Abstract
The form-stable composite phase change material of lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite was prepared by vacuum impregnation method for thermal energy storage. The maximum mass fraction of lauric–palmitic–stearic acid ternary eutectic mixture retained in vermiculite was determined as 50 wt% without melted phase change material seepage from the composite phase change material. Fourier transformation infrared spectroscope and scanning electron microscope were used to characterize the structure and morphology of the prepared lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite form-stable composite phase change material, and the results indicate that lauric–palmitic–stearic acid ternary eutectic mixture was well confined into the layer porous structure of vermiculite by physical reaction. The melting and freezing temperatures and latent heats were measured by differential scanning calorimeter as 31.4°C and 30.3°C, and 75.8 and 73.2 J/g, respectively. Thermal cycling test showed that there was no significant change in the thermal properties of lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite form-stable composite phase change material after 1000 thermal cycles. Moreover, 2 wt% expanded graphite was added to improve the thermal conductivity of lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite form-stable composite phase change material. All results indicated that the prepared lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite form-stable composite phase change material had suitable thermal properties and good thermal reliability for the application of thermal energy storage in building energy efficiency.
Introduction
Thermal energy storage (TES) has attracted much attention in the field of energy application due to the energy shortage and environment problem. Phase change materials (PCMs) have been recognized as a satisfactory choice for TES since they can store/release latent thermal energy during their melting/freezing process. PCMs have been widely applied in the fields of solar energy storage, 1 waste heat recovery, 2 air-conditioning systems,3,4 and building energy efficiency5,6 owing to the high energy storage density and nearly isothermal processes during energy storage and retrieval.7,8
PCMs are usually classified as organic and inorganic materials based on their components. A wide range of candidate inorganic and organic PCMs, including salt hydrates, paraffin waxes and non-paraffin organic compounds, fatty acids, and their mixtures, have been investigated for latent heat TES applications.9–12 Among the investigated PCMs, fatty acids have attracted great attention because of the good thermal properties, non-toxic, and good compatibility with other materials. However, the phase change temperature of single fatty acid is always too high to use in low-temperature TES application such as building energy efficiency. Fortunately, binary and ternary fatty acids can form as eutectic mixture according to the law of eutectic effect. The phase change temperature of polynary fatty acid eutectic mixture is lower than any one of the components in eutectic mixture. Yuan et al. 13 predicted the melting temperature and latent heat of binary fatty acid eutectic mixture based on theoretic formula, and the minimum melting temperature is 10.2°C suitable to use in low-temperature heating application. Song et al. 14 prepared stearic–capric acid eutectic mixture (SA-CA) to decrease the phase change temperature to a suitable value for passive solar houses. He et al. 15 prepared five different kinds of polynary fatty acid eutectic mixture according to the Schroeder–Van Laar equation. One of the eutectic mixtures was absorbed into sludge ceramsite by vacuum impregnation method for regulating room temperature. The fatty acid eutectic mixture should be prepared to expand the phase change temperature for more TES application.
Nevertheless, two major drawbacks limit the application of fatty acids and their eutectic mixtures. One is the leakage of melted PCM during melting process which increases the heat resistance in the practical use. And the other is the low thermal conductivity which causes a poor heat transfer rate. To prevent the fatty acids from leakage, many methods such as microencapsulation,16,17 electrospun fibers, 18 and form-stable composites19–21 have been introduced. Numerous form-stable composites have been successfully prepared by impregnating PCMs into porous substances. Song et al. 14 used activated-attapulgite (a-ATP), which is an open-ended tubular capillary with large specific surface area, as supporting material to prepare novel form-stable PCMs for latent heat TES in low temperature with the SA-CA as PCM. Tang et al. 22 prepared shape-stabilized fatty acid eutectics and diatomite composites by absorbing liquid fatty acid eutectics into diatomite. Wei et al. 23 used CA and CA-SA as PCMs and expanded perlite (EP) as porous supporting material to prepare form-stable composite PCM. The results show that the form-stable composite PCM has an optimal mass ratio of 50% fatty acid. The fatty acid is impregnated into EP by physical attraction. Vermiculite (VMT) is a kind of porous building material which has good chemical compatibility with organic PCMs such as fatty acids and their eutectic mixtures.24,25 VMT is one of the most suitable candidates as building material to prepare form-stable composite PCMs for latent heat TES in buildings. So, the research on PCM/VMT form-stable composite is meaningful.
The limitation of low thermal conductivity can be partially solved by introducing high thermal conductivity materials.26,27 Carbon materials are the most common additions to improve the thermal conductivity of PCMs. Li 28 applied nano-graphite (NG) to improve the thermal conductivity of paraffin. Thermal conductivity of the material containing 10% NG was 0.9362 W/(m K). Tang et al. 22 added expanded graphite (EG) to help prevent the eutectic mixtures from leakage and enhance thermal conductivity of the composite PCMs.
In this article, lauric acid–palmitic acid–SA eutectic mixture (LA-PA-SA) with the suitable phase change temperature for building energy efficiency application in some summer burning hot area was prepared first. Then, the form-stable composite PCM of LA-PA-SA/VMT was prepared using the vacuum impregnation method with LA-PA-SA as PCM and VMT as supporting material. The structure and thermal properties of LA-PA-SA/VMT were characterized by Fourier transformation infrared spectroscopy (FT-IR), scanning electronic microscope (SEM), and differential scanning calorimetry (DSC). Thermal reliability of LA-PA-SA/VMT was investigated by 1000 thermal cycles. The thermal conductivity of the form-stable composite PCM was improved by adding EG. And the effect of the increased thermal conductivity on melting and freezing rates was also studied.
Experimental
Materials
LA (98% purity, melting temperature: 44°C), PA (Analytical Reagent, melting temperature: 62°C), and SA (98% purity, melting temperature: 68.5°C) were purchased from Aladdin Industrial Corporation (Shanghai China). VMT (7–18 meshes) was supplied by Dalian Zhongde Perlite Factory (China). EG (carbon content: 99%) was obtained by microwave treatment of expansible graphite using a microwave oven at microwave irradiation power of 700 W for 30 s.
Preparation of LA-PA-SA/VMT composite PCM
LA-PA-SA was prepared with the method of combination of theoretical calculation (formula (1)) 15 and experimental procedure. Based on the results of calculation, LA, PA, and SA were weighed and put in a 100-mL beaker which was placed in a thermostatic water bath at 70°C. When the fatty acids melted completely, they were stirred at 400 r/min for 30 min to ensure a homogeneous mixing and then cooled down to the room temperature. Afterward, phase change temperature and latent heat of the mixture were measured by DSC. The eutectic mass ratio of LA-PA-SA was determined as LA:PA:SA = 62.2:24.6:13.2
where Ti and Hi are the phase change temperature and latent heat of the ith fatty acid, respectively; Tm is the phase change temperature of the eutectic mixture; Xi is the content of the ith substance in the eutectic mixture, and R is the gas constant.
The application range of formula (1) is eutectic system with three assumptions: 29 (1) components do not react with each other, (2) different components are completely immiscible in solid state while completely miscible in liquid state, and (3) phase transition of every single component occurs only when it reaches the specific phase transition temperatures, especially for solid–liquid phase transition. The ternary eutectic mixture can be inferred by the formula in a pseudo-binary fatty acid system with a binary eutectic mixture as one component. 30
LA-PA-SA/VMT composite PCMs were obtained by the vacuum impregnation method using the experiment setup which is shown in Figure 1. LA-PA-SA was dissolved in ethyl alcohol; VMT was placed in the three-necked flask. The ethyl alcohol solution was dropped into the flask under vacuum environment. Then, ethyl alcohol was evaporated after vacuum impregnation for 1 h. The LA-PA-SA/VMT composite PCMs were obtained after being dried for 3 h at 40°C. A series of LA-PA-SA/VMT composite in different mass fractions (40, 45, 50, 55, and 60 wt% for LA-PA-SA) were prepared to determine the maximum absorption ratio without leakage of melted PCM. Moreover, EG (2 wt%) was added into the form-stable PCM to increase thermal conductivity. The EG was mixed homogeneously with the VMT and the vacuum impregnation was carried out.
Experiment setup for preparation of composite PCM.
Characterization
The phase change temperature and latent heat of samples were measured by a DSC (TA Q20) at the heating rate of 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. The phase change temperature corresponds to the onset temperature obtained by drawing a line at the point of maximum slope of the leading edge of the DSC peak and extrapolating baseline on the same side as the leading edge of the peak. The latent heat of phase change was calculated by numerical integration of the area under the peaks. They can be obtained by the special software of the DSC. And every sample was tested three times to decrease the error. The structure and morphology of VMT, LA-PA-SA/VMT composite PCM, and LA-PA-SA were observed by a Fourier transform infrared spectrometer (FT-IR; Nicolet 6700) and an SEM (Fei Inspect FEI). Thermal conductivity was measured by a thermal property analyzer (Hot Disk 2500) and the samples were prepared as thin plates with the thickness of 10 mm, diameter of 30 mm, and density of 10 g/cm3. The accelerated thermal cycling test was carried out by a thermal cycler (BIOER CHB-T2-E). The thermal cycler can control the temperature at a range of 0°C–100°C by the software. The prepared LA-PA-SA/VMT composite PCM was heated at 50°C for 1 min and then cooled at 10°C for 1 min for 1000 thermal cycles.
Thermal performance test
Thermal performance test was investigated by the constant temperature water bath method. LA-PA-SA/VMT (30 g) and LA-PA-SA/VMT/EG (30 g) were filled into two glass test tubes, respectively. Thermocouples were placed in the centers of the test tubes to measure the temperature. When the temperatures of the materials were at a constant temperature of 20°C, the two test tubes were placed into a water bath at a constant temperature of 45°C. Then, the heat storage process was started. After the heat storage was finished, the test tubes were immediately moved to a water bath at a constant temperature of 20°C for the heat release process. The temperature variations in the samples were automatically recorded by a PC by data logger (Agilent 34980A) with a temperature measuring accuracy of ±0.25°C at a time interval of 10 s.
Results and discussion
Thermal properties of LA-PA-SA
It is an effective way to decrease the phase change temperature of fatty acids as PCM using the eutectic method. Binary or ternary fatty acid eutectic mixtures are always more suitable than the single fatty acid used in low-temperature application. Figure 2 shows the DSC curves of LA, PA, and SA. The melting temperature of LA, PA, and SA was measured as 44°C, 62°C, and 68.5°C, respectively. It can be found that the phase change temperatures of LA, PA, and SA are too high for using in the application of building energy efficiency. In this article, the LA-PA-SA with the mass ratio of LA:PA:SA = 62.2:24.6:13.2 was prepared. The melting and freezing temperatures and latent heats of LA-PA-SA were measured as 32.1°C and 30.4°C, and 151.6 and 150.6 J/g, respectively. From Figure 2, the DSC curve of the prepared LA-PA-SA was similar to that of the LA, PA, and SA and showed a good eutectic performance. Compared with the phase change temperature of LA, PA, SA, and LA-PA-SA, LA-PA-SA is more suitably applied in building energy efficiency.
DSC curves of LA, PA, SA, and LA-PA-SA.
Morphology characterization of the form-stable composite PCM
Based on the prepared LA-PA-SA/VMT composites in different mass fractions, it was found that LA-PA-SA could be retained as a maximum mass ratio of 50 wt% in VMT. In order to research the seepage state of the LA-PA-SA/VMT composite with the maximum mass ratio, the samples of LA-PA-SA, LA-PA-SA/VMT, and LA-PA-SA/VMT/EG were treated at 50°C for 2 h. Figure 3 shows the states of the LA-PA-SA, LA-PA-SA/VMT, and LA-PA-SA/VMT/EG at (a) room temperature and (b) after heat treatment. Compared with Figure 3(a) and (b), it can be seen that the LA-PA-SA was melted to liquid state because of the high temperature, but the LA-PA-SA/VMT and LA-PA-SA/VMT/EG maintained the shape even after heat treatment without leakage of melted PCMs. Therefore, the LA-PA-SA/VMT composite with the 50 wt% LA-PA-SA was defined as form-stable composite PCM since it did not allow the melted PCM seepage from the composite. And it should, thanks to the layer porous structure of VMT. The SEM images of VMT and LA-PA-SA/VMT form-stable PCM are shown in Figure 4. It can be observed from Figure 4(a) that VMT showed a layer porous structure, and the special structure increases the specific surface area of VMT. And a Brunauer–Emmet–Teller (BET)-specific area of VMT was measured as 5.21 m2/g by a specific surface area analyzer (Beishide Instrument 3H-2000PS2). The layer porous structure of VMT helps it to absorb the LA-PA-SA. Figure 4(b) shows that the LA-PA-SA was dispersed uniformly into the pores of VMT. And the layer porous structure prevented the leakage of PCMs during the phase change process.
Photographs of samples (a) before and (b) after heat treatment.
SEM image of (a) VMT and (b) LA-PA-SA/VMT.
Structural characterization of form-stable composite PCM
The chemical structure of the prepared LA-PA-SA/VMT form-stable composite PCM was characterized by FT-IR spectroscopy. Figure 5 shows the FT-IR spectra of LA-PA-SA, VMT, LA-PA-SA/VMT form-stable composite PCM, and LA-PA-SA/VMT/EG. In the FT-IR spectroscopy of LA-PA-SA, the peaks at 2930 and 2850 cm−1 represent the stretching vibration of −CH3 and −CH2 group of the LA-PA-SA, respectively. The peak at 1700 cm−1 is the characteristic absorption peak for the stretching vibration of C=O. The peak at 1470 cm−1 is the −CH2 bending peak, 1300 cm−1 represents C–H and C–C bending, and 935 and 721 cm−1 respond to rocking vibration and bending, respectively, which are all characteristics for aliphatic chain of LA-PA-SA. There are some characteristic absorption peaks of VMT at 2430, 1380, and 1020 cm−1 in the FT-IR spectroscopy of VMT.
FT-IR spectra of LA-PA-SA, VMT, LA-PA-SA/VMT, and LA-PA-SA/VMT/EG.
It is clearly found that the FT-IR spectrum of LA-PA-SA/VMT form-stable composite PCM just contained the characteristic peaks of LA-PA-SA and VMT, and there are no significant new peaks. The slight frequency shifts were caused by the physical interactions between the carboxyl group of the fatty acids and the alkaline region in VMT. And the physical interactions prevent the leakage of melted PCM from the surface of LA-PA-SA/VMT composite PCM. It can also be found from the FT-IR spectrum of LA-PA-SA/VMT/EG; LA-PA-SA is retained in VMT and EG pores by physical interaction. EG helps to prevent the leakage of LA-PA-SA.
Thermal properties of form-stable composite PCM
The DSC curve and thermal properties of LA-P-SA/VMT form-stable composite PCM are shown in Figure 6 and Table 1. The melting and freezing temperatures of LA-PA-SA/VMT form-stable composite PCM were 31.4°C and 30.3°C, respectively. The phase change temperatures of LA-PA-SA/VMT were a little lower than those of LA-PA-SA. This may be contributed by the weak interaction between the fatty acid and VMT. The melting and freezing latent heats of LA-PA-SA/VMT were measured as 75.8 and 73.2 J/g, respectively. In theory, the latent heats of LA-PA-SA/VMT form-stable composite PCM are proportional to the content of LA-PA-SA. The measured values of phase change latent heats of LA-PA-SA/VMT were similar to the theoretical values in this study. The melting and freezing temperatures and latent heats of LA-PA-SA/VMT/EG were measured as 30.6°C and 30.4°C, and 72.7 and 68.8 J/g, respectively. It can be found that the additive of EG has a little effect on the thermal properties of composite PCM. Moreover, the comparison of thermal properties of the prepared form-stable PCM with those of different composite PCMs in the literature is shown in Table 2. It can be seen that the prepared LA-PA-SA/VMT form-stable composite PCM has similar thermal properties. The prepared LA-PA-SA/VMT form-stable composite PCM is a potential material in the application of building energy efficiency.
DSC curves of LA-PA-SA/VMT before and after 1000 thermal cycles and LA-PA-SA/VMT/EG.
Thermal properties of prepared samples.
LA-PA-SA: lauric–palmitic–stearic acid eutectic mixture; VMT: vermiculite; EG: expanded graphite.
Comparison of the thermal properties of the LA-PA-SA/VMT and some other composite PCMs.
PCM: phase change material; MA: myristic acid; PA: palmitic acid; SA: stearic acid; EG: expanded graphite; CA: capric acid; EP: expanded perlite; VMT: vermiculite.
Thermal reliability of form-stable composite PCM
The thermal reliability of the prepared LA-PA-SA/VMT form-stable composite PCM was studied by the thermal cycling test after 1000 thermal cycles. Figure 6 shows the DSC curve of LA-PA-SA/VMT after 1000 thermal cycles. The melting and freezing temperatures and latent heats of LA-PA-SA/VMT form-stable composite PCM after 1000 thermal cycles were measured as 31.7°C and 30.6°C, and 71.8 and 70.0 J/g, respectively. It can be found that the phase change temperatures of LA-PA-SA/VMT have no significant change after 1000 thermal cycles. The melting and freezing latent heats of LA-PA-SA/VMT drop slightly by 5.28% and 4.37%, respectively, after 1000 thermal cycles. The results showed that the prepared LA-PA-SA/VMT form-stable composite PCM has a good thermal reliability due to the little changes in the phase change temperature and latent heat. And the changes would not influence its application.
Thermal conductivity and performance test of form-stable composite PCM
Table 3 lists the thermal conductivity of LA-PA-SA, LA-PA-SA/VMT, and LA-PA-SA/VMT/EG. It can be seen that the thermal conductivities of LA-PA-SA and LA-PA-SA/VMT form-stable composite PCM were measured as 0.21 and 0.56 W/(m K), respectively, which are too low for the application. Thermal conductivity influences the heat transfer rate, so EG was used as additive to improve the thermal conductivity of LA-PA-SA/VMT form-stable composite PCM in this study. The thermal conductivity of LA-PA-SA/VMT/EG was measured as 0.94 W/(m K), increased by 68% compared with that of LA-PA-SA/VMT form-stable composite PCM. The increase in the thermal conductivity was also verified by comparing the melting and freezing performance. Figure 7 illustrates the melting and freezing temperature curves of LA-PA-SA/VMT and LA-PA-SA/VMT/EG. From Figure 7(a), it took 35 min for LA-PA-SA/VMT and only 20 min for LA-PA-SA/VMT/EG when the center temperature increased from 20°C to 45°C. From Figure 7(b), it took about 25 min for the latent heat release of LA-PA-SA/VMT and only about 15 min for LA-PA-SA/VMT/EG. The reason was that the EG had a high thermal conductivity, which can greatly improve the heat transfer rate of LA-PA-SA/VMT/EG in the heat storage/release process. Thus, the heat storage/release time was reduced. All the results indicated that the thermal storage and release rate were significantly increased due to the high thermal conductivity of EG.
Thermal conductivity of samples.
LA-PA-SA: lauric–palmitic–stearic acid eutectic mixture; VMT: vermiculite; EG: expanded graphite.
Temperature curves of LA-PA-SA/VMT and LA-PA-SA/VMT/EG: (a) melting temperature curves and (b) freezing temperature curves.
Conclusion
In this article, LA-PA-SA was prepared with a mass ratio of LA:PA:SA = 62.2:24.6:13.2. Then, LA-PA-SA/VMT form-stable PCM was prepared with the maximum mass ratio of LA-PA-SA as 50 wt% by the vacuum impregnation method. LA-PA-SA was distributed in the layer pore structure of VMT by physical interaction. The melting and freezing temperatures and latent heats of LA-PA-SA/VMT were measured as 31.4°C and 30.3°C, and 75.8 and 73.2 J/g, respectively. The prepared LA-PA-SA/VMT had a good thermal stability. The thermal property of LA-PA-SA/VMT had a slight change after 1000 thermal cycles, and the result showed that LA-PA-SA/VMT had a good thermal reliability. Thermal conductivity of LA-PA-SA/VMT was increased by 68% by adding 2 wt% EG. In conclusion, the prepared LA-PA-SA/VMT form-stable PCM is a potential material for building energy reservation.
Footnotes
Academic Editor: Mohammad Reza Salimpour
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Funding
This work was supported by the Natural Science Foundation of China (No. 51378426), Youth Science and Technology Innovation Team of Sichuan Province of Building Environment and Energy Efficiency (No. 2015TD0015), Fundamental Research Funds for Central Universities (No. 2682015CX038), and 2014 Cultivation program for the Excellent Doctoral Dissertation of Southwest Jiaotong University.