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nickel foam composite phase change materials for thermal energy storage
Journal of Energy Storage 28 (2020) 101179
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Facile synthesis and thermal performance of cety palmitate/nickel foam composite phase change materials for thermal energy storage
T
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Chaoming Wanga,b, , Tingjun Wanga,1, Zhanjiang Hua,1, Zhengyu Caia a
Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China b Engineering Research Center of Clinical Functional Materials and Diagnosis & Treatment Devices of Zhejiang Province, Wenzhou Institute, University of Chinese Academy of Sciences, Wenzhou, Zhejiang 325001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cetyl palmitate Thermal performance Thermal conductivity Nickel foam Phase change material
Cetyl palmitate (CP) was thermally synthesized using binary 1-hexadecanol (HD) and palmitic acid (PA) mixture as the starting materials in nitrogen with the absence of any catalyst. The molecule structure, thermal property, thermal stability and reliability of as-prepared cetyl palmitate was studied by 1H nuclear magnetic resonance (1H NMR), Fourier transformation infrared (FT-IR) spectrometer, differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA), respectively. 1HNMR and FT-IR confirmed the as-prepared product under 350 °C for 30 min was CP with high purity. According to the DSC measurements, the onset melting temperature and latent heat of fusion for the as-prepared CP was 52.3 °C and 180.9 J/g, respectively. Results showed that the synthesized cetyl palmitate was favorable used as phase change material (PCM) owing to its desirable thermal properties, excellent thermal stability and reliability. Due to the low thermal conductivity of CP, it was impregnating into nickel (Ni) foams with different pore sizes to make CP/Ni foam composite PCMs (CPCMs). Compared with pure CP, thermal conductivity of CP/Ni foam (70, 90, and 110PPI) CPCMs were increased by 1.88, 2.02 and 4.86 times, respectively. The CP/Ni foam CPCMs displayed appropriate thermal properties for latent heat energy storage applications.
1. Introduction Phase change materials (PCMs) were a kind of materials that have high latent heat of fusion/solidification and can storing/releasing large amount of heat energy during their melting/freezing process, which played important role and can provide a way to match thermal energy supplying and utilization [1]. When considering various application areas, such as solar energy storage and transfer [2,3], building materials [4], waste heat recovery [5], electricity peak shaving and valley filling [6], and thermal management of spacecraft [7], PCMs should have appropriate phase transition temperature, high latent heat, excellent chemical stability, good thermal stability and reliability. According to the chemical composition, the PCMs can be classified into two major types as inorganics and organics. In those organic PCMs, paraffins and fatty acids attract extensive attentions of scientific researchers all over the world [8–13]. Compared to inorganic PCMs, fatty acids and paraffin have some advantages, such as low supercooling,
high latent heat, good chemical stability, nontoxicity and non-corrosiveness. Although fatty acids and paraffin have lots of advantages, sometimes it is still hard to be utilized in practical applications. Therefore, it is necessary to develop new types of PCMs. In the past few years, many efforts have been tried to explore new types of PCMs, which was including high-chain fatty acid esters [14–17]. Generally, alkanol-fatty acid esters were synthesized by Fischer esterification. On the one hand, the synthesis process was complicated and catalysis was needed [18]. On the other hand, owing to the slow reaction speed and the need to heat up the medium to evaporate the generated water, the high product yields are almost impossible to be reserved by Fischer esterification [19]. Therefore, developing easy and low cost procedure for high-chain fatty acid esters is necessary. Besides, the organic PCMs have low thermal conductivities, which lead to the lower energy storage/release rate and restrict their practical applications in real case. With the aim to increase the thermal
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Corresponding author at: Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China. E-mail address: [email protected] (C. Wang). 1 These authors contributed equally to this article. https://doi.org/10.1016/j.est.2019.101179 Received 8 October 2019; Received in revised form 17 December 2019; Accepted 23 December 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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2.3. 1H NMR analysis
conductivity of those organic PCMs, various nanofillers, such as carbon additives [20–23], metal nanoparticles [24–26], metal oxide nanoparticles [27–29], and metal foams [30–32], have been developed and added. Except those nanoadditives, metal foam (such as nickel foam) is one type of promising material because of its high porosity, large surface area, solid skeleton structure, strong mechanical strength, light weight, and excellent thermal conductivities, which brings it into practical applications in latent heat energy storage systems, solar energy collector, and battery thermal management and so on [33,34]. Owing to the interconnected path of metal foam and good compatibility with many organic PCMs, it allows the heat can transfer throughout the PCM uniformly and rapidly. In this study, cetyl palmitate (CP) was synthesized by a facile thermal method in nitrogen without any catalyst using 1-hexadecanol (HD) and palmitic acid (PA) binary mixture (abbreviated as HP) as the starting material. The molecular structures of the as-prepared products under different thermal treatment temperatures were studied by FT-IR and 1H NMR. The thermal performance, thermal stability and reliability of the synthesized CP were determined by DSC, TGA and repeated thermal cycling test, respectively. Results showed that the synthesized CP (with the onset melting temperature of 52.3 °C and the latent heat of 180.9 J/g) were favorable for using as long-term thermal energy storage material owing to its desirable thermal properties, excellent thermal stability and reliability. For the purpose of increasing the thermal conductivity of CP, it was impregnating into nickel foams with different pore sizes to make CP/Ni foam composite PCMs (CPCMs). Compared with pure CP, thermal conductivity of CP/Ni foam (70, 90, and 110PPI) CPCMs were increased by 1.88, 2.02 and 4.86 times, respectively. The CP/Ni foam CPCMs displayed an appropriate thermal performance for latent heat energy storage applications.
The 1H NMR analyses were performed on a 400 MHz nuclear magnetic resonance spectrometer (AVANCE III, BRUKER, Germany). The as-prepared ester samples were dissolved in CDCl3 and the 1H NMR spectra were attained by using tetramethylsilane (TMS) as an internal reference standard.
2.4. FT-IR measurements Fourier Transform infrared spectroscopy (FT-IR) was utilized to investigate molecular structure and chemical composition of as-prepared samples using an ATR (attenuated total reflection) spectrophotometer (Nicolet iS50, Thermo Scientific, USA) in the 4000–600 cm−1 spectral range with a resolution of 2 cm−1 (referred to a dry air background) using thermoelectrically cooled DTGS detector.
2.5. Preparation of the CP/Ni foam CPCMs CP/Ni foam CPCMs were prepared via melting infiltration method. Briefly, enough amount of as-prepared CP (solid state) was placed at the bottom of a glass bottle, and the nickel foam with different pore size (70, 90, and 110PPI) and circular shape (12.7 mm in diameter) was placed onto the solid CP. Thereafter, the bottle was immersed in a 60 °C water bath for 1 h until the melted CP was infiltrated into the nickel foam fully. Afterwards, the glass bottle was taken out from the water bath and cooled naturally to room temperature until all of the CP solidified thoroughly. Lastly, extra CP was got rid of via micro–thermal separation. The as-prepared three different types of samples made from nickel foam with different pore size (70, 90 and 110PPI) were labelled as CPCM1 (70PPI), CPCM2 (90PPI) and CPCM3 (110PPI) in the following text without further mentioned.
2. Experimental 2.1. Chemicals Palmitic acid (C16H32O2, PA) and 1-hexadecanol (C16H34O, HD) with analytical purity were purchased from Tianjin Zhiyuan Reagent Co., Ltd (Tianjin, China). The nickel foams with different pore sizes (70, 90 and 110PPI) were supplied by Kunshan Jiayisheng Electronics Co., Ltd (Kunshan, China). All of chemicals were used as received.
2.6. Thermal analysis The thermal properties (such as melting temperature, latent heat of melting or solidifying) of the binary HD-PA mixture and thermal treated samples were measured by a differential scanning calorimetry (DSC, DSC3+, METTLER TOLEDO). Briefly, about 5 mg samples was sealed in an aluminum pan and placed inside the DSC chamber with an empty aluminum pan as a reference under continuously purged nitrogen gas with a flow rate of 40 ml/min at heating rate of 5 °C/min from 20 to 100 °C. The thermal stability of the samples thermally treated at different temperatures were investigated by a thermogravimetric analyzer (TGA, DSC3+, METTLER TOLEDO) scanning from 30 to 600 °C at 10 °C/min heating rate under a constant nitrogen stream rate of 40 ml/min. The repeated cycling heating and cooling test was performed to study the thermal reliability of as-prepared CP. Briefly, certain amount of as-prepared CP was loaded into a plastic vial and put in a metal bath (CHB-T2-E, BIOERThermoQ) to run repeated heating and cooling cycles from 0–100 °C for 0–500 times.
2.2. Preparation of the binary HD-PA mixture and CP Binary HD-PA mixture was firstly prepared by melting blending method and used as the starting material to synthesize the CP. In a typical procedure, a certain amount of HD and PA with molar ratio of 1:1 was sealed in a glass bottle and heated by immersing in a 70 °C water bath and stirred at 500 rpm/min for 30 min. Then binary HD-PA mixture (abbreviate as HP) was obtained by cooling the mixture to room temperature. Instead of using the commonly known procedure of Fischer's esterification in the presence of acid catalyst, the presented cetyl palmitate (CP) was synthesized under inert gas in the absence of catalyst according to a modified procedure used by Baykut and Aydın [35]. Briefly, the as-prepared HP, which was sealed in a quartz crucible, was thermally treated at different temperatures in a pipe furnace under purging a constant stream of inert nitrogen. The thermal treated HP at 150, 200, 250, 300 and 350 °C for 30 min were denoted as HP150, HP200, HP250, HP300, and HP350, respectively. After 30 min, the temperature was cooled down naturally and the new formed products were taken out. The high purity ester was obtained after several crystallizations of the solution with acetone and ether. The reaction scheme for CP synthesis was shown in Scheme. 1.
2.7. Thermal conductivity measurements The thermal conductivities of the as-prepared CP and CP/Ni foam CPCMs were carried on a thermal conductivity meter (HotDisk TPS2200, Sweeden) using the transient plane source technique equipped with a 7577 probe (2.001 mm in radius), which had a maximum uncertainty of 4%. The samples were cut into circular cylinder with diameter of 12.7 mm and thickness of about 3 mm for the measurement. Each test was performed for three times. 2
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Fig. 1. FT-IR spectra of HP, HP150, HP200, HP250, HP300, and HP350 (A) and 1HNMR spectra of HP 350 (B).
3. Results and discussions
3.2. Thermal properties of HP and HP150-350
3.1. FT-IR and 1H NMR measurement
The thermal properties of HP and thermally treated HP samples were studied by DSC. Fig. 2A displayed the cycling DSC curves of HP, HP150, HP200, HP250, HP300 and HP350, respectively. For the binary mixture, it showed a downward endothermal peak (melting peak) with onset temperature at 44.8 °C and an upward exothermal peak (freezing peak). As the thermal treatment temperature (from 150 °C to 300 °C) increased, the melting peak moved to high temperature. For the DSC curve of HP350 (also the as-prepared CP), it showed the onset melting temperature and latent heat were 52.3 °C and 180.9 J/g, respectively. The thermal stability of as-prepared products was studied by TGA. Fig. 2B presented the thermogravimetric (TG) curves of HP and HP150350, respectively. From the TG curves of HP and HP150-HP250, it was seen that the decomposition process was divided into two stages. The first stage attributed to the decomposition of HP binary mixture and the second stage represented the decomposition of as-produced CP. For HP sample, the first and second weight loss stages occurred at 180–320 °C and 320–400 °C, respectively. For HP350 (also the as-prepared CP), the decomposition process was only showed one stage and the initial decomposition temperature (about 280 °C) was much higher than its melting temperature (55.3 °C), which indicated that CP had excellent thermal stability. For HP150-HP300, the temperature at the first maximum mass loss rate was almost the same. However, for the temperatures at the second stage, they were moved to higher position when the thermal treatment temperature increased, which was attributed to more and more CP produced at higher thermal treated temperature.
Fig. 1A showed the FT-IR spectra of HP, HP150, HP200, HP250, HP300, and HP350, respectively. For HP, strong absorption peaks at 2913, 2848 and 1704 cm−1 were belonging to the symmetrical stretching vibration of -CH3 and -CH2 group, and the -C ] O stretching vibration of PA, respectively. In addition, the absorption peaks at 1471 and 717 cm−1 were attributed to the bending vibration of -CH2 group and the in-planes winging vibration of –OH group of HD. Compared to the spectra of HP, all of the above-mentioned absorption peaks were also appearing in HP150-350 samples' spectra. However, when the thermal treatment temperatures increased from 150 °C to 350 °C, the absorption peak intensity of the as-prepared samples at 1704 cm−1 and 1059 cm−1 decreased and two new absorption peaks at 1732 cm−1 and 1182 cm−1, which were assigned for the -C ] O stretching vibration of acyclic saturated esters, were appearing and the intensity increased gradually. Meanwhile, the absorption peaks of –OH stretching vibration of PA and HD located at 2500–2700 cm−1 and 3230–3550 cm−1were not observed for HP350, which suggested high ester rate after the thermal treatment. The 1H NMR characteristic peaks of HP350 were shown in Fig. 1B, and no peaks of –OH groups at about 2.24 ppm for HD, and 9.78 ppm for PA were observed [36]. Besides, the peak attributed to the protons of –COO–CH2– group appearing at 4.07 ppm for HP350. According to the peak integration, the purity of the product (HP350) was larger than 90%. Those results indicated that the cetyl palmitate (CP) was successfully synthesized starting with binary HD-PA mixture (abbreviate as HP), which was treated under 350°C for 30 min without using any catalyst.
3.3. Thermal reliability of HP350 (also as the as-prepared CP) The thermal reliability of the as-prepared CP after experiencing repeated heating and cooling cycles was assessed by DSC. Fig. 3A depicted the cycling DSC curves of the as-prepared CP after experiencing 0, 100, and 500 heating and cooling cycles in a metal bath from 0–100
Fig. 2. DSC (A) and TG (B) curves of HP, HP150, HP200, HP250, HP300, and HP350. 3
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Fig. 3. DSC curves (A) and FT-IR spectra (B) of HP350 after running 0, 100, and 500 repeated heating and cooling cycles with heating rate of 5 °C/min from 20–100 °C. Table 1 DSC data of CP after running 0, 100, and 500 times. Name Melting
Solidifying
Ton(°C) Tm(°C) Te(°C) ΔHf(J/g) Ton(°C) Tm(°C) Te(°C) ΔHs(J/g)
0
100
500
53.5 55.2 57.2 178.3 50.0 49.3 45.5 171.9
53.6 55.3 57.1 177.6 50.3 49.1 45.1 174.9
52.5 54.9 56.8 177.1 50.1 48.9 45.8 171.6
°C. The onset melting temperature (Ton), melting temperature (Tm), end melting temperatures (Te), latent heat of fusion (ΔHf) and solidifying (ΔHs) determined from those DSC measurements were collected and listed in Table 1. For the as-prepared CP, the onset melting and solidifying temperatures maintained nearly the same even experienced 100, and 500 heating and cooling cycles. Compared to synthesized pristine CP, the latent heat of fusion and solidifying changed by −0.4, −0.7% and 1.7, −0.2% after experiencing 100 and 500 reduplicative thermal cycles, which were in negligible level. Moreover, the molecular structural stability of CP after thermal cycles was also investigated by FT-IR technique and the results were presented in Fig. 3B. It was found that the characteristic absorption peaks of CP were in consistent well with that going through 100 and 500 thermal cycles, which implied that CP had fabulous thermal stability. Those results suggested that facile synthesized CP was a prospective PCM for long term latent heat energy storage applications. 3.4. Microstructure analysis and impregnation ratios The optical images of nickel foams and CP/Ni foam CPCMs after impregnation of CP were presented in Fig. 4A-F, respectively. Fig. 4A, C and E displayed the morphologies of the pure nickel foams with pore size at 70, 90 and 110PPI and Fig. 4B, D and F showed the optical images of the CP/Ni foam (with pore size at 70, 90 and 110PPI) CPCMs after impregnated with CP. As shown in Fig. 4B, D and F, the CP were trapped well in nickel foams. Since the high porosity of nickel foam, the ratio of CP impregnated into the nickel foam can be determined from the following equation:
m′ − m m′ − m = α= vth ρPCM εvtotal ρPCM
Fig. 4. Optical images of nickel foams with different pore sizes before (A: 70PPI; C: 90PPI; E: 110PPI) and after (B: 70PPI; D: 90PPI; F: 110PPI) impregnated with CP.
given from the supplier. According the calculation from Eq. (1), the impregnation ratios for different CP/Ni foam composites were calculated and presented in Fig. 5. It was shown that the mass fraction of CP impregnated in 70, 90 and 110PPI nickel foams were 83.86, 82.85 and 76.62%, respectively. The reason for this was that the outward of the composites solidified quicker than the inward during the cooling process, which caused a decrease in the inner volume of the composite and led to the impregnation ratios less than 100%.
(1)
In the above Eq. (1), α was impregnation ratio, m and m’ were the mass of metal foam and metal foam composite after impregnating with the PCM. vtotal and vth represented whole volume of the composite and practical volume of the PCM, respectively. ε was the porosity of metal foam, ρPCM was the density of PCM. In our experiment, the density (ρ) of CP was 0.858 g/cm3 and the porosity (ε) of the nickel foam was 98% 4
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Table 2 TGA and DTG data of the as-prepared CP and CP/Ni foam composites. Sample Property
CP
CPCM1
CPCM2
CPCM3
T-5% Tmax Residual at 600 °C
281.2 395.8 −0.23
281.2 354.7 22.77
286.3 364.9 18.52
302.3 370.3 31.79
Fig. 5. Mass impregnation ratio of CP/Ni foam CPCMs made from different pore size at 70, 90, and 110PPI, respectively.
3.5. Thermal stability of CP/Ni foam CPCMs The thermal stability of the as-prepared CP/Ni foam CPCMs (CPCM1, CPCM2 and CPCM3) were tested by TG and DTG. Fig. 6A and B showed the TG and DTG curves of CP, CPCM1, CPCM2 and CPCM3, respectively. The temperature at 5wt% weight loss (T-5wt%), the maximum weight loss rate (Tmax), and the charred residue at 600 °C were recorded and listed in Table 2. From the TG and DTG curves of CP and CP/Ni foam composites, it was seen that the decomposition process only showed one weight loss stage in the range of 230–420 °C, which was far above the melting temperature of the as-prepared CP (55.3 °C). This property was a precondition for PCM to be used for latent heat energy storage. As shown in Fig. 6B and Table 2, the Tmax for CP and CP/Ni foam composites was 395.8 °C (CP), 354.7 °C (CPCM1), 364.9 °C (CPCM2), and 370.3 °C (CPCM3), which were ascribed to the decomposition of CP. In addition, the residue at 600 °C was close to the actual content of the nickel foam in the composites, which implied that the CP and nickel foam had good compatibility.
Fig. 7. Thermal conductivities of the as-prepared CP and CP/Ni foam composites.
Scheme 1. The esterification reaction of the palmitic acid and 1-hexadecanol. Table 3 Thermal conductivity, density, metal/PCM ratio of CP and CP/Ni foam (with different pore size) composites PCMs.
3.6. Thermal conductivity analysis The effect of nickel foam with different pore sizes (70, 90 and 110PPI) on the thermal conductivity of CP/Ni foam CPCMs were investigated by using a thermal conductivity meter. The measured thermal conductivities of the as-prepared CP and CP/Ni foam CPCMs were displayed in Fig. 7. The thermal conductivity, density, metal/PCM ratio of CP and CP/Ni foam (with different pore size) CPCMs were collected and presented in Table 3. Compared to pure CP (0.3432 W/ m•K), the thermal conductivity of CPCM1, CPCM2 and CPCM3 were measured as 0.6465, 0.6942, and 1.6687 W/m•K, which enhanced by 1.88, 2.02 and 4.86 times, respectively. From Fig. 7 and Table 3, it implied that larger pore size of the nickel foam would result in lower the thermal conductivity of the composites. The reason for this was because that the bigger pore size of nickel foam, the smaller interface
Samples
Pore size (PPI)
Density (g/ cm3)
Metal/CP ratio
Thermal conductivity (W/m•K)
CP CPCM1 CPCM2 CPCM3
/ 70 90 110
0.858 0.8599 0.8609 0.8963
1 0.2621 0.2357 0.3912
0.3432 0.6465 0.6942 1.6687
contact area, which resulted in the decrease of thermal conductivity of the composites. In addition, as the density increased, the thermal conductivity of CP/Ni foam composites increased since the heat exchange was enhanced per volume unit. Table 4 collected and listed thermal conductivities of some typical
Fig. 6. TGA (A) and DTG (B) curves of the as-prepared CP and CP/Ni foam composites. 5
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Supplementary materials
Table 4 Comparison of the thermal conductivity of composite PCMs with published literature. Samples
Thermal conductivity (W/m•K)
References
PEG (90 wt%)/EG Paraffin+Al foam Paraffin+Cu foam MA+Ni foam CP+Ni foam
1.324 0.4 7.6 0.4768 1.6687
[37] [38] [39] [40] This work
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composite PCMs with other additives (such as carbon materials and metal foam) from published literatures. In comparison with other fillers, metal foam had excellent thermal conductivity enhancement and showed promising potential in the practical applications. 4. Conclusions In this study, a high-chain fatty acid ester, cetyl palmitate (CP), was synthesized by a facile one-step thermal method in the absence of any catalyst using binary HD-PA mixture with molar ratio of 1:1. The chemical structure, thermal stability and reliability of as-prepared CP were studied by 1H NMR, FT-IR, DSC and TGA, respectively. 1H NMR and FTIR confirmed the as-prepared HP350 was CP with high purity. According to the DSC measurement, the onset melting temperature and latent heat of fusion for the as-prepared CP was determined as 52.3 °C and 180.9 J/g, respectively. Results showed the synthesized CP was favorable used as long-term energy storage materials owing to its desirable thermal properties, excellent thermal stability and reliability. With the aim to increase the thermal conductivity of the CP, it was impregnating into nickel foams with different pore sizes (70, 90 and 110PPI) to make CP/Ni foam CPCMs. TG results displayed that the CP/ Ni foam composites retained good stability and good compatibility between CP and Ni foam. Moreover, in comparison with pure CP, thermal conductivity of CP/Ni foam (70, 90, and 110PPI) composites were increased by 1.88, 2.02 and 4.86 times respectively. The CP/Ni foam CPCMs exhibited appropriate thermal behavior for latent heat energy storage applications. Data availability All of the data were available. CRediT authorship contribution statement Chaoming Wang: Conceptualization, Methodology, Resources, Supervision, Writing - original draft, Writing - review & editing, Funding acquisition. Tingjun Wang: Investigation, Data curation, Methodology, Software, Visualization. Zhanjiang Hu: Investigation, Data curation, Methodology, Software, Visualization. Zhengyu Cai: Investigation, Data curation, Validation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51672227), Fundamental Research Funds for the Central Universities (Grant No. 2682017CY08 and 2682017CX089), and Engineering Research Center of Clinical Functional Materials and Diagnosis & Treatment Devices of Zhejiang Province (Grant No: WIUCASK19004). 6
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Facile synthesis and thermal performance of cety palmitate/nickel foam composite phase change materials for thermal energy storage
T
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Chaoming Wanga,b, , Tingjun Wanga,1, Zhanjiang Hua,1, Zhengyu Caia a
Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China b Engineering Research Center of Clinical Functional Materials and Diagnosis & Treatment Devices of Zhejiang Province, Wenzhou Institute, University of Chinese Academy of Sciences, Wenzhou, Zhejiang 325001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cetyl palmitate Thermal performance Thermal conductivity Nickel foam Phase change material
Cetyl palmitate (CP) was thermally synthesized using binary 1-hexadecanol (HD) and palmitic acid (PA) mixture as the starting materials in nitrogen with the absence of any catalyst. The molecule structure, thermal property, thermal stability and reliability of as-prepared cetyl palmitate was studied by 1H nuclear magnetic resonance (1H NMR), Fourier transformation infrared (FT-IR) spectrometer, differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA), respectively. 1HNMR and FT-IR confirmed the as-prepared product under 350 °C for 30 min was CP with high purity. According to the DSC measurements, the onset melting temperature and latent heat of fusion for the as-prepared CP was 52.3 °C and 180.9 J/g, respectively. Results showed that the synthesized cetyl palmitate was favorable used as phase change material (PCM) owing to its desirable thermal properties, excellent thermal stability and reliability. Due to the low thermal conductivity of CP, it was impregnating into nickel (Ni) foams with different pore sizes to make CP/Ni foam composite PCMs (CPCMs). Compared with pure CP, thermal conductivity of CP/Ni foam (70, 90, and 110PPI) CPCMs were increased by 1.88, 2.02 and 4.86 times, respectively. The CP/Ni foam CPCMs displayed appropriate thermal properties for latent heat energy storage applications.
1. Introduction Phase change materials (PCMs) were a kind of materials that have high latent heat of fusion/solidification and can storing/releasing large amount of heat energy during their melting/freezing process, which played important role and can provide a way to match thermal energy supplying and utilization [1]. When considering various application areas, such as solar energy storage and transfer [2,3], building materials [4], waste heat recovery [5], electricity peak shaving and valley filling [6], and thermal management of spacecraft [7], PCMs should have appropriate phase transition temperature, high latent heat, excellent chemical stability, good thermal stability and reliability. According to the chemical composition, the PCMs can be classified into two major types as inorganics and organics. In those organic PCMs, paraffins and fatty acids attract extensive attentions of scientific researchers all over the world [8–13]. Compared to inorganic PCMs, fatty acids and paraffin have some advantages, such as low supercooling,
high latent heat, good chemical stability, nontoxicity and non-corrosiveness. Although fatty acids and paraffin have lots of advantages, sometimes it is still hard to be utilized in practical applications. Therefore, it is necessary to develop new types of PCMs. In the past few years, many efforts have been tried to explore new types of PCMs, which was including high-chain fatty acid esters [14–17]. Generally, alkanol-fatty acid esters were synthesized by Fischer esterification. On the one hand, the synthesis process was complicated and catalysis was needed [18]. On the other hand, owing to the slow reaction speed and the need to heat up the medium to evaporate the generated water, the high product yields are almost impossible to be reserved by Fischer esterification [19]. Therefore, developing easy and low cost procedure for high-chain fatty acid esters is necessary. Besides, the organic PCMs have low thermal conductivities, which lead to the lower energy storage/release rate and restrict their practical applications in real case. With the aim to increase the thermal
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Corresponding author at: Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China. E-mail address: [email protected] (C. Wang). 1 These authors contributed equally to this article. https://doi.org/10.1016/j.est.2019.101179 Received 8 October 2019; Received in revised form 17 December 2019; Accepted 23 December 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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2.3. 1H NMR analysis
conductivity of those organic PCMs, various nanofillers, such as carbon additives [20–23], metal nanoparticles [24–26], metal oxide nanoparticles [27–29], and metal foams [30–32], have been developed and added. Except those nanoadditives, metal foam (such as nickel foam) is one type of promising material because of its high porosity, large surface area, solid skeleton structure, strong mechanical strength, light weight, and excellent thermal conductivities, which brings it into practical applications in latent heat energy storage systems, solar energy collector, and battery thermal management and so on [33,34]. Owing to the interconnected path of metal foam and good compatibility with many organic PCMs, it allows the heat can transfer throughout the PCM uniformly and rapidly. In this study, cetyl palmitate (CP) was synthesized by a facile thermal method in nitrogen without any catalyst using 1-hexadecanol (HD) and palmitic acid (PA) binary mixture (abbreviated as HP) as the starting material. The molecular structures of the as-prepared products under different thermal treatment temperatures were studied by FT-IR and 1H NMR. The thermal performance, thermal stability and reliability of the synthesized CP were determined by DSC, TGA and repeated thermal cycling test, respectively. Results showed that the synthesized CP (with the onset melting temperature of 52.3 °C and the latent heat of 180.9 J/g) were favorable for using as long-term thermal energy storage material owing to its desirable thermal properties, excellent thermal stability and reliability. For the purpose of increasing the thermal conductivity of CP, it was impregnating into nickel foams with different pore sizes to make CP/Ni foam composite PCMs (CPCMs). Compared with pure CP, thermal conductivity of CP/Ni foam (70, 90, and 110PPI) CPCMs were increased by 1.88, 2.02 and 4.86 times, respectively. The CP/Ni foam CPCMs displayed an appropriate thermal performance for latent heat energy storage applications.
The 1H NMR analyses were performed on a 400 MHz nuclear magnetic resonance spectrometer (AVANCE III, BRUKER, Germany). The as-prepared ester samples were dissolved in CDCl3 and the 1H NMR spectra were attained by using tetramethylsilane (TMS) as an internal reference standard.
2.4. FT-IR measurements Fourier Transform infrared spectroscopy (FT-IR) was utilized to investigate molecular structure and chemical composition of as-prepared samples using an ATR (attenuated total reflection) spectrophotometer (Nicolet iS50, Thermo Scientific, USA) in the 4000–600 cm−1 spectral range with a resolution of 2 cm−1 (referred to a dry air background) using thermoelectrically cooled DTGS detector.
2.5. Preparation of the CP/Ni foam CPCMs CP/Ni foam CPCMs were prepared via melting infiltration method. Briefly, enough amount of as-prepared CP (solid state) was placed at the bottom of a glass bottle, and the nickel foam with different pore size (70, 90, and 110PPI) and circular shape (12.7 mm in diameter) was placed onto the solid CP. Thereafter, the bottle was immersed in a 60 °C water bath for 1 h until the melted CP was infiltrated into the nickel foam fully. Afterwards, the glass bottle was taken out from the water bath and cooled naturally to room temperature until all of the CP solidified thoroughly. Lastly, extra CP was got rid of via micro–thermal separation. The as-prepared three different types of samples made from nickel foam with different pore size (70, 90 and 110PPI) were labelled as CPCM1 (70PPI), CPCM2 (90PPI) and CPCM3 (110PPI) in the following text without further mentioned.
2. Experimental 2.1. Chemicals Palmitic acid (C16H32O2, PA) and 1-hexadecanol (C16H34O, HD) with analytical purity were purchased from Tianjin Zhiyuan Reagent Co., Ltd (Tianjin, China). The nickel foams with different pore sizes (70, 90 and 110PPI) were supplied by Kunshan Jiayisheng Electronics Co., Ltd (Kunshan, China). All of chemicals were used as received.
2.6. Thermal analysis The thermal properties (such as melting temperature, latent heat of melting or solidifying) of the binary HD-PA mixture and thermal treated samples were measured by a differential scanning calorimetry (DSC, DSC3+, METTLER TOLEDO). Briefly, about 5 mg samples was sealed in an aluminum pan and placed inside the DSC chamber with an empty aluminum pan as a reference under continuously purged nitrogen gas with a flow rate of 40 ml/min at heating rate of 5 °C/min from 20 to 100 °C. The thermal stability of the samples thermally treated at different temperatures were investigated by a thermogravimetric analyzer (TGA, DSC3+, METTLER TOLEDO) scanning from 30 to 600 °C at 10 °C/min heating rate under a constant nitrogen stream rate of 40 ml/min. The repeated cycling heating and cooling test was performed to study the thermal reliability of as-prepared CP. Briefly, certain amount of as-prepared CP was loaded into a plastic vial and put in a metal bath (CHB-T2-E, BIOERThermoQ) to run repeated heating and cooling cycles from 0–100 °C for 0–500 times.
2.2. Preparation of the binary HD-PA mixture and CP Binary HD-PA mixture was firstly prepared by melting blending method and used as the starting material to synthesize the CP. In a typical procedure, a certain amount of HD and PA with molar ratio of 1:1 was sealed in a glass bottle and heated by immersing in a 70 °C water bath and stirred at 500 rpm/min for 30 min. Then binary HD-PA mixture (abbreviate as HP) was obtained by cooling the mixture to room temperature. Instead of using the commonly known procedure of Fischer's esterification in the presence of acid catalyst, the presented cetyl palmitate (CP) was synthesized under inert gas in the absence of catalyst according to a modified procedure used by Baykut and Aydın [35]. Briefly, the as-prepared HP, which was sealed in a quartz crucible, was thermally treated at different temperatures in a pipe furnace under purging a constant stream of inert nitrogen. The thermal treated HP at 150, 200, 250, 300 and 350 °C for 30 min were denoted as HP150, HP200, HP250, HP300, and HP350, respectively. After 30 min, the temperature was cooled down naturally and the new formed products were taken out. The high purity ester was obtained after several crystallizations of the solution with acetone and ether. The reaction scheme for CP synthesis was shown in Scheme. 1.
2.7. Thermal conductivity measurements The thermal conductivities of the as-prepared CP and CP/Ni foam CPCMs were carried on a thermal conductivity meter (HotDisk TPS2200, Sweeden) using the transient plane source technique equipped with a 7577 probe (2.001 mm in radius), which had a maximum uncertainty of 4%. The samples were cut into circular cylinder with diameter of 12.7 mm and thickness of about 3 mm for the measurement. Each test was performed for three times. 2
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Fig. 1. FT-IR spectra of HP, HP150, HP200, HP250, HP300, and HP350 (A) and 1HNMR spectra of HP 350 (B).
3. Results and discussions
3.2. Thermal properties of HP and HP150-350
3.1. FT-IR and 1H NMR measurement
The thermal properties of HP and thermally treated HP samples were studied by DSC. Fig. 2A displayed the cycling DSC curves of HP, HP150, HP200, HP250, HP300 and HP350, respectively. For the binary mixture, it showed a downward endothermal peak (melting peak) with onset temperature at 44.8 °C and an upward exothermal peak (freezing peak). As the thermal treatment temperature (from 150 °C to 300 °C) increased, the melting peak moved to high temperature. For the DSC curve of HP350 (also the as-prepared CP), it showed the onset melting temperature and latent heat were 52.3 °C and 180.9 J/g, respectively. The thermal stability of as-prepared products was studied by TGA. Fig. 2B presented the thermogravimetric (TG) curves of HP and HP150350, respectively. From the TG curves of HP and HP150-HP250, it was seen that the decomposition process was divided into two stages. The first stage attributed to the decomposition of HP binary mixture and the second stage represented the decomposition of as-produced CP. For HP sample, the first and second weight loss stages occurred at 180–320 °C and 320–400 °C, respectively. For HP350 (also the as-prepared CP), the decomposition process was only showed one stage and the initial decomposition temperature (about 280 °C) was much higher than its melting temperature (55.3 °C), which indicated that CP had excellent thermal stability. For HP150-HP300, the temperature at the first maximum mass loss rate was almost the same. However, for the temperatures at the second stage, they were moved to higher position when the thermal treatment temperature increased, which was attributed to more and more CP produced at higher thermal treated temperature.
Fig. 1A showed the FT-IR spectra of HP, HP150, HP200, HP250, HP300, and HP350, respectively. For HP, strong absorption peaks at 2913, 2848 and 1704 cm−1 were belonging to the symmetrical stretching vibration of -CH3 and -CH2 group, and the -C ] O stretching vibration of PA, respectively. In addition, the absorption peaks at 1471 and 717 cm−1 were attributed to the bending vibration of -CH2 group and the in-planes winging vibration of –OH group of HD. Compared to the spectra of HP, all of the above-mentioned absorption peaks were also appearing in HP150-350 samples' spectra. However, when the thermal treatment temperatures increased from 150 °C to 350 °C, the absorption peak intensity of the as-prepared samples at 1704 cm−1 and 1059 cm−1 decreased and two new absorption peaks at 1732 cm−1 and 1182 cm−1, which were assigned for the -C ] O stretching vibration of acyclic saturated esters, were appearing and the intensity increased gradually. Meanwhile, the absorption peaks of –OH stretching vibration of PA and HD located at 2500–2700 cm−1 and 3230–3550 cm−1were not observed for HP350, which suggested high ester rate after the thermal treatment. The 1H NMR characteristic peaks of HP350 were shown in Fig. 1B, and no peaks of –OH groups at about 2.24 ppm for HD, and 9.78 ppm for PA were observed [36]. Besides, the peak attributed to the protons of –COO–CH2– group appearing at 4.07 ppm for HP350. According to the peak integration, the purity of the product (HP350) was larger than 90%. Those results indicated that the cetyl palmitate (CP) was successfully synthesized starting with binary HD-PA mixture (abbreviate as HP), which was treated under 350°C for 30 min without using any catalyst.
3.3. Thermal reliability of HP350 (also as the as-prepared CP) The thermal reliability of the as-prepared CP after experiencing repeated heating and cooling cycles was assessed by DSC. Fig. 3A depicted the cycling DSC curves of the as-prepared CP after experiencing 0, 100, and 500 heating and cooling cycles in a metal bath from 0–100
Fig. 2. DSC (A) and TG (B) curves of HP, HP150, HP200, HP250, HP300, and HP350. 3
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Fig. 3. DSC curves (A) and FT-IR spectra (B) of HP350 after running 0, 100, and 500 repeated heating and cooling cycles with heating rate of 5 °C/min from 20–100 °C. Table 1 DSC data of CP after running 0, 100, and 500 times. Name Melting
Solidifying
Ton(°C) Tm(°C) Te(°C) ΔHf(J/g) Ton(°C) Tm(°C) Te(°C) ΔHs(J/g)
0
100
500
53.5 55.2 57.2 178.3 50.0 49.3 45.5 171.9
53.6 55.3 57.1 177.6 50.3 49.1 45.1 174.9
52.5 54.9 56.8 177.1 50.1 48.9 45.8 171.6
°C. The onset melting temperature (Ton), melting temperature (Tm), end melting temperatures (Te), latent heat of fusion (ΔHf) and solidifying (ΔHs) determined from those DSC measurements were collected and listed in Table 1. For the as-prepared CP, the onset melting and solidifying temperatures maintained nearly the same even experienced 100, and 500 heating and cooling cycles. Compared to synthesized pristine CP, the latent heat of fusion and solidifying changed by −0.4, −0.7% and 1.7, −0.2% after experiencing 100 and 500 reduplicative thermal cycles, which were in negligible level. Moreover, the molecular structural stability of CP after thermal cycles was also investigated by FT-IR technique and the results were presented in Fig. 3B. It was found that the characteristic absorption peaks of CP were in consistent well with that going through 100 and 500 thermal cycles, which implied that CP had fabulous thermal stability. Those results suggested that facile synthesized CP was a prospective PCM for long term latent heat energy storage applications. 3.4. Microstructure analysis and impregnation ratios The optical images of nickel foams and CP/Ni foam CPCMs after impregnation of CP were presented in Fig. 4A-F, respectively. Fig. 4A, C and E displayed the morphologies of the pure nickel foams with pore size at 70, 90 and 110PPI and Fig. 4B, D and F showed the optical images of the CP/Ni foam (with pore size at 70, 90 and 110PPI) CPCMs after impregnated with CP. As shown in Fig. 4B, D and F, the CP were trapped well in nickel foams. Since the high porosity of nickel foam, the ratio of CP impregnated into the nickel foam can be determined from the following equation:
m′ − m m′ − m = α= vth ρPCM εvtotal ρPCM
Fig. 4. Optical images of nickel foams with different pore sizes before (A: 70PPI; C: 90PPI; E: 110PPI) and after (B: 70PPI; D: 90PPI; F: 110PPI) impregnated with CP.
given from the supplier. According the calculation from Eq. (1), the impregnation ratios for different CP/Ni foam composites were calculated and presented in Fig. 5. It was shown that the mass fraction of CP impregnated in 70, 90 and 110PPI nickel foams were 83.86, 82.85 and 76.62%, respectively. The reason for this was that the outward of the composites solidified quicker than the inward during the cooling process, which caused a decrease in the inner volume of the composite and led to the impregnation ratios less than 100%.
(1)
In the above Eq. (1), α was impregnation ratio, m and m’ were the mass of metal foam and metal foam composite after impregnating with the PCM. vtotal and vth represented whole volume of the composite and practical volume of the PCM, respectively. ε was the porosity of metal foam, ρPCM was the density of PCM. In our experiment, the density (ρ) of CP was 0.858 g/cm3 and the porosity (ε) of the nickel foam was 98% 4
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Table 2 TGA and DTG data of the as-prepared CP and CP/Ni foam composites. Sample Property
CP
CPCM1
CPCM2
CPCM3
T-5% Tmax Residual at 600 °C
281.2 395.8 −0.23
281.2 354.7 22.77
286.3 364.9 18.52
302.3 370.3 31.79
Fig. 5. Mass impregnation ratio of CP/Ni foam CPCMs made from different pore size at 70, 90, and 110PPI, respectively.
3.5. Thermal stability of CP/Ni foam CPCMs The thermal stability of the as-prepared CP/Ni foam CPCMs (CPCM1, CPCM2 and CPCM3) were tested by TG and DTG. Fig. 6A and B showed the TG and DTG curves of CP, CPCM1, CPCM2 and CPCM3, respectively. The temperature at 5wt% weight loss (T-5wt%), the maximum weight loss rate (Tmax), and the charred residue at 600 °C were recorded and listed in Table 2. From the TG and DTG curves of CP and CP/Ni foam composites, it was seen that the decomposition process only showed one weight loss stage in the range of 230–420 °C, which was far above the melting temperature of the as-prepared CP (55.3 °C). This property was a precondition for PCM to be used for latent heat energy storage. As shown in Fig. 6B and Table 2, the Tmax for CP and CP/Ni foam composites was 395.8 °C (CP), 354.7 °C (CPCM1), 364.9 °C (CPCM2), and 370.3 °C (CPCM3), which were ascribed to the decomposition of CP. In addition, the residue at 600 °C was close to the actual content of the nickel foam in the composites, which implied that the CP and nickel foam had good compatibility.
Fig. 7. Thermal conductivities of the as-prepared CP and CP/Ni foam composites.
Scheme 1. The esterification reaction of the palmitic acid and 1-hexadecanol. Table 3 Thermal conductivity, density, metal/PCM ratio of CP and CP/Ni foam (with different pore size) composites PCMs.
3.6. Thermal conductivity analysis The effect of nickel foam with different pore sizes (70, 90 and 110PPI) on the thermal conductivity of CP/Ni foam CPCMs were investigated by using a thermal conductivity meter. The measured thermal conductivities of the as-prepared CP and CP/Ni foam CPCMs were displayed in Fig. 7. The thermal conductivity, density, metal/PCM ratio of CP and CP/Ni foam (with different pore size) CPCMs were collected and presented in Table 3. Compared to pure CP (0.3432 W/ m•K), the thermal conductivity of CPCM1, CPCM2 and CPCM3 were measured as 0.6465, 0.6942, and 1.6687 W/m•K, which enhanced by 1.88, 2.02 and 4.86 times, respectively. From Fig. 7 and Table 3, it implied that larger pore size of the nickel foam would result in lower the thermal conductivity of the composites. The reason for this was because that the bigger pore size of nickel foam, the smaller interface
Samples
Pore size (PPI)
Density (g/ cm3)
Metal/CP ratio
Thermal conductivity (W/m•K)
CP CPCM1 CPCM2 CPCM3
/ 70 90 110
0.858 0.8599 0.8609 0.8963
1 0.2621 0.2357 0.3912
0.3432 0.6465 0.6942 1.6687
contact area, which resulted in the decrease of thermal conductivity of the composites. In addition, as the density increased, the thermal conductivity of CP/Ni foam composites increased since the heat exchange was enhanced per volume unit. Table 4 collected and listed thermal conductivities of some typical
Fig. 6. TGA (A) and DTG (B) curves of the as-prepared CP and CP/Ni foam composites. 5
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Supplementary materials
Table 4 Comparison of the thermal conductivity of composite PCMs with published literature. Samples
Thermal conductivity (W/m•K)
References
PEG (90 wt%)/EG Paraffin+Al foam Paraffin+Cu foam MA+Ni foam CP+Ni foam
1.324 0.4 7.6 0.4768 1.6687
[37] [38] [39] [40] This work
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composite PCMs with other additives (such as carbon materials and metal foam) from published literatures. In comparison with other fillers, metal foam had excellent thermal conductivity enhancement and showed promising potential in the practical applications. 4. Conclusions In this study, a high-chain fatty acid ester, cetyl palmitate (CP), was synthesized by a facile one-step thermal method in the absence of any catalyst using binary HD-PA mixture with molar ratio of 1:1. The chemical structure, thermal stability and reliability of as-prepared CP were studied by 1H NMR, FT-IR, DSC and TGA, respectively. 1H NMR and FTIR confirmed the as-prepared HP350 was CP with high purity. According to the DSC measurement, the onset melting temperature and latent heat of fusion for the as-prepared CP was determined as 52.3 °C and 180.9 J/g, respectively. Results showed the synthesized CP was favorable used as long-term energy storage materials owing to its desirable thermal properties, excellent thermal stability and reliability. With the aim to increase the thermal conductivity of the CP, it was impregnating into nickel foams with different pore sizes (70, 90 and 110PPI) to make CP/Ni foam CPCMs. TG results displayed that the CP/ Ni foam composites retained good stability and good compatibility between CP and Ni foam. Moreover, in comparison with pure CP, thermal conductivity of CP/Ni foam (70, 90, and 110PPI) composites were increased by 1.88, 2.02 and 4.86 times respectively. The CP/Ni foam CPCMs exhibited appropriate thermal behavior for latent heat energy storage applications. Data availability All of the data were available. CRediT authorship contribution statement Chaoming Wang: Conceptualization, Methodology, Resources, Supervision, Writing - original draft, Writing - review & editing, Funding acquisition. Tingjun Wang: Investigation, Data curation, Methodology, Software, Visualization. Zhanjiang Hu: Investigation, Data curation, Methodology, Software, Visualization. Zhengyu Cai: Investigation, Data curation, Validation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51672227), Fundamental Research Funds for the Central Universities (Grant No. 2682017CY08 and 2682017CX089), and Engineering Research Center of Clinical Functional Materials and Diagnosis & Treatment Devices of Zhejiang Province (Grant No: WIUCASK19004). 6
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