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A novel low-temperature phase change material based on stearic acid and hexanamide eutectic mixture for thermal energy storage
Chemical Physics Letters 714 (2019) 166–171
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
A novel low-temperature phase change material based on stearic acid and hexanamide eutectic mixture for thermal energy storage ⁎
Guixiang Maa,b,c, Jinhe Suna,b, , Yue Zhanga,b,c, Yan Jinga,b, Yongzhong Jiaa,b,
T
⁎
a Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China b Qinghai Engineering and Technology Research Center of Comprehensive Utilization of Salt Lake Resources, 810008 Xining, China c University of Chinese Academy of Sciences, 100049 Beijing, China
H I GH L IG H T S
diagram and Tammann plot of stearic acid and hexanamide binary system were developed. • Phase stearic acid and hexanamide eutectic point located at χ = 0.462 and melted at 331.75 K with latent heat of 176.62 J·g • The • The stearic acid/hexanamide eutectic mixture had good thermal stability and reliability. SA
−1
.
A R T I C LE I N FO
A B S T R A C T
Keywords: Stearic acid Hexanamide Eutectic mixture Phase change materials Thermal energy storage
The stearic acid (SA) – hexanamide (HA) binary mixtures were prepared, and the phase diagram and Tammann plot were constructed. The differential scanning calorimetry results indicated that a eutectic composition formed by 46.2 mol% SA and 53.8 mol% HA melted at the temperature of 331.15 K with a latent heat of 176.62 J·g−1. Fourier transform infrared spectroscopy and X-ray diffraction results revealed that SA and HA were just physical mixture. The results of thermogravimetric analysis and 100 thermal cycles test respectively manifested that eutectic mixture had good thermal stability and reliability. Therefore, eutectic mixture is promising for thermal energy storage application.
1. Introduction Thermal energy storage systems (TESS) with phase change materials (PCMs) as a cost-effective and relatively matured energy technologies, which has gained great worldwide attention since it can make energy sources use more efficiency and sustainable. TESS includes sensible heat storage, latent heat storage and chemical heat storage. By comparison, latent heat storage is regarded as the most promising energy storage technology at present, since it has many advantages such as large energy storage density, small temperature span during heat storage/retrieval, extensive temperature range for different application and relatively low cost [1–6]. In recent years, eutectic PCM get more and more attention due to its temperature adjustability. And eutectic PCM is obtained by mixing two or more phase change materials with different melting points and its temperature lower than that of any of the materials in the mixture. Fatty acids such as caprylic acid, capric acid (CA), lauric acid (LA),
myristic acid (MA), palmitic acid (PA), steatric acid (SA), oleic acid (OA) and their eutectic mixtures are particularly promising candidates for use in thermal energy storage applications owing to their melting congruency, good thermal and chemical stability, low toxicity, no phase separation, low corrosion, small volume change during phase transition, high latent heat of fusion and suitable melting temperature range [7–13]. As one of the widely studied fatty acids PCMs, stearic acid displays a suitable phase change temperature range of 330.88–343.98 K and relatively high latent heat of 180.79–210.00 J·g−1 for low temperature thermal energy storage. What’s more, stearic acid based eutectic PCMs also got much attention for thermal energy storage. Baran et al. [14], Sari et al. [15] and Ding [16] et al. prepared SA and other fatty acids three binary mixtures, i.e., SA-PA, SA-CA, and SA-LA, with melting temperatures of 325.45 K, 299.19 K, and 307.31 K, respectively; these melting temperatures are less than that of each fatty acid. Wu [17] and François [18] et al. explored two binary mixtures of the SA and alkanols (hexadecanol and stearyl alcohol). The phase change
⁎ Corresponding authors at: Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China. E-mail addresses: [email protected] (J. Sun), [email protected] (Y. Jia).
https://doi.org/10.1016/j.cplett.2018.11.003 Received 22 September 2018; Received in revised form 4 November 2018; Accepted 5 November 2018 Available online 07 November 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.
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temperature and enthalpy of fusion of the SA-hexadecanol and SAstearyl alcohol of eutectic mixtures were 318.98 K and 259.4 J·g−1, and 322.45 K and 202.2 J·g−1, respectively. Su [19] et al studied SA and noctadecane binary mixture, the results indicated that the SA were found to form a eutectic mixture at SA to n-octadecane mass ratio of 2: 98, which melts at around 300.96 K with latent heat of 223.68 J·g−1. However, the availability of potential materials with essential properties that can be used for solar energy application is very few in number. In our previous work, SA/adipic acid, SA/suberic acid and SA/ sebacic acid were investigated [20]. Eutectic mixtures with melting point and latent heat of (340.60 K, 200.30 J·g−1), (339.52 K, 191.38 J·g−1) and (340.23 K, 196.63 J·g−1) were identified respectively in SA/adipic acid, SA/suberic acid and SA/sebacic acid systems. Besides, the eutectic mixture displayed good cyclic, thermal, and chemical stability compared to its individual components. Moreover, we also explored four different SA-amides mixtures, SA-acetamide [21], SA-nbutyramide, SA-n-octanamide [22], and SA-acetanilide [23], respectively, and calculated their eutectic ratios by using the Schrader equation. The above studies show that SA can form eutectic mixtures with fatty acids, alkanols, alkanes, dicarboxylic acids and amides, and these SA-based binary mixtures has relatively high latent heat, good thermal stability and chemical stability, and SA-based binary mixtures have great potential as PCMs for the thermal energy storage. Besides, these researches show that amide compounds are attractive PCMs that can be used as latent thermal energy storage materials in solar heating application. Thus, in this work, the well-investigated PCMs of stearic acid and hexanamide were chosed as raw materials and their binary mixture was prepared. Then, the thermal properties and thermal reliability of two binary mixtures were identified by using differential scanning calorimeter (DSC), thermal conductivity analyzer (TC), X-ray diffractometer (XRD), fourier transform infrared (FT-IR) spectrophotometer, thermogravimeter (TGA) and 100 accelerate thermal cycle test, and explored their LHTES potential in low temperature.
2.3. Characterization
2. Experimental
ΔHi ⎞ ⎛ Tm − Ti ⎞ ⎤ χi γi = exp ⎡ ⎛ × ⎢ ⎝ RTi ⎠ ⎝ Tm ⎠ ⎥ ⎦ ⎣
The thermal properties and thermal cycle stability of the samples were determined by using a Mettler Toledo DSC2. All experiments were carried out under nitrogen with the flow rate of 100 mL. The heating and cooling rates of 2 K·min−1 and 10 K·min−1 were employed for the DSC experiment to entabilished the phase diagram of binary mixture and 100 accelerated thermal cycle test, respectively. The instrument was calibrated by using indium standard. The phase change enthalpy and temperature measurements have accuracy within ± 5% and ± 1.0 K, respectively. The melting points and heats of fusion were obtained from the second heating cycle of three independent DSC experiments. The thermal conductivity of samples at room temperature (293.15 K) was tested according to transient hot wire method by using a thermal conductivity analyzer (TC 3000E, Xiatech Electronic Technology Co., Ltd., China). Before the measurement, the cylindrical samples were ground and pressed with a molding under 15 MPa to obtain discs (diameter of 20 mm). The chemical structure stability of samples was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. XRD experiments were carried out on PANalytical X’PRO Pert diffractometer fitted with Cu Kα radiation with a powder setting of 40 kV and 30 mA. FT-IR experiments were performed on on a FT-IR spectrometer (Nicolet Nexus 670, Thermo Nicolet Corporation, USA) and scanned from (4000 to 400) cm−1 using KBr pellet. Thermal stability of the samples was determined by using thermogravimetric analysis (TGA) on a thermogravimetric analyzer STA 449F3 (NETZSCH, Germany) with a heating rate of 5 K·min−1 from room temperature to 900 K under nitrogen atmosphere. 3. Thermodynamic modeling The liquidus temperature of phase diagram of SA-HA binary mixture at different composition can be calculated by the following equations [24]: ⎜
2.1. Materials
⎟
⎜
⎟
(1)
Here, χi, γi, Tm, Ti, ΔHi, R represent the mole fraction of substance i in the liquid phase, activity coefficient of substance i in the liquid phase, the melting temperature of the system (K), the temperature of substance i (K), the molar enthalpy of fusion for substance i (J·mol−1), and idea gas constant, respectively. The binary system is approximated as an ideal solution, γi is equal to unity.
Stearic acid (SA, CAS number: 57–11-4, 98% for purity) was provided by Sinopharm Chemical Reagent Co., Ltd and hexanamide (HA, CAS number: 628-02-4, 98% for purity) was obtained from Tokyo Chemical Industry Co., Ltd (Tokyo, Japan). All reagents were used as received without further purification. Moreover, the thermophysical properties of SA and HA are listed in Table 1.
4. Results and discussion 2.2. Preparation of the binary mixtures 4.1. Thermophysical properties of SA, HA, and their mixtures The binary mixtures with different molar fraction of SA were prepared as following: SA and HA was mixed and stirred for 15 min in a round-bottomed flask which was immersed in silicone oil bath with heat temperature of 353.15 K. And then the obtained melted liquid mixtures were slowly cooled to room temperature.
4.1.1. The SA/HA binary mixtures A series of DSC melting curves of the SA/HA binary mixture with different molar fraction of SA is shown in Fig. 1, and corresponding data are summarized in Table 2. As can be seen in Fig. 1, for SA, there is only one endothermic peak observed. The onset temperature of melting is 342.62 K. The latent heat of fusion is 212.22 J·g−1. For HA, two endothermic peaks are displayed on the melting curve. And the first endothermic peak at lower temperature is referred to solid-solid phase transition of HA, the second endothermic peak at higher temperature corresponds to solid-liquid phase transition of HA. Thus, the onset temperature of melting of the solid-solid phase transition is 308.17 K, and the corresponding latent heat of melting of solid-solid phase transition is 33.10 J·g−1. On the other hand, the onset temperature of the solid-liquid phase transition is 373.44 K. The latent heat of solid-liquid phase transition is 130.15 J·g−1. The results indicates that the experimental data is close to the literature value [25]. For SA-HA binary mixture, the DSC thermograms presents multiple peaks, the first peak
Table 1 Chemical data and thermal properties for the individual PCM (Ttrs, the temperature of solid-solid phase transition; Tm, the temperature of solid-liquid phase transition; ΔHtrs, the enthalpy of solid-solid phase transition; ΔHm, the enthalpy of solid- liquid phase transition). Name
Molecular Formula
Ttrsa (K)
ΔtrsHmb (J·g−1)
Tma (K)
ΔHmb (J·g−1)
Stearic acid Hexanamide
C18H36O2 C6H13NO
n.a 308.17
n.a 33.10
342.62 373.44
212.22 130.15
a b
Standard uncertainty, u(T) = 1.00 K. Relative standard uncertainty, ur(ΔH) = 5%. 167
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Fig. 1. DSC melting curves of the SA, BA and their mixture as a function of the molar fraction of SA (where χSA represents the mole fraction of SA in binary mixture).
occurs at lower temperature corresponds to solid-solid phase transition of HA. The second peak at slightly higher temperature at fixed temperature corresponds to the eutectic transition of binary system, and it can be used to construct the solidus line of phase diagram. However, the third peak at highest temperature gradually varies corresponds to the solution of the remaining solid in the melt, and it can be used to determine the liquidus line of phase diagram [26]. Besides, it can be found that only two endothermic peaks are observed in χSA equals to 0.429, 0.451, 0.462, and 0.486.
Table 2 DSC results of SA, HA, and SA-HA binary mixture in melting process (χSA, the mole fraction of SA in binary mixture; Ttrs, the temperature of solid-solid phase transition; TE, the eutectic temperature; ΔHE, the enthalpy of the eutectic reaction; TL, the liquidus temperature; ΔHm, the total enthalpy of melting), at pressure 77.0 kPa.a χSAb
Ttrsc (K)
ΔtrsHmd (J·g−1)
0.000 0.043 0.092 0.148 0.213 0.249 0.288 0.331 0.378 0.398 0.419 0.429 0.451 0.462 0.486 0.510 0.548 0.618 0.696 0.785 0.843 0.907 1.000
309.55 309.60 309.48 309.52 309.50 309.74 309.75 309.64 309.75 310.09 310.19 309.91 310.15 310.25 310.05 309.83 309.75 309.87 309.77 309.85 310.14 309.90
33.10 31.95 27.90 23.83 18.48 15.85 13.56 13.73 11.21 11.74 10.82 9.28 10.57 9.85 8.07 9.21 6.93 4.98 4.17 1.68 1.86 0.99
TEc (K)
ΔEHmd (J·g−1)
330.42 330.56 330.67 330.68 331.07 331.05 330.82 330.98 331.21 331.37 330.82 330.97 331.15 330.92 331.14 330.81 330.86 330.56 330.31 330.37 329.59
26.34 49.76 76.29 98.51 104.43 128.14 136.46 151.71 158.00 164.15 172.92 176.27 176.62 172.46 162.45 158.27 118.87 88.67 53.37 39.31 23.62
TLc (K)
ΔHmd (J·g−1)
374.75 371.51 368.10 364.10 359.32 356.06 352.37 347.66 343.06 340.15 337.70
130.15 165.91 162.85 154.06 149.63 149.37 170.35 161.77 173.02 179.82 182.10 182.20 186.84 186.47 180.53 187.10 190.39 194.53 193.72 194.06 203.81 211.12 212.22
332.16 333.76 336.31 338.11 340.04 341.24 342.05 343.52
4.1.2. Phase diagram and Tammann plot of the SA/HA binary system As mentioned above, previous studies of this binary system gave a rather wide range for the eutectic composition. Therefore, we need to construct the phase diagram (Fig. 2a) and Tammann plot (Fig. 2b) of the binary system to accurately determine the composition of the eutectic mixture. The phase diagram is obtained by plotting the peak temperature of the endotherms of binary mixture with different molar fraction of SA ranging from 0 to 1. Seen from Fig. 2a, the binary system is the simple eutectic type, the melting points of binary mixtures decrease firstly and then increase with the increase of molar fraction of SA, the temperature of lowest point of the binary mixture is 331.05 K, and corresponding eutectic composition is χSA = 0.462. According to the Schroder equation, the predictive eutectic composition is χSA = 0.463, and corresponding eutectic temperature is 331.62 K. Comparing the experimental and theoretical data for the studied mixtures in Fig. 2a, it can be found that experimental values are in good agreement with the theoretical predictive values. In order to obtain the Tammann plot, the melting enthalpy of eutectic transition of the DSC thermograms as a function of composition was evaluated. The results are shown in Fig. 2b. As can be observed, the melting eutectic enthalpies form two ascending lines that start from the extremes of the composition values and extend until they reach a
a Experimental pressure was not controlled beyond the typical range of atmospheric pressure, (77.0 ± 1.0) kPa. b SA mole fraction with standard uncertainty, u = 0.005. c Standard uncertainty, u(T) = 1.00 K. d Relative standard uncertainty, ur(ΔH) = 5%.
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Fig. 2. (a) Phase diagram of the SA-HA binary mixture with various SA molar ratio (dash line ┄: the predicted results obtained; black line —: the experimental results obtained) and (b) Tammann plot (the variation of enthalpy of the binary mixture as a function of χSA) for SA-HA binary system. (χSA, the molar fraction of SA in binary mixture).
Fig. 3. (a) XRD patterns and (b) FT-IR spectras of SA, HA and SA-HA eutectic mixture.
results indicate that theoretical calculated value is in well consistent with the experimental measured value.
maximum at a certain composition (χSA = 0.462), and the maximum melting eutectic enthalpy is 176.62 J·g−1. Therefore, according to the phase diagram and the Tammann plot, the eutectic composition and enthalpy of fusion at the eutectic point of the SA-HA binary system are accurately determined: χSA = 0.462, and corresponding eutectic temperature and latent heat of fusion are 331.15 K and 176.62 J·g−1.
4.2. Chemical structure analysis 4.2.1. XRD analysis The XRD patterns of SA, HA and the SA-HA eutectic mixture are presented in Fig. 3a. As Fig. 3a shows that SA displays two intense diffraction peak at 2θ = 6.64° and 2θ = 21.56° and a mild peak at 2θ = 24.18°, and HA indicates one intense diffraction peaks at 2θ = 6.35° and two mild peaks at 2θ = 12.74° and 2θ = 25.65°. Although the diffraction intensities is changed due to the varied contents, all the diffraction peaks of SA and HA could be found on the XRD pattern of the eutectic mixture, indicating that the eutectic mixture is a physical mixture of SA and HA and their crystal structures remained.
4.1.3. Thermal conductivity Thermal conductivity plays a very important role in thermal energy storage application. Since the higher the thermal conductivity, the faster the heat transfer rate. And the thermal conductivities of SA-HA binary mixtures were determined by transient hot wire method at solid state (293.15 K). The thermal conductivities of SA, HA and SA-HA eutectic mixture were measured as 0.3362 W·m−1·K−1, −1 −1 −1 −1 0.2456 W·m ·K , and 0.2728 W·m ·K , respectively. On the other hand, the thermal conductivity of SA-HA eutectic mixture can be calculated by using the equation κSA-HA = χSA·κSA + (1 − χSA) ·κHA, where χSA is the molar fraction of SA in the eutectic composition, κSA-HA, κSA and κHA are the thermal conductivity of SA-HA eutectic mixture, pure SA and pure HA, respectively. Besides, in this work, χSA equal to 0.462. Therefore, the calculated thermal conductivity of SA-HA eutectic mixture is 0.2874 W·m−1·K−1, which is only 0.0146 W·m−1 ·K−1 higher than experimental measured value. Based on the above analysis, the
4.2.2. FTIR analysis The FT-IR spectra of SA, HA and the eutectic mixture are displayed in Fig. 3b. As can be seen from the SA-HA eutectic mixture FT-IR spectrum, the peaks at 3362.02 cm−1 and 3197.38 cm−1 correspond to the stretching vibration of eNH2 group. The peaks at 2955.64 cm−1 represents asymmetric stretching vibration of eCH3 group. The peaks at 2916.83 cm−1 and 2848.90 cm−1 are attributed to asymmetric and symmetric stretching vibration of eCH2 group, respectively. The broad peak observed at 3000–2600 cm−1 is resulted from OeH stretching 169
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Fig. 4. Thermogravimetric analysis of SA, HA and SA-HA eutectic mixture. Fig. 5. DSC curves of SA-HA eutectic mixture with heating and cooling rate of 10 K·min−1 before and after 100 thermal cycles.
vibration, which usually overlaps with the absorption band of aliphatic CeH stretching vibration in eCH3 and eCH2 group. The C]O stretching vibration peak is observed at 1703.23 cm−1 and 1660.79 cm−1 in O]CeOH and O]CeNH2 group. The peaks at 1633.13 cm−1 represents the rocking vibration of eNH2 group. The peak at 1469.45 cm−1 is the bending vibration of CeH in eCH3 group. The peak at 1426.46 cm−1 represents stretching vibration of C]O in O]CeOH. The peak at 1295.98 cm−1 is the CeH rocking and twisting vibration of eCH2 groups from SA. The peak at 942.98 cm−1 represents OeH out-of-plane bending vibration of O]CeOH. The peak at 718.96 cm−1 represents the eCH2 rocking vibration. Compared with the FT-IR spectra of SA and HA, we can find that the characteristic peaks of SA and HA all display in that of the eutectic mixture. The results indicates that there is no chemical interaction except strong hydrogen bond exists between SA and HA.
Table 3 Phase change temperature and latent heat of tested eutectic PCM after 100 thermal cycles (Ttrs, onset temperature of solid–solid phase transition; Tm, onset temperature of melting; ΔHm, the total latent heat of melting; Tf, onset temperature of freezing; ΔHf, the total latent heat of freezing), at pressure 77.0 kPa.a Number of cycles
0 20 40 60 80 100
4.3. Thermal stability and reliability of SA-HA binary mixture
Melting process
Freezing process
Ttrsb (K)
Tmb (K)
ΔHmc (J·g−1)
Ttrsb (K)
Tfb (K)
ΔHfc (J·g−1)
308.31 307.98 307.90 308.01 308.07 308.06
329.37 329.29 329.29 329.31 329.30 329.32
167.54 166.38 165.78 165.23 164.50 164.21
310.74 310.80 310.90 310.92 310.89 310.92
324.88 324.30 324.47 324.54 325.06 325.28
164.26 161.98 161.04 159.96 160.26 159.68
a Experimental pressure was not controlled beyond the typical range of atmospheric pressure, (77.0 ± 1.0) kPa. b Standard uncertainty, u(T) = 1.00 K. c Relative standard uncertainty, ur(ΔH) = 5%.
4.3.1. Thermal stability The TG curves of SA, HA, and the eutectic mixture are exhibited in Fig. 4. As can be seen from Fig. 4, SA shows a two-step weight loss that started at 397.65 K, followed at 505.55 K and finished at 547.35 K, corresponding to the decomposition of SA. HA shows a one-step mass loss that started at 413.55 K and terminated at 476.85 K, representing the decomposition of HA. The TG curve of the eutectic mixture also shows a two-step mass loss feature that is similar to SA. The first step of mass loss begins at approximately 421.35 K, followed by the second step of mass loss begins at approximately 493.15 K. Besides, the initial mass loss temperature of eutectic mixture is higher than both SA and HA. Which indicates that the eutectic mixture has better thermal stability in working temperature range than pure PCMs for it to be applied in thermal energy storage.
and latent heat are acceptable in thermal energy storage application. The results indicates that the eutectic mixture has good stability during long-term application as a PCM.
5. Conclusion In this study, a series of SA/HA binary mixtures were prepared and determined by DSC. The phase diagram and Tammann plot of the binary system were established, and corresponding the eutectic point of the eutectic mixture was 331.15 K and the latent heat of melting of the eutectic mixture was 176.62 J·g−1 when the molar fraction of SA was 0.462. The thermal conductivity of the eutectic mixture was 0.2728 W·m−1·K−1. Moreover, the eutectic mixture had good thermal stability and reliability. Accordingly, the SA-HA eutectic mixture has great potential as PCM for solar energy storage.
4.3.2. Thermal reliability The long-time stability of the SA-HA eutectic mixture was determined by 100 accelerated thermal cycles from 298.15 K to 373.15 K. The melting and freezing curves of the 1st, 20th, 40th, 60th, 80th and 100th cycles are shown in Fig. 5, and corresponding phase change temperature and latent heats are summarized in Table 3. As shown in Fig. 5, the freezing peak becomes wider with the increases number of thermal cycles. However, the melting peaks show slight changes in shape or magnitude. After 100 cycles, it can be found from Table 3 that the phase change temperature of melting and freezing of the eutectic mixture change −0.05 K and 0.4 K respectively. And the melting and freezing latent heats of the eutectic mixture decrease by 1.99% and 2.79% respectively. The small changes in phase change temperature
Acknowledgements This work was supported by the National Natural Science Foundation of China (U1407205), and Youth Innovation Promotion Association of Chinese Academy of Sciences (2015351).
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
A novel low-temperature phase change material based on stearic acid and hexanamide eutectic mixture for thermal energy storage ⁎
Guixiang Maa,b,c, Jinhe Suna,b, , Yue Zhanga,b,c, Yan Jinga,b, Yongzhong Jiaa,b,
T
⁎
a Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China b Qinghai Engineering and Technology Research Center of Comprehensive Utilization of Salt Lake Resources, 810008 Xining, China c University of Chinese Academy of Sciences, 100049 Beijing, China
H I GH L IG H T S
diagram and Tammann plot of stearic acid and hexanamide binary system were developed. • Phase stearic acid and hexanamide eutectic point located at χ = 0.462 and melted at 331.75 K with latent heat of 176.62 J·g • The • The stearic acid/hexanamide eutectic mixture had good thermal stability and reliability. SA
−1
.
A R T I C LE I N FO
A B S T R A C T
Keywords: Stearic acid Hexanamide Eutectic mixture Phase change materials Thermal energy storage
The stearic acid (SA) – hexanamide (HA) binary mixtures were prepared, and the phase diagram and Tammann plot were constructed. The differential scanning calorimetry results indicated that a eutectic composition formed by 46.2 mol% SA and 53.8 mol% HA melted at the temperature of 331.15 K with a latent heat of 176.62 J·g−1. Fourier transform infrared spectroscopy and X-ray diffraction results revealed that SA and HA were just physical mixture. The results of thermogravimetric analysis and 100 thermal cycles test respectively manifested that eutectic mixture had good thermal stability and reliability. Therefore, eutectic mixture is promising for thermal energy storage application.
1. Introduction Thermal energy storage systems (TESS) with phase change materials (PCMs) as a cost-effective and relatively matured energy technologies, which has gained great worldwide attention since it can make energy sources use more efficiency and sustainable. TESS includes sensible heat storage, latent heat storage and chemical heat storage. By comparison, latent heat storage is regarded as the most promising energy storage technology at present, since it has many advantages such as large energy storage density, small temperature span during heat storage/retrieval, extensive temperature range for different application and relatively low cost [1–6]. In recent years, eutectic PCM get more and more attention due to its temperature adjustability. And eutectic PCM is obtained by mixing two or more phase change materials with different melting points and its temperature lower than that of any of the materials in the mixture. Fatty acids such as caprylic acid, capric acid (CA), lauric acid (LA),
myristic acid (MA), palmitic acid (PA), steatric acid (SA), oleic acid (OA) and their eutectic mixtures are particularly promising candidates for use in thermal energy storage applications owing to their melting congruency, good thermal and chemical stability, low toxicity, no phase separation, low corrosion, small volume change during phase transition, high latent heat of fusion and suitable melting temperature range [7–13]. As one of the widely studied fatty acids PCMs, stearic acid displays a suitable phase change temperature range of 330.88–343.98 K and relatively high latent heat of 180.79–210.00 J·g−1 for low temperature thermal energy storage. What’s more, stearic acid based eutectic PCMs also got much attention for thermal energy storage. Baran et al. [14], Sari et al. [15] and Ding [16] et al. prepared SA and other fatty acids three binary mixtures, i.e., SA-PA, SA-CA, and SA-LA, with melting temperatures of 325.45 K, 299.19 K, and 307.31 K, respectively; these melting temperatures are less than that of each fatty acid. Wu [17] and François [18] et al. explored two binary mixtures of the SA and alkanols (hexadecanol and stearyl alcohol). The phase change
⁎ Corresponding authors at: Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China. E-mail addresses: [email protected] (J. Sun), [email protected] (Y. Jia).
https://doi.org/10.1016/j.cplett.2018.11.003 Received 22 September 2018; Received in revised form 4 November 2018; Accepted 5 November 2018 Available online 07 November 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.
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temperature and enthalpy of fusion of the SA-hexadecanol and SAstearyl alcohol of eutectic mixtures were 318.98 K and 259.4 J·g−1, and 322.45 K and 202.2 J·g−1, respectively. Su [19] et al studied SA and noctadecane binary mixture, the results indicated that the SA were found to form a eutectic mixture at SA to n-octadecane mass ratio of 2: 98, which melts at around 300.96 K with latent heat of 223.68 J·g−1. However, the availability of potential materials with essential properties that can be used for solar energy application is very few in number. In our previous work, SA/adipic acid, SA/suberic acid and SA/ sebacic acid were investigated [20]. Eutectic mixtures with melting point and latent heat of (340.60 K, 200.30 J·g−1), (339.52 K, 191.38 J·g−1) and (340.23 K, 196.63 J·g−1) were identified respectively in SA/adipic acid, SA/suberic acid and SA/sebacic acid systems. Besides, the eutectic mixture displayed good cyclic, thermal, and chemical stability compared to its individual components. Moreover, we also explored four different SA-amides mixtures, SA-acetamide [21], SA-nbutyramide, SA-n-octanamide [22], and SA-acetanilide [23], respectively, and calculated their eutectic ratios by using the Schrader equation. The above studies show that SA can form eutectic mixtures with fatty acids, alkanols, alkanes, dicarboxylic acids and amides, and these SA-based binary mixtures has relatively high latent heat, good thermal stability and chemical stability, and SA-based binary mixtures have great potential as PCMs for the thermal energy storage. Besides, these researches show that amide compounds are attractive PCMs that can be used as latent thermal energy storage materials in solar heating application. Thus, in this work, the well-investigated PCMs of stearic acid and hexanamide were chosed as raw materials and their binary mixture was prepared. Then, the thermal properties and thermal reliability of two binary mixtures were identified by using differential scanning calorimeter (DSC), thermal conductivity analyzer (TC), X-ray diffractometer (XRD), fourier transform infrared (FT-IR) spectrophotometer, thermogravimeter (TGA) and 100 accelerate thermal cycle test, and explored their LHTES potential in low temperature.
2.3. Characterization
2. Experimental
ΔHi ⎞ ⎛ Tm − Ti ⎞ ⎤ χi γi = exp ⎡ ⎛ × ⎢ ⎝ RTi ⎠ ⎝ Tm ⎠ ⎥ ⎦ ⎣
The thermal properties and thermal cycle stability of the samples were determined by using a Mettler Toledo DSC2. All experiments were carried out under nitrogen with the flow rate of 100 mL. The heating and cooling rates of 2 K·min−1 and 10 K·min−1 were employed for the DSC experiment to entabilished the phase diagram of binary mixture and 100 accelerated thermal cycle test, respectively. The instrument was calibrated by using indium standard. The phase change enthalpy and temperature measurements have accuracy within ± 5% and ± 1.0 K, respectively. The melting points and heats of fusion were obtained from the second heating cycle of three independent DSC experiments. The thermal conductivity of samples at room temperature (293.15 K) was tested according to transient hot wire method by using a thermal conductivity analyzer (TC 3000E, Xiatech Electronic Technology Co., Ltd., China). Before the measurement, the cylindrical samples were ground and pressed with a molding under 15 MPa to obtain discs (diameter of 20 mm). The chemical structure stability of samples was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. XRD experiments were carried out on PANalytical X’PRO Pert diffractometer fitted with Cu Kα radiation with a powder setting of 40 kV and 30 mA. FT-IR experiments were performed on on a FT-IR spectrometer (Nicolet Nexus 670, Thermo Nicolet Corporation, USA) and scanned from (4000 to 400) cm−1 using KBr pellet. Thermal stability of the samples was determined by using thermogravimetric analysis (TGA) on a thermogravimetric analyzer STA 449F3 (NETZSCH, Germany) with a heating rate of 5 K·min−1 from room temperature to 900 K under nitrogen atmosphere. 3. Thermodynamic modeling The liquidus temperature of phase diagram of SA-HA binary mixture at different composition can be calculated by the following equations [24]: ⎜
2.1. Materials
⎟
⎜
⎟
(1)
Here, χi, γi, Tm, Ti, ΔHi, R represent the mole fraction of substance i in the liquid phase, activity coefficient of substance i in the liquid phase, the melting temperature of the system (K), the temperature of substance i (K), the molar enthalpy of fusion for substance i (J·mol−1), and idea gas constant, respectively. The binary system is approximated as an ideal solution, γi is equal to unity.
Stearic acid (SA, CAS number: 57–11-4, 98% for purity) was provided by Sinopharm Chemical Reagent Co., Ltd and hexanamide (HA, CAS number: 628-02-4, 98% for purity) was obtained from Tokyo Chemical Industry Co., Ltd (Tokyo, Japan). All reagents were used as received without further purification. Moreover, the thermophysical properties of SA and HA are listed in Table 1.
4. Results and discussion 2.2. Preparation of the binary mixtures 4.1. Thermophysical properties of SA, HA, and their mixtures The binary mixtures with different molar fraction of SA were prepared as following: SA and HA was mixed and stirred for 15 min in a round-bottomed flask which was immersed in silicone oil bath with heat temperature of 353.15 K. And then the obtained melted liquid mixtures were slowly cooled to room temperature.
4.1.1. The SA/HA binary mixtures A series of DSC melting curves of the SA/HA binary mixture with different molar fraction of SA is shown in Fig. 1, and corresponding data are summarized in Table 2. As can be seen in Fig. 1, for SA, there is only one endothermic peak observed. The onset temperature of melting is 342.62 K. The latent heat of fusion is 212.22 J·g−1. For HA, two endothermic peaks are displayed on the melting curve. And the first endothermic peak at lower temperature is referred to solid-solid phase transition of HA, the second endothermic peak at higher temperature corresponds to solid-liquid phase transition of HA. Thus, the onset temperature of melting of the solid-solid phase transition is 308.17 K, and the corresponding latent heat of melting of solid-solid phase transition is 33.10 J·g−1. On the other hand, the onset temperature of the solid-liquid phase transition is 373.44 K. The latent heat of solid-liquid phase transition is 130.15 J·g−1. The results indicates that the experimental data is close to the literature value [25]. For SA-HA binary mixture, the DSC thermograms presents multiple peaks, the first peak
Table 1 Chemical data and thermal properties for the individual PCM (Ttrs, the temperature of solid-solid phase transition; Tm, the temperature of solid-liquid phase transition; ΔHtrs, the enthalpy of solid-solid phase transition; ΔHm, the enthalpy of solid- liquid phase transition). Name
Molecular Formula
Ttrsa (K)
ΔtrsHmb (J·g−1)
Tma (K)
ΔHmb (J·g−1)
Stearic acid Hexanamide
C18H36O2 C6H13NO
n.a 308.17
n.a 33.10
342.62 373.44
212.22 130.15
a b
Standard uncertainty, u(T) = 1.00 K. Relative standard uncertainty, ur(ΔH) = 5%. 167
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Fig. 1. DSC melting curves of the SA, BA and their mixture as a function of the molar fraction of SA (where χSA represents the mole fraction of SA in binary mixture).
occurs at lower temperature corresponds to solid-solid phase transition of HA. The second peak at slightly higher temperature at fixed temperature corresponds to the eutectic transition of binary system, and it can be used to construct the solidus line of phase diagram. However, the third peak at highest temperature gradually varies corresponds to the solution of the remaining solid in the melt, and it can be used to determine the liquidus line of phase diagram [26]. Besides, it can be found that only two endothermic peaks are observed in χSA equals to 0.429, 0.451, 0.462, and 0.486.
Table 2 DSC results of SA, HA, and SA-HA binary mixture in melting process (χSA, the mole fraction of SA in binary mixture; Ttrs, the temperature of solid-solid phase transition; TE, the eutectic temperature; ΔHE, the enthalpy of the eutectic reaction; TL, the liquidus temperature; ΔHm, the total enthalpy of melting), at pressure 77.0 kPa.a χSAb
Ttrsc (K)
ΔtrsHmd (J·g−1)
0.000 0.043 0.092 0.148 0.213 0.249 0.288 0.331 0.378 0.398 0.419 0.429 0.451 0.462 0.486 0.510 0.548 0.618 0.696 0.785 0.843 0.907 1.000
309.55 309.60 309.48 309.52 309.50 309.74 309.75 309.64 309.75 310.09 310.19 309.91 310.15 310.25 310.05 309.83 309.75 309.87 309.77 309.85 310.14 309.90
33.10 31.95 27.90 23.83 18.48 15.85 13.56 13.73 11.21 11.74 10.82 9.28 10.57 9.85 8.07 9.21 6.93 4.98 4.17 1.68 1.86 0.99
TEc (K)
ΔEHmd (J·g−1)
330.42 330.56 330.67 330.68 331.07 331.05 330.82 330.98 331.21 331.37 330.82 330.97 331.15 330.92 331.14 330.81 330.86 330.56 330.31 330.37 329.59
26.34 49.76 76.29 98.51 104.43 128.14 136.46 151.71 158.00 164.15 172.92 176.27 176.62 172.46 162.45 158.27 118.87 88.67 53.37 39.31 23.62
TLc (K)
ΔHmd (J·g−1)
374.75 371.51 368.10 364.10 359.32 356.06 352.37 347.66 343.06 340.15 337.70
130.15 165.91 162.85 154.06 149.63 149.37 170.35 161.77 173.02 179.82 182.10 182.20 186.84 186.47 180.53 187.10 190.39 194.53 193.72 194.06 203.81 211.12 212.22
332.16 333.76 336.31 338.11 340.04 341.24 342.05 343.52
4.1.2. Phase diagram and Tammann plot of the SA/HA binary system As mentioned above, previous studies of this binary system gave a rather wide range for the eutectic composition. Therefore, we need to construct the phase diagram (Fig. 2a) and Tammann plot (Fig. 2b) of the binary system to accurately determine the composition of the eutectic mixture. The phase diagram is obtained by plotting the peak temperature of the endotherms of binary mixture with different molar fraction of SA ranging from 0 to 1. Seen from Fig. 2a, the binary system is the simple eutectic type, the melting points of binary mixtures decrease firstly and then increase with the increase of molar fraction of SA, the temperature of lowest point of the binary mixture is 331.05 K, and corresponding eutectic composition is χSA = 0.462. According to the Schroder equation, the predictive eutectic composition is χSA = 0.463, and corresponding eutectic temperature is 331.62 K. Comparing the experimental and theoretical data for the studied mixtures in Fig. 2a, it can be found that experimental values are in good agreement with the theoretical predictive values. In order to obtain the Tammann plot, the melting enthalpy of eutectic transition of the DSC thermograms as a function of composition was evaluated. The results are shown in Fig. 2b. As can be observed, the melting eutectic enthalpies form two ascending lines that start from the extremes of the composition values and extend until they reach a
a Experimental pressure was not controlled beyond the typical range of atmospheric pressure, (77.0 ± 1.0) kPa. b SA mole fraction with standard uncertainty, u = 0.005. c Standard uncertainty, u(T) = 1.00 K. d Relative standard uncertainty, ur(ΔH) = 5%.
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Fig. 2. (a) Phase diagram of the SA-HA binary mixture with various SA molar ratio (dash line ┄: the predicted results obtained; black line —: the experimental results obtained) and (b) Tammann plot (the variation of enthalpy of the binary mixture as a function of χSA) for SA-HA binary system. (χSA, the molar fraction of SA in binary mixture).
Fig. 3. (a) XRD patterns and (b) FT-IR spectras of SA, HA and SA-HA eutectic mixture.
results indicate that theoretical calculated value is in well consistent with the experimental measured value.
maximum at a certain composition (χSA = 0.462), and the maximum melting eutectic enthalpy is 176.62 J·g−1. Therefore, according to the phase diagram and the Tammann plot, the eutectic composition and enthalpy of fusion at the eutectic point of the SA-HA binary system are accurately determined: χSA = 0.462, and corresponding eutectic temperature and latent heat of fusion are 331.15 K and 176.62 J·g−1.
4.2. Chemical structure analysis 4.2.1. XRD analysis The XRD patterns of SA, HA and the SA-HA eutectic mixture are presented in Fig. 3a. As Fig. 3a shows that SA displays two intense diffraction peak at 2θ = 6.64° and 2θ = 21.56° and a mild peak at 2θ = 24.18°, and HA indicates one intense diffraction peaks at 2θ = 6.35° and two mild peaks at 2θ = 12.74° and 2θ = 25.65°. Although the diffraction intensities is changed due to the varied contents, all the diffraction peaks of SA and HA could be found on the XRD pattern of the eutectic mixture, indicating that the eutectic mixture is a physical mixture of SA and HA and their crystal structures remained.
4.1.3. Thermal conductivity Thermal conductivity plays a very important role in thermal energy storage application. Since the higher the thermal conductivity, the faster the heat transfer rate. And the thermal conductivities of SA-HA binary mixtures were determined by transient hot wire method at solid state (293.15 K). The thermal conductivities of SA, HA and SA-HA eutectic mixture were measured as 0.3362 W·m−1·K−1, −1 −1 −1 −1 0.2456 W·m ·K , and 0.2728 W·m ·K , respectively. On the other hand, the thermal conductivity of SA-HA eutectic mixture can be calculated by using the equation κSA-HA = χSA·κSA + (1 − χSA) ·κHA, where χSA is the molar fraction of SA in the eutectic composition, κSA-HA, κSA and κHA are the thermal conductivity of SA-HA eutectic mixture, pure SA and pure HA, respectively. Besides, in this work, χSA equal to 0.462. Therefore, the calculated thermal conductivity of SA-HA eutectic mixture is 0.2874 W·m−1·K−1, which is only 0.0146 W·m−1 ·K−1 higher than experimental measured value. Based on the above analysis, the
4.2.2. FTIR analysis The FT-IR spectra of SA, HA and the eutectic mixture are displayed in Fig. 3b. As can be seen from the SA-HA eutectic mixture FT-IR spectrum, the peaks at 3362.02 cm−1 and 3197.38 cm−1 correspond to the stretching vibration of eNH2 group. The peaks at 2955.64 cm−1 represents asymmetric stretching vibration of eCH3 group. The peaks at 2916.83 cm−1 and 2848.90 cm−1 are attributed to asymmetric and symmetric stretching vibration of eCH2 group, respectively. The broad peak observed at 3000–2600 cm−1 is resulted from OeH stretching 169
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Fig. 4. Thermogravimetric analysis of SA, HA and SA-HA eutectic mixture. Fig. 5. DSC curves of SA-HA eutectic mixture with heating and cooling rate of 10 K·min−1 before and after 100 thermal cycles.
vibration, which usually overlaps with the absorption band of aliphatic CeH stretching vibration in eCH3 and eCH2 group. The C]O stretching vibration peak is observed at 1703.23 cm−1 and 1660.79 cm−1 in O]CeOH and O]CeNH2 group. The peaks at 1633.13 cm−1 represents the rocking vibration of eNH2 group. The peak at 1469.45 cm−1 is the bending vibration of CeH in eCH3 group. The peak at 1426.46 cm−1 represents stretching vibration of C]O in O]CeOH. The peak at 1295.98 cm−1 is the CeH rocking and twisting vibration of eCH2 groups from SA. The peak at 942.98 cm−1 represents OeH out-of-plane bending vibration of O]CeOH. The peak at 718.96 cm−1 represents the eCH2 rocking vibration. Compared with the FT-IR spectra of SA and HA, we can find that the characteristic peaks of SA and HA all display in that of the eutectic mixture. The results indicates that there is no chemical interaction except strong hydrogen bond exists between SA and HA.
Table 3 Phase change temperature and latent heat of tested eutectic PCM after 100 thermal cycles (Ttrs, onset temperature of solid–solid phase transition; Tm, onset temperature of melting; ΔHm, the total latent heat of melting; Tf, onset temperature of freezing; ΔHf, the total latent heat of freezing), at pressure 77.0 kPa.a Number of cycles
0 20 40 60 80 100
4.3. Thermal stability and reliability of SA-HA binary mixture
Melting process
Freezing process
Ttrsb (K)
Tmb (K)
ΔHmc (J·g−1)
Ttrsb (K)
Tfb (K)
ΔHfc (J·g−1)
308.31 307.98 307.90 308.01 308.07 308.06
329.37 329.29 329.29 329.31 329.30 329.32
167.54 166.38 165.78 165.23 164.50 164.21
310.74 310.80 310.90 310.92 310.89 310.92
324.88 324.30 324.47 324.54 325.06 325.28
164.26 161.98 161.04 159.96 160.26 159.68
a Experimental pressure was not controlled beyond the typical range of atmospheric pressure, (77.0 ± 1.0) kPa. b Standard uncertainty, u(T) = 1.00 K. c Relative standard uncertainty, ur(ΔH) = 5%.
4.3.1. Thermal stability The TG curves of SA, HA, and the eutectic mixture are exhibited in Fig. 4. As can be seen from Fig. 4, SA shows a two-step weight loss that started at 397.65 K, followed at 505.55 K and finished at 547.35 K, corresponding to the decomposition of SA. HA shows a one-step mass loss that started at 413.55 K and terminated at 476.85 K, representing the decomposition of HA. The TG curve of the eutectic mixture also shows a two-step mass loss feature that is similar to SA. The first step of mass loss begins at approximately 421.35 K, followed by the second step of mass loss begins at approximately 493.15 K. Besides, the initial mass loss temperature of eutectic mixture is higher than both SA and HA. Which indicates that the eutectic mixture has better thermal stability in working temperature range than pure PCMs for it to be applied in thermal energy storage.
and latent heat are acceptable in thermal energy storage application. The results indicates that the eutectic mixture has good stability during long-term application as a PCM.
5. Conclusion In this study, a series of SA/HA binary mixtures were prepared and determined by DSC. The phase diagram and Tammann plot of the binary system were established, and corresponding the eutectic point of the eutectic mixture was 331.15 K and the latent heat of melting of the eutectic mixture was 176.62 J·g−1 when the molar fraction of SA was 0.462. The thermal conductivity of the eutectic mixture was 0.2728 W·m−1·K−1. Moreover, the eutectic mixture had good thermal stability and reliability. Accordingly, the SA-HA eutectic mixture has great potential as PCM for solar energy storage.
4.3.2. Thermal reliability The long-time stability of the SA-HA eutectic mixture was determined by 100 accelerated thermal cycles from 298.15 K to 373.15 K. The melting and freezing curves of the 1st, 20th, 40th, 60th, 80th and 100th cycles are shown in Fig. 5, and corresponding phase change temperature and latent heats are summarized in Table 3. As shown in Fig. 5, the freezing peak becomes wider with the increases number of thermal cycles. However, the melting peaks show slight changes in shape or magnitude. After 100 cycles, it can be found from Table 3 that the phase change temperature of melting and freezing of the eutectic mixture change −0.05 K and 0.4 K respectively. And the melting and freezing latent heats of the eutectic mixture decrease by 1.99% and 2.79% respectively. The small changes in phase change temperature
Acknowledgements This work was supported by the National Natural Science Foundation of China (U1407205), and Youth Innovation Promotion Association of Chinese Academy of Sciences (2015351).
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