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expanded graphite composite with high thermal conductivity as a shape-stabilized phase change material for thermal energy storage
Energy Conversion and Management 101 (2015) 164–171
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Hydrated salts/expanded graphite composite with high thermal conductivity as a shape-stabilized phase change material for thermal energy storage Yuping Wu, Tao Wang ⇑ State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 6 March 2015 Accepted 2 May 2015
Keywords: Phase change material Hydrated salts Expanded graphite Thermal energy storage
a b s t r a c t A novel shape-stabilized phase change material (PCM) was developed by impregnation of hydrated salts into expanded graphite (EG) and further coated with paraffin wax. It was displayed by scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR) measurements that the hydrated salts were filled into pores or adhered onto the flakes of EG by physical interactions, including capillary forces and surface tension. It was revealed from differential scanning calorimetry (DSC) analysis that phase segregation was inhibited and subcooling weakened in the coated composite PCM. The melting and freezing enthalpy of the coated composite PCM can reach 172.3 kJ/kg at 32.05 °C and 140.8 kJ/kg at 17.11 °C, respectively. Also, the results of thermal gravimetric analysis (TG) suggested that the coated composite PCM had good thermal stability in the working temperature range from 25 to 50 °C. Furthermore, the enthalpy loss of melting and freezing was negligible after 100 cycles, indicating its good thermal reliabilities. The thermal conductivity of the coated composite PCM can be as high as 3.643 W/(m K). According to the obtained results, the coated hydrated salts/EG composite PCM enjoys high latent heat, good thermal reliability and high thermal conductivity. Apart from its favorable thermal properties, the cost of the coated hydrated salts/EG composite PCM was quite low, making it promising for low temperature thermal energy storage applications. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Thermal energy storage (TES) is considered as one of the most important energy storage technologies, due to that it can even out the imbalance between heat supply and consumption and help in saving of capital costs. Generally, TES can be accomplished by using sensible heat storage, thermochemical energy storage and latent heat storage [1–3]. Latent heat storage using phase change materials (PCMs) is carried out by storing and releasing latent heat during the phase change (solid–liquid, solid–solid, gas–liquid) [4– 6]. It has been the most attractive choice for TES applications because of its advantages of convenient use, high storage density and constant temperature during phase change. Latent heat storage by solid–liquid PCMs, including organic and inorganic PCMs, is widely used due to their larger number of substances and potential for wider temperature applications, compared with the solid–solid ones. Nevertheless, the leakage of the melted PCMs and the corrosion limited the applications of solid–liquid PCMs. In order to solve these problems, the ⇑ Corresponding author. Tel./fax: +86 10 62784877. E-mail address: [email protected] (T. Wang). http://dx.doi.org/10.1016/j.enconman.2015.05.006 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
shape-stabilized PCMs, which are prepared by incorporating solid–liquid PCMs into supporting materials through encapsulation or impregnation, emerged. The shape-stabilized PCMs based on organic PCMs, including paraffin [7,8], fatty acids [9–11] and poly (ethylene glycol) (PEG) [12,13] have been extensively studied owing to the fact that they have suitable energy storage density, no corrosivity and no subcooling. Nevertheless, the inorganic PCMs, especially the hydrated salts, are rarely reported to be prepared into shape-stabilized PCMs, although they enjoy the advantages of high storage density, high thermal conductivity and low cost. This may be attributed to the drawbacks of their phase segregation and subcooling. The phase segregation has been prevented by adding thickening agents or using rotating storage devices, while the subcooling has been improved by the addition of nucleating agents or using the cold finger technique [14,15]. Also, it was effective to embed the hydrated salts into the mesopores to solve these problems mentioned above. We have tried to impregnate the hydrated salts into the mesopores of silica and it turned out that phase segregation was inhibited and subcooling was slightly mitigated [16]. In addition, the crystal water of hydrated salts was hard to hold due to the dehydration during heating. Therefore, microencapsulation became the first choice of the
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researchers. Zhang et al. [17] encapsulated the hydrated salts into the nanoscale pores of silica by sol–gel method, which improved the thermal performance of Na2SO410H2O. Huang et al. [18] prepared the microcapsules loaded by Na2HPO47H2O by means of suspension copolymerization–solvent volatile method. However, the process of microencapsulation was complex, and it may be adverse to the thermal conductivity. Moreover, it should be noted that organic PCMs suffer from the disadvantage of low thermal conductivity, preventing them from being applied in practical use. Therefore, high thermal conductive materials, including metal materials and carbon based materials [19], were added into the composites to improve their thermal conductivities. However, introducing additives inevitably bring the loss of energy storage density. Therefore, researchers directly incorporated the PCMs into the abundant pore structures of carbon based materials [20–22]. Expanded graphite (EG) is a kind of carbon material produced by heating expandable graphite in a furnace at high temperature or treated by microwave irradiation [23]. Due to its desirable properties of high thermal conductivity, low density, low cost and good chemical stability, EG has been a promising candidate to increase the thermal conductivity of PCMs. Meanwhile, EG has a unique network-like pore structure with large surface area and high surface activity. This enables EG to increase the thermal conductivities of PCMs without much reduction in the latent heat energy storage density. As a result, EG has been popular in the preparation of shape-stabilized PCMs. Lee et al. [24] prepared erythritol/EG composites with different interlayer distances by a simple blending and impregnating method and found that the thermal conductivity of the composite increased with increasing EG interlayer distance. In particular, the thermal conductivity can be as high as 3.56 W/(m K). Wang et al. [25] reported that the thermal conductivity of the PEG/EG composites increased four times compared with that of pure PEG and it can reach a large latent heat of 161.2 kJ/kg when the mass percentage of EG was 10%. It was mentioned above that both EG and hydrated salts have the advantage of high thermal conductivity and the combination of them may be beneficial to prepare the PCM with high thermal conductivity. Duan et al. [26] impregnated CaCl26H2O into EG through vacuum impregnation and found that the thermal conductivity of the composite with 50 mass% CaCl26H2O (8.796 W/(m K)) was 14 times as that of pure CaCl26H2O (0.596 W/(m K)). Moreover, due to the low cost of EG and hydrated salts, the combination of them are advantageous for reducing cost. This work aims at the preparation of the shape-stabilized PCMs, which adopted the mixture of hydrated salts (sodium sulfate decahydrate (Na2SO410H2O) and sodium phosphate dibasic dodecahydrate (Na2HPO412H2O) in mass ratio of 1:1) as PCMs and the EG, treated by microwave irradiation beforehand, as supporting material. The hydrated salts/EG composite PCM was prepared by impregnation method and further coated with paraffin wax to inhibit the phase segregation as well as enhance the thermal stability. The morphology, chemical compatibilities, phase change properties, thermal stabilities and thermal conductivities of the hydrated salts/EG composite PCM were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermal gravimetric analysis (TG) and laser flash thermal analyzer (LFA). Moreover, the thermal cycling performance of the coated hydrated salts/EG composite PCM was also investigated.
China. Prior to use, 1 g of it was dried in an oven at 70 °C for 20 h, followed by microwave treatment using a domestic microwave oven (Midea, G70D20CN1P-D2(S0), China), which was held at 500 W for 90 s. Thus, the expanded graphite (EG) was obtained and employed as supporting material. The contact angles of obtained EG with water were measured, and found to be 71.0° on the left and 70.3° on the right. This suggested that the pure EG was hydrophilic, which enabled the hydrated salts solution to be impregnated into the pores of EG. Sodium sulfate decahydrate (Na2SO410H2O, analytical grade) and sodium phosphate dibasic dodecahydrate (Na2HPO412H2O, analytical grade) were purchased from Beijing Modern Eastern Fine Chemicals, China and used as phase change materials (PCMs). Paraffin wax (melting point Tm = 50–60 °C) and n-hexane were supplied by Tianjin Guangfu company, China.
2.2. Preparation of uncoated hydrated salts/EG composite PCM Uncoated hydrated salts/EG composite PCM was prepared by a physical blending and impregnation method. Firstly, 3.5 g of sodium sulfate decahydrate (Na2SO410H2O) and 3.5 g of sodium phosphate dibasic dodecahydrate (Na2HPO412H2O) were dissolved in 3 mL of deionized water to obtain a homogeneous solution. Subsequently, 1 g of EG was added into the hydrated salts solution and stirred vigorously in a beaker. Then, the mixture was held in 40 °C in thermostat water bath for 10 h, ensuring that the hydrated salts were fully impregnated into the EG. After that, the mixture was dried at 10 °C in an oven until it kept the same weight, thus the uncoated hydrated salts/EG composite PCM was obtained, as displayed in Fig. 1. No hydrated salts were observed on the surface of EG and the wall of the beaker. Therefore, it can be considered that the hydrated salts were fully impregnated into the pores of EG or adhered to the surface of EG. The content of hydrated salts was obtained by means of immersing the composite into deionized water repeatedly. Before immersing, the weight of the composite PCM was recorded as M1. Then, the composite was immersed into deionized water repeatedly to remove the hydrated salts. At the same time, the concentration of Na+ in deionized water after immersing was measured by atomic absorption spectrophotometer. If the concentration of Na+ was below 10 5 mol/L, it was considered that the hydrated salts had been completely removed. Subsequently, the composite was dried in room temperature until it kept the weight constant. The weight of the composite PCM after immersing was recorded as M2. Therefore, the content of hydrated salts was (M1 M2)/M1.
2. Experimental section 2.1. Materials Raw expandable graphite (300 meshes, expansion coefficient: 200 mL/g) was supplied by Qingdao Xingyuan Shimo Company,
1 cm
Fig. 1. The image of uncoated hydrated salts/EG composite PCM.
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2.3. Preparation of coated hydrated salts/EG composite PCM Paraffin wax was chosen to coat hydrated salts/EG composite on the basis of the following reasons. On the one hand, it was widely reported that paraffin can be adsorbed by EG, so the paraffin was capable to coat the hydrated salts/EG composite. On the other hand, the paraffin can form a solid layer, which may prevent the evaporation of water when the hydrated salts transformed into liquid state. Furthermore, the coating process was aided by n-hexane, in which the hydrated salts could not be dissolved. Actually, the paraffin wax was used only as coated. It may fill into the pores of the EG that unfilled by the hydrated salts, too. 0.2 g of paraffin wax was added and dissolved completely in 20 mL of n-hexane in a beaker before the addition of 1 g of uncoated hydrated salts/EG composite PCM. Subsequently, the mixture was sealed by plastic wrap and stirred at 500 rpm at room temperature for 5 h. After that, the mixture was filtered to remove n-hexane and the coated hydrated salts/EG composite PCM was obtained, as shown in Fig. 3(a).
nitrogen atmosphere. The thermal cycling tests were carried out in an oven by heating and cooling the samples from 1 to 60 °C repeatedly. Thermal gravimetric analysis was conducted by a thermo analyzer instrument (TGA, STA409PC, Netzsch, Germany). The samples were heated from 20 to 200 °C at a rate of 5 °C/min under the nitrogen atmosphere at a flow rate of 20 mL/min. Thermal conductivities were determined by a laser flash thermal analyzer (LFA, LFA 447, Netzsch, Germany). Before the measurement, samples were pressed into a tablet under the pressure of 2 MPa. The tablets with a diameter of 12.7 mm and a thickness of 2 mm were obtained and the densities of them were 1.33 g/cm3 and 1.26 g/cm3 for uncoated and coated hydrated salts/EG composites, respectively. Each sample was measured three times and the average results were chosen. The pore structure of EG was measured by a mercury porosimetry (Micromeritics AutoPore IV 9500 series pore size analyzer). 3. Results and discussion 3.1. Morphology of hydrated salts/EG composite PCM
2.4. Characterization of hydrated salts/EG composite PCM The morphologies of EG and hydrated salts/EG composite PCMs were observed by a scanning electron microscope (SEM, JSM7401, Shimadzu, Japan). Samples were coated with gold before testing. The chemical compatibility between EG and hydrated salts was investigated by a Fourier transform infrared spectroscopy (FT-IR, Tensor27, Bruker, Germany) with the wavenumber range from 400 to 4000 cm 1 at room temperature. Samples were mixed with KBr and pressed into a pellet before analysis. The crystalline phase of samples was examined by X-ray diffraction (XRD, D8ADVANCE, Bruker, Germany) using Cu Ka radiation (k = 1.5406 Å). Diffraction patterns were collected in the 2h ranges from 10° to 80°. Thermal properties were measured using a differential scanning calorimetry (DSC, Q2000, TA, USA). Samples were sealed in an alumina pan and heated from 20 to 60 °C at a rate of 5 °C/min in purified
Fig. 2 illustrates the SEM images of EG and uncoated hydrated salts/EG composite PCMs with different contents of hydrated salts. As shown in Fig. 2(a), EG consists of overlapped graphite flakes, forming abundant crevice-like and net-like pores. It was determined from mercury intrusion method that the porosity of EG was 92.78% and the pore volume was as high as 28.00 ml/g. Owing to the porous structure, the melted hydrated salts can be easily adsorbed and filled into pores or adhered onto the flakes of EG. The capillary forces and surface tension between hydrated salts and EG can prevent melted hydrated salts from leaking. It can be seen from Fig. 2(b) that EG was completely dispersed in the superfluous hydrated salts, when the content of hydrated salts was 93.69%. This means that this content of hydrated salts may be in excess of the saturated adsorption amount of EG. When the content of hydrated salts was only 85.45%, it was observed that some
Fig. 2. SEM images of expanded graphite with different contents of hydrated salts: (a) 0%; (b) 93.69%; (c) 87.36%; and (d) 85.45%.
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inner pores were not filled with hydrated salts. These unfilled pores led to a lower loading capacity, indicating that more hydrated salts can be adsorbed. In comparison, when the content of hydrated salts was 87.36%, both the pores and the surfaces were thoroughly occupied by hydrated salts. This suggested that this content may be the optimum content of the hydrated salts. Therefore, the hydrated salts/EG composite with this content was studied in the following. To confirm the shape stabilization ability of the composite PCM mentioned above, the coated hydrated salts/EG composite PCM in powder state was heated to 60 °C in an oven, which is higher than the phase transition temperature of the hydrated salts. The images of them before and after heating were shown in Fig. 3(a) and (b). It can be seen that no leakage was observed from the powder after heating. Furthermore, the coated composite PCM was pressed into a tablet, as displayed in Fig. 3(c). Then, this tablet was put into an oven and heated to 60 °C to observe the shape stability. It was shown in Fig. 3(d) that the shape of the composite PCM kept the same with that before heating. This further indicated that the coated composite PCM had good shape stabilization ability.
3.2. Chemical compatibility of hydrated salts/EG composite PCM Fig. 4 displays the FTIR spectra of EG, hydrated salts, uncoated and coated hydrated salts/EG composite PCM. The FTIR spectrum of EG showed the characteristic absorption peaks at 1636 cm 1 and 3453 cm 1, corresponding to the stretching vibrations of C@O and OH [27], respectively. The peaks at 1163 cm 1 and 619 cm 1 represented SAO stretching vibration and symmetric SAO stretching vibration [28], respectively. The stretching vibrations of HPO24 , that is, mas (PAO), ms (PAO) and mas (PAOH), were
167
Fig. 4. FTIR spectra of EG, hydrated salts, uncoated and coated hydrated salts/EG composite PCMs.
observed at 1071, 953, and 861 cm 1, respectively [29]. As displayed, the FTIR spectrum of uncoated hydrated salts/EG composite PCM showed all the characteristic peaks of hydrated salts and EG and no new peaks appeared. This indicated that there were no chemical interactions between them. In addition, it should be noted that there was a down shift of the peak of C@O in that of uncoated composite PCM. This may be due to the hydrogen bonding between C@O and OH. In comparison with the FTIR spectrum of the uncoated composite, the FTIR spectrum of coated composite showed all the same peaks except three new peaks at 2930, 2860
Fig. 3. Images of coated hydrated salts/EG composite PCMs in powder (a) before heating, (b) after heating and in tablet, (c) before heating and (d) after heating.
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and 1265 cm 1, respectively. These three peaks were attributed to the stretching vibrations of ACH3 and ACH2 group and CAC bonding, confirming the existence of paraffin wax. 3.3. XRD pattern of hydrated salts/EG composite PCM To investigate the crystalline phase of the hydrated salts, uncoated and coated hydrated salts/EG composite PCM, X-ray diffraction was carried out and the results were shown in Fig. 5. A strong diffraction peak located at 26.49° was observed in the XRD pattern of EG, which is corresponding to the feature peak of graphite. The same diffraction peak also appeared in the XRD patterns of uncoated and coated composite, confirming the existence of EG in the composite. It was found that the XRD pattern of the hydrated salts was not the simple overlay of two hydrated salts, because it was not indexable to the XRD pattern of Na2SO410H2O and Na2HPO412H2O in the database. Therefore, it was assumed that hydrated salts with different water content may form as Na4(SO4)(HPO4)X H2O. In addition, the XRD patterns of the composite demonstrated that the hydrated salts in the pores of EG were different from those in bulk. This may be due to the confinement effects of the hydrated salts crystals within the graphite sheets [30]. Further study should be done to make clear of the X in bulk and in the pores of EG. Moreover, it should be noted that the XRD patterns of uncoated and coated composite were the same. This revealed that the coating of paraffin wax did not interfere the crystallization of the hydrated salts.
Fig. 6. DSC curves of hydrated salts, uncoated and coated composite PCMs.
Table 1 Phase change enthalpies of hydrated salts, uncoated and coated composite PCMs. Samples
Melting enthalpy (kJ/kg)
±95% Conf. interval
Freezing enthalpy (kJ/kg)
±95% Conf. interval
Hydrated salts
32.52 226.90
±5.63 ±5.98
67.26 78.97
±7.81 ±1.93
RSD (%)
3.93 1.06
3.4. Thermal properties of hydrated salts/EG composite PCM DSC analysis was performed to investigate the phase change properties of the hydrated salts/EG composite PCMs. DSC curves of hydrated salts, uncoated and coated hydrated salts/EG composite PCMs are presented in Fig. 6 with detailed data given in Table 1. It was noticeable that there were two peaks in the melting DSC curve of hydrated salts [16] with the temperature at 0.61 and 36.68 °C, respectively. This indicated the phase segregation of hydrated salts, which may be due to the existence of free water in bulk. It was stated that heterogeneous nucleation together with mesoporous confinement effects in pores may be responsible for the inhibition of phase segregation [15]. Although the pore diameter of EG ranges from 2 nm to 100 lm (see Fig. 7), there were also confinement effects that inhibited the phase segregation. As can be seen from Fig. 6, only one set of melting and freezing peaks appeared in the DSC curves of both uncoated and coated composite PCM. This means that phase segregation was inhibited.
Uncoated composite
166.3
RSD (%)
0.12
Coated composite
172.3
RSD (%)
1.81
1.68 0.60 ±0.51
125.3
±5.38
1.73 ±3.88
140.8
±4.72
2.70
Fig. 7. Pore size distribution of EG obtained by the mercury porosimetry.
Fig. 5. XRD patterns of hydrated salts, uncoated and coated composite PCMs.
In addition, it was noteworthy that the melting temperatures of hydrated salts in the uncoated and coated composite PCMs shifted to a lower temperature in comparison with that of hydrated salts. This can be ascribed to the confinement effects [31,32]. That is, the melting temperature of PCMs confined in the pores may be lower compared to those in bulk. Also, it should be noted that the melting temperature of coated composite PCM was higher than that of uncoated one. This may be due to the interactions in different
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degree between pore wall and PCMs. It was shown from the FTIR spectra that there were some interactions between hydrated salts and EG in the uncoated composite PCM, while the interactions in the coated one was hardly detected. As reported [33,34], the strong interactions between PCMs and porous materials will lead to a declined phase change temperature. Therefore, the melting temperature of uncoated composite PCM was lower than that of coated one. Moreover, it was surprisingly found that the subcooling degree remarkably decreased in coated composite PCM. The subcooling degree of hydrated salts was as high as 27.66 °C, while it was only 14.94 °C for the coated composite PCM. As shown in Table 1, the melting enthalpies of uncoated and coated composite PCMs were smaller than that of hydrated salts. The melting enthalpy of the hydrated salts was as high as 226.90 kJ/kg, while that of uncoated and coated composite PCMs were 166.3 kJ/kg and 172.3 kJ/kg, respectively. The decrease of the mass fraction of hydrated salts may account for the enthalpy decrease of the composites. Also, the confinement effects contributed to the enthalpy reduction. It was mentioned above that the hydrated salts in the composite PCM was confined in pores and there were interactions between hydrated salts and the functional groups of the pore wall of EG. Both of them led to the enthalpy reduction. Additionally, it was worth noting that the melting enthalpy of the coated composite PCM was higher than that of uncoated ones. This may be attributed to the weaker interactions between pore wall and hydrated salts as mentioned above. As shown in Fig. 4, the intensities of C@O in the FTIR spectrum of the coated composite PCM were much weaker than that of uncoated ones, suggesting the weaker interactions between pore wall and hydrated salts in coated composite PCM. Perhaps the presence of paraffin wax changed surface properties that may weaken the intensities of C@O and may have aided nucleation at the same time. Nevertheless, the enthalpies of both the uncoated and coated composite PCM were comparable or superior to those reported in other literatures (Table 2), demonstrating the favorable thermal properties of the prepared composite PCMs.
Fig. 8. TG curves of hydrated salts, uncoated and coated composite PCMs.
salts and uncoated ones. This revealed that paraffin coating was advantageous to delay the thermal decomposition of hydrated salts. Thus, the thermal stability of PCMs was improved. Nevertheless, it should be admitted that there was still weight loss of coated composite PCMs, indicating that it was not coated completely by the paraffin wax. If the hydrated salts in the pores of EG was the mix of Na2SO410H2O and Na2HPO412H2O in mass ratio of 1:1, the weight loss should be 58.04%. However, the TG analysis of the hydrated salts in bulk, uncoated and coated hydrated salts/EG composite PCMs gave different results. As presented in Fig. 8, it was calculated that the contents of water for the hydrated salts in bulk, in uncoated and coated composite PCM were 56.89%, 55.21% and 52.61%, respectively. This suggested that the hydrated salts in bulk and the pores of EG (uncoated and coated) were not the simple mix of Na2SO410H2O and Na2HPO412H2O. This was consistent with the results of XRD.
3.5. Thermal stability of hydrated salts/EG composite PCM The thermal stabilities of the hydrated salts, uncoated and coated hydrated salts/EG composite PCMs were evaluated by TG analysis and the results were presented in Fig. 8. As shown in Fig. 8, all the samples experienced a single weight loss between 25 °C and 100 °C. In the working temperature range from 25 °C to 50 °C, the weight loss may be mainly caused by the moisture release and the dehydration of hydrated salts. It was determined that the weight loss of coated composite PCM in this temperature range was less than 10%, implying that it has a good thermal stability in the working temperature range. The most significant degradation took place at the stage from 50 °C to 100 °C, which belonged to the evaporation of water. The onset temperatures of weight loss (Tonset) for hydrated salts, uncoated and coated hydrated salts/EG were 63.0 °C, 63.1 °C and 70.9 °C, respectively. That is, the Tonset of the coated composite PCM was higher than those of hydrated
3.6. Thermal reliability of coated hydrated salts/EG composite PCM For composite PCMs, it is essential to have good thermal reliabilities over a number of thermal cycles. Therefore, thermal cycling tests were carried out to investigate the thermal reliabilities of the coated composite PCMs. Thermal cycling tests were conducted up to 100 cycles and DSC analysis was performed before and after different thermal cycles. As can be seen from Fig. 9(a), the enthalpy changes of the coated composite PCMs after 100 cycles were negligible. After 100 cycles, the enthalpies of melting and freezing were 162.5 kJ/kg and 134.9 kJ/kg, dropping by 5.69% and 4.19%, respectively. In addition, it can be seen from Fig. 9(b) that no significant differences were found among melting temperatures after different cycles. For the freezing temperatures, there was a small rise with the increasing cycles. Nevertheless, these changes were
Table 2 Thermal properties comparison of some composites in literatures. Sample
Melting temperature (°C)
Melting enthalpy (kJ/kg)
Freezing temperature (°C)
Freezing enthalpy (kJ/kg)
Thermal conductivity (W/(m K))
Ref.
Paraffin/SiO2/7.2 wt%EG CaCl26H2O/10 wt%EG MA–PA–SA/7 wt%EG PA/20 wt%EG PEG/10 wt%EG Coated hydrated salts/13 wt%EG
27.72 29.48 41.64 60.88 61.46 32.05
104.41 98.30 153.50 148.36 161.20 172.3
– 42.99 60.81 46.91 17.11
– 151.4 149.66 – 140.8
0.25 3.437 2.510 0.600 1.324 3.643
[35] [26] [27] [36] [25] Present study
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Fig. 9. Thermal properties of coated hydrated salts/EG composite PCMs after different cycles (a) phase change enthalpy and (b) phase change temperature.
Table 3 Thermal conductivities of prepared composite PCMs. Sample
Thermal conductivity (W/(m K))
EG Uncoated hydrated salts/EG Coated hydrated salts/EG
11.000 3.615 3.643
in insignificant level for TES applications. In other words, the coated hydrated salts/EG composite PCM showed good thermal reliabilities. 3.7. Thermal conductivities of hydrated salts/EG composite PCM One of the important factors in TES applications is the rates of energy storage and release, which substantially depend on the thermal conductivity of the composite PCMs. Thermal conductivities of the uncoated and coated hydrated salts/EG composite PCMs were measured at 25 °C and the results were presented in Table 3. EG is generally adopted to enhance the conductive performance of organic PCMs to make up its disadvantage of low thermal conductivity. Admittedly, their thermal conductivity can significantly increase with the addition of EG, but they are still inferior to the hydrated salts. As shown in Table 3, the thermal conductivity of hydrated salts/EG could be as high as 3.615 W/(m K) (uncoated) or 3.643 W/(m K)(coated), which was much higher than most of the organic PCMs (Table 2). This may be attributed to the high thermal conductivity of both hydrated salts and EG. This verified that the combination of hydrated salts and EG was an effective way to improve the thermal conductive performance of PCMs. Additionally, it was noted that there was no big difference between the uncoated and coated composite PCM. This indicated that the addition of little paraffin wax into the composite PCM did not affect their thermal conductive performance. 4. Conclusions A novel shape-stabilized phase change material, adopting hydrated salts as phase change material and expanded graphite (EG) as supporting material, was prepared by impregnation method and further coated with paraffin. The phase segregation of hydrated salts was inhibited and the subcooling reduced to a large extent. The melting enthalpy of the coated hydrated
salts/EG composite PCM can reach 172.3 kJ/kg with a melting temperature at 32.05 °C. The coated composite PCM had good thermal stability in the working temperature range from 25 to 50 °C, too. Additionally, the thermal conductivity of the coated hydrated salts/EG composite PCM can be as high as 3.643 W/(m K), indicating its good thermal conductive performance. Moreover, the enthalpy loss of the coated composite PCM was negligible after 100 thermal cycles, showing its good thermal reliabilities. In a word, the coated hydrated salts/EG composite PCM enjoys favorable thermal performance, including high latent heat, good thermal stability, high thermal conductivity and good thermal reliability. Moreover, the combination of hydrated salts and EG can significantly reduce cost due to their low cost. Due to these favorable properties and advantages, the coated composite PCM can be considered as a good candidate for low temperature thermal energy storage applications in many fields, such as greenhouse, building energy conservation, textile fibers and thermal protection of electrical devices.
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Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Hydrated salts/expanded graphite composite with high thermal conductivity as a shape-stabilized phase change material for thermal energy storage Yuping Wu, Tao Wang ⇑ State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 6 March 2015 Accepted 2 May 2015
Keywords: Phase change material Hydrated salts Expanded graphite Thermal energy storage
a b s t r a c t A novel shape-stabilized phase change material (PCM) was developed by impregnation of hydrated salts into expanded graphite (EG) and further coated with paraffin wax. It was displayed by scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR) measurements that the hydrated salts were filled into pores or adhered onto the flakes of EG by physical interactions, including capillary forces and surface tension. It was revealed from differential scanning calorimetry (DSC) analysis that phase segregation was inhibited and subcooling weakened in the coated composite PCM. The melting and freezing enthalpy of the coated composite PCM can reach 172.3 kJ/kg at 32.05 °C and 140.8 kJ/kg at 17.11 °C, respectively. Also, the results of thermal gravimetric analysis (TG) suggested that the coated composite PCM had good thermal stability in the working temperature range from 25 to 50 °C. Furthermore, the enthalpy loss of melting and freezing was negligible after 100 cycles, indicating its good thermal reliabilities. The thermal conductivity of the coated composite PCM can be as high as 3.643 W/(m K). According to the obtained results, the coated hydrated salts/EG composite PCM enjoys high latent heat, good thermal reliability and high thermal conductivity. Apart from its favorable thermal properties, the cost of the coated hydrated salts/EG composite PCM was quite low, making it promising for low temperature thermal energy storage applications. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Thermal energy storage (TES) is considered as one of the most important energy storage technologies, due to that it can even out the imbalance between heat supply and consumption and help in saving of capital costs. Generally, TES can be accomplished by using sensible heat storage, thermochemical energy storage and latent heat storage [1–3]. Latent heat storage using phase change materials (PCMs) is carried out by storing and releasing latent heat during the phase change (solid–liquid, solid–solid, gas–liquid) [4– 6]. It has been the most attractive choice for TES applications because of its advantages of convenient use, high storage density and constant temperature during phase change. Latent heat storage by solid–liquid PCMs, including organic and inorganic PCMs, is widely used due to their larger number of substances and potential for wider temperature applications, compared with the solid–solid ones. Nevertheless, the leakage of the melted PCMs and the corrosion limited the applications of solid–liquid PCMs. In order to solve these problems, the ⇑ Corresponding author. Tel./fax: +86 10 62784877. E-mail address: [email protected] (T. Wang). http://dx.doi.org/10.1016/j.enconman.2015.05.006 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
shape-stabilized PCMs, which are prepared by incorporating solid–liquid PCMs into supporting materials through encapsulation or impregnation, emerged. The shape-stabilized PCMs based on organic PCMs, including paraffin [7,8], fatty acids [9–11] and poly (ethylene glycol) (PEG) [12,13] have been extensively studied owing to the fact that they have suitable energy storage density, no corrosivity and no subcooling. Nevertheless, the inorganic PCMs, especially the hydrated salts, are rarely reported to be prepared into shape-stabilized PCMs, although they enjoy the advantages of high storage density, high thermal conductivity and low cost. This may be attributed to the drawbacks of their phase segregation and subcooling. The phase segregation has been prevented by adding thickening agents or using rotating storage devices, while the subcooling has been improved by the addition of nucleating agents or using the cold finger technique [14,15]. Also, it was effective to embed the hydrated salts into the mesopores to solve these problems mentioned above. We have tried to impregnate the hydrated salts into the mesopores of silica and it turned out that phase segregation was inhibited and subcooling was slightly mitigated [16]. In addition, the crystal water of hydrated salts was hard to hold due to the dehydration during heating. Therefore, microencapsulation became the first choice of the
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researchers. Zhang et al. [17] encapsulated the hydrated salts into the nanoscale pores of silica by sol–gel method, which improved the thermal performance of Na2SO410H2O. Huang et al. [18] prepared the microcapsules loaded by Na2HPO47H2O by means of suspension copolymerization–solvent volatile method. However, the process of microencapsulation was complex, and it may be adverse to the thermal conductivity. Moreover, it should be noted that organic PCMs suffer from the disadvantage of low thermal conductivity, preventing them from being applied in practical use. Therefore, high thermal conductive materials, including metal materials and carbon based materials [19], were added into the composites to improve their thermal conductivities. However, introducing additives inevitably bring the loss of energy storage density. Therefore, researchers directly incorporated the PCMs into the abundant pore structures of carbon based materials [20–22]. Expanded graphite (EG) is a kind of carbon material produced by heating expandable graphite in a furnace at high temperature or treated by microwave irradiation [23]. Due to its desirable properties of high thermal conductivity, low density, low cost and good chemical stability, EG has been a promising candidate to increase the thermal conductivity of PCMs. Meanwhile, EG has a unique network-like pore structure with large surface area and high surface activity. This enables EG to increase the thermal conductivities of PCMs without much reduction in the latent heat energy storage density. As a result, EG has been popular in the preparation of shape-stabilized PCMs. Lee et al. [24] prepared erythritol/EG composites with different interlayer distances by a simple blending and impregnating method and found that the thermal conductivity of the composite increased with increasing EG interlayer distance. In particular, the thermal conductivity can be as high as 3.56 W/(m K). Wang et al. [25] reported that the thermal conductivity of the PEG/EG composites increased four times compared with that of pure PEG and it can reach a large latent heat of 161.2 kJ/kg when the mass percentage of EG was 10%. It was mentioned above that both EG and hydrated salts have the advantage of high thermal conductivity and the combination of them may be beneficial to prepare the PCM with high thermal conductivity. Duan et al. [26] impregnated CaCl26H2O into EG through vacuum impregnation and found that the thermal conductivity of the composite with 50 mass% CaCl26H2O (8.796 W/(m K)) was 14 times as that of pure CaCl26H2O (0.596 W/(m K)). Moreover, due to the low cost of EG and hydrated salts, the combination of them are advantageous for reducing cost. This work aims at the preparation of the shape-stabilized PCMs, which adopted the mixture of hydrated salts (sodium sulfate decahydrate (Na2SO410H2O) and sodium phosphate dibasic dodecahydrate (Na2HPO412H2O) in mass ratio of 1:1) as PCMs and the EG, treated by microwave irradiation beforehand, as supporting material. The hydrated salts/EG composite PCM was prepared by impregnation method and further coated with paraffin wax to inhibit the phase segregation as well as enhance the thermal stability. The morphology, chemical compatibilities, phase change properties, thermal stabilities and thermal conductivities of the hydrated salts/EG composite PCM were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermal gravimetric analysis (TG) and laser flash thermal analyzer (LFA). Moreover, the thermal cycling performance of the coated hydrated salts/EG composite PCM was also investigated.
China. Prior to use, 1 g of it was dried in an oven at 70 °C for 20 h, followed by microwave treatment using a domestic microwave oven (Midea, G70D20CN1P-D2(S0), China), which was held at 500 W for 90 s. Thus, the expanded graphite (EG) was obtained and employed as supporting material. The contact angles of obtained EG with water were measured, and found to be 71.0° on the left and 70.3° on the right. This suggested that the pure EG was hydrophilic, which enabled the hydrated salts solution to be impregnated into the pores of EG. Sodium sulfate decahydrate (Na2SO410H2O, analytical grade) and sodium phosphate dibasic dodecahydrate (Na2HPO412H2O, analytical grade) were purchased from Beijing Modern Eastern Fine Chemicals, China and used as phase change materials (PCMs). Paraffin wax (melting point Tm = 50–60 °C) and n-hexane were supplied by Tianjin Guangfu company, China.
2.2. Preparation of uncoated hydrated salts/EG composite PCM Uncoated hydrated salts/EG composite PCM was prepared by a physical blending and impregnation method. Firstly, 3.5 g of sodium sulfate decahydrate (Na2SO410H2O) and 3.5 g of sodium phosphate dibasic dodecahydrate (Na2HPO412H2O) were dissolved in 3 mL of deionized water to obtain a homogeneous solution. Subsequently, 1 g of EG was added into the hydrated salts solution and stirred vigorously in a beaker. Then, the mixture was held in 40 °C in thermostat water bath for 10 h, ensuring that the hydrated salts were fully impregnated into the EG. After that, the mixture was dried at 10 °C in an oven until it kept the same weight, thus the uncoated hydrated salts/EG composite PCM was obtained, as displayed in Fig. 1. No hydrated salts were observed on the surface of EG and the wall of the beaker. Therefore, it can be considered that the hydrated salts were fully impregnated into the pores of EG or adhered to the surface of EG. The content of hydrated salts was obtained by means of immersing the composite into deionized water repeatedly. Before immersing, the weight of the composite PCM was recorded as M1. Then, the composite was immersed into deionized water repeatedly to remove the hydrated salts. At the same time, the concentration of Na+ in deionized water after immersing was measured by atomic absorption spectrophotometer. If the concentration of Na+ was below 10 5 mol/L, it was considered that the hydrated salts had been completely removed. Subsequently, the composite was dried in room temperature until it kept the weight constant. The weight of the composite PCM after immersing was recorded as M2. Therefore, the content of hydrated salts was (M1 M2)/M1.
2. Experimental section 2.1. Materials Raw expandable graphite (300 meshes, expansion coefficient: 200 mL/g) was supplied by Qingdao Xingyuan Shimo Company,
1 cm
Fig. 1. The image of uncoated hydrated salts/EG composite PCM.
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2.3. Preparation of coated hydrated salts/EG composite PCM Paraffin wax was chosen to coat hydrated salts/EG composite on the basis of the following reasons. On the one hand, it was widely reported that paraffin can be adsorbed by EG, so the paraffin was capable to coat the hydrated salts/EG composite. On the other hand, the paraffin can form a solid layer, which may prevent the evaporation of water when the hydrated salts transformed into liquid state. Furthermore, the coating process was aided by n-hexane, in which the hydrated salts could not be dissolved. Actually, the paraffin wax was used only as coated. It may fill into the pores of the EG that unfilled by the hydrated salts, too. 0.2 g of paraffin wax was added and dissolved completely in 20 mL of n-hexane in a beaker before the addition of 1 g of uncoated hydrated salts/EG composite PCM. Subsequently, the mixture was sealed by plastic wrap and stirred at 500 rpm at room temperature for 5 h. After that, the mixture was filtered to remove n-hexane and the coated hydrated salts/EG composite PCM was obtained, as shown in Fig. 3(a).
nitrogen atmosphere. The thermal cycling tests were carried out in an oven by heating and cooling the samples from 1 to 60 °C repeatedly. Thermal gravimetric analysis was conducted by a thermo analyzer instrument (TGA, STA409PC, Netzsch, Germany). The samples were heated from 20 to 200 °C at a rate of 5 °C/min under the nitrogen atmosphere at a flow rate of 20 mL/min. Thermal conductivities were determined by a laser flash thermal analyzer (LFA, LFA 447, Netzsch, Germany). Before the measurement, samples were pressed into a tablet under the pressure of 2 MPa. The tablets with a diameter of 12.7 mm and a thickness of 2 mm were obtained and the densities of them were 1.33 g/cm3 and 1.26 g/cm3 for uncoated and coated hydrated salts/EG composites, respectively. Each sample was measured three times and the average results were chosen. The pore structure of EG was measured by a mercury porosimetry (Micromeritics AutoPore IV 9500 series pore size analyzer). 3. Results and discussion 3.1. Morphology of hydrated salts/EG composite PCM
2.4. Characterization of hydrated salts/EG composite PCM The morphologies of EG and hydrated salts/EG composite PCMs were observed by a scanning electron microscope (SEM, JSM7401, Shimadzu, Japan). Samples were coated with gold before testing. The chemical compatibility between EG and hydrated salts was investigated by a Fourier transform infrared spectroscopy (FT-IR, Tensor27, Bruker, Germany) with the wavenumber range from 400 to 4000 cm 1 at room temperature. Samples were mixed with KBr and pressed into a pellet before analysis. The crystalline phase of samples was examined by X-ray diffraction (XRD, D8ADVANCE, Bruker, Germany) using Cu Ka radiation (k = 1.5406 Å). Diffraction patterns were collected in the 2h ranges from 10° to 80°. Thermal properties were measured using a differential scanning calorimetry (DSC, Q2000, TA, USA). Samples were sealed in an alumina pan and heated from 20 to 60 °C at a rate of 5 °C/min in purified
Fig. 2 illustrates the SEM images of EG and uncoated hydrated salts/EG composite PCMs with different contents of hydrated salts. As shown in Fig. 2(a), EG consists of overlapped graphite flakes, forming abundant crevice-like and net-like pores. It was determined from mercury intrusion method that the porosity of EG was 92.78% and the pore volume was as high as 28.00 ml/g. Owing to the porous structure, the melted hydrated salts can be easily adsorbed and filled into pores or adhered onto the flakes of EG. The capillary forces and surface tension between hydrated salts and EG can prevent melted hydrated salts from leaking. It can be seen from Fig. 2(b) that EG was completely dispersed in the superfluous hydrated salts, when the content of hydrated salts was 93.69%. This means that this content of hydrated salts may be in excess of the saturated adsorption amount of EG. When the content of hydrated salts was only 85.45%, it was observed that some
Fig. 2. SEM images of expanded graphite with different contents of hydrated salts: (a) 0%; (b) 93.69%; (c) 87.36%; and (d) 85.45%.
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inner pores were not filled with hydrated salts. These unfilled pores led to a lower loading capacity, indicating that more hydrated salts can be adsorbed. In comparison, when the content of hydrated salts was 87.36%, both the pores and the surfaces were thoroughly occupied by hydrated salts. This suggested that this content may be the optimum content of the hydrated salts. Therefore, the hydrated salts/EG composite with this content was studied in the following. To confirm the shape stabilization ability of the composite PCM mentioned above, the coated hydrated salts/EG composite PCM in powder state was heated to 60 °C in an oven, which is higher than the phase transition temperature of the hydrated salts. The images of them before and after heating were shown in Fig. 3(a) and (b). It can be seen that no leakage was observed from the powder after heating. Furthermore, the coated composite PCM was pressed into a tablet, as displayed in Fig. 3(c). Then, this tablet was put into an oven and heated to 60 °C to observe the shape stability. It was shown in Fig. 3(d) that the shape of the composite PCM kept the same with that before heating. This further indicated that the coated composite PCM had good shape stabilization ability.
3.2. Chemical compatibility of hydrated salts/EG composite PCM Fig. 4 displays the FTIR spectra of EG, hydrated salts, uncoated and coated hydrated salts/EG composite PCM. The FTIR spectrum of EG showed the characteristic absorption peaks at 1636 cm 1 and 3453 cm 1, corresponding to the stretching vibrations of C@O and OH [27], respectively. The peaks at 1163 cm 1 and 619 cm 1 represented SAO stretching vibration and symmetric SAO stretching vibration [28], respectively. The stretching vibrations of HPO24 , that is, mas (PAO), ms (PAO) and mas (PAOH), were
167
Fig. 4. FTIR spectra of EG, hydrated salts, uncoated and coated hydrated salts/EG composite PCMs.
observed at 1071, 953, and 861 cm 1, respectively [29]. As displayed, the FTIR spectrum of uncoated hydrated salts/EG composite PCM showed all the characteristic peaks of hydrated salts and EG and no new peaks appeared. This indicated that there were no chemical interactions between them. In addition, it should be noted that there was a down shift of the peak of C@O in that of uncoated composite PCM. This may be due to the hydrogen bonding between C@O and OH. In comparison with the FTIR spectrum of the uncoated composite, the FTIR spectrum of coated composite showed all the same peaks except three new peaks at 2930, 2860
Fig. 3. Images of coated hydrated salts/EG composite PCMs in powder (a) before heating, (b) after heating and in tablet, (c) before heating and (d) after heating.
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and 1265 cm 1, respectively. These three peaks were attributed to the stretching vibrations of ACH3 and ACH2 group and CAC bonding, confirming the existence of paraffin wax. 3.3. XRD pattern of hydrated salts/EG composite PCM To investigate the crystalline phase of the hydrated salts, uncoated and coated hydrated salts/EG composite PCM, X-ray diffraction was carried out and the results were shown in Fig. 5. A strong diffraction peak located at 26.49° was observed in the XRD pattern of EG, which is corresponding to the feature peak of graphite. The same diffraction peak also appeared in the XRD patterns of uncoated and coated composite, confirming the existence of EG in the composite. It was found that the XRD pattern of the hydrated salts was not the simple overlay of two hydrated salts, because it was not indexable to the XRD pattern of Na2SO410H2O and Na2HPO412H2O in the database. Therefore, it was assumed that hydrated salts with different water content may form as Na4(SO4)(HPO4)X H2O. In addition, the XRD patterns of the composite demonstrated that the hydrated salts in the pores of EG were different from those in bulk. This may be due to the confinement effects of the hydrated salts crystals within the graphite sheets [30]. Further study should be done to make clear of the X in bulk and in the pores of EG. Moreover, it should be noted that the XRD patterns of uncoated and coated composite were the same. This revealed that the coating of paraffin wax did not interfere the crystallization of the hydrated salts.
Fig. 6. DSC curves of hydrated salts, uncoated and coated composite PCMs.
Table 1 Phase change enthalpies of hydrated salts, uncoated and coated composite PCMs. Samples
Melting enthalpy (kJ/kg)
±95% Conf. interval
Freezing enthalpy (kJ/kg)
±95% Conf. interval
Hydrated salts
32.52 226.90
±5.63 ±5.98
67.26 78.97
±7.81 ±1.93
RSD (%)
3.93 1.06
3.4. Thermal properties of hydrated salts/EG composite PCM DSC analysis was performed to investigate the phase change properties of the hydrated salts/EG composite PCMs. DSC curves of hydrated salts, uncoated and coated hydrated salts/EG composite PCMs are presented in Fig. 6 with detailed data given in Table 1. It was noticeable that there were two peaks in the melting DSC curve of hydrated salts [16] with the temperature at 0.61 and 36.68 °C, respectively. This indicated the phase segregation of hydrated salts, which may be due to the existence of free water in bulk. It was stated that heterogeneous nucleation together with mesoporous confinement effects in pores may be responsible for the inhibition of phase segregation [15]. Although the pore diameter of EG ranges from 2 nm to 100 lm (see Fig. 7), there were also confinement effects that inhibited the phase segregation. As can be seen from Fig. 6, only one set of melting and freezing peaks appeared in the DSC curves of both uncoated and coated composite PCM. This means that phase segregation was inhibited.
Uncoated composite
166.3
RSD (%)
0.12
Coated composite
172.3
RSD (%)
1.81
1.68 0.60 ±0.51
125.3
±5.38
1.73 ±3.88
140.8
±4.72
2.70
Fig. 7. Pore size distribution of EG obtained by the mercury porosimetry.
Fig. 5. XRD patterns of hydrated salts, uncoated and coated composite PCMs.
In addition, it was noteworthy that the melting temperatures of hydrated salts in the uncoated and coated composite PCMs shifted to a lower temperature in comparison with that of hydrated salts. This can be ascribed to the confinement effects [31,32]. That is, the melting temperature of PCMs confined in the pores may be lower compared to those in bulk. Also, it should be noted that the melting temperature of coated composite PCM was higher than that of uncoated one. This may be due to the interactions in different
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degree between pore wall and PCMs. It was shown from the FTIR spectra that there were some interactions between hydrated salts and EG in the uncoated composite PCM, while the interactions in the coated one was hardly detected. As reported [33,34], the strong interactions between PCMs and porous materials will lead to a declined phase change temperature. Therefore, the melting temperature of uncoated composite PCM was lower than that of coated one. Moreover, it was surprisingly found that the subcooling degree remarkably decreased in coated composite PCM. The subcooling degree of hydrated salts was as high as 27.66 °C, while it was only 14.94 °C for the coated composite PCM. As shown in Table 1, the melting enthalpies of uncoated and coated composite PCMs were smaller than that of hydrated salts. The melting enthalpy of the hydrated salts was as high as 226.90 kJ/kg, while that of uncoated and coated composite PCMs were 166.3 kJ/kg and 172.3 kJ/kg, respectively. The decrease of the mass fraction of hydrated salts may account for the enthalpy decrease of the composites. Also, the confinement effects contributed to the enthalpy reduction. It was mentioned above that the hydrated salts in the composite PCM was confined in pores and there were interactions between hydrated salts and the functional groups of the pore wall of EG. Both of them led to the enthalpy reduction. Additionally, it was worth noting that the melting enthalpy of the coated composite PCM was higher than that of uncoated ones. This may be attributed to the weaker interactions between pore wall and hydrated salts as mentioned above. As shown in Fig. 4, the intensities of C@O in the FTIR spectrum of the coated composite PCM were much weaker than that of uncoated ones, suggesting the weaker interactions between pore wall and hydrated salts in coated composite PCM. Perhaps the presence of paraffin wax changed surface properties that may weaken the intensities of C@O and may have aided nucleation at the same time. Nevertheless, the enthalpies of both the uncoated and coated composite PCM were comparable or superior to those reported in other literatures (Table 2), demonstrating the favorable thermal properties of the prepared composite PCMs.
Fig. 8. TG curves of hydrated salts, uncoated and coated composite PCMs.
salts and uncoated ones. This revealed that paraffin coating was advantageous to delay the thermal decomposition of hydrated salts. Thus, the thermal stability of PCMs was improved. Nevertheless, it should be admitted that there was still weight loss of coated composite PCMs, indicating that it was not coated completely by the paraffin wax. If the hydrated salts in the pores of EG was the mix of Na2SO410H2O and Na2HPO412H2O in mass ratio of 1:1, the weight loss should be 58.04%. However, the TG analysis of the hydrated salts in bulk, uncoated and coated hydrated salts/EG composite PCMs gave different results. As presented in Fig. 8, it was calculated that the contents of water for the hydrated salts in bulk, in uncoated and coated composite PCM were 56.89%, 55.21% and 52.61%, respectively. This suggested that the hydrated salts in bulk and the pores of EG (uncoated and coated) were not the simple mix of Na2SO410H2O and Na2HPO412H2O. This was consistent with the results of XRD.
3.5. Thermal stability of hydrated salts/EG composite PCM The thermal stabilities of the hydrated salts, uncoated and coated hydrated salts/EG composite PCMs were evaluated by TG analysis and the results were presented in Fig. 8. As shown in Fig. 8, all the samples experienced a single weight loss between 25 °C and 100 °C. In the working temperature range from 25 °C to 50 °C, the weight loss may be mainly caused by the moisture release and the dehydration of hydrated salts. It was determined that the weight loss of coated composite PCM in this temperature range was less than 10%, implying that it has a good thermal stability in the working temperature range. The most significant degradation took place at the stage from 50 °C to 100 °C, which belonged to the evaporation of water. The onset temperatures of weight loss (Tonset) for hydrated salts, uncoated and coated hydrated salts/EG were 63.0 °C, 63.1 °C and 70.9 °C, respectively. That is, the Tonset of the coated composite PCM was higher than those of hydrated
3.6. Thermal reliability of coated hydrated salts/EG composite PCM For composite PCMs, it is essential to have good thermal reliabilities over a number of thermal cycles. Therefore, thermal cycling tests were carried out to investigate the thermal reliabilities of the coated composite PCMs. Thermal cycling tests were conducted up to 100 cycles and DSC analysis was performed before and after different thermal cycles. As can be seen from Fig. 9(a), the enthalpy changes of the coated composite PCMs after 100 cycles were negligible. After 100 cycles, the enthalpies of melting and freezing were 162.5 kJ/kg and 134.9 kJ/kg, dropping by 5.69% and 4.19%, respectively. In addition, it can be seen from Fig. 9(b) that no significant differences were found among melting temperatures after different cycles. For the freezing temperatures, there was a small rise with the increasing cycles. Nevertheless, these changes were
Table 2 Thermal properties comparison of some composites in literatures. Sample
Melting temperature (°C)
Melting enthalpy (kJ/kg)
Freezing temperature (°C)
Freezing enthalpy (kJ/kg)
Thermal conductivity (W/(m K))
Ref.
Paraffin/SiO2/7.2 wt%EG CaCl26H2O/10 wt%EG MA–PA–SA/7 wt%EG PA/20 wt%EG PEG/10 wt%EG Coated hydrated salts/13 wt%EG
27.72 29.48 41.64 60.88 61.46 32.05
104.41 98.30 153.50 148.36 161.20 172.3
– 42.99 60.81 46.91 17.11
– 151.4 149.66 – 140.8
0.25 3.437 2.510 0.600 1.324 3.643
[35] [26] [27] [36] [25] Present study
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Fig. 9. Thermal properties of coated hydrated salts/EG composite PCMs after different cycles (a) phase change enthalpy and (b) phase change temperature.
Table 3 Thermal conductivities of prepared composite PCMs. Sample
Thermal conductivity (W/(m K))
EG Uncoated hydrated salts/EG Coated hydrated salts/EG
11.000 3.615 3.643
in insignificant level for TES applications. In other words, the coated hydrated salts/EG composite PCM showed good thermal reliabilities. 3.7. Thermal conductivities of hydrated salts/EG composite PCM One of the important factors in TES applications is the rates of energy storage and release, which substantially depend on the thermal conductivity of the composite PCMs. Thermal conductivities of the uncoated and coated hydrated salts/EG composite PCMs were measured at 25 °C and the results were presented in Table 3. EG is generally adopted to enhance the conductive performance of organic PCMs to make up its disadvantage of low thermal conductivity. Admittedly, their thermal conductivity can significantly increase with the addition of EG, but they are still inferior to the hydrated salts. As shown in Table 3, the thermal conductivity of hydrated salts/EG could be as high as 3.615 W/(m K) (uncoated) or 3.643 W/(m K)(coated), which was much higher than most of the organic PCMs (Table 2). This may be attributed to the high thermal conductivity of both hydrated salts and EG. This verified that the combination of hydrated salts and EG was an effective way to improve the thermal conductive performance of PCMs. Additionally, it was noted that there was no big difference between the uncoated and coated composite PCM. This indicated that the addition of little paraffin wax into the composite PCM did not affect their thermal conductive performance. 4. Conclusions A novel shape-stabilized phase change material, adopting hydrated salts as phase change material and expanded graphite (EG) as supporting material, was prepared by impregnation method and further coated with paraffin. The phase segregation of hydrated salts was inhibited and the subcooling reduced to a large extent. The melting enthalpy of the coated hydrated
salts/EG composite PCM can reach 172.3 kJ/kg with a melting temperature at 32.05 °C. The coated composite PCM had good thermal stability in the working temperature range from 25 to 50 °C, too. Additionally, the thermal conductivity of the coated hydrated salts/EG composite PCM can be as high as 3.643 W/(m K), indicating its good thermal conductive performance. Moreover, the enthalpy loss of the coated composite PCM was negligible after 100 thermal cycles, showing its good thermal reliabilities. In a word, the coated hydrated salts/EG composite PCM enjoys favorable thermal performance, including high latent heat, good thermal stability, high thermal conductivity and good thermal reliability. Moreover, the combination of hydrated salts and EG can significantly reduce cost due to their low cost. Due to these favorable properties and advantages, the coated composite PCM can be considered as a good candidate for low temperature thermal energy storage applications in many fields, such as greenhouse, building energy conservation, textile fibers and thermal protection of electrical devices.
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