paraffin as composite shape

Keywords: Phase change material; Calcium chloride hexahydrate; Diatomite;. Thermal energy storage. Page 1 of 16. ACS Paragon Plus Environment. Energy ...
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Calcium Chloride Hexahydrate/Diatomite/Paraffin as Composite Shape-Stabilized Phase-Change Material for Thermal Energy Storage Xinxing Zhang,†,‡,§ Xiang Li,*,†,§ Yuan Zhou,*,†,§ Chunxi Hai,†,§ Yue Shen,†,§ Xiufeng Ren,†,§ and Jinbo Zeng†,§ †

Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China ‡ University of Chinese Academy of Sciences, Beijing, 100049, China § Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining 810008, China ABSTRACT: In this work, the calcium chloride hexahydrate/diatomite/paraffin composite phase-change material (PCM) was fabricated by impregnating calcium chloride hexahydrate into diatomite and further coating with paraffin. Scanning electron microscope (SEM) results showed that the hydrated salt could be impregnated into a diatomite well and the paraffin could be coated on a diatomite surface. The Fourier transform infrared spectroscopy (FT−IR) demonstrated that no chemical interactions were found between calcium chloride hexahydrate and diatomite matrix except that of hydrogen bonds. The melting and crystallizing enthalpy of the coated composite PCM are 108.2 and 98.5 J/g, respectively. The supercooling of hydrated salts was weakened due to the nucleation positions on the huge surface of diatomite and the coating effect of the paraffin, and phase segregation was eliminated in 100 cycles by microvolume effect in the pores of the diatomite. Additionally, coated composite PCM and composite PCM exhibited better thermal stability and reliability than hydrated salts due to the interactions of hydrated bonds, the capillary force and the surface tension as well as the coating of the paraffin. After the 100 cycles, the melting and crystallizing enthalpy of coated composite PCM declined to 106.2 and 93.8 J/g from 108.2 and 98.5 J/g, dropping by 1.8% and 4.8%, respectively. The coated composite PCM with well thermal properties, thermal reliability, and chemical stability was a promising PCM candidate for heat-energy storage applications.

1. INTRODUCTION Energy utilization and management get much attention due to continuous energy crises and environmental issues around the world, and thermal energy storage and saving are considered the key technologies for efficient energy utilization in the future.1−3 Phase-change materials (PCMs), the latent heat energy storage materials that can absorb and release energy in the process of phase change (solid−liquid, solid−solid, and gas−liquid), have been studied in practical applications such as solar energy collection, waste-heat recovering, and building materials because of the high energy storage density and the cost saving.4−8 Phase-change materials can be classified into organic and inorganic PCMs. The organic PCM includes paraffin, PEG, nhexadecane, etc., and the inorganic PCM can be divided into alloys, metals, and hydrates salts.9 As a very important lowtemperature PCM system, hydrated salts obtained a lot of attractions due to their numerous advantageous properties such as high enthalpy, incombustibility, and low price. Calcium chloride hexahydrate (CaCl2·6H2O) is one of the most important hydrated salts PCMs, and it has been used in solar energy heating10 and building materials11 due to its suitable phase-change temperature of 29 °C and high thermal storage capacity around 190 J/g.12 However, it greatly suffered supercooling and special segregation problems in the applications due to its property of semi congruent melting in phase change.13 The nucleation can be used to solve © XXXX American Chemical Society

supercooling and the thickness has been utilized to deal with segregation with excellent results obtained by the previous results.14,15 Meanwhile, the method by incorporating hydrated salts into the porous material as supporting matrix to prepare shape-stabilized composite PCMs is also a promising way to solve the problems mentioned above. It has been reported that hydrated salts that experience ion hydration in the micropore perform very different properties in the aspects of energetics of physical and chemical systems, energy levels and stability of the ion hydrations compared with samples in bulk.16−18 Additionally, the nucleation positions on the huge surface area of porous material could inhibit the supercooling, the effect of micro volume in pores could restrain the phase separation, and the capillary force and the surface tension of the porous structure can help to improve the stability and reliability of hydrated salts in practical applications in thermal energy storage system.19,20 Diatomite as a supporting material has received extensive attentions in the thermal storage for its high porosity, high adsorption capability, high specific surface area, and low price.21,22 Many organic composite PCMs have been studied such as stearic acid and diatomite,23 erythritol tetrapalmitate and diatomite,24 and decanoic−dodecanoic acid and diatomReceived: September 22, 2017 Revised: December 11, 2017 Published: December 11, 2017 A

DOI: 10.1021/acs.energyfuels.7b02866 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels ite,25 and it is reported that diatomite is a hydrophilic mineral due to its abundant −OH functional groups on the surface;26 it has a high capacity of absorption to water,27 which helps to enhance the stability and reliability of hydrated salts through forming hydrated bonds and benefits the absorption of liquid hydrated salts,28 there are few studies on the composite of hydrated salts and diatomite as the shape-stabilized phasechange material. In this work, the shape-stabilized composite PCM with CaCl2·6H2O as the PCM and diatomite as the supporting material was prepared by vacuum impregnation method, and to further enhance the stability, paraffin was used to coat the surface of composite PCM. A series of thermal properties including supercooling, segregation, stability and reliability were measured to support the research and the results show that the prepared coated composite PCM with excellent thermal properties, chemical stability, and thermal reliability is a promising PCM candidate for heat energy storage applications.

Figure 1. Schematic diagram of the preparation of coated composite PCM.

Table 1. Mean Pore Size, Pore Surface Area, and Pore Volume of Diatomite before and after Treatment

2. MATERIALS AND EXPERIMENTS 2.1. Materials. Raw diatomite was supplied by the Jilin Wanzhong Company. Calcium chloride anhydrous (purity >96%, analytical grade) and n-hexane (purity >95%, analytical grade) were supplied by the Tianjin Damao Company. Paraffin (melting point Tm = 50−60 °C) was supplied by to Lanzhou Wotu Company. 2.2. Characterization. Morphologies of samples were observed by a scanning electron microscope (SEM, JSM-5610LV). The pore characters of diatomite were measured by a specific surface and pore size analyzer (Autosorb-iQ2-MP).The crystalline phases of samples were got from an X-ray diffraction (X’PRO Pert). Thermal stability texting was performed by thermogravimetric analysis (TGA, STA449F3). The samples were heated from 25 to 300 °C at a rate of 5 °C/min under the nitrogen atmosphere. The chemical compatibilities of PCM were determined by a Fourier transform infrared spectroscopy (NEXUS). Thermal properties were measured by using a differential scanning calorimetry (DSC, TA Q20). Samples were sealed in an alumina pan at a rate of 10 °C/min in purified nitrogen atmosphere. 2.3. Preparations of Diatomite and Calcium Chloride Hexahydrate. The raw diatomite was treated with sulfuric acid (H2SO4, 60 wt %) for 4 h after being calcined at 450 °C for 3 h. Subsequently treated diatomite was obtained by drying at 105 °C for 24 h. Additionally, the CaCl2·6H2O crystal was obtained by the crystallization of the aqueous solution of 45 wt % CaCl2 to ensure its purity. 2.4. Preparations of Composite PCM and Coated Composite PCM. The diatomite (M0) was vacuumed for 30 min and liquid CaCl2·6H2O (7M0) was added into a suction flask through a separatory funnel to conduct the process of absorption. Subsequently, composite PCM (M1) obtained by filtering through a suction filtration for 20 min at 40 °C. The load of hydrated salts in composite PCM is 67 wt % by the formula of M1 − M0/M1.29 After the process of absorption, the paraffin (0.8 M2) was added into n-hexane (25 M2) to dissolve completely in a beaker before adding composite PCM (M2), and the mixture was stirred at 400 rpm for 6 h at 25 °C. Subsequently the coated composite PCM was obtained through filtering for 20 min at 25 °C and drying for 1 h at 25 °C. The schematic diagram of the preparation of coated composite PCM is shown in Figure 1.

materials

Rmean (nm)

Pvolume (cm3/g)

Sarea (m2/g)

raw diatomite treated diatomite

1.779 1.782

0.081 0.115

19.117 33.243

that the treatments have a little effect on mean pore size, while the pore volume is from 0.081 to 0.115 (cm3/g), and the surface area is from 19.117 to 33.243 (m2/g) after treatment. Those results show that calcination and acid treatment can remove the components such as organic carbon substance, Fe2O3, and Al2O3 to increase the surface area. The SEM images of diatomite before and after treatment are shown in Figure 2. As shown in Figure 2a, the raw diatomite is

Figure 2. SEM images of (a) raw diatomite and (b) treated diatomite.

a porous mineral and numerous porous structure could be observed on the surface of diatomite, indicating its high porosity and high surface area. It is also obvious that a lot of impurities block the channels of pores on a large scale, suggesting that the further treatment for raw diatomite is necessary to remove the impurities blocking the pores to enhance its surface area. After calcination and acid treatment, the basic morphology of porous structure is not be destroyed, and there were fewer impurities in the pores with more clear channels of pores shown in Figure 2b, which will guarantee a good adsorbent effect in the process of absorption for hydrated salts. 3.2. Characterization of the Effect of Absorption and Coating. The effects of time and temperature were investigated to obtain a good effect of absorption. In Figure 3, the phase-transition enthalpies of composite PCM are 111.5, 110.6, and 111.8 J/g at 40, 45, and 50 °C, respectively. The enthalpies at 60, 80, and 100 min are 117.1, 118.0, and 117.2 J/g, respectively. As the data shown, the phase-transition enthalpies showed a negligible change with temperature and

3. RESULTS AND DISCUSSION 3.1. Characterization of the Pores of the Diatomite. The raw diatomite has a lot of impurities that block the channels of pores and decline the pore surface area.26,30 Table 1 shows the mean pore size, pore volume, and pore surface area of diatomite before and after treatment. The results reveal B

DOI: 10.1021/acs.energyfuels.7b02866 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Effects of (a) temperature and (b) time on absorption.

time, and the error of DSC may also cause the slight fluctuations of phase-transition enthalpies due to the tiny testing quality.31 Therefore, the adsorption of diatomite to hydrated salts was approximately saturated. Taking into account the time and energy, the 40 °C and 60 min could be considered as the optimized conditions. The SEM images of composite PCM and coated composite PCM are shown in Figure 4. As shown in Figure 4a, hydrated salts are

Figure 4. SEM images of (a) composite PCM and (b) coated composite PCM.

Figure 5. XRD patterns of calcium chloride hexahydrate, composite PCM, and coated composite PCM.

impregnated well into the pores of diatomite and the channels of pores are filled by the PCM, which indicates a good adsorbent effect of hydrated salts by the diatomite. After being coated by the paraffin, the surface of composite PCM has been sealed by a distinct layer of paraffin, as displayed in Figure 4b. These results illustrate that the process of absorption and coating can achieve the goal of making shape-stabilized PCM. 3.3. Chemical Compatibilities of Coated Composite PCM. To investigate the structural integrity of hydrated salts in composite, an X-ray was performed, and the XRD patterns of CaCl 2 ·6H 2 O, diatomite, composite PCM, and coated composite PCM are shown in Figure 5. It is noted that all the sharp and intense diffraction peaks of CaCl2·6H2O can be observed in composite PCM and coated composite PCM, which demonstrates that the crystal structure of CaCl2·6H2O is not destroyed, and no other forms of hydrated salts exist after impregnation in composite PCM and coated composite PCM. It is important that the different component of composite PCM has a good compatibility to ensure the good performance of PCM. Therefore, FT−IR was utilized to test the interactions of different components of the composite. Figure 6a displays the FT−IR spectra of CaCl2·6H2O, diatomite, and composite PCM. The spectrum of diatomite illustrates that peaks at 466, 794, and 1094 cm−1 are caused by the bending vibration of Si− O, the Si−O−Si symmetric stretching vibration and the Si−

O−Si asymmetry stretching vibration, respectively. In addition, the peaks at 1636 and 3432 cm−1 relate to the stretching vibration and the bending vibration of −OH functional group, respectively.32 These characteristic peaks prove that the main component of diatomite is silicon dioxide. The FT−IR spectrum of CaCl2·6H2O shows its characteristic peak at 509 cm−1, while the peaks at 1627 and 3369 cm−1 also relate to the vibration of −OH functional group. As shown in Figure 6a, it is noted that the stretching vibration and the bending vibration of the −OH shift to 1629 and 3444 cm−1 in composite PCM from 1636 and 3432 cm−1 of the diatomite due to the formation of hydrogen bonds between the −OH group and the crystal H2O molecules, and all of the characteristic peaks of CaCl2·6H2O are presented in the spectrum of composite PCM with no special new peaks appearing, which proves the good chemical compatibility between CaCl2·6H2O and the diatomite. Similarly, Figure 6b shows the FT−IR spectra of paraffin, composite PCM, and coated composite PCM, and all of the same characteristic peaks of composite PCM exist in coated composite PCM except for four new ones of paraffin at 749, 1461, 2848, and 2912 cm−1, which relate to the rocking vibration of −CH2 (749 cm−1), the deformation vibration of −CH2 and −CH3 (1461 cm−1), the stretching vibration of −CH2 and −CH3 (2848 and C

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Figure 6. (a) FTIR spectra of calcium chloride hexahydrate, diatomite, and composite PCM and (b) FTIR spectra of paraffin, composite PCM, and coated composite PCM.

2912 cm−1). These results indicate the good compatibilities of different components in the coated composite PCM. 3.4. Thermal Properties of Coated Composite PCM. DSC instrument was performed to test the thermal properties of PCMs with the results shown in Figure 7 and Table 2. As

experience phase change.33 The crystallizing enthalpies of CaCl2·6H2O, composite PCM, and coated composite PCM are 161.8, 104.3, and 98.5 J/g, respectively, which are lower than the melting enthalpies; this may be attributed to the special crystallization process of CaCl2·6H2O, which has two steps with the first formation of CaCl2·4H2O and a second peritectic reaction of the mixture. These two steps can also be observed from the phase diagram of CaCl2−H2O,34 and the step of formation of CaCl2·4H2O is a metastable state in which the DSC instrument may not be so sensible enough to detect the change of enthalpy that there is no peak of the first step of crystallization in DSC curves.13 Additionally, the crystallizing temperature of composite PCM and coated composite PCM is 0.9 and 3.2 °C, respectively, compared to −10.5 °C (CaCl2· 6H2O). This inhibition of supercooling attributes to the nucleation positions on the huge surface area of diatomite and partly attachment between paraffin and CaCl2·6H2O, which help to intrigue crystallization of hydrated salts. The melting temperatures are 28.4, 28.7, and 28.9 °C for CaCl2·6H2O, composite PCM, and coated composite PCM, respectively. The DSC curves of melting temperatures shift slightly toward high temperature, which attributes to that the diatomite and paraffin in the composite PCM have the function of reducing the heat-transfer rate due to the lower thermal conductivities of diatomite and paraffin compared with that of hydrated salts, and a lower heat-transfer rate will make the testing melting temperatures of composite PCM and coated composite PCM increase slightly.35−37 3.5. Thermal Stability of Coated Composite PCM. The thermal stability can be used as a measure to evaluate the adaptive ability of temperature changes in applications. Therefore, the thermogravimetric analysis (TGA) was performed to test the stability of PCM in the continuous heating-up. Figure 8 shows the TGA curves of CaCl2·6H2O, composite PCM, and coated composite PCM. It is shown that the samples have processes of mass loss from 25 to 265 °C and the mass loss between 25 and 40 °C relates to the loss of attached water. From the TGA curve of CaCl2·6H2O, three distinct steps can be identified, which are the decomposition processes of CaCl2·6H2O to CaCl2·4H2O, CaCl2·4H2O to CaCl2·2H2O and CaCl2·2H2O to CaCl2 from 25 to 150 °C, and the same steps of mass loss can be observed in composite PCM and coated composite PCM.38 However, It is noted that the temperatures ranges of mass loss of coated composite PCM and composite PCM were 58.6−265.0 °C and 52.9−

Figure 7. DSC curves of calcium chloride hexahydrate, composite PCM, and coated composite PCM.

Table 2. Thermal Properties of Calcium Chloride Hexahydrate, Composite PCM, and Coated Composite PCM samples calcium chloride hexahydrate composite PCM coated composite PCM

melting enthalpy (J/g)

crystallizing enthalpy (J/g)

melting temperature (°C)

crystallizing temperature (°C)

185.6

161.8

28.4

−10.5

117.1

104.3

28.7

0.9

108.2

98.5

28.9

3.2

shown in Table 2, the melting enthalpies of CaCl2·6H2O, composite PCM, and coated composite PCM are 185.6, 117.1, and 108.2 J/g, respectively. The declines of enthalpies attribute to the part of the diatomite and the paraffin that do not D

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different cycles do not change significantly. These results indicate that the capillary force, surface tension, the interactions of hydrogen bond and the coating of paraffin help to restrain the loss of crystal water to some extent. So the technology of adsorption and coating is an efficient way to enhance the reliability of hydrated salts. After 100 cycles, XRD was performed to test the components of the samples with results showed in Figure 10, it is noted that all the sharp and

Figure 8. TG curves of calcium chloride hexahydrate, composite PCM, and coated composite PCM̀ .

148.7 °C, respectively, compared to CaCl2·6H2O (50.2−145.1 °C), suggesting that the porous structure and −OH groups of the diatomite as well as the paraffin coating have the advantages to enhance the stability of hydrated salts. These results show that the coated composite PCM has a relatively good thermal stability. 3.6. Thermal Reliability of Coated Composite PCM. The thermal reliability is a key influencing factor in practical applications of PCM, which helps to reduce the cost of maintenance and system update. Therefore, the samples were investigated its thermal reliability for 100 cycles, and the results are shown in Figure 9. The melting and crystallizing enthalpies of hydrated salts decline to 178.9 and 147.8 J/g from 185.8 and 161.8 J/g after 100 cycles, dropping by 3.7% and 8.7%, respectively, with composite PCM declining to 114.2 and 99.7 J/g from 117.1 and 104.3 J/g, dropping by 2.5% and 5.4%, respectively, for coated composite PCM declining to 106.2 and 93.8 J/g from 108.2 and 98.5 J/g, dropping by 1.8% and 4.8%, respectively. The decreases of enthalpies are caused by the loss of the crystal water in the process of temperature ascending and descending repeated in the cycling. The data listed above shows that the proportions of enthalpy reduction of coated composite PCM and composite PCM are lower than the hydrated salts. In addition, it can be observed from Figure 9b that the melting and crystallizing temperatures of samples after

Figure 10. XRD patterns of composite PCM and coated composite PCM after 100 cycles.

intense diffraction peaks of CaCl2·6H2O can be observed in composite PCM and coated composite PCM after 100 cycles, which demonstrates the existing of CaCl2·6H2O and no presenting of other forms of hydrated salts, suggesting that no phase segregations occur in the cycling. These results indicate that the coated composite PCM has a relatively good thermal reliability. In further studies, the technic of microcapsule with higher strength materials such as the organic polymer for composite PCM may be a good way to improve thermal reliability further.39

4. CONCLUSIONS In this study, the diatomite as a supporting material was introduced into hydrated salt PCM systems to make shapestabilized PCM, which was made by vacuum impregnation and a further coating with paraffin. The melting and crystallizing

Figure 9. Thermal properties of samples after different cycles: (a) phase-change enthalpy and (b) phase-change temperature. E

DOI: 10.1021/acs.energyfuels.7b02866 Energy Fuels XXXX, XXX, XXX−XXX

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enthalpy of the coated composite PCM are 108.2 and 98.5 J/g, respectively. As shown above, the supercooling of coated composite PCM was inhibited with the crystallization temperatures enhanced from −10.5 °C (CaCl2·6H2O) to 3.2 °C due to the nucleation positions on the huge surface of the diatomite and the coating effect of the paraffin, and the phasesegregation problem was eliminated because of the microvolume effect in the pores of the diatomite. After 100 cycles, the coated composite PCM exhibited a relatively good thermal reliability due to the interactions of hydrated bonds, the capillary force, the surface tension, and the coating of the paraffin. Based on these results, the prepared coated composite PCM in this work can be a promising PCM candidate for heat energy storage applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-971-6338403. Fax: +86971-6338403. *E-mail: [email protected]. ORCID

Xiang Li: 0000-0002-2682-7853 Chunxi Hai: 0000-0002-0430-2757 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Financial support from the Natural Science Foundation of Qinghai province of China (no. 2017-ZJ-938Q), Qinghai Province International Cooperation Projects (no. 2017-HZ805), Major Science and Technology Projects of Qinghai Province of China (no. 2015-GX-A1A), and Qinghai Provincial Thousand Talents Program for High-level Innovative Professionals is gratefully acknowledged.

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DOI: 10.1021/acs.energyfuels.7b02866 Energy Fuels XXXX, XXX, XXX−XXX