatty acid mixture were calculated by the Schrader equation [3,15,17].
T ¼ 1=ð1=T i R ln X i =DHi Þ
ð1Þ
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where Ti and DHi are the phase change temperature and latent heat of the ith fatty acid, T is the phase change temperature of the eutectic mixture, Xi is the content of the ith substance in the eutectic mixture, and R is ideal gas constant. Through the above equation, the mass ratio of CA–PA binary eutectic mixture was calculated first and was acted as a kind of ‘‘single” fatty acid, and then the mass ratio of single fatty acid (i.e., CA–PA) and SA was calculated again. Finally, the mass ratios for CA–PA–SA ternary eutectic mixture were determined at 79.3:14.7:6.0 and the calculated melting point of the mixture was 25.71 °C. The CA–PA–SA ternary eutectic mixture was prepared through melt-blending. Three fatty acids (CA, PA and SA) with the calculated eutectic mass ratio were first mixed in a sealed beaker, then placed into an oven at 70 °C for 2 h until totally melted. Subsequently, the molten mixture was stirred in a magnetic stirrer for 2 h at 300 r min1 to ensure the homogeneity of the mixture, and then slowly cooled down to ambient temperature. 2.3. Preparation of CA–PA–SA/EG composite PCM The CA–PA–SA/EG composite PCMs containing 85 wt.%, 90 wt.%, and 95 wt.% CA–PA–SA were prepared by absorbing different amounts of liquid CA–PA–SA into the pores of EG. To determine the maximum mass fraction of CA–PA–SA that can be absorbed in the composite PCM, the leakage test was carried out as follows. Three CA–PA–SA/EG composite PCM samples containing 85 wt.%, 90 wt.%, and 95 wt.% CA–PA–SA were placed on three pieces of filter paper, and then inserted into an oven at a temperature of 40 °C (above the melting point of CA–PA–SA) for 8 h to make sure the phase change of CA–PA–SA from solid to liquid takes place. After cooling to ambient temperature, the samples were removed from the corresponding filter paper. Finally, a careful examination was given to each piece of filter paper to find out whether there were any traces of liquid CA–PA–SA left on it. Furthermore, the weights of the samples were compared before and after the thermal treatment to further judge whether the leakage happened. 2.4. Fabrication of compressed CA–PA–SA/EG composite PCM The CA–PA–SA/EG composite PCM powders were formed into several round blocks by dry pressing with a cylindrical mold (4 cm inside diameter and 1 cm height) under the pressure of 100 kg cm2. Actual packing densities were calculated by the actual masses to volumes of the formed blocks, which were 435.7, 526.7, 615.4, 710.4 and 743.3 kg m3, respectively. Hot disk thermal constant analyzer (TPS2500, Hot Disk Inc., Sweden) was applied to measure the thermal conductivities of the prepared round blocks with different packing densities, with a type 5501 probe acting as both heat source and sensor. A transient plane source method, in which two samples (round blocks) of the test materials with a similar thickness were required to be placed in contact with the probe and heated at constant power for a setting scanning time, was selected for these measurements. The type of the probe, heating power and scanning time for these tests were chosen based upon the diameter, thickness and range of thermal conductivity of each sample. 2.5. Characterization The phase change temperatures and latent heat of CA, PA, SA, CA–PA–SA and CA–PA–SA/EG composite PCM were analyzed by DSC (DSC Q20, TA Instruments, USA) under nitrogen atmosphere at a flow rate of 50 mL min1. The heating/cooling rate was set at 5 °C min1. Crystalline phases of EG, CA–PA–SA and CA–PA–SA/ EG composite PCM were characterized by an X-ray diffractometer
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3. Results and discussion 3.1. Thermal properties of individual fatty acids and the prepared CA– PA–SA eutectic mixture The DSC curves of CA, PA, SA and CA–PA–SA are displayed in Fig. 1, and their thermal properties are summarized in Table 1. From the DSC curve of CA–PA–SA, only one solid–liquid phase change peak can be observed, which indicating a successful preparation of CA–PA–SA ternary eutectic mixture. The results in Table 1 indicate the phase transition temperatures of CA–PA–SA are much lower than those of individual fatty acids, while latent heat of the eutectic mixture still remains high, thus making CA–PA–SA eutectic mixture promising in potential applications such as building heating/cooling, indoor temperature controlling, and thermoregulating fibers and textiles. Fig. 1. DSC curves of CA, PA, SA and CA–PA–SA ternary eutectic mixture. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(XRD, D8-ADVANCE, Bruker, German) using Cu Ka radiation (k = 1.5406 Å). Fourier transformation infrared (FT-IR) spectra of CA–PA–SA, EG and CA–PA–SA/EG composite PCM were measured at ambient temperature on a spectrometer (Tensor 27, Bruke, Germany). The FT-IR spectra were recorded on a KBr pellet at the frequency range of 4000 –400 cm1. The morphology and microstructure of EG and CA–PA–SA/EG composite PCM were observed by using a scanning electron microscope (SEM, ZEISS Merlin, Germany). The thermal stability of CA–PA–SA and CA– PA–SA/EG composite PCM were investigated by the thermogravimetric analysis (TGA) using a thermal analyzer (Q600 SDT, TA, URT100). The measurements were conducted by heating the samples from ambient temperature to 600 °C at a heating rate of 10 °C min1 under nitrogen atmosphere with a flow rate of 100 mL min1. The thermal reliability of CA–PA–SA/EG composite was studied by placing the sample sealed in an aluminum box into a high-low temperature chamber to experience 100 and 500 heating–cooling cycles, followed by the characterizations by DSC measured under the same conditions as measured before the cycling test. The thermal performances of CA–PA–SA and CA–PA–SA/EG composite PCM during thermal energy storage and retrieval process were also investigated in order to evaluate the effects of EG on heat transfer behavior. Firstly, 20 g samples was put in a 100 ml test tube. Subsequently, a K-type thermocouple was inserted into the center of the test tube and tightly surrounded with the samples. The test tube was then placed in a water bath at a constant temperature of 35 °C for being heated. When the temperature reached balance, the bottle was transferred to another water bath at a constant temperature of 10 °C immediately, where the samples performed process of heat retrieval. After the heat retrieval was finished, the bottle was rapidly subjected to a constant temperature of 35 °C, where the samples performed process of heat storage. The temperature variations (at the time interval of 2 s) were automatically recorded by a data logger (Agilent34970A, USA) with the accuracy of ±0.1 °C.
3.2. Maximum absorption ratio of CA–PA–SA ternary eutectic mixture in the EG Application of a PCM is mainly decided by its phase change temperature and latent heat. And usually the higher latent heat under a proper phase change temperature, the better the PCM performs. It is obvious that the higher the mass ratio of CA–PA– SA in composite the larger latent heat the obtained form-stable composite PCM exhibits. While the content of EG has little influence on the phase change temperature, but greatly affects thermal conductivity, latent heat and solid–liquid leakage phenomenon of composite PCM. Therefore, determining the optimal mass ratio of CA–PA–SA in the composite PCM is of great importance. The photographs of CA–PA–SA/EG composite PCM before (a) and after (b) the thermal treatment are illustrated in Fig. 2. By comparing Fig. 2(a) and (b), it can be found that leakage happened to samples containing 95 wt.% CA–PA–SA, not to those containing 85 wt.% and 90 wt.% CA–PA–SA. Hence, to effectively prevent PCM leakage from EG, the maximum mass percentage of CA–PA– SA in CA–PA–SA/EG composite PCM is about 90%, which is also verified by mass analysis as shown in Table 2. In addition, maximum mass percentages around 90% of fatty acids or their eutectic mixtures in EG-based composites have also been reported [15–17,19]. For convenience, the composite sample containing 90 wt.% CA–PA–SA is in the following called CA–PA–SA/EG composite PCM. 3.3. Chemical characterizations and microstructure of CA–PA–SA/EG composite PCM Fig. 3 shows the XRD patterns of CA–PA–SA, EG and CA–PA–SA/ EG composite PCM. It can be seen that EG has one strong diffraction peak centered at 26.42° with a lattice spacing of 3.3579 Å, and CA–PA–SA has three strong diffraction peaks located at 8.01°, 21.49° and 23.60° with lattice spacing of 7.6938, 4.1294 and 3.77673 Å, respectively. The CA–PA–SA/EG composite exhibits four diffraction peaks, which are just the combination of the peaks of EG and CA–PA–SA. No new diffraction peak is found in the XRD
Table 1 Thermal properties of CA, PA, SA, CA–PA–SA ternary eutectic mixture and CA–PA–SA/EG composite PCM. Fatty acids
Melting temperature (°C)
Melting latent heat (kJ kg1)
Freezing temperature (°C)
Freezing latent heat (kJ kg1)
CA PA SA CA–PA–SA CA–PA–SA/EG
30.98 62.04 68.74 18.9 21.33
162.7 209.9 214.7 147.2 131.7
29.94 60.86 66.73 16.73 19.01
163.2 213.3 216.9 142.3 127.2
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Fig. 2. Photographs of CA–PA–SA/EG composite PCM before (a) and after (b) the thermal treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 Weight of CA–PA–SA/EG composite PCM before and after the thermal treatment. Mass ratio (CA– PA–SA/EG)
Before thermal treatment (g)
After thermal treatment (g)
Percentage (%)
85:15 90:10 95:05
0.2061 0.2125 0.2144
0.2029 0.2081 0.1923
98.4 97.9 89.7
Fig. 4. FT-IR spectra of CA–PA–SA, EG and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. XRD patterns of EG, CA–PA–SA and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
pattern of the composite, indicating that CA–PA–SA/EG composite PCM is just the physical integration of CA–PA–SA with EG. Fig. 4 illustrates the FT-IR spectra of CA–PA–SA, EG and CA–PA– SA/EG composite. In the CA–PA–SA FT-IR spectrum, the peaks at 2929 cm1 and 2857 cm1 represent the stretching vibration of ACH3 and ACH2 group. The absorption band of OAH stretching vibration is in the interval of 3000–2750 cm1 which usually overlaps with the absorption band of aliphatic CAH stretching vibration. The peak at 1710 cm1 is the characteristic absorption peak
for the stretching vibration of C@O. The peak at 1462 cm1 is the ACH2 bending peak, 1280 cm1 represents CAH and CAC bending, 940 cm1 and 722 cm1 responds to rocking vibration and bending, which are all characteristics for aliphatic chain of CA–PA–SA. For CA–PA–SA/EG composite PCM, the characteristic peaks at 2929, 2857, 1710, 1462, 1280, 940 and 722 cm1 are also existed with no distinct new absorption peak appears. These results prove again that CA–PA–SA/EG composite PCM is just the combination of CA–PA–SA and EG without any chemical interaction between the two components. Fig. 5 displays the SEM images of EG (a) and CA–PA–SA/EG composite PCM (b). The SEM image of EG in Fig. 5(a) shows that EG has porous structure. Such porous structure increases its specific surface area, thus making molten CA–PA–SA adsorbed easily to a high value of 90 wt.%. As shown in Fig. 5(b), the SEM image of CA–PA– SA/EG composite PCM indicates that CA–PA–SA distributes uniformly in the porous structure of EG. Based on the above results, it can be concluded that the capillary and surface tension between
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Fig. 5. SEM images of EG (a) and CA–PA–SA/EG composite PCM (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. DSC curves of CA–PA–SA and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
CA–PA–SA and the porous structure of EG can prevent molten CA– PA–SA from leaking [8]. 3.4. Thermal properties of CA–PA–SA/EG composite PCM Fig. 6 illustrates DSC curves of CA–PA–SA and CA–PA–SA/EG composite PCM. And their thermal properties are listed in Table 1. The temperatures of melting and freezing of CA–PA–SA/EG composite are 21.33 °C and 19.01 °C as shown in Table 1. It can be seen that the phase transition temperatures (both melting and freezing temperature) of CA–PA–SA/EG composite are slightly higher than CA–PA–SA. The similar results have also been reported in literatures [17,20,21]. It can be found in Table 1, CA–PA–SA/EG composite PCM possesses a melting latent heat as high as 131.7 kJ kg1
Fig. 8. Relationship between the packing density and thermal conductivity of the CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and a freezing latent heat about 127.2 kJ kg1. Theoretically, the latent heat of composite PCMs can be calculated by multiplying latent heat of PCMs and mass ratio of PCMs in composite PCMs. The calculated melting and freezing latent heat of CA–PA–SA/EG composite PCM are 133.29 kJ kg1 and 127.44 kJ kg1, both slightly higher than the experiment results. This result may be attributed to the prevention of supercooling in the CA–PA–SA/EG composite. 3.5. Thermal conductivity and thermal storage/retrieval behavior of CA–PA–SA/EG composite PCM Thermal conductivity is an important parameter of PCM because it affects the heat transfer rates. In this paper, the cylindri-
Fig. 7. Photographs of the round blocks fabricated from the CA–PA–SA powders by dry pressing (a), together with that of the CA–PA–SA/EG composite PCM samples, whose packing densities were calculated to be 435.7, 526.7, 615.4, 710.4 and 743.3 kg m3 (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Schematic of the setup for thermal performance measurement.
Fig. 10. Melting temperature curves (a) and freezing temperature curves (b) of CA–PA–SA and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
cal compressed CA–PA–SA and CA–PA–SA/EG composite PCMs were formed by dry pressing. All the round blocks have smooth surface without any cracks, indicating that CA–PA–SA and CA– PA–SA/EG composite PCM have good formability, as shown in Fig. 7. The thermal conductivity of CA–PA–SA samples (as shown in Fig. 7(a)) with the density of 850.9 kg m3 is measured to be 0.3407 W (m K)1. Fig. 8 shows the thermal conductivity of CA–
PA–SA/EG composite PCM round blocks with different packing densities. As the packing density of the blocks is increased from 435.7, 526.7, 615.4 and 710.4 to 743.3 kg m3, their thermal conductivity accordingly increases from 1.125, 2.410, 3.677 and 4.844 to 5.225 W (m K)1. The relationship between the packing density (x) and the thermal conductivity (y) of the CA–PA–SA/EG blocks can be fitted into a linear equation: y = 4.641 + 0.0133x, indicating an obvious increase in the thermal conductivity with
Fig. 11. Mass loss curves of EG, CA–PA–SA and CA–PA–SA/EG composite PCM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. DSC curves of CA–PA–SA and CA–PA–SA/EG composite PCM before and after thermal cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 3 Phase change temperatures and latent heat of the CA–PA–SA/EG composite PCM before and after thermal cycles. Samples
Melting temperatures (°C)
Melting latent heat (kJ kg1)
Freezing temperatures (°C)
Freezing latent heat (kJ kg1)
CA–PA–SA/EG CA–PA–SA/EG after 100 cycles CA–PA–SA/EG after 500 cycles
21.33 21.24 22.22
131.7 129.0 124.3
19.01 19.88 19.97
127.2 125.6 123.3
the packing density. This phenomenon may be resulted from the reduction of void space within the composite PCM and extension of contact surface area for the composite particles [7]. The linear equation between the thermal conductivity and the packing density of EG-based PCM composite has been demonstrated in previous works [8,22,23]. The addition of EG into CA–PA–SA increased the thermal conductivity is verified by comparing the melting/freezing times of CA–PA–SA with those of CA–PA–SA/EG composite PCM. Fig. 9 displays the experimental setup. The temperature–time curves of CA– PA–SA and CA–PA–SA/EG composite PCM for melting/freezing process are shown in Fig. 10(a) and (b). As seen in Fig. 10(a), it takes 3400 s by heating CA–PA–SA from 10 °C to 35 °C and for CA–PA– SA/EG composite PCM only 300 s. It can be found from Fig. 10(b), when temperature decreases from 35 °C to 10 °C, it takes 6710 s for CA–PA–SA and just 1022 s for CA–PA–SA/EG composite PCM. These results illustrate that the melting/freezing times of CA–PA– SA/EG composite PCM are significantly shorter than CA–PA–SA as a result of the increase in the thermal conductivity. In other words, the thermal energy storage/retrieval rates as well as the heat transfer rates of CA–PA–SA/EG composite PCM are largely increased due to the increased thermal conductivity. 3.6. Thermal stability and reliability of CA–PA–SA/EG composite PCM Fig. 11 illustrates the mass loss curves of EG, CA–PA–SA and CA– PA–SA/EG composite PCM. There is a single degradation process from 30 °C to 600 °C for CA–PA–SA and CA–PA–SA/EG composite because of the decomposition of fatty acids. The EG decomposes rarely in the temperature range, and the initial decomposition temperatures of CA–PA–SA and CA–PA–SA/EG composite are around 108.8 °C and 106.9 °C. Moreover, no decomposition is observed for both CA–PA–SA and CA–PA–SA/EG composite within 100 °C, showing high thermal stability in low-temperature applications. Furthermore, the weight-loss ratio of the CA–PA–SA/EG composite PCM is about 90%, which is equivalent to the mass ratio of CA–PA–SA in the composite. For composite PCMs, good thermal reliabilities over a number of thermal cycles are necessary. In this paper, a thermal cycle experiment was conducted to test the thermal reliabilities of CA–PA–SA/ EG composite PCM. The DSC curves and thermal properties of CA– PA–SA/EG composite before and after thermal cycles are showed in Fig. 12 and Table 3. As it can be seen in Fig. 12, the DSC curves of CA–PA–SA/EG composite PCM after experiencing 100 and 500 heating/cooling cycles are very close to that of the composite PCM before experiencing the test. From Table 3, it can be found that there are no obvious difference in the melting and freezing temperatures, while just a slight reduction in the melting and freezing latent heat from 131.7 kJ kg1 to 129.0 kJ kg1 and 124.3 kJ kg1, and 127.2 kJ kg1 to 125.6 kJ kg1 and 123.3 kJ kg1, respectively. All the results indicate that CA–PA–SA/EG composite PCM has a good thermal reliability. 4. Conclusion In this work, through combining CA–PA–SA as PCM and EG as matrix material, a novel form-stable CA–PA–SA/EG composite
PCM was fabricated. The maximum mass percentage of CA–PA– SA was determined to be 90%. The CA–PA–SA/EG composite PCM powders can be compressed into round blocks by dry pressing. The SEM results showed that CA–PA–SA uniformly distributed into the porous structure of EG and the XRD and FT-IR results indicated that there were no chemical interactions but physical adsorption action between CA–PA–SA and EG. The DSC results revealed that the melting temperature and latent heat of CA–PA–SA/EG composite PCM were 21.33 °C and 131.7 kJ kg1. TG analysis revealed that the prepared CA–PA–SA/EG composite PCM has a high thermal stability. Good thermal reliability of CA–PA–SA/EG composite PCM was found by thermal cycling test. The thermal conductivities of all CA–PA–SA/EG composite PCM round blocks were much higher than CA–PA–SA and linearly increased with the packing density. The addition of EG into CA–PA–SA resulted in a remarkable improvement in the thermal conductivity, which was verified by comparing the melting/freezing times of CA–PA–SA with those of the CA–PA–SA/EG composite PCM. These excellent properties make the CA–PA–SA/EG composite PCM promising in lowtemperature latent heat storage applications such as building energy conservation, solar energy storage, low temperature refrigeration and indoor temperature controlling.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. U1507201) and Guangdong Natural Science Foundation (2014A030312009).
References [1] Hyun DC, Levinson NS, Jeong U, Xia Y. Emerging applications of phase-change materials (PCMs): teaching an old dog new tricks. Angew Chem Int Ed 2014;53 (15):3780–95. [2] Yuan Y, Zhang N, Tao W, Cao X, He Y. Fatty acids as phase change materials: a review. Renew Sustain Energy Rev 2014;29:482–98. [3] Yanping Y, Wenquan T, Xiaoling C, Li B. Theoretic prediction of melting temperature and latent heat for a fatty acid eutectic mixture. J Chem Eng Data 2011;56(6):2889–91. [4] Wang L, Meng D. Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change material for thermal energy storage. Appl Energy 2010;87(8):2660–5. [5] Sarı A. Eutectic mixtures of some fatty acids for low temperature solar heating applications: thermal properties and thermal reliability. Appl Therm Eng 2005;25(14–15):2100–7. [6] Cai Y, Ke H, Lin L, Fei X, Wei Q, Song L, et al. Preparation, morphology and thermal properties of electrospun fatty acid eutectics/polyethylene terephthalate form-stable phase change ultrafine composite fibers for thermal energy storage. Energy Convers Manage 2012;64:245–55. [7] Huang Z, Gao X, Xu T, Fang Y, Zhang Z. Thermal property measurement and heat storage analysis of LiNO3/KCl – expanded graphite composite phase change material. Appl Energy 2014;115:265–71. [8] Wang S, Qin P, Fang X, Zhang Z, Wang S, Liu X. A novel sebacic acid/expanded graphite composite phase change material for solar thermal mediumtemperature applications. Sol Energy 2014;99:283–90. [9] Sarı A, Karaipekli A, Alkan C. Preparation, characterization and thermal properties of lauric acid/expanded perlite as novel form-stable composite phase change material. Chem Eng J 2009;155(3):899–904. [10] Towata A, Hwang H, Yasuoka M, Sando M, Niihara K. Preparation of polycrystalline YAG/alumina composite fibers and YAG fiber by sol–gel method. Compos A: Appl Sci Manuf 2001;32(8):1127–31. [11] Fang G, Li H, Chen Z, Liu X. Preparation and properties of palmitic acid/SiO2 composites with flame retardant as thermal energy storage materials. Sol Energy Mater Sol Cells 2011;95(7):1875–81.
H. Zhang et al. / Composites: Part A 87 (2016) 138–145 [12] Wang Z, Zhang Z, Jia L, Yang L. Paraffin and paraffin/aluminum foam composite phase change material heat storage experimental study based on thermal management of Li-ion battery. Appl Therm Eng 2015;78:428–36. [13] Cai Y, Gao C, Zhang T, Zhang Z, Wei Q, Du J, et al. Influences of expanded graphite on structural morphology and thermal performance of composite phase change materials consisting of fatty acid eutectics and electrospun PA6 nanofibrous mats. Renew Energy 2013;57:163–70. [14] Zhang Z, Zhang N, Peng J, Fang X, Gao X, Fang Y. Preparation and thermal energy storage properties of paraffin/expanded graphite composite phase change material. Appl Energy 2012;91(1):426–31. [15] Zhang N, Yuan Y, Wang X, Cao X, Yang X, Hu S. Preparation and characterization of lauric–myristic–palmitic acid ternary eutectic mixtures/expanded graphite composite phase change material for thermal energy storage. Chem Eng J 2013;231:214–9. [16] Liu C, Yuan Y, Zhang N, Cao X, Yang X. A novel PCM of lauric–myristic–stearic acid/expanded graphite composite for thermal energy storage. Mater Lett 2014;120:43–6. [17] Yang X, Yuan Y, Zhang N, Cao X, Liu C. Preparation and properties of myristic– palmitic–stearic acid/expanded graphite composites as phase change materials for energy storage. Sol Energy 2014;99:259–66.
145
[18] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sustain Energy Rev 2009;13(2):318–45. [19] Zhang J, Guan X, Song X, Hou H, Yang Z, Zhu J. Preparation and properties of gypsum based energy storage materials with capric acid–palmitic acid/expanded perlite composite PCM. Energy Buildings 2015;92:155–60. [20] Li M, Wu Z, Kao H. Study on preparation and thermal properties of binary fatty acid/diatomite shape-stabilized phase change materials. Sol Energy Mater Sol Cells 2011;95(8):2412–6. [21] Karaipekli A, Sarı A. Preparation, thermal properties and thermal reliability of eutectic mixtures of fatty acids/expanded vermiculite as novel form-stable composites for energy storage. J Ind Eng Chem 2010;16(5):767–73. [22] Zhang Q, Wang H, Ling Z, Fang X, Zhang Z. RT100/expand graphite composite phase change material with excellent structure stability, photo-thermal performance and good thermal reliability. Sol Energy Mater Sol Cells 2015;140:158–66. [23] Ling Z, Chen J, Xu T, Fang X, Gao X, Zhang Z. Thermal conductivity of an organic phase change material/expanded graphite composite across the phase change temperature range and a novel thermal conductivity model. Energy Convers Manage 2015;102:202–8.