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expanded graphite composite as phase change material for energy storage
Energy xxx (2014) 1e7
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Preparation and properties of palmitic-stearic acid eutectic mixture/ expanded graphite composite as phase change material for energy storage Nan Zhang a, Yanping Yuan a, *, Yanxia Du b, Xiaoling Cao a, Yaguang Yuan a a b
School of Mechanical Engineering, Southwest Jiaotong University, 610031 Chengdu, China State Key Laboratory of Aerodynamics, China Aerodynamics Research and Development Center, 621000 Mianyang, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2014 Received in revised form 27 October 2014 Accepted 30 October 2014 Available online xxx
A novel composite PCM (phase change material) with PA-SA (palmitic-stearic acid) eutectic mixture as PCM and EG (expanded graphite) as supporting material was prepared. The optimum absorption ratio of PA-SA/EG (Palmitic-stearic acid/expanded graphite) composite PCM was determined as PA-SA:EG ¼ 13:1 (by mass). Scanning electron microscope and Fourier transformation infrared spectroscopy results show that PA-SA was uniformly distributed in the porous network structure of EG due to the physical action. Thermal property and thermal stability of the PA-SA/EG composite PCM were characterized by DSC (differential scanning calorimetry) and TGA (thermogravimetric analysis). DSC results indicated that the melting and freezing temperatures and latent heats of PA-SA/EG were measured as 53.89 C and 54.37 C, and 166.27 J/g and 166.13 J/g. TGA test results revealed that PA-SA/EG had a good thermal stability in working temperature range. Thermal cycling test results showed PA-SA/EG had a good thermal reliability after 720 thermal cycles. Thermal conductivity of the composite PCM was measured as 2.51 W/m K, much higher than that of PA-SA. The thermal energy storage and release rates of PA-SA/EG were also increased due to the high thermal conductivity of EG. In conclusion, the prepared PA-SA/EG composite PCM can be acted as a potential material for thermal energy storage due to the acceptable thermal properties, good thermal reliability and stability, high thermal conductivity. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Palmitic-stearic acid eutectic mixture Composite phase change materials Optimum absorption ratio Thermal properties
1. Introduction TES (thermal energy storage) is one of promising methods to solve the contradiction between energy supply and demand in time and space. This technique has been considered as a potential solution to the problem of energy shortage and environmental issues [1]. Thermal energy can be stored in the form of sensible heat storage, latent heat storage and chemical reaction heat storage [2]. Among these forms, LHTES (latent heat thermal energy storage) is realized by using PCMs (phase change materials) which can store or release heat energy during the phase change process when the surrounding temperature increases or decreases. PCMs feature the advantages of high energy density and small temperature variation from storage to retrieval [3]. These features make the LHTES highly concerned and widely employed in many applications such as solar
* Corresponding author. Tel./fax: þ86 28 87634937. E-mail address: [email protected] (Y. Yuan).
thermal energy storage [4,5], air conditioning condensation heat recovery system [6], temperature regulating textiles [7], building energy saving engineering [8,9]. Among the investigated PCMs, fatty acids have been paid much attention because of the high heat capacity, low supercooling and vapor pressure, non-toxic, good thermal and chemical stability, small volume change and self-nucleating behavior [10]. The raw materials of fatty acids can be obtained from the fat of animals and vegetables, so the source of fatty acids is abundant. And the prices of PA (palmitic acid) and SA (stearic acid) are cheaper than the other inorganic PCMs in the market. Furthermore, the phase change temperature of binary and ternary fatty acids eutectic mixture is lower than that of the component of eutectic mixture, which means a wider phase change temperature range of fatty acids [11]. In order to use PA and LA (lauric acid) in low temperature solar application such as solar space heating and greenhouse heating, Tuncbilek et al. [12] mixed 69 wt% LA and 31 wt% PA to form a eutectic mixture with the melting temperature of 35.2 C. They also consider the experimental determination of the thermal characteristics of the
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eutectic mixture during the heat charging and discharging processes and the results indicate the LAePA eutectic mixture can be a potential material for low temperature thermal energy storage application. However, there is a major drawback of low thermal conductivity restricting the application of fatty acids in LHTES. In order to overcome the shortcoming, various techniques enhancing the thermal conductivity of PCMs have been investigated. The thermal conductivity of PCMs could be enhanced by using finned heat exchangers [13], adding materials with high thermal conductivity [14e17], and impregnating PCMs into high thermal conductivity with porous structure materials [18,19]. Each of those methods has its own advantages. Moreover, high thermal conductivity with porous structure materials not only can enhance the thermal conductivity but also can prevent the leakage of PCMs during the phase change process. EG (expanded graphite) has been widely used in enhancing the thermal conductivity of PCMs due to the high thermal conductivity and unique network pore structure. Zhao et al. [20] prepared a series of paraffin/EG composites by taking EG as supporting material. The maximum content of the paraffin was 95 wt% in the composite and there was no exudation of paraffin liquid during the solideliquid phase change. The results of heat storage and release tests showed that the storage (release) times of paraffin/EG composite PCMs were greatly reduced compared with paraffin. And the reason was that the EG in the composites had higher thermal conductivity, which can greatly improve the heat transfer rate of the paraffin/EG in the heat storage (release) process. Fang et al. [21] prepared the SA/EG composites with different mass ratios were prepared by absorbing liquid SA into the EG. It was found that the SA/EG composite with a mass ratio of SA:EG ¼ 5:1 had a significantly increase in thermal conductivity without much reduction in its latent heat energy storage capacity. The thermal diffusivity of the composites was 10 times higher than that of the SA. And the leakage of the melted SA form the composite can be prevented even it was heated over the melting temperature of the SA by capillary and surface tension forces. In recent years, using PCMs in the air conditioning condensation heat recovery system for preparing domestic hot water has a remarkable economy, and can save energy and reduce thermal pollution to environment. This paper aims to prepare a novel composite PCM of PA-SA/EG (palmitic-stearic acid eutectic mixture/expanded graphite) with a suitable phase change temperature for air conditioning condensation heat recovery system. The optimum absorption ratio of PA-SA/EG composite PCM was determined. The microstructure, chemical structure and thermal properties of the prepared composite PCM were comprehensively characterized. The thermal reliability of the composite PCM was studied by the thermal cycling test. Moreover, the improvement of thermal conductivity and thermal performance of the prepared PASA/EG composite PCM was also investigated. 2. Experimental 2.1. Materials Palmitic acid (PA, AR) and stearic acid (SA, 98%pure) were purchased from Aladdin Industrial Corporation, Shanghai China. Expansible graphite (80meshes, expansion coefficient: 200 mL/g, Carbon content: 99%) was supplied by Jinrilai Electronic Materials Factory, Qingdao China. 2.2. Preparation of PA-SA binary eutectic mixture A certain amount of PA and SA were put in a 100 ml beaker, and then heated in a thermostatic water bath at 80 C. When the
two fatty acids melted completely, they were stirred in a magnetic stirrer at 400 r/min for 30 min to make sure a homogenous mixing and then cooled down to the room temperature. Then phase change temperature and latent heat of PA-SA eutectic mixture were determined by DSC (differential scanning calorimetry).
2.3. Preparation of PA-SA/EG composite PCM EG was obtained by microwave treatment of expansible graphite using a microwave oven at a microwave irradiation power 700 W for 30 s. In order to determine the optimum absorption ratio of PA-SA to EG, a series of PA-SA/EG composites with different mass fractions were prepared by using the obtained PA-SA eutectic mixture as PCM and EG as supporting material as the following process. EG was put into ten 50 ml beakers with 0.2 g for each beaker, then PA-SA eutectic mixtures with different weight were placed on the EG in the ten beakers. Then, the beakers were put in an oven for 24 h at 75 C. The mixtures in each beaker were stirred every 8 h to make sure a homogenous absorption of PA-SA in EG. The composites were obtained when the samples cooled down to the room temperature.
2.4. Characterization The phase change temperatures and latent heats of PA, SA, PASA eutectic mixture and PA-SA/EG composite PCM were obtained by using a differential scanning calorimeter (DSC, TA Q20 USA) at 5 C/min under a constant stream of argon at a flow rate of 50 ml/ min. DSC instrument was calibrated with indium as a standard reference material, and the accuracy of enthalpy measurement was ±4%. The morphology and microstructure of the EG and PASA/EG composite were observed by using a scanning electronic microscope (SEM, Fei Inspect FEI, the Netherlands). The structural analysis of the EG, PA-SA and PA-SA/EG composite was carried out by using a FT-IR (Fourier transform infrared spectrometer, Nicolet 6700, USA). The FT-IR spectra were recorded on a KBr pellet at the frequency range of 4000 cm1 to 400 cm1. The accelerated thermal cycling test was carried out by a metal bath (CHB-T2-E, BIOER, China). Thermogravimetric analysis was determined by a thermo analyzer instrument (TGA (thermogravimetic analysis), NETZSCH TG 209F1, German). The tests were carried out under an inert nitrogen atmosphere at a flow of 60 ml/min and a heating rate of 20 C/min from 35 C to 400 C. Thermal conductivities of PA-SA and PA-SA/EG were measured by a thermal property analyzer (Hot Disk 2500, Swedish) at the room temperature (25 C) which is lower than the phase change temperature of the samples. In order to investigate the thermal performance of the prepared PA-SA/EG, 30 g PA-SA and 30 g PA-SA/EG were separately filled into the same glass test tubes. And two thermocouples were placed in the center of the test tubes respectively. When temperature of the materials inside the test tubes at a constant temperature of 18 C, the two test tubes were placed into a water bath at temperature of 75 C. Then the heat storage process was measured. After the heat storage finished, the test tubes were immediately moved to solidification process at a constant temperature of 18 C. The heat release curve was obtained once the temperature variation was recorded. The temperature variations of the samples were automatically recorded by a PC via data logger (Agilent 34980A) with a temperature measuring accuracy of ±0.25 C at time intervals of 10 s.
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3. Results and discussion 3.1. Determination of the mass ratio and thermal properties of PA-SA The thermal properties of PA and SA are listed in Table 1. As seen from Table 1, the phase change temperatures of PA and SA are higher than 60 C. The phase change temperatures of PA and SA are too high to use in the air conditioning condensation heat recovery system. There is an efficient way to reduce the phase change temperature of fatty acids by making eutectic mixture. Therefore, in this work, PA-SA eutectic mixture as PCM with a proper phase change temperature for air conditioning condensation heat recovery system was firstly prepared. The eutectic mixture mass ratio and eutectic melting temperature of PA-SA curve was developed (as Fig. 1) based on Schroder's equation which is listed as formula (1) and the measured thermal properties of PA and SA. It can be seen from Fig. 1 that the mass ratio of PA to SA was 58.8 to 41.2, and the melting temperature was 53.24 C. Referring to the calculated results, a PA-SA binary eutectic mixture was prepared with the eutectic mixture mass ratio of PA:SA ¼ 62:38, which was similar to that reported in the literature [22]. Baran et al. [22] found that the PA-SA binary system in the mixture ratio of 64.2:35.8 wt% forms a eutectic, which melts at 52.3 C and has a latent heat of 181.7 J/g. The DSC curves of the PA, SA and PA-SA eutectic mixture are showed in Fig. 2 and the thermal properties of PA, SA and PA-SA eutectic mixture are showed in Table 1 with RSD (Relative Standard Deviation) and 95% confidence interval of the measured data. It can be seen from Fig. 2 that the DSC curve of PA-SA was similar to that of PA and SA, that is, each of them showed only one endothermic peak during the melting process and only one exothermic peak on the freezing process. The melting temperature and freezing temperature of PA-SA were measured as 53.95 C and 53.72 C, and the melting and freezing latent heats were measured as 177.67 J/g and 178.10 J/g. The phase change temperature of the mixture was very close to the result calculated by formula (1).
Tm ¼
1 R lnXi Ti Hi
Fig. 1. Melting temperatures of PA-SA mixtures versus composition of the components.
1 (1)
where Ti and Hi are the phase change temperature and latent heat of the ith fatty acid; Tm is the phase change temperature of the eutectic mixture, Xi is the content of the ith composite contained in the eutectic mixture, and R is the gas constant, is 8.314 J/mol K. 3.2. Determination of the optimum mass ratio of PA-SA to EG In the practical application of PCMs, those with a higher latent heat are preferred because it means a lower cost. For PA-SA/EG composite PCM, the latent heat capacity is affected on the content of PA-SA. In this study, 10 kinds of PA-SA/EG composites with different mass ratios named as S1eS10 were prepared. The mass ratios of PA-SA to EG are listed in Table 2. The optimum absorption ratio of the PA-SA eutectic mixture to EG was determined by making use of the absorption of molten PA-SA in the filter paper.
Fig. 2. DSC curves of PA, SA and PA-SA eutectic mixture.
0.200 g each of the 10 kinds of the prepared PA-SA/EG composites was placed on the corresponding spots of the filter paper as shown in Fig. 3(a). Then the samples were heated at 75 C for 1 h Fig. 3(b) shows the results of the state of samples after a high temperature heat treatment. The weight and percentage loss of the samples before and after the thermal treatment are listed in Table 2. From Fig. 3(b), it can be found that there was no imprints appeared around the samples after the thermal treatment, which could illustrate there was no liquid PA-SA leakage from S1, S2 and S3. There were some imprints around S4eS10, and the size of the imprint increased with the increase of content of PA-SA in S4eS10. Table 2 shows that the percentage losses of S1, S2 and S3 were below 3%, which may be caused by measuring and operation errors; and the percentage losses of S4eS10 we from 9.50% to 25.50%. The
Table 1 Thermal properties of PA, SA and PA-SA eutectic mixture. PCMs
Melting temperature ( C)
Latent heat of melting (J/g)
RSD (%)
95% confidence interval
Freezing temperature ( C)
Latent heat of freezing (J/g)
RSD (%)
95% confidence interval
PA SA PA-SA
61.99 68.54 53.95
198.43 201.83 177.67
1.81 2.35 0.64
±2.25 ±2.92 ±0.79
60.60 67.42 53.72
198.30 202.03 178.10
2.23 2.47 0.54
±2.77 ±3.07 ±0.67
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Table 2 Mass ratio and weight of before and after the thermal treatment of PA-SA/EG. Samples
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
Mass ratio (PA-SA:EG) Before heat treatment (g) After heat treatment (g) Percentage loss %
11:1
12:1
13:1
14:1
15:1
16:1
17:1
18:1
19:1
20:1
0.200 0.198 1.00
0.200 0.197 1.50
0.200 0.194 3.00
0.200 0.181 9.50
0.200 0.177 11.50
0.200 0.175 12.50
0.200 0.170 15.00
0.200 0.168 16.00
0.200 0.149 25.50
0.200 0.149 25.50
Fig. 5 shows the FT-IR spectra of PA-SA, EG and PA-SA/EG composite PCM. The FT-IR spectra of PA-SA shows that the peak observed at 3430 cm1 represents the stretching vibration of eOH group, the peaks at 2920 cm1 and 2850 cm1 represent the stretching vibration of eCH3 and eCH2 group. The absorption band of OeH stretching vibration is in the interval of 3000e2750 cm1 which usually overlaps with the absorption band of aliphatic CeH stretching vibration. The peak at 1700 cm1 is the characteristic absorption peak for the stretching vibration of C]O. The peak at 1460 cm1 is the eCH2 bending peak, 1300 cm1 represents CeH and CeC bending, 933 cm1 and 721 cm1 responds to rocking vibration and bending, which are all characteristics for aliphatic chain of PA-SA. The FT-IR spectra of EG shows that the peak observed at 1640 cm1 represents its characteristic absorption peak. The FT-IR spectra of PA-SA/EG composite PCM shows that the characteristic
Fig. 3. Photographs of PA-SA/EG composite PCM (a) before and (b) after the thermal treatment.
percentage losses of samples are corresponding to the sizes of the imprint shown in Fig. 3(b). The results indicated that liquid PA-SA would leak from the PA-SA/EG composites after the thermal treatment when the mass ratio of PA-SA to EG was more than 13:1. It was because that the absorption capacity of EG to PA-SA was limited [23], the excess PA-SA adsorbed in EG will flow out and form an imprint on the filter paper when the surrounding temperature is higher than the melting temperature of PCM. Based on the above results, the optimum mass ratio of PA-SA to EG was determined as 13:1, meaning the mass fraction of PA-SA in PA-SA/ EG was 92.86 wt%. Therefore, in the following research, S3 was taken as the PA-SA/EG composite PCM.
3.3. Microstructure of EG and PA-SA/EG composite PCM The morphologies of (a) EG and (b) PA-SA/EG composite PCM (PA-SA: EG ¼ 13:1, by mass) prepared in this work are showed in Fig. 4. Fig. 4(a), the SEM of EG indicates that EG has a worm-like porous structure which expands its specific surface area and curved lamella edges which make lamellas link tighter. As a result, molten PA-SA can be absorbed by EG between the lamellas easier with a maximum absorption ratio of 92.86 wt%. Fig. 4(b), the SEM of PA-SA/EG composite PCM indicates that PA-SA is distributed uniformly in the porous structure of EG. It can be concluded that the porous structure of EG provides the mechanical strength for the whole composite materials; PA-SA and EG are closely connected at their two-phase interfaces due to high infiltrating capacity of PASA, showing a good compatibility between PA-SA and EG. And as Zhang et al. [24] mentioned that the capillary tubes and surface tension between PA-SA and the porous structure of EG can prevent molten PA-SA from leaking. The chemical compatibility between PA-SA and EG was determined by FT-IR to characterize the interaction among components.
Fig. 4. SEM images of (a) EG and (b) PA-SA/EG composite PCM.
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composite PCM was proportional to the content of PA-SA. Via formula (2), the melting and freezing latent heats of the composite PCM were calculated as 164.98 J/g and 165.38 J/g, which were slightly higher than the values measured by DSC due to the abnormal interactions between the PCM and the inner surface of pore of EG [25] on one hand, and the DSC test error on the other hand. Moreover, the comparison of thermal properties of the prepared composite PCM with those of some composite PCMs developed for thermal energy storage in the literature is shown in Table 4. It can be found that the latent heat of PA-SA/EG is as high as comparable with that of composite PCMs in literature. In conclusion, the PA-SA/EG composite PCM, with a proper phase change temperature and a higher latent heat, is a good choice for air conditioning condensation heat recovery system, solar thermal energy storage and other potential applications.
DHPASA=EG ¼ DHPASA Fig. 5. FT-IR spectra of PA-SA, EG and PA-SA/EG composite PCM.
absorption peak of EG at 1640 cm1 and the characteristic absorption peaks of PA-SA at 2,920, 2,850, 1,700, 1,460, 1,300, 933, 721 cm1 exist simultaneously with no distinct new absorption peak appears, indicating that there was no chemical but physical action-surface tension and capillary effect between LA-MA-PA and EG in the composite PCM. The absorption peak at 3430 cm1 in the FT-IR spectra was caused by the air moisture during the test.
3.4. Thermal properties of PA-SA/EG composite PCM Fig. 6 shows the DSC curve of PA-SA/EG composite PCM. Table 3 lists the thermal properties of PA-SA/EG composite PCM with standard deviation and 95% confidence interval of the measured data. It can be found from Table 3 that the melting and freezing temperatures of the composite PCM were measured as 53.89 C and 54.37 C. By comparing the melting and freezing temperatures of PA-SA and PA-SA/EG composite PCM, it is easy to find that EG had no significant effect on the phase change temperatures of composite PCM. The melting and freezing latent heats of the PA-SA/EG composite PCM were 166.27 J/g and 166.13 J/g. In the composite PCM, only PA-SA could absorb and release heat during the melting and freezing process, therefore, the latent heat capacity of
Fig. 6. DSC curves of PA-SA/EG composite PCM before and after thermal cycling.
mPASA mPASA=EG
(2)
where DHPA-SA/EG is the latent heat of PA-SA/EG composite PCM, DHPA-SA is the latent heat of PA-SA, mPA-SA/EG and mPA-SA are the mass values of PA-SA/EG composite PCM and PA-SA. 3.5. Thermal reliability of PA-SA/EG composite PCM A good thermal reliability is very important for the application of composite PCMs, for it makes sure the stable phase change temperature and latent heat of composite PCMs after a large number of thermal cycles. Therefore, there should be no or little change in the thermal property after long-term utility period. In this paper, an accelerated thermal cycle experiment was conducted to investigate the changes of the phase change temperature and latent heat of the PA-SA/EG composite PCM after 720 thermal cycles. In Fig. 6, the dotted line represents the DSC test results of the composite PCM after the thermal cycles. Table 3 shows that after 720 thermal cycles, the melting and freezing temperatures of the composite PCM changed to 0.11 C and 0.14 C. The latent heats dropped by 2.93% in melting and 2.97% in freezing. As can be seen from these results, PA-SA/EG composite PCM just changed slightly in phase change temperature and latent heat after 720 thermal cycles, showing a good thermal reliability. And the changes would not affect the LHTES application. 3.6. Thermal conductivity and performance improvement of PA-SA/ EG composite PCM The thermal conductivity is an important parameter of PCM in LHTES applications because it affects the heat storage and release rate. In this paper, the thermal conductivities of the PA-SA and PASA/EG composite PCM were measured by hot-wire technique. The thermal conductivity of the PA-SA was measured as 0.26 W/m K, which was too low for thermal energy storage. The thermal conductivity of the prepared PA-SA/EG composite PCM was measured as 2.51 W/m K, much higher than that of PA-SA. The results showed that it is an effective way to improve the thermal conductivity of PCMs by impregnating PCMs into high thermal conductivity with porous structure material, such as EG. The improvement of the thermal conductivity was also verified by comparing the melting and freezing performance of the PA-SA with that of the PA-SA/EG composite PCM. Temperature curves for melting and freezing of the PA-SA and PA-SA/EG are shown in Fig. 7. As seen from Fig. 7(a), when the center temperature increased from 18 C to 53 C, it took 12.5 min for the PA-SA and only 3.5 min for the PA-SA/EG composite PCM. The freezing time was also determined by the freezing curves in Fig. 7(b). It took
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Table 3 Thermal properties of PA-SA/EG composite PCM before and after thermal cycling. PCMs
Melting temperature ( C)
Latent heat of melting (J/g)
RSD (%)
95% Confidence interval
Freezing temperature ( C)
Latent heat of freezing (J/g)
RSD (%)
95% Confidence interval
PA-SA/EG PA-SA/EG after 720 cycles
53.89 53.78
166.27 161.40
0.16 0.45
±0.19 ±0.56
54.37 54.23
166.13 161.19
0.57 0.55
±0.71 ±0.69
Table 4 Comparison of thermal properties of PA-SA/EG with that of some composite PCMs in literature. Composite PCM
Melting Freezing Latent Reference temperature/ C temperature/ C heat/J/g
90wt%paraffin/EG Lauric/activated carbon (1:2) Stearic acid/EG (5:1) 60wt%Capric acid/ halloysite nanotube MA-PA-SA/EG PA-SA/EG
40.2 44.07
e 42.83
178.3 65.14
[18] [19]
53.12 29.34
54.28 25.28
155.5 75.52
[21] [26]
41.64 53.89
42.99 54.37
153.5 166.27
[27] Present work
29 min for PA-SA and only 18 min for PA-SA/EG composite PCM to complete the solidification process. All the results showed that the thermal storage and release rates were significantly increased due to the increased thermal conductivity.
3.7. Thermal stability of PA-SA/EG composite PCM The thermal stability of PA-SA and PA-SA/EG composite PCM were evaluated by TGA method. TGA curves of the PA-SA and PASA/EG composite PCM are shown in Fig. 8. The 5% weight loss temperature (T5 wt.%), onset temperature of weight loss (Tonset), temperature of maximum decomposition (Tmax) and the charred residue at 400 C are presented in Table 5. It can be seen from Fig. 8 that there was a single degradation process from 35 C to 400 C for the PA-SA and PA-SA/EG composite PCM because of the decomposition of fatty acids. It can be found from Table 5 that T5 wt.%, Tonset, Tmax and the charred residue amount of the PA-SA/EG composite PCM were all higher than those of the PA-SA. The results indicated that EG was advantageous to slow the degradation process during the thermal decomposition. The 5% weight loss temperature and onset temperature of weigh loss of the PA-SA/EG composite PCM are higher than 200 C. That means that the PA-SA/ EG composite PCM has a good thermal stability in its working temperature range. 4. Conclusions In this paper, a novel composite PCM of PA-SA/EG with an optimum absorption ratio (PA-SA:EG ¼ 13:1, by mass) was prepared. SEM and FT-IR results showed that PA-SA was uniformly distributed in the network porous structure of EG by the capillary force and surface tension force. The melting and freezing temperatures and
Fig. 8. TG curves of the PA-SA and PA-SA/EG composite PCM.
Table 5 Characteristic temperatures and charred residue at 400 C of the PA-SA and PA-SA/ EG composite PCM.
Fig. 7. (a) Melting temperature curves and (b) solidification temperature curves of PASA and PA-SA/EG composite PCM.
Samples
T5
PA-SA PA-SA/EG
194.9 209.3
wt.%
( C)
Tonset ( C)
Tmax ( C)
Charred residue amount (%) (400 C)
215.1 243.7
256.7 273.3
1.21 7.15
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N. Zhang et al. / Energy xxx (2014) 1e7
latent heats of PA-SA/EG composite PCM were measured as 53.89 C and 54.37 C, 166.27 J/g and 166.13 J/g by DSC. Thermal cycling teat results indicated a good thermal reliability of PA-SA/EG because of the melting and freezing temperatures of PA-SA/EG composite PCM dropped by merely 0.11 C and 0.14 C, and the melting and freezing latent heats of the composite PCM decreased by only 2.93% and 2.97% after 720 thermal cycles. The thermal conductivity of the composite PCM at room temperature was measured as 2.51 W/m K, which was much higher than that of pure PA-SA. The thermal performance test results showed that the thermal storage and release rates of PA-SA/EG composite PCM were greatly increased due to the improvement of the thermal conductivity. TGA test revealed that the prepared composite PCM had a good thermal stability in working temperature range. However, even as a novel PCM, the PA-SA/EG composite also has some limitations in the practical applications. The phase change temperature is fixed, the latent heat is not very high, and the thermal conductivity is just enough. Therefore, the prepared PA-SA/EG composite PCM can be just applied in the air conditioning condensation heat recovery system. Acknowledgments The work is supported by the Natural Science Foundation of China (NO: 51378426), Program for New Century Excellent Talents in University of Ministry of Education (NO: NCET-11-0714), Program for Outstanding Young Science Leaders of Sichuan Province (NO: 2010A14-449) and 2014 Cultivation program for the Excellent Doctoral Dissertation of Southwest Jiaotong University. References [1] Wang Y, Zheng H, Feng H, Zhang D. Effect of preparation methods on the structure and thermal properties of stearic acid/activated montmorillonite phase change materials. Energy Build 2012;47:467e73. [2] Chen C, Wang L, Huang Y. Morphology and thermal properties of electrospun fatty acids/polyethylene terephthalate composite fibers as novel form-stable phase change materials. Sol Energy Mater Sol C 2008;92:1382e7. [3] Tumirah K, Hussein MZ, Zulkarnain Z, Rafeadah R. Nano-encapsulated organic phase change material based on copolymer nanocomposites for thermal energy storage. Energy 2014;66:881e90. [4] Cai Y, Gao C, Xu X, Fu Z, Fei X, Zhao Y, et al. Electrospun ultrafine composite fibers consisting of lauric acid and polyamide 6 as form-stable phase change materials for storage and retrieval of solar thermal energy. Sol Energy Mater Sol C 2012;103:53e61. [5] Koca A, Oztop HF, Koyun T, Varol Y. Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector. Renew Energy 2008;33:567e8. [6] Gu Z, Liu H, Li Y. Thermal energy recovery of air conditioning systemeeheat recovery system calculation and phase change materials development. Appl Therm Eng 2004;24:2511e26.
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[7] Li W, Zhang X, Wang X, Tang G, Shi H. Fabrication and morphological characterization of microencapsulated phase change materials (MicroPCMs) and macrocapsules containing MicroPCMs for thermal energy storage. Energy 2012;38:249e54. [8] Tyagi VV, Buddhi D. PCM thermal storage in buildings: a state of art. Renew Sustain Energy Rev 2007;11:1146e66. [9] Zhang Y, Zhou G, Lin K, Zhang Q, Di H. Application of latent heat thermal energy storage in buildings: state-of-the-art and outlook. Build Environ 2007;42:2197e209. [10] Yuan Y, Zhang N, Tao W, Cao X, He Y. Fatty acids as phase change materials: a review. Renew Sustain Energy Rev 2014;19:482e98. [11] Yuan Y, Tao W, Cao X, Bai L. Theoretic prediction of melting temperature and latent heat for a fatty acid eutectic mixture. J Chem Eng Data 2011;56: 2889e91. [12] Tuncbilek K, Sarı A, Tarhan S, Ergunes G, Kaygusuz K. Lauric palmitic acids eutectic mixture as latent heat storage material for low temperature heating applicationsvol. 30; 2005. p. 677e92. [13] Shatikian V, Ziskand G, Letan R. Numerical investigation of a PCM-based heat sink with internal fins. Int J Heat Mass Transf 2005;48:3689e706. [14] Fan L, Khodadadi JM. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew Sustain Energy Rev 2011;15:24e46. [15] Karaipekli A, Sarı A, Kaygusuz K. Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications. Renew Energy 2007;32:2201e10. [16] Li T, Lee JH, Wang R, Kang YT. Enhancement of heat transfer for thermal energy storage application using stearic acid nanocomposite with multiwalled carbon nanotubes. Energy 2013;55:752e61. [17] Wang J, Xie H, Xin Z, Li Y. Increasing the thermal conductivity of palmitic acid by the addition of carbon nanotubes. Carbon 2010;48:3979e86. [18] Sarı A, Karaipekli A. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Appl Therm Eng 2007;27:1271e7. [19] Chen Z, Shan F, Cao L, Fang G. Synthesis and thermal properties of shapestabilized lauric acid/activated carbon composites as phase change materials for thermal energy storage. Sol Energy Mater Sol C 2012;102:131e6. [20] Zhao J, Guo Y, Feng F, Tong Q, Qv W, Wang H. Microstructure and thermal properties of a paraffin/expanded graphite phaseechange composite for thermal storage. Renew Energy 2011;36:1339e42. [21] Fang G, Li H, Chen Z, Liu X. Preparation and characterization of stearic acid/ expanded graphite composites as thermal energy storage materials. Energy 2010;35:4622e6. [22] Baran G, Sarı A. Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system. Energ Convers Manage 2003;44:3227e46. [23] Celzard A, Mareche JF, Furdin G. Surface area of compressed expanded graphite. Carbon 2002;40:2713e8. [24] Zhang D, Tian S, Xiao D. Experimental study on the phase change behavior of phase change material confined in pores. Sol Energy 2007;81:653e60. [25] 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;16:767e73. [26] Mei D, Zhang B, Liu R, Zhang Y, Liu J. Preparation of capric acid/halloysite nanotube composite as form-stable phase change material for thermal energy storage. Sol Energy Mater Sol C 2011;95:2772e7. [27] Yang X, Yuan Y, Zhang N, Cao X, Liu C. Preparation and properties of myristicpalmitic-stearic acid/expanded graphite composites as phase change materials for energy storage. Sol Energy 2014;99:259e66.
Please cite this article in press as: Zhang N, et al., Preparation and properties of palmitic-stearic acid eutectic mixture/expanded graphite composite as phase change material for energy storage, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.10.092
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Energy journal homepage: www.elsevier.com/locate/energy
Preparation and properties of palmitic-stearic acid eutectic mixture/ expanded graphite composite as phase change material for energy storage Nan Zhang a, Yanping Yuan a, *, Yanxia Du b, Xiaoling Cao a, Yaguang Yuan a a b
School of Mechanical Engineering, Southwest Jiaotong University, 610031 Chengdu, China State Key Laboratory of Aerodynamics, China Aerodynamics Research and Development Center, 621000 Mianyang, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2014 Received in revised form 27 October 2014 Accepted 30 October 2014 Available online xxx
A novel composite PCM (phase change material) with PA-SA (palmitic-stearic acid) eutectic mixture as PCM and EG (expanded graphite) as supporting material was prepared. The optimum absorption ratio of PA-SA/EG (Palmitic-stearic acid/expanded graphite) composite PCM was determined as PA-SA:EG ¼ 13:1 (by mass). Scanning electron microscope and Fourier transformation infrared spectroscopy results show that PA-SA was uniformly distributed in the porous network structure of EG due to the physical action. Thermal property and thermal stability of the PA-SA/EG composite PCM were characterized by DSC (differential scanning calorimetry) and TGA (thermogravimetric analysis). DSC results indicated that the melting and freezing temperatures and latent heats of PA-SA/EG were measured as 53.89 C and 54.37 C, and 166.27 J/g and 166.13 J/g. TGA test results revealed that PA-SA/EG had a good thermal stability in working temperature range. Thermal cycling test results showed PA-SA/EG had a good thermal reliability after 720 thermal cycles. Thermal conductivity of the composite PCM was measured as 2.51 W/m K, much higher than that of PA-SA. The thermal energy storage and release rates of PA-SA/EG were also increased due to the high thermal conductivity of EG. In conclusion, the prepared PA-SA/EG composite PCM can be acted as a potential material for thermal energy storage due to the acceptable thermal properties, good thermal reliability and stability, high thermal conductivity. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Palmitic-stearic acid eutectic mixture Composite phase change materials Optimum absorption ratio Thermal properties
1. Introduction TES (thermal energy storage) is one of promising methods to solve the contradiction between energy supply and demand in time and space. This technique has been considered as a potential solution to the problem of energy shortage and environmental issues [1]. Thermal energy can be stored in the form of sensible heat storage, latent heat storage and chemical reaction heat storage [2]. Among these forms, LHTES (latent heat thermal energy storage) is realized by using PCMs (phase change materials) which can store or release heat energy during the phase change process when the surrounding temperature increases or decreases. PCMs feature the advantages of high energy density and small temperature variation from storage to retrieval [3]. These features make the LHTES highly concerned and widely employed in many applications such as solar
* Corresponding author. Tel./fax: þ86 28 87634937. E-mail address: [email protected] (Y. Yuan).
thermal energy storage [4,5], air conditioning condensation heat recovery system [6], temperature regulating textiles [7], building energy saving engineering [8,9]. Among the investigated PCMs, fatty acids have been paid much attention because of the high heat capacity, low supercooling and vapor pressure, non-toxic, good thermal and chemical stability, small volume change and self-nucleating behavior [10]. The raw materials of fatty acids can be obtained from the fat of animals and vegetables, so the source of fatty acids is abundant. And the prices of PA (palmitic acid) and SA (stearic acid) are cheaper than the other inorganic PCMs in the market. Furthermore, the phase change temperature of binary and ternary fatty acids eutectic mixture is lower than that of the component of eutectic mixture, which means a wider phase change temperature range of fatty acids [11]. In order to use PA and LA (lauric acid) in low temperature solar application such as solar space heating and greenhouse heating, Tuncbilek et al. [12] mixed 69 wt% LA and 31 wt% PA to form a eutectic mixture with the melting temperature of 35.2 C. They also consider the experimental determination of the thermal characteristics of the
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eutectic mixture during the heat charging and discharging processes and the results indicate the LAePA eutectic mixture can be a potential material for low temperature thermal energy storage application. However, there is a major drawback of low thermal conductivity restricting the application of fatty acids in LHTES. In order to overcome the shortcoming, various techniques enhancing the thermal conductivity of PCMs have been investigated. The thermal conductivity of PCMs could be enhanced by using finned heat exchangers [13], adding materials with high thermal conductivity [14e17], and impregnating PCMs into high thermal conductivity with porous structure materials [18,19]. Each of those methods has its own advantages. Moreover, high thermal conductivity with porous structure materials not only can enhance the thermal conductivity but also can prevent the leakage of PCMs during the phase change process. EG (expanded graphite) has been widely used in enhancing the thermal conductivity of PCMs due to the high thermal conductivity and unique network pore structure. Zhao et al. [20] prepared a series of paraffin/EG composites by taking EG as supporting material. The maximum content of the paraffin was 95 wt% in the composite and there was no exudation of paraffin liquid during the solideliquid phase change. The results of heat storage and release tests showed that the storage (release) times of paraffin/EG composite PCMs were greatly reduced compared with paraffin. And the reason was that the EG in the composites had higher thermal conductivity, which can greatly improve the heat transfer rate of the paraffin/EG in the heat storage (release) process. Fang et al. [21] prepared the SA/EG composites with different mass ratios were prepared by absorbing liquid SA into the EG. It was found that the SA/EG composite with a mass ratio of SA:EG ¼ 5:1 had a significantly increase in thermal conductivity without much reduction in its latent heat energy storage capacity. The thermal diffusivity of the composites was 10 times higher than that of the SA. And the leakage of the melted SA form the composite can be prevented even it was heated over the melting temperature of the SA by capillary and surface tension forces. In recent years, using PCMs in the air conditioning condensation heat recovery system for preparing domestic hot water has a remarkable economy, and can save energy and reduce thermal pollution to environment. This paper aims to prepare a novel composite PCM of PA-SA/EG (palmitic-stearic acid eutectic mixture/expanded graphite) with a suitable phase change temperature for air conditioning condensation heat recovery system. The optimum absorption ratio of PA-SA/EG composite PCM was determined. The microstructure, chemical structure and thermal properties of the prepared composite PCM were comprehensively characterized. The thermal reliability of the composite PCM was studied by the thermal cycling test. Moreover, the improvement of thermal conductivity and thermal performance of the prepared PASA/EG composite PCM was also investigated. 2. Experimental 2.1. Materials Palmitic acid (PA, AR) and stearic acid (SA, 98%pure) were purchased from Aladdin Industrial Corporation, Shanghai China. Expansible graphite (80meshes, expansion coefficient: 200 mL/g, Carbon content: 99%) was supplied by Jinrilai Electronic Materials Factory, Qingdao China. 2.2. Preparation of PA-SA binary eutectic mixture A certain amount of PA and SA were put in a 100 ml beaker, and then heated in a thermostatic water bath at 80 C. When the
two fatty acids melted completely, they were stirred in a magnetic stirrer at 400 r/min for 30 min to make sure a homogenous mixing and then cooled down to the room temperature. Then phase change temperature and latent heat of PA-SA eutectic mixture were determined by DSC (differential scanning calorimetry).
2.3. Preparation of PA-SA/EG composite PCM EG was obtained by microwave treatment of expansible graphite using a microwave oven at a microwave irradiation power 700 W for 30 s. In order to determine the optimum absorption ratio of PA-SA to EG, a series of PA-SA/EG composites with different mass fractions were prepared by using the obtained PA-SA eutectic mixture as PCM and EG as supporting material as the following process. EG was put into ten 50 ml beakers with 0.2 g for each beaker, then PA-SA eutectic mixtures with different weight were placed on the EG in the ten beakers. Then, the beakers were put in an oven for 24 h at 75 C. The mixtures in each beaker were stirred every 8 h to make sure a homogenous absorption of PA-SA in EG. The composites were obtained when the samples cooled down to the room temperature.
2.4. Characterization The phase change temperatures and latent heats of PA, SA, PASA eutectic mixture and PA-SA/EG composite PCM were obtained by using a differential scanning calorimeter (DSC, TA Q20 USA) at 5 C/min under a constant stream of argon at a flow rate of 50 ml/ min. DSC instrument was calibrated with indium as a standard reference material, and the accuracy of enthalpy measurement was ±4%. The morphology and microstructure of the EG and PASA/EG composite were observed by using a scanning electronic microscope (SEM, Fei Inspect FEI, the Netherlands). The structural analysis of the EG, PA-SA and PA-SA/EG composite was carried out by using a FT-IR (Fourier transform infrared spectrometer, Nicolet 6700, USA). The FT-IR spectra were recorded on a KBr pellet at the frequency range of 4000 cm1 to 400 cm1. The accelerated thermal cycling test was carried out by a metal bath (CHB-T2-E, BIOER, China). Thermogravimetric analysis was determined by a thermo analyzer instrument (TGA (thermogravimetic analysis), NETZSCH TG 209F1, German). The tests were carried out under an inert nitrogen atmosphere at a flow of 60 ml/min and a heating rate of 20 C/min from 35 C to 400 C. Thermal conductivities of PA-SA and PA-SA/EG were measured by a thermal property analyzer (Hot Disk 2500, Swedish) at the room temperature (25 C) which is lower than the phase change temperature of the samples. In order to investigate the thermal performance of the prepared PA-SA/EG, 30 g PA-SA and 30 g PA-SA/EG were separately filled into the same glass test tubes. And two thermocouples were placed in the center of the test tubes respectively. When temperature of the materials inside the test tubes at a constant temperature of 18 C, the two test tubes were placed into a water bath at temperature of 75 C. Then the heat storage process was measured. After the heat storage finished, the test tubes were immediately moved to solidification process at a constant temperature of 18 C. The heat release curve was obtained once the temperature variation was recorded. The temperature variations of the samples were automatically recorded by a PC via data logger (Agilent 34980A) with a temperature measuring accuracy of ±0.25 C at time intervals of 10 s.
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3. Results and discussion 3.1. Determination of the mass ratio and thermal properties of PA-SA The thermal properties of PA and SA are listed in Table 1. As seen from Table 1, the phase change temperatures of PA and SA are higher than 60 C. The phase change temperatures of PA and SA are too high to use in the air conditioning condensation heat recovery system. There is an efficient way to reduce the phase change temperature of fatty acids by making eutectic mixture. Therefore, in this work, PA-SA eutectic mixture as PCM with a proper phase change temperature for air conditioning condensation heat recovery system was firstly prepared. The eutectic mixture mass ratio and eutectic melting temperature of PA-SA curve was developed (as Fig. 1) based on Schroder's equation which is listed as formula (1) and the measured thermal properties of PA and SA. It can be seen from Fig. 1 that the mass ratio of PA to SA was 58.8 to 41.2, and the melting temperature was 53.24 C. Referring to the calculated results, a PA-SA binary eutectic mixture was prepared with the eutectic mixture mass ratio of PA:SA ¼ 62:38, which was similar to that reported in the literature [22]. Baran et al. [22] found that the PA-SA binary system in the mixture ratio of 64.2:35.8 wt% forms a eutectic, which melts at 52.3 C and has a latent heat of 181.7 J/g. The DSC curves of the PA, SA and PA-SA eutectic mixture are showed in Fig. 2 and the thermal properties of PA, SA and PA-SA eutectic mixture are showed in Table 1 with RSD (Relative Standard Deviation) and 95% confidence interval of the measured data. It can be seen from Fig. 2 that the DSC curve of PA-SA was similar to that of PA and SA, that is, each of them showed only one endothermic peak during the melting process and only one exothermic peak on the freezing process. The melting temperature and freezing temperature of PA-SA were measured as 53.95 C and 53.72 C, and the melting and freezing latent heats were measured as 177.67 J/g and 178.10 J/g. The phase change temperature of the mixture was very close to the result calculated by formula (1).
Tm ¼
1 R lnXi Ti Hi
Fig. 1. Melting temperatures of PA-SA mixtures versus composition of the components.
1 (1)
where Ti and Hi are the phase change temperature and latent heat of the ith fatty acid; Tm is the phase change temperature of the eutectic mixture, Xi is the content of the ith composite contained in the eutectic mixture, and R is the gas constant, is 8.314 J/mol K. 3.2. Determination of the optimum mass ratio of PA-SA to EG In the practical application of PCMs, those with a higher latent heat are preferred because it means a lower cost. For PA-SA/EG composite PCM, the latent heat capacity is affected on the content of PA-SA. In this study, 10 kinds of PA-SA/EG composites with different mass ratios named as S1eS10 were prepared. The mass ratios of PA-SA to EG are listed in Table 2. The optimum absorption ratio of the PA-SA eutectic mixture to EG was determined by making use of the absorption of molten PA-SA in the filter paper.
Fig. 2. DSC curves of PA, SA and PA-SA eutectic mixture.
0.200 g each of the 10 kinds of the prepared PA-SA/EG composites was placed on the corresponding spots of the filter paper as shown in Fig. 3(a). Then the samples were heated at 75 C for 1 h Fig. 3(b) shows the results of the state of samples after a high temperature heat treatment. The weight and percentage loss of the samples before and after the thermal treatment are listed in Table 2. From Fig. 3(b), it can be found that there was no imprints appeared around the samples after the thermal treatment, which could illustrate there was no liquid PA-SA leakage from S1, S2 and S3. There were some imprints around S4eS10, and the size of the imprint increased with the increase of content of PA-SA in S4eS10. Table 2 shows that the percentage losses of S1, S2 and S3 were below 3%, which may be caused by measuring and operation errors; and the percentage losses of S4eS10 we from 9.50% to 25.50%. The
Table 1 Thermal properties of PA, SA and PA-SA eutectic mixture. PCMs
Melting temperature ( C)
Latent heat of melting (J/g)
RSD (%)
95% confidence interval
Freezing temperature ( C)
Latent heat of freezing (J/g)
RSD (%)
95% confidence interval
PA SA PA-SA
61.99 68.54 53.95
198.43 201.83 177.67
1.81 2.35 0.64
±2.25 ±2.92 ±0.79
60.60 67.42 53.72
198.30 202.03 178.10
2.23 2.47 0.54
±2.77 ±3.07 ±0.67
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Table 2 Mass ratio and weight of before and after the thermal treatment of PA-SA/EG. Samples
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
Mass ratio (PA-SA:EG) Before heat treatment (g) After heat treatment (g) Percentage loss %
11:1
12:1
13:1
14:1
15:1
16:1
17:1
18:1
19:1
20:1
0.200 0.198 1.00
0.200 0.197 1.50
0.200 0.194 3.00
0.200 0.181 9.50
0.200 0.177 11.50
0.200 0.175 12.50
0.200 0.170 15.00
0.200 0.168 16.00
0.200 0.149 25.50
0.200 0.149 25.50
Fig. 5 shows the FT-IR spectra of PA-SA, EG and PA-SA/EG composite PCM. The FT-IR spectra of PA-SA shows that the peak observed at 3430 cm1 represents the stretching vibration of eOH group, the peaks at 2920 cm1 and 2850 cm1 represent the stretching vibration of eCH3 and eCH2 group. The absorption band of OeH stretching vibration is in the interval of 3000e2750 cm1 which usually overlaps with the absorption band of aliphatic CeH stretching vibration. The peak at 1700 cm1 is the characteristic absorption peak for the stretching vibration of C]O. The peak at 1460 cm1 is the eCH2 bending peak, 1300 cm1 represents CeH and CeC bending, 933 cm1 and 721 cm1 responds to rocking vibration and bending, which are all characteristics for aliphatic chain of PA-SA. The FT-IR spectra of EG shows that the peak observed at 1640 cm1 represents its characteristic absorption peak. The FT-IR spectra of PA-SA/EG composite PCM shows that the characteristic
Fig. 3. Photographs of PA-SA/EG composite PCM (a) before and (b) after the thermal treatment.
percentage losses of samples are corresponding to the sizes of the imprint shown in Fig. 3(b). The results indicated that liquid PA-SA would leak from the PA-SA/EG composites after the thermal treatment when the mass ratio of PA-SA to EG was more than 13:1. It was because that the absorption capacity of EG to PA-SA was limited [23], the excess PA-SA adsorbed in EG will flow out and form an imprint on the filter paper when the surrounding temperature is higher than the melting temperature of PCM. Based on the above results, the optimum mass ratio of PA-SA to EG was determined as 13:1, meaning the mass fraction of PA-SA in PA-SA/ EG was 92.86 wt%. Therefore, in the following research, S3 was taken as the PA-SA/EG composite PCM.
3.3. Microstructure of EG and PA-SA/EG composite PCM The morphologies of (a) EG and (b) PA-SA/EG composite PCM (PA-SA: EG ¼ 13:1, by mass) prepared in this work are showed in Fig. 4. Fig. 4(a), the SEM of EG indicates that EG has a worm-like porous structure which expands its specific surface area and curved lamella edges which make lamellas link tighter. As a result, molten PA-SA can be absorbed by EG between the lamellas easier with a maximum absorption ratio of 92.86 wt%. Fig. 4(b), the SEM of PA-SA/EG composite PCM indicates that PA-SA is distributed uniformly in the porous structure of EG. It can be concluded that the porous structure of EG provides the mechanical strength for the whole composite materials; PA-SA and EG are closely connected at their two-phase interfaces due to high infiltrating capacity of PASA, showing a good compatibility between PA-SA and EG. And as Zhang et al. [24] mentioned that the capillary tubes and surface tension between PA-SA and the porous structure of EG can prevent molten PA-SA from leaking. The chemical compatibility between PA-SA and EG was determined by FT-IR to characterize the interaction among components.
Fig. 4. SEM images of (a) EG and (b) PA-SA/EG composite PCM.
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composite PCM was proportional to the content of PA-SA. Via formula (2), the melting and freezing latent heats of the composite PCM were calculated as 164.98 J/g and 165.38 J/g, which were slightly higher than the values measured by DSC due to the abnormal interactions between the PCM and the inner surface of pore of EG [25] on one hand, and the DSC test error on the other hand. Moreover, the comparison of thermal properties of the prepared composite PCM with those of some composite PCMs developed for thermal energy storage in the literature is shown in Table 4. It can be found that the latent heat of PA-SA/EG is as high as comparable with that of composite PCMs in literature. In conclusion, the PA-SA/EG composite PCM, with a proper phase change temperature and a higher latent heat, is a good choice for air conditioning condensation heat recovery system, solar thermal energy storage and other potential applications.
DHPASA=EG ¼ DHPASA Fig. 5. FT-IR spectra of PA-SA, EG and PA-SA/EG composite PCM.
absorption peak of EG at 1640 cm1 and the characteristic absorption peaks of PA-SA at 2,920, 2,850, 1,700, 1,460, 1,300, 933, 721 cm1 exist simultaneously with no distinct new absorption peak appears, indicating that there was no chemical but physical action-surface tension and capillary effect between LA-MA-PA and EG in the composite PCM. The absorption peak at 3430 cm1 in the FT-IR spectra was caused by the air moisture during the test.
3.4. Thermal properties of PA-SA/EG composite PCM Fig. 6 shows the DSC curve of PA-SA/EG composite PCM. Table 3 lists the thermal properties of PA-SA/EG composite PCM with standard deviation and 95% confidence interval of the measured data. It can be found from Table 3 that the melting and freezing temperatures of the composite PCM were measured as 53.89 C and 54.37 C. By comparing the melting and freezing temperatures of PA-SA and PA-SA/EG composite PCM, it is easy to find that EG had no significant effect on the phase change temperatures of composite PCM. The melting and freezing latent heats of the PA-SA/EG composite PCM were 166.27 J/g and 166.13 J/g. In the composite PCM, only PA-SA could absorb and release heat during the melting and freezing process, therefore, the latent heat capacity of
Fig. 6. DSC curves of PA-SA/EG composite PCM before and after thermal cycling.
mPASA mPASA=EG
(2)
where DHPA-SA/EG is the latent heat of PA-SA/EG composite PCM, DHPA-SA is the latent heat of PA-SA, mPA-SA/EG and mPA-SA are the mass values of PA-SA/EG composite PCM and PA-SA. 3.5. Thermal reliability of PA-SA/EG composite PCM A good thermal reliability is very important for the application of composite PCMs, for it makes sure the stable phase change temperature and latent heat of composite PCMs after a large number of thermal cycles. Therefore, there should be no or little change in the thermal property after long-term utility period. In this paper, an accelerated thermal cycle experiment was conducted to investigate the changes of the phase change temperature and latent heat of the PA-SA/EG composite PCM after 720 thermal cycles. In Fig. 6, the dotted line represents the DSC test results of the composite PCM after the thermal cycles. Table 3 shows that after 720 thermal cycles, the melting and freezing temperatures of the composite PCM changed to 0.11 C and 0.14 C. The latent heats dropped by 2.93% in melting and 2.97% in freezing. As can be seen from these results, PA-SA/EG composite PCM just changed slightly in phase change temperature and latent heat after 720 thermal cycles, showing a good thermal reliability. And the changes would not affect the LHTES application. 3.6. Thermal conductivity and performance improvement of PA-SA/ EG composite PCM The thermal conductivity is an important parameter of PCM in LHTES applications because it affects the heat storage and release rate. In this paper, the thermal conductivities of the PA-SA and PASA/EG composite PCM were measured by hot-wire technique. The thermal conductivity of the PA-SA was measured as 0.26 W/m K, which was too low for thermal energy storage. The thermal conductivity of the prepared PA-SA/EG composite PCM was measured as 2.51 W/m K, much higher than that of PA-SA. The results showed that it is an effective way to improve the thermal conductivity of PCMs by impregnating PCMs into high thermal conductivity with porous structure material, such as EG. The improvement of the thermal conductivity was also verified by comparing the melting and freezing performance of the PA-SA with that of the PA-SA/EG composite PCM. Temperature curves for melting and freezing of the PA-SA and PA-SA/EG are shown in Fig. 7. As seen from Fig. 7(a), when the center temperature increased from 18 C to 53 C, it took 12.5 min for the PA-SA and only 3.5 min for the PA-SA/EG composite PCM. The freezing time was also determined by the freezing curves in Fig. 7(b). It took
Please cite this article in press as: Zhang N, et al., Preparation and properties of palmitic-stearic acid eutectic mixture/expanded graphite composite as phase change material for energy storage, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.10.092
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N. Zhang et al. / Energy xxx (2014) 1e7
Table 3 Thermal properties of PA-SA/EG composite PCM before and after thermal cycling. PCMs
Melting temperature ( C)
Latent heat of melting (J/g)
RSD (%)
95% Confidence interval
Freezing temperature ( C)
Latent heat of freezing (J/g)
RSD (%)
95% Confidence interval
PA-SA/EG PA-SA/EG after 720 cycles
53.89 53.78
166.27 161.40
0.16 0.45
±0.19 ±0.56
54.37 54.23
166.13 161.19
0.57 0.55
±0.71 ±0.69
Table 4 Comparison of thermal properties of PA-SA/EG with that of some composite PCMs in literature. Composite PCM
Melting Freezing Latent Reference temperature/ C temperature/ C heat/J/g
90wt%paraffin/EG Lauric/activated carbon (1:2) Stearic acid/EG (5:1) 60wt%Capric acid/ halloysite nanotube MA-PA-SA/EG PA-SA/EG
40.2 44.07
e 42.83
178.3 65.14
[18] [19]
53.12 29.34
54.28 25.28
155.5 75.52
[21] [26]
41.64 53.89
42.99 54.37
153.5 166.27
[27] Present work
29 min for PA-SA and only 18 min for PA-SA/EG composite PCM to complete the solidification process. All the results showed that the thermal storage and release rates were significantly increased due to the increased thermal conductivity.
3.7. Thermal stability of PA-SA/EG composite PCM The thermal stability of PA-SA and PA-SA/EG composite PCM were evaluated by TGA method. TGA curves of the PA-SA and PASA/EG composite PCM are shown in Fig. 8. The 5% weight loss temperature (T5 wt.%), onset temperature of weight loss (Tonset), temperature of maximum decomposition (Tmax) and the charred residue at 400 C are presented in Table 5. It can be seen from Fig. 8 that there was a single degradation process from 35 C to 400 C for the PA-SA and PA-SA/EG composite PCM because of the decomposition of fatty acids. It can be found from Table 5 that T5 wt.%, Tonset, Tmax and the charred residue amount of the PA-SA/EG composite PCM were all higher than those of the PA-SA. The results indicated that EG was advantageous to slow the degradation process during the thermal decomposition. The 5% weight loss temperature and onset temperature of weigh loss of the PA-SA/EG composite PCM are higher than 200 C. That means that the PA-SA/ EG composite PCM has a good thermal stability in its working temperature range. 4. Conclusions In this paper, a novel composite PCM of PA-SA/EG with an optimum absorption ratio (PA-SA:EG ¼ 13:1, by mass) was prepared. SEM and FT-IR results showed that PA-SA was uniformly distributed in the network porous structure of EG by the capillary force and surface tension force. The melting and freezing temperatures and
Fig. 8. TG curves of the PA-SA and PA-SA/EG composite PCM.
Table 5 Characteristic temperatures and charred residue at 400 C of the PA-SA and PA-SA/ EG composite PCM.
Fig. 7. (a) Melting temperature curves and (b) solidification temperature curves of PASA and PA-SA/EG composite PCM.
Samples
T5
PA-SA PA-SA/EG
194.9 209.3
wt.%
( C)
Tonset ( C)
Tmax ( C)
Charred residue amount (%) (400 C)
215.1 243.7
256.7 273.3
1.21 7.15
Please cite this article in press as: Zhang N, et al., Preparation and properties of palmitic-stearic acid eutectic mixture/expanded graphite composite as phase change material for energy storage, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.10.092
N. Zhang et al. / Energy xxx (2014) 1e7
latent heats of PA-SA/EG composite PCM were measured as 53.89 C and 54.37 C, 166.27 J/g and 166.13 J/g by DSC. Thermal cycling teat results indicated a good thermal reliability of PA-SA/EG because of the melting and freezing temperatures of PA-SA/EG composite PCM dropped by merely 0.11 C and 0.14 C, and the melting and freezing latent heats of the composite PCM decreased by only 2.93% and 2.97% after 720 thermal cycles. The thermal conductivity of the composite PCM at room temperature was measured as 2.51 W/m K, which was much higher than that of pure PA-SA. The thermal performance test results showed that the thermal storage and release rates of PA-SA/EG composite PCM were greatly increased due to the improvement of the thermal conductivity. TGA test revealed that the prepared composite PCM had a good thermal stability in working temperature range. However, even as a novel PCM, the PA-SA/EG composite also has some limitations in the practical applications. The phase change temperature is fixed, the latent heat is not very high, and the thermal conductivity is just enough. Therefore, the prepared PA-SA/EG composite PCM can be just applied in the air conditioning condensation heat recovery system. Acknowledgments The work is supported by the Natural Science Foundation of China (NO: 51378426), Program for New Century Excellent Talents in University of Ministry of Education (NO: NCET-11-0714), Program for Outstanding Young Science Leaders of Sichuan Province (NO: 2010A14-449) and 2014 Cultivation program for the Excellent Doctoral Dissertation of Southwest Jiaotong University. References [1] Wang Y, Zheng H, Feng H, Zhang D. Effect of preparation methods on the structure and thermal properties of stearic acid/activated montmorillonite phase change materials. Energy Build 2012;47:467e73. [2] Chen C, Wang L, Huang Y. Morphology and thermal properties of electrospun fatty acids/polyethylene terephthalate composite fibers as novel form-stable phase change materials. Sol Energy Mater Sol C 2008;92:1382e7. [3] Tumirah K, Hussein MZ, Zulkarnain Z, Rafeadah R. Nano-encapsulated organic phase change material based on copolymer nanocomposites for thermal energy storage. Energy 2014;66:881e90. [4] Cai Y, Gao C, Xu X, Fu Z, Fei X, Zhao Y, et al. Electrospun ultrafine composite fibers consisting of lauric acid and polyamide 6 as form-stable phase change materials for storage and retrieval of solar thermal energy. Sol Energy Mater Sol C 2012;103:53e61. [5] Koca A, Oztop HF, Koyun T, Varol Y. Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector. Renew Energy 2008;33:567e8. [6] Gu Z, Liu H, Li Y. Thermal energy recovery of air conditioning systemeeheat recovery system calculation and phase change materials development. Appl Therm Eng 2004;24:2511e26.
7
[7] Li W, Zhang X, Wang X, Tang G, Shi H. Fabrication and morphological characterization of microencapsulated phase change materials (MicroPCMs) and macrocapsules containing MicroPCMs for thermal energy storage. Energy 2012;38:249e54. [8] Tyagi VV, Buddhi D. PCM thermal storage in buildings: a state of art. Renew Sustain Energy Rev 2007;11:1146e66. [9] Zhang Y, Zhou G, Lin K, Zhang Q, Di H. Application of latent heat thermal energy storage in buildings: state-of-the-art and outlook. Build Environ 2007;42:2197e209. [10] Yuan Y, Zhang N, Tao W, Cao X, He Y. Fatty acids as phase change materials: a review. Renew Sustain Energy Rev 2014;19:482e98. [11] Yuan Y, Tao W, Cao X, Bai L. Theoretic prediction of melting temperature and latent heat for a fatty acid eutectic mixture. J Chem Eng Data 2011;56: 2889e91. [12] Tuncbilek K, Sarı A, Tarhan S, Ergunes G, Kaygusuz K. Lauric palmitic acids eutectic mixture as latent heat storage material for low temperature heating applicationsvol. 30; 2005. p. 677e92. [13] Shatikian V, Ziskand G, Letan R. Numerical investigation of a PCM-based heat sink with internal fins. Int J Heat Mass Transf 2005;48:3689e706. [14] Fan L, Khodadadi JM. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew Sustain Energy Rev 2011;15:24e46. [15] Karaipekli A, Sarı A, Kaygusuz K. Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications. Renew Energy 2007;32:2201e10. [16] Li T, Lee JH, Wang R, Kang YT. Enhancement of heat transfer for thermal energy storage application using stearic acid nanocomposite with multiwalled carbon nanotubes. Energy 2013;55:752e61. [17] Wang J, Xie H, Xin Z, Li Y. Increasing the thermal conductivity of palmitic acid by the addition of carbon nanotubes. Carbon 2010;48:3979e86. [18] Sarı A, Karaipekli A. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Appl Therm Eng 2007;27:1271e7. [19] Chen Z, Shan F, Cao L, Fang G. Synthesis and thermal properties of shapestabilized lauric acid/activated carbon composites as phase change materials for thermal energy storage. Sol Energy Mater Sol C 2012;102:131e6. [20] Zhao J, Guo Y, Feng F, Tong Q, Qv W, Wang H. Microstructure and thermal properties of a paraffin/expanded graphite phaseechange composite for thermal storage. Renew Energy 2011;36:1339e42. [21] Fang G, Li H, Chen Z, Liu X. Preparation and characterization of stearic acid/ expanded graphite composites as thermal energy storage materials. Energy 2010;35:4622e6. [22] Baran G, Sarı A. Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system. Energ Convers Manage 2003;44:3227e46. [23] Celzard A, Mareche JF, Furdin G. Surface area of compressed expanded graphite. Carbon 2002;40:2713e8. [24] Zhang D, Tian S, Xiao D. Experimental study on the phase change behavior of phase change material confined in pores. Sol Energy 2007;81:653e60. [25] 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;16:767e73. [26] Mei D, Zhang B, Liu R, Zhang Y, Liu J. Preparation of capric acid/halloysite nanotube composite as form-stable phase change material for thermal energy storage. Sol Energy Mater Sol C 2011;95:2772e7. [27] Yang X, Yuan Y, Zhang N, Cao X, Liu C. Preparation and properties of myristicpalmitic-stearic acid/expanded graphite composites as phase change materials for energy storage. Sol Energy 2014;99:259e66.
Please cite this article in press as: Zhang N, et al., Preparation and properties of palmitic-stearic acid eutectic mixture/expanded graphite composite as phase change material for energy storage, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.10.092