calcium carbonate phase change microcapsules with carbon network

Applied Energy 179 (2016) 601–608

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Enhancement on thermal properties of paraffin/calcium carbonate phase change microcapsules with carbon network Tingyu Wang, Shuangfeng Wang ⇑, Lixia Geng, Yutang Fang Key Laboratory of Enhanced Heat Transfer & Energy Conservation, Ministry of Education, South China University of Technology, Guangzhou 510640, Guangdong, China

h i g h l i g h t s  A sort of phase change composites with double-layer network was developed.  The composites thermal conductivity and thermal stability were enhanced.  The heat transfer areas were increased due to spindle microcapsules morphologies.  A distinct carbon network structure was detected with 20 wt% expanded graphite.

a r t i c l e

i n f o

Article history: Received 7 June 2016 Received in revised form 9 July 2016 Accepted 9 July 2016

Keywords: Thermal energy storage Phase change composites Network structure Thermal conductivity Thermal stability

a b s t r a c t For latent heat storage with phase change materials (PCM), heat transfer rate and energy storage efficiency are often limited by the low PCM thermal conductivity. Therefore, this paper develops a sort of new phase change composites (PCC) with double-layer network to enhance the thermal conductivity and thermal stability. Different mass fractions of expanded graphite (EG) as heat transfer promoter were added in the spindle microencapsulated phase change materials (MicroPCM). The relationship between the PCC thermal conductivity and carbon network structure was investigated. The thermal conductivity was measured by transient plane source method. The carbon network structure of PCC was detected by energy dispersive spectroscopy. Temperature-regulated property was captured by infrared imager. As a result, distinct carbon network structure in PCC was observed with 20% mass fraction of EG, the corresponding thermal conductivity was increased up to 7.5 times of the pristine paraffin. Negligible change in thermal properties of the PCC was confirmed after 100 times thermal cycling and 7 days serving durability tests. The enhancement on thermal properties of the PCC is a promising route to achieve high energy storage efficiency targets of numerous thermal applications. Ó 2016 Published by Elsevier Ltd.

1. Introduction Thermal energy storage (TES) technologies have played a critical role in sustainable energy infrastructure. Application of phase change materials (PCM) to improve the efficiency of energy storage is under active investigations [1–3]. PCM can absorb or release thermal energy during phase transition at a constant temperature range [4]. It has shown potential to be applied in many fields, such as battery thermal management [5–6], industrial waste heat recovery [7], building energy conservation [8] and solar energy utilization [9]. However, traditional PCM generally suffer from leakage during the solid-liquid phase transition and low thermal conduc-

⇑ Corresponding author. E-mail address: [email protected] (S. Wang). http://dx.doi.org/10.1016/j.apenergy.2016.07.026 0306-2619/Ó 2016 Published by Elsevier Ltd.

tivity (k) that to some extent limited in practical applications [10,11]. Microencapsulation technique has attracted more and more attentions in recent years. Since it can avoid the leakage of PCM from their location and also increase the heat transfer areas [12,13]. Various shapes and structures of the microencapsulated phase change materials (MicroPCM) are generated by different preparation methods with different properties of the shell materials. It is well known that CaCO3 has three crystalline polymorphs, i.e., calcite, aragonite, and vaterite, which correspond to different morphologies and fit for different application fields [14]. The morphologies of MicroPCMs with CaCO3 shells depend on the surfactant concentrations and synthetic temperature, it showed spherical morphology when synthesized under 35 °C in our previous work [15]. The thermal properties of spherical MicroPCM are isotropous which result in better stability and dispersibility, while

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the spindle ones possess larger heat transfer areas. Hence, they are suitable for different utilization fields according to their own advantages. Moreover, MicroPCM can achieve a stable microstructure by wrapping pristine PCM inside organic/inorganic shells [16]. For the past decades, MicroPCM with polymer shells have been widely studied, including melamine formaldehyde resin [17], poly (methyl methacrylate) (PMMA) [18], polystyrene [19], calcium alginate [20]. They have good structural stability but low thermal conductivity [21]. In order to overcome this problem, some attempts have recently been taken to encapsulate PCM with appropriate inorganic shells, such as aluminum hydroxide (AlOOH) [22], silicon dioxide (SiO2) [23], germanium dioxide (ZrO2) [24], titanium dioxide (TiO2) [25] and calcium carbonate (CaCO3) [26]. In consequence, these MicroPCM with inorganic shells were observed significant improvement in thermal conductivity and thermal stability than that of the pristine PCM. Nevertheless, thermal conductivities of MicroPCM with inorganic shells are still inadequate for meeting many energy storage efficiency targets. The heat transfer rate governing the power capacity, service life and energy storage efficiency is dominated by thermal conductivity [27]. For a certain quality of PCM, higher heat transfer rate result in more total thermal energy that could be stored/released within a unit time. In this way, energy storage efficiency can be enhanced by improving thermal conductivity. Hence, in order to accelerate heat transfer and improve energy storage efficiency, the necessarity to further explore PCM with high thermal conductivity and high thermal stability is evident. A literature survey indicates that high thermal conductivity fillers or inserts are sought to achieve high energy storage efficiency [28]. Liu et al. [29] demonstrated that the heat storing/releasing rate is effectively enhanced by adding expanded graphite (EG) to the MicroPCM with melamine resin shell. Li et al. [30] focused on improving thermal conductivity by using the MicroPCM with urea formaldehyde resin shell that merged into modified carbon nanotubes. Yang et al. [31] strengthened the heat transfer through employing the MicroPCM with polymethyl methacrylate shell that supplemented with modified silicon nitride powders. Wang et al. [32] significantly enhanced thermal conductivity by 22 times with 20 wt% EG loaded via utilizing the MicroPCM with melamine formaldehyde shell. Wang et al. [33] enhanced thermal conductivity by incorporating reduced graphene oxide in the MicroPCM with silica shell. Thereinto, carbon materials are the best fillers for enhancing thermal conductivity since high thermal conductivity can be achieved with low density. Till now, researchers mainly focus on the methods, varieties and optimal mass fractions of adding high thermal conductivity fillers into pristine PCM or MicroPCM with organic shells. Each study has presented remarkable increase in the thermal conductivity, which is due to the inherent high thermal conductivity of carbon fillers. Nevertheless, it is more important to form an effective filler network to obtain high thermal conductivity with less filler content. However, few studies have deeply investigated the principles and methods of forming the filler network, especially based on the MicroPCM with inorganic shells. In this paper, we reported an experimental investigation to show that the MicroPCM with calcium carbonate shell and loading with 20 wt% EG could significantly increase thermal conductivity by up to 7.5 times compared to the pristine paraffin, heat storing/releasing rates were accelerated effectively as well. The phase change enthalpy and thermal conductivities of the MicroPCM/EG composites changed less than 3% over 100 solid-liquid phase transition cycles and 7 days serving durability tests, which indicating the phase change composites (PCC) possess good thermal stability during thermal cycling. With these prominent thermal properties, the PCC designed by this work will be a potential candidate for the thermal storage applications.

2. Experimental 2.1. Materials Paraffin (RT 42) was used as PCM provided by ZDJN PCMS Co., Ltd., China. Calcium chloride (CaCl2) and sodium carbonate (Na2CO3) were acted as monomers of the MicroPCM shell materials. Sodium dodecyl benzene sulfonate (SDBS) was treated as surfactant. The above reagents were all of analytical grade and obtained from Tianjin Kemiou Chemical Reagent Co., Ltd., China. Deionized water was homemade. EG was served as the high thermal conductivity fillers and prepared from expandable graphite powder by the microwave method. It was supplied by Qingdao Graphite Co., Ltd., China. 2.2. Preparation Fig. 1(a) shows the procedures to synthesize the MicroPCM via self-assembly precipitation method. The preparation process was carried out in a 500 ml three-neck flask equipped with a thermostatic water bath (60 °C) and a mechanical stirring (300 rpm). Firstly, paraffin (10 g) was melted into liquid state and mixed with the surfactant solution that containing SDBS (1.046 g) in deionized water (60 ml). The solution was stirred for 20 min and paraffin droplets gradually aggregate to form spindle shapes. Next, an aqueous solution of CaCl2 (5.55 g) in deionized water (70 ml) was added into the above mixture dropwise and followed with agitation for 4 h. At this stage, a stable oil-in-water emulsion was formed. Afterwards, an aqueous solution of Na2CO3 (5.3 g) in deionized water (70 ml) was added dropwise in the flask and stirring for 6 h. Finally, the precipitation reaction was completed and the resultant microcapsules were filtered, washed and dried at 50 °C for 12 h. Fig. 1(b) illustrates the PCC containing 10 wt%, 20 wt% and 30 wt% EG (denoted as S1, S2 and S3 respectively) were prepared by conventional dispersion method. The as prepared MicroPCM and EG according to the proportion were mixed in the whirlpool mixer and agitated for 30 min. Then, the mixture were poured into a cylindrical stainless steel mould (U 20 mm). The cylindrical compressed MicroPCM/EG composites were formed by dry pressing of MicroPCM and EG powders. Ultimately, the PCC products were cut and polished to form sheets. 2.3. Characterization 2.3.1. Fourier transform infrared spectroscopy (FTIR) The FTIR spectra was obtained using a Bruker Tensor-27 instrument on a KBr disk at room temperature. The specimens were mixed with KBr sheets and the wave numbers ranging from 4000 to 500 cm 1. 2.3.2. Energy dispersive microscopy (EDS) The carbon network structure of the PCC was observed by Oxford Inca Energy-350 energy dispersive X-ray spectrometers under the view of a SEM. The PCC specimens were polished with #2000 emery paper and sprayed coating with a thin layer of gold-palladium alloy. 2.3.3. Scanning electron microscopy (SEM) The morphology of the MicroPCM and PCC were measured by JEOL JSM-7400F scanning electron microscope with 20 kV acceleration voltage. 2.3.4. Transmission electron microscopy (TEM) The microstructures of the MicroPCM was tested by JEOL JEM2100 transmission electron microscope. The samples were dis-

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Fig. 1. Schematic diagram for producing the MicroPCM and PCC.

persed in ethanol solution by ultrasonicator and randomly collected on carbon-coated copper grids for observation.

3. Results and discussion 3.1. Chemical composition and crystallography

2.3.5. Thermal conductivity The thermal conductivities of MicroPCM and PCC were tested by transient plane source method using Sweden Hot Disk thermal conductivity meter. The specimens were polished to reduce the thermal contact resistance.

2.3.6. Differential scanning calorimeter (DSC) The phase change properties were measured by TA instrument Q20 differential scanning calorimeter with a heating or cooling rate of 10 °C/min under nitrogen atmosphere.

2.3.7. Heat storing/releasing test The heat storing/releasing performances of the MicroPCM and PCC were tested by American Agilent 34970A Data acquisition instrument. The samples were placed in the water bath with heating and cooling rates were about 1 °C/min, the temperature was maintained at 70 °C or 30 °C corresponding to heating and cooling process.

Fig. 2 shows the FT-IR spectra of RT42, CaCO3, MicroPCM, EG and PCC (S1-S3). RT42 exhibits three strong absorption peaks around 1466, 2850 and 2920 cm 1, which attributed to ACH2 variable angle vibration, ACH2 symmetrical stretching vibration and ACH2 asymmetrical stretching vibration, respectively. The vibration bands appeared at 712, 876 and 1418 cm 1 in CaCO3 spectra were caused by in-plane bending vibration of CAOAC in calcite, out-of-plane bending vibration and asymmetrical stretching vibration of CO23 , respectively. It should be mentioned that the absorbance peaks in the spectrum of MicroPCM appeared at 715, 858, 1493, 2850 and 2921 cm 1, which evidently shows both the characteristic peaks of RT42 and CaCO3 in Fig. 2a. In addition, three intensive absorption peak at 3429, 2920 and 2853 cm 1 are corresponded to the AOH stretching mode, stretching vibration of ACH2 groups and asymmetrical stretching vibration of ACH3 groups in EG spectrum. It is notable that the PCC spectra curves of S1-S3 are quite similar, which show both the characteristic peaks of MicroPCM and EG in Fig 2b. These results indicated the successful microencapsulation of paraffin with CaCO3 shell and integration of MicroPCM with EG. 3.2. Microstructure and morphology

2.3.8. Thermal gravimetric analysis (TGA) The thermal decomposition behaviors of MicroPCM and PCC were determined by TA instrument SDT Q600 thermal gravimetric analyzer at a heating rate of 10 °C/min under nitrogen atmosphere.

2.3.9. Infrared thermography The temperature-regulated properties were captured by FLIR SC3000 thermacam in a temperature range of 25–60 °C. The specimens were formed cylindrical sheets (U 10 mm  6 mm) by dry pressing of the uniformly mixed MicroPCM and EG powders according to the proportion. The bottom surface of the cylindrical sheets were heated and the central point temperature were measured in time.

Fig. 3 exhibits the EDS-mappings of the PCC (S1-S3) which contain (a) 10 wt%; (b) 20 wt% and (c) 30 wt% EG, respectively. MicroPCM is consisted of paraffin core and calcium carbonate shell, which contain calcium (Ca), carbon (C), oxygen (O) and hydrogen (H), while EG only contains carbon (C). So the distribution of MicroPCM and EG can be distinguished in PCC by the different colors of Ca (red) and C (green). The red areas stand for the distributed MicroPCM, while the independent green regions but not covered by red could represent the EG. Thereinto, significant differences of the EG structure in PCC were detected. The percolating carbon network was clearly observed in S2 (Fig. 3b) with EG contents about 20 wt%. On the contrary, S1 (Fig. 3a) and S3 (Fig. 3c) failed to form the legible network of EG within 500 lm unit scope. Since

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Fig. 2. FT-IR spectrum of (a) RT42, CaCO3 and MicroPCM; (b) MicroPCM, EG and PCC.

(a)

(b)

(c)

100 μm

100 μm

100 μm

Fig. 3. EDS-mappings of PCC: (a) S1contains 10 wt% EG; (b) S2 contains 20 wt% EG; (c) S3 contains 30 wt% EG.

the MicroPCM were easy to agglomerate and dispersed unequally when its mass fraction was more than 90 wt%, which was similar to the EG as its loading was more than 30 wt%. The phenomena indicated that the formation of carbon network within microsized scope in the PCC is based on proper contents of EG. The microstructure and morphology of MicroPCM are demonstrated in Fig. 4(a). A typical core-shell structure of the microcapsule with spindle shape and length about 1–6 lm can be further confirmed. Thus, calcium carbonate shells were the first layer network of the paraffin core. At the same time, Fig. 4(b) demonstrates that the intact microcapsules are uniformly dispersed and surrounded by EG network [29]. Consequently, EG formed the second layer percolated network for enhancement on heat transfer and thermal stability. It should be mentioned that the as prepared microcapsules without any damage after compression for preparing the PCC. This is because of the inherent flexibility of EG that can protect the MicroPCM from the pressure. Moreover, the spindle microcapsules were randomly oriented in the plane and vertical to

(a)

500 nm

the direction of the pressure for the PCC prepared. Since the microcapsules and EG both have a high aspect ratios, which tends to lie within a plane perpendicular to the stress direction in dispersed state [28]. 3.3. Thermal conductivity Table 1 presents the thermal conductivities of the RT42, MicroPCM and PCC (S1-S3) at room temperature. It is anticipated that the encapsulation of RT42 with CaCO3 shell can effectively enhance heat transfer of the resulting MicroPCM, which could be owed to the inherent good thermal conductivity of the CaCO3. Meanwhile, the thermal conductivities of PCC are increased with the mass fractions of EG, such as S1-S3 were significantly enhanced 3.7, 7.5, 9.0 times compared to the pristine paraffin, respectively. As a result, EG is excellent additive to enhance the heat transfer of MicroPCM since it has lager specific surface and also good dispersity in the spindle microcapsules. Furthermore, the heat

(b)

10 μm

MicroPCM

EG

Fig. 4. (a) TEM image of the MicroPCM; (b) SEM-EDS image of S2 contains 20 wt% EG.

T. Wang et al. / Applied Energy 179 (2016) 601–608 Table 1 Phase change properties of the RT42, MicroPCM and PCC. Sample code

Sample composition

Phase change temperature Tm (°C)

Phase change enthalpy DHm (J/g)

a Thermal conductivity k (W m 1 K 1)

– – – S0 S1

RT42 CaCO3 EG MicroPCM MicroPCM/ EG (10 wt%) MicroPCM/ EG (20 wt%) MicroPCM/ EG (30 wt%)

48.69 – – 48.62 49.94

233.0 – – 143.6 117.4

0.369 2.167 49.120 0.814 1.363

48.71

113.9

2.755

48.46

95.48

3.326

605

per unit volume/mass. Therefore, the phase change enthalpies and thermal conductivities should be balanced based on the needs of practical application. 3.5. Thermal energy storage performance

transfer areas are increased as the spindle morphologies of microcapsules which have more contact areas with the compressed EG plane. It is noteworthy that the S2 specimen with 20 wt% EG strengthen heat transfer more effectively compared to the S1 and S3. This is because of effective carbon network structure was formed in S2, and thus heat transfer areas can be significantly increased [28]. Therefore, the core PCM would continuously absorb heat from carbon network structures, which resulted in faster heat storing/releasing rate [34].

The heat storing/releasing performances of the MicroPCM and PCC are illustrated in Fig. 6. The samples were placed in the water bath with heating and cooling rates being as about 1 °C/min. The time span of the heat storage and release processes are shown in Table 2. In the heat storage process, the required time from 35 °C heated to 60 °C of S1, S2 and S3 decreased by 0.56%, 2.95% and 5.61% compared to the ones of S0, respectively. In the same way, during the heat release process, the time duration from 60 °C cooling to 35 °C of S1, S2 and S3 decreased by 11.72%, 27.00% and 34.89%, respectively. In addition, two continuous phase change platforms were observed in the temperature variation curves during heat storing/releasing process, which fully confirm to the DSC results. The length of the phase transformation platform depends on the amount of PCM while energy storage efficiency hinges on the heat storing and releasing rates. The faster heat storing and releasing rates lead to the more total thermal energy stored/ released in a unit time. It is evident that the heat storing and releasing rates of S1-S3 were faster than that of MicroPCM, which promoted to accelerate heat transfer and resulted in higher energy storage efficiency.

3.4. Phase change performance

3.6. Thermal stability

The energy density of the PCC is dominated by phase change latent heat (DHm), which derived from the corresponding contents packed PCM inside it. The phase change performances of PCC with different mass fractions of EG are presented in Fig. 5 and Table 1. All the samples exhibit two broad phase change peaks for melting and solidification processes, which is probably owing to the reason that RT42 is a mixture of paraffinic hydrocarbons with different carbon atoms. This also shows that negligible change in the phase change temperature (Tm) of the MicroPCM and PCC were detected compared to the pristine paraffin. Hence, there is no reactions between the paraffin, MicroPCM and EG fillers, which is in accordance with the FTIR results. However, the encapsulation of paraffin with CaCO3 shell relatively reduce the absolute phase change enthalpies of the pristine paraffin core. It is indicated that the latent heat of PCC decrease with the increasing mass fractions of EG fillers. Adding thermal conductive fillers result in less PCM

Fig. 7(a) illustrates the mass loss curves of RT42, MicroPCM and PCC (S1-S3). It is observed that the weight loss profiles of the samples all occurred through one-step degradation. The maximum weight loss of the MicroPCM, S1, S2 and S3 appeared around 227.0 °C, 217.9 °C, 199.6 °C and 188.8 °C, respectively. Which are greatly improved compare to 124.8 °C of the pristine RT42. The final weight loss ratios of the MicroPCM, S1-S3 are 60.25%, 58.23%, 35.77% and 19.24%, which are apparently superior to 10.61% of RT42. It means that the MicroPCM with calcium carbonate shell as protective layer can effectively increase heat-resistance temperature and prevent leakage. Better still, PCC with doublelayer network of calcium carbonate and EG can further improve the thermal stability. What counts is PCC must be stable and characteristics unchanged after thermal recycling in practical applications. We evaluated the thermal cycling stability of the PCC by measuring

S2 S3

a Thermal conductivities were measured in perpendicular to the direction of the pressure.

Fig. 5. DSC (a) heating and (b) cooling thermograms of MicroPCM and PCC.

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Fig. 6. Temperature variations of the MicroPCM and PCC during (a) storing and (b) releasing process.

Table 2 Time required for heat storing/releasing processes of MicroPCM and PCC. Sample code

Sample composition

Time required for heat storage process (s)

Time required for heat release process (s)

S0 S1

MicroPCM MicroPCM/ EG (10 wt%) MicroPCM/ EG (20 wt%) MicroPCM/ EG (30 wt%)

1426 1418

1416 1250

1384

1034

1346

922

S2 S3

phase change latent heat and thermal conductivity (k) after multiple melting-solidification cycles and serving durability tests. As demonstrated in Fig. 7(b), the phase change latent heat of the MicroPCM/EG composites changed less than 3% over 100 solidliquid phase transition cycles and 7 days serving durability tests. Likewise, negligible change was observed in thermal conductivity and phase change temperature. This results suggests that the MicroPCM incorporating with EG are remarkably stable during thermal cycling.

3.7. Temperature-regulated property Fig. 8 presents the effect on thermal performance of the MicroPCM (denoted as S0) and PCC (S1-S3) loading with different mass fractions of EG. The surface temperature distributions of the PCC were captured by infrared camera when the samples heated in 60 °C water bath. Firstly, temperature of the PCC increased with the environmental temperature and ultimately reached Tm (about 48 °C), heat conduction occurred in this stage. Then, the PCC began to absorb thermal energy at a constant temperature (Tm) during the phase transition. As illustrated in Fig. 8, it took 70 s, 100 s, 150 s and 180 s for the S3, S2, S1 and S0 heated to Tm, respectively. These results reveal that PCC possess excellent thermal management and temperature regulated properties as it can lead to more homogeneous constant temperature distribution (Tm) for a certain time. Moreover, temperature increase and heat penetrate more quickly for the PCC than MicroPCM. So the PCC induce a higher heat transfer rate essentially in the first stage. To put these results into perspective, the MicroPCM incorporating with EG is a promising route to improve energy storage efficiency in thermal energy storage applications.

Fig. 7. (a) TGA of thermograms of MicroPCM and PCC; (b) thermal properties of S2 after 100 thermal cycling and 7 days serving durability tests.

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t=70 s

t=100 s

S0

S1

S3

S0

S1

S2

t=150 s

S3

S2

t=180 s

S0

S1

S3

S2

S0

S1

S3

S2

Fig. 8. Thermal images of MicroPCM and PCC heated at different times: t = 70 s; t = 100 s; t = 150 s; t = 180 s.

4. Conclusion

References

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1. The heat transfer areas were increased as the spindle morphologies of microcapsules have more contact areas with the compressed EG plane. 2. EG as an excellent thermal fillers possess inherent high thermal conductivity, lager specific surface areas and also good dispersity in the spindle microcapsules. 3. The heat transfer areas of the MicroPCM/EG composites significantly increase when the carbon network structure is formed with 20 wt% EG. In addition, negligible change in thermal properties of the MicroPCM/EG (20 wt%) after 100 times thermal cycling and 7 days serving durability tests, indicating the as prepared PCC with double-layer network also possess excellent thermal cycling stability. In conclusion, such enhancement can result in an improvement of energy storage efficiency to meet the requirements of numerous thermal applications.

Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 51536003) and Fundamental Research Funds for Central Universities (Grant No. 20152P031).

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