- Email: [email protected]
expanded graphite
Journal of Molecular Liquids 277 (2019) 577–583
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Preparation and thermophysical properties of low temperature composite phase change material octanoic-lauric acid/ expanded graphite Yuyang Li, Xuelai Zhang ⁎, Jotham Muthoka Munyalo, Zhen Tian, Jun Ji Institute of Thermal Storage Technology, Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China
a r t i c l e
i n f o
Article history: Received 3 September 2018 Received in revised form 10 November 2018 Accepted 20 December 2018 Available online 21 December 2018 Keywords: Expanded graphite Composite phase change materials Thermodynamic properties Stability
a b s t r a c t A new composite phase change material (PCM) was developed with octanoic acid and lauric acid as a base fluid and expanded graphite (OA-LA/EG) as matrix. The OA-LA/EG base fluid was determined with mass ratio 81:19 and the optimal ratio of EG was 7 wt%. The transition temperature and latent heat of OA-LA/EG were measured by differential scanning calorimetry (DSC). The thermal conductivity of OA-LA/EG was measured by Hot Disk thermal constant analyzer. In addition, the cool storage and discharge cycle tests were carried out for 100 times to validate the stability of OA-LA/EG. The experimental results show that, the phase transition temperature of OA-LA/EG is 3.6 °C and the latent heat of phase change is 132.8 J·g−1. The thermal conductivity of OA-LA/EG is 1.275 W·(m·K)−1 which is 2.8 times higher than that of OA-LA. The phase change temperature, latent heat and thermal conductivity demonstrated good reliability. The composite PCM developed has great prospect in the practical application to medical refrigeration transportation system and air conditioning cold storage system with temperature range of 2–8 °C. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Energy crisis is getting more and more serious in recent years. LHTES (Latent heat thermal energy storage) with PCMs utilize the latent heat during phase change process to realize energy storage and utilization, which can achieve “shift peak fill valley” and consequently alleviate the pressure on power grid [1,2]. At present, PCMs have been widely used in LHTES systems, such as energy saving in buildings [3–6], solar thermal storage [7–9], food and medicine cooling [10–12]. In particular, the mobility of PCMs can solve the application limitation of energy in space and time. Therefore, it is of great significance to develop efficient PCMs to meet the requirements of engineering applications. Organic PCMs have shown advantages in low super-cooling degree, non-toxicity, low corrosiveness and good thermal reliability in practical applications [13]. Taguchi et al. [14] prepared pentadecane methyl methacrylic acid micro-capsule with a phase transition temperature of 9.5 °C and latent heat of 97 J·g−1. Ying et al. [15] studied lauric acidtetradecane binary composite PCM with a phase transition temperature of 4.03 °C and latent heat of 207.05 J·g−1. The latent heat of these organic PCMs is promising while the thermal conductivity is generally low. Zhang et al. [16] found that the thermal conductivity of capricpalmitic-stearic acid was 0.3407 W·(m·K)−1. Siahpush et al. [17] ⁎ Corresponding author. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.molliq.2018.12.111 0167-7322/© 2018 Elsevier B.V. All rights reserved.
studied organic PCM eicosane and found that the thermal conductivity was 0.423 W·(m·K)−1. With respect to the enhancement of thermal conductivity, many researchers have done a lot of work. A series of modified techniques have been proposed to improve thermal conductivity, such as high thermal conductivity materials being added into the PCMs. The additives mainly include metal particles and their oxides, carbon nanofiber, carbon nanotubes and expanded graphite. Sharma et al. [18] found that the thermal conductivity of palmitic acid was increased by 80% after adding TiO2 with a ratio of 5 wt%. Cui et al. [19] found that if the thermal conductivity of pure soy wax was 0.324 W·(m·K)−1, the thermal conductivities of 10% carbon nanofiber/soy wax and carbon nanotubes/soy wax were 0.469 and 0.403 W·(m·K)−1. Huang and Zhang [20] reported that the thermal conductivity of lauryl alcoholcapric acid was increased by 20.5% after adding multiwalled carbon nanotubes (MWNTs) with a ratio of 0.1 wt%. Zeng et al. [21] found that when the loading of MWNTs increased to 5% and no surfactant was added, the thermal conductivity of the composite PCM was enhanced to be 26% higher than that of PA. Zhang et al. [22] selected nine kinds of high thermal conductivity mediums such as kieselguhr, organic bentonite and EG to improve the thermal conductivity of the shape-stabilized PCM. They found that EG demonstrated the best performance in thermal conductivity enhancement. Yuan task group [23–27] studied the thermal conductivity of compound PCMs of stearic-palmitic acid, lauric-palmitic-stearic acid, lauric-myristicpalmitic acid, capric-myristic-palmitic acid, lauric-myristic-stearic acid
578
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
Fig. 1. SEM images of: (a) EG without expansion; (b) EG; (c) OA-LA/EG; (d) OA-LA/EG after circulation.
and so on with EG. The experimental results show that the thermal conductivities of the materials all obviously improved by the addition of EG. Zhou et al. [28] obtained adipic acid with a thermal conductivity of 4.35 W·(m·K) −1 by adding of 10 wt% EG, the thermal conductivity of which is 8 times higher than pure adipic acid. Yu et al. [29] found that when the composite PCM was prepared with
10% EG, the time required for heat storage/retrieval with the stearic acid decreased by 34.8 and 57.3%, respectively. However, the temperature of PCMs in the above study is not suitable for medical refrigerated transportation and air conditioning cold storage. Furthermore, they did not carry out a complete cool storage and discharge experiment after the cycle.
Fig. 2. OA-LA/EG images with different EG mass ratio.
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
579
The aim of this paper is to study the new developed binary organic PCM OA-LA/EG, which can be used in medical refrigeration transport system and cold-storage air-conditioning system at 2–8 °C. In order to enhance the thermal conductivity of OA-LA, EG with micrometer-scale grid structure was used as additive. EG is worm-like and its surface has micrometer-scale grid structure, which can adsorb OA-LA. The composite phase change material OA-LA/EG improves the thermal conductivity through the overlap of EG. The thermodynamic properties and cyclic stability of OA-LA/EG were investigated. 2. Experiments 2.1. Experimental technique 2.1.1. Reagents and instruments The ideal PCMs should have the following characteristics [30]: high latent heat of phase transformation, high crystallization rate, appropriate phase transition temperature, chemical stability, non-toxic, nonexplosive, non-corrosive, less costly and easily available. After comprehensive consideration, OA and LA were selected as the main raw materials for composite PCM in this study. OA and LA are analytical reagents and the expansion volume of EG is 250 mL·g−1. Experimental instruments mainly include box-type resistance furnace (SX2-4-10A), scanning electron microscope (KYKY-EM6000), SBC12 ion sputter, differential scanning calorimetry DSC (Model 200F3, temperature accuracy of 0.1 °C, enthalpy accuracy 0.1%), Hot Disk thermal constant analyzer (TPS2500s type, accuracy of 2%), magnetic stirrer, low-temperature thermostat bath (DC-6515), thermocouple (accuracy 0.01 °C), electrothermal blowing dry box, Agilent temperature-time recorder (34970A, recording time interval of 3 s), electronic analytical balance (FA2004, accuracy 0.1 mg), high and low temperature alternating test box (YSGJW-100C). 2.1.2. Preparation of OA-LA OA and LA were weighed, and placed them in the beaker. Notably, the mass ratio of OA and LA was determined as 81:19 in this study. Then, the beaker was heated to 40 °C and mixed by magnetic stirring apparatus for 30 min to guarantee the uniformity. The solutions with the mass of 29.1 g, 28.8 g, 28.5 g, 28.2 g, 27.9 g, 27.6 g, 27.3 g, and 27 g were prepared for the experiment. 2.1.3. Preparation and SEM scanning experiment of EG EG has a flake structure with a smooth surface and no voids and is necessary to be expanded before it can be used in experiments. EG was expanded in an electric box-type resistance furnace, which was set at 800 °C. The duration of expansion process was set 10 min. The SEM micrographs of EG was demonstrated in Fig. 1. Comparing Fig. 1 (a) with Fig. 1(b), it is clear that the surface of EG is relatively smooth before being expanded, while the surface of EG is worm-like with a clear porous structure after the high temperature expansion. The porous structure in expanded EG contributes to adsorbing more OA-LA base solution, which can achieve the purpose of enhancing the thermal conductivity. 2.1.4. Preparation of OA-LA/EG EG with porous structure was used as the matrix of OA-LA solutions in order to enhance the thermal conductivity of OA-LA. Eight groups of 30 g OA-LA/EG were prepared. The mass of OA-LA base solutions were 29.1 g, 28.8 g, 28.5 g, 28.2 g, 27.9 g, 27.6 g, 27.3 g, and 27 g. The mass of EG were 0.9 g, 1.2 g, 1.5 g, 1.8 g, 2.1 g, 2.4 g, 2.7 g, and 3 g, respectively. Thus, the mass proportions of EG in the eight groups were 3%, 4%, 5%, 6%, 7%, 8%, 9% and 10%, respectively. The prepared OA-LA/EG was stirred every half an hour for six times so that EG and OA-LA mixed uniformly. Eight groups of OA-LA/EG images with different EG mass ratio are shown in Fig. 2. As the proportion of EG increases, it can be seen from the images that the volume of OA-LA/EG are 32.1 mL, 38.6 mL, 48.7 mL, 56.3 mL,
Fig. 3. Schematic diagram of experimental setup for cool storage and discharge test 1polyurethane sealing cover; 2-beaker; 3-PCM; 4-thermocouple; 5-low temperature thermostat bath; 6-Agilent temperature-time recorder; 7-data display.
67.4 mL, 76.6 mL, 90.1 mL and 98.5 mL, respectively. However, all the mass is 30 g. The addition of EG makes the liquid OA-LA change into a solid form.
2.2. Experimental methods 2.2.1. The optimal ratio of EG Since EG does not participate phase transition at the phase change temperature, the higher the content of OA-LA is, the greater the latent heat is. Nevertheless, the content of EG can affect the thermal conductivity of the material and the speed of energy storage and release in practical applications. In this paper, the maximum adsorption ratio of EG to OA-LA mixed solution is defined as the optimal ratio of OA-LA to EG. The OA-LA/EG samples were placed in an electrothermal blowing dry box at 50 °C for 60 min. By comparing the mass before and after drying, the optimal proportion of EG was qualitatively determined by the smallest mass loss.
2.2.2. Cool storage and discharge experiment The performance of the material generally was validated by the cool storage and discharge experiments, which were carried out with two low-temperature thermostat baths. Experimental setup for cool storage and discharge of PCMs is shown in Fig. 3. One of the low-temperature thermostat baths was maintained at −15 °C and the other lowtemperature thermostat bath was kept at 25 °C. The configured 30 g OA-LA and OA-LA/EG were placed in a low-temperature thermostat bath with the temperature of −15 °C to cool the materials. The temperature of the material was measured by thermocouple and recorded by an Agilent temperature-time recorder. When the temperature is near −15 °C, the beaker was quickly removed and placed in another lowtemperature thermostat bath with the temperature of 25 °C. The cool storage and discharge curve of the materials were plotted by experimental data.
Table 1 Weight changes of OA-LA/EG before and after drying. Mass ratio of EG/% 3 4 5 6 7 8 9 10
Weight of material (before drying)/g
Weight of material (After drying)/g
Mass loss/g
Mass loss rate/%
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
0.1408 0.1846 0.2216 0.2591 0.2846 0.2848 0.2852 0.2849
0.1592 0.1154 0.0784 0.0409 0.0154 0.0152 0.0148 0.0151
53.1 38.5 26.1 13.6 5.1 5.1 4.9 5.0
580
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
2.2.5. Testing of material stability Low temperature composite PCM (30 g) of OA-LA/EG was weighed, and put it in the high and low temperature alternating test box. The temperature range of high and low temperature alternating test box was set at −40–60 °C. The period of stability test was 60 min and totally 100 cycles were conducted. The melting temperature, latent heat and mass loss of materials were measured. To validate the stability of OALA/EG, the results were compared with those of the original OA-LA/EG which were not treated in high and low temperature alternating test box. 3. Results and discussion 3.1. The optimal proportional of EG
Fig. 4. The fitting curve of OA-LA/EG mass loss.
2.2.3. Measurement of phase transition temperature and latent heat The phase transition temperature of material determines the range of application. The latent heat of material plays a key role in determining whether it is suitable for practical application. Therefore, experiments for testing phase transition temperature and latent heat were performed by DSC. Liquid nitrogen was used to cool the PCM and nitrogen vapor was used as protective gas and purge gas. Indium was performed as a reference material for calibration. The temperature was in the range of −20 to 30 °C and the purge gas flow rate was 20 mL·min−1. The protective gas flow rate was 60 mL·min−1 and heating and cooling rate was 5 K·min−1.
2.2.4. Measurement of thermal conductivity Thermal conductivity of PCMs has a great influence in the practical application. TPS2500s thermal constant analyzer was used to test the thermal conductivity of the material. Before testing, the machine was turn on and preheated for 30 min. The C5465 probe was inserted into the middle of the mixed solution and the vertical distances from the probe center to the liquid level and the wall of the beaker were recorded. In order to avoid the influence of indoor air flow and reduce the measurement error, the test probe and beaker was closed inside a container. The test temperature is 18 °C, which is the ambient temperature.
The comparison of weight changes of the composite PCM before and after electrothermal blowing dry box treatment is shown in Table 1. As can be seen from Table 1, the mass loss of the composite PCM with EG mass ratio of 3%, 4%, 5% and 6% decreased linearly. The mass loss rates were 53.07%, 38.47%, 26.13% and 13.63%, respectively. The insufficient addition of expanded graphite leading to some OA-LA cannot be adsorbed, which results in a higher mass loss rate. The mass loss rates of composite PCMs with EG mass ratios of 7%, 8%, 9% and 10% are between 4.9% and 5.1%, respectively. The mass loss rates of these four groups of materials are almost equal and stable. Therefore, the optimal mass ratio of EG is determined at 7%. As can be seen from Fig. 4, when the proportion of EG is less than the optimal addition ratio of 7%, the mass loss rate of OA-LA/EG has its regularity, which is approximately fitting to logarithmic function: y ¼ –57:26 ln ðxÞ–146:73 R2 ¼ 0:9971
3.2. Phase transition temperature and latent heat DSC results of OA-LA and OA-LA/EG are shown in Figs. 5 and 6. The phase transition temperature of OA-LA/EG is 3.6, which is reduced by 0.2 °C compared to OA-LA. The latent heat of OA-LA is 132.8 J·g−1, which is reduced 8.9 J·g−1 compared to OA-LA. The reason is that EG just acts as a skeleton without phase transformation in the composite PCM and does not contribute to its latent heat. The proportion of EG (7 wt%) matches the decrease of the latent heat of 6.3%.
Fig. 5. DSC curve of OA-LA.
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
581
Fig. 6. DSC curve of OA-LA/EG.
3.3. Thermal conductivity of OA-LA/EG The thermal conductivity of common organic PCMs is very low, which is only 0.2–0.4 W·(m·K)−1. Thermal conductivity is an important factor affecting the use of PCMs. Higher thermal conductivity can speed up the cool storage and discharge of PCMs, which can ensure uniform and stable temperature of materials in application and improve the utilization of materials. The step-cooling curves of OA-LA and OALA/EG are shown in Fig. 8. During sensible heat release, OA-LA/EG takes 2.40 min and OA-LA/EG takes 1.45 min for the temperature dropped from 20 to 10 °C. The addition of EG increased the cooling rate of sensible heat by 65.5%. By adding OA-LA into EG matrix, the perfect heat conduction grid structure can be formed due to the overlap joint of EG, which can improve the way of heat conduction. Hence, heat transfer is given a priority to heat conduction, whose transmission speed is high. Which makes the thermal conductivity of OA-LA increased significantly. During the absorption of latent heat of fusion, the time of OA-LA is 8.45 min and the time of OA-LA/EG is 4.95 min when the temperature increased from 5 °C to 7 °C. In the period of latent heat release, the content of OA-LA was 69.25 min and OA-LA/EG was 9.40 min when the temperature of material declined from 5 to 3 °C. The addition of EG improves the release rate of OA-LA latent heat by 636.7%. After adding EG, OA-LA is divided into several small units to
increase the contact area of the material. EG can not only increase the thermal conductivity, but also greatly enhance the coagulation and nucleation rate of the material. The thermal conductivity of OA-LA and OALA/EG are 0.3357 W·(m·K)−1 and 1.275 W·(m·K)−1, respectively. The temperature of the test environment is 18 °C at room temperature. The addition of EG increased the thermal conductivity by 2.8 times, so material can achieve rapid storage and discharge. Therefore, OA-LA/EG has a broader application space in medical refrigeration transport system and cold-storage air-conditioning system. 3.4. Thermal reliability of OA-LA/EG Stability is an important performance indicator of the material. The results of 100 cycles of cool storage and discharge experiments on OA-LA/ EG before and after the storage and discharge are shown in Fig. 9. DSC results of OA-LA/EG and OA-LA/EG after 100 cycles are shown in Fig. 7. The phase transition temperature of OA-LA/EG after 100 cycles is 3.7, which is increased by 0.1 °C compared to that before the cycle. The latent heat of OA-LA/EG after 100 cycles is 132.3 J·g−1, which was 0.6 J·g−1 lower than that before cycling. The latent heat and phase transition temperature of OA-LA/EG did not change obviously before and after the cycle. The results of 100 cycles of cool storage and discharge experiments on OA-LA/EG before and after the storage and discharge are shown in
Fig. 7. DSC curve of OA-LA/EG after 100 cycles.
582
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
4. Conclusion
Fig. 8. Step cooling curve of OA-LA and OA-LA/EG.
Fig. 9. In the phase of sensible heat release, when the temperature of material is decreased from 20 to 10 °C, the original OA-LA/EG takes 1.45 min and the OA-LA/EG after 100 cycles takes 1.64 min. After 100 cycles, the rate of sensible cooling of OA-LA/EG is decreased 13.1%. During the stage of latent heat release, when the temperature of the material is decreased from 5 to 3 °C, the original OA-LA/EG takes 9.4 min and the OA-LA/EG after 100 cycles takes 11.50 min. After 100 cycles, the release rate of latent heat of OA-LA/EG is reduced by 22.3%. During the phase of latent heat absorption, the temperature of the material increased from 5 to 7 °C, the time for OA-LA/EG was 4.95 min and the time for OA-LA/EG after 100 cycles was 5.82 min. After circulation, the absorption rate of melting latent heat of OA-LA/ EG decreased by 17.6%. The thermal conductivity of OA-LA/EG after 100 cycles is 1.132 W·(m·K)−1, which decreased by 11.2% compared with that before cycling. After circulation, the mass loss rate was 6.5% which was greater than the mass loss rate range 4.9 to 5.1%. The reason the sensible heat, latent heat and the thermal conductivity of OA-LA/EG are less than those before the cycling of 100 times, is that a small part of the OA-LA spill from the EG, which makes the grid structure cut off by it, and then lead to the slowdown of heat transfer. After 100 cycles of OA-LA/EG, the phase change temperature, latent heat and thermal conductivity did not change obviously.
Fig. 9. Cold storage and discharge curves before and after OA-LA/EG 100 cycles.
(1) A novel low-temperature composite PCM OA-LA/EG used in medical refrigeration transport system and cold-storage airconditioning system with temperature range of 2–8 °C has been developed. The optimal mass ratio of EG was determined as 7% for OA-LA by electrothermal blowing dry box. The phase transition temperature of OA-LA/EG is 3.8 °C and the latent heat is 141.7 J·g−1. (2) The thermal conductivity of OA-LA was increased 2.8 times after addition of EG. Compared to the base OA-LA solution, the cooling rate of sensible heat of OA-LA/EG was increased by 65.5%, the heat absorption rate of latent heat was increased by 70.7% and the release rate of latent heat was increased by 636.7%. (3) Both the phase change temperature and latent heat of the OA-LA/ EG were not changed obviously after 100 cycles of cool storage and discharge contrast experiment. Due to the stable thermal storage and discharge properties of the material, the newly developed OA-LA/EG can maintain stable thermodynamic properties in practical application. In further work, the experiments with vacuum insulation incubator to simulate the properties of the material in application will be conducted.
Acknowledgements This study benefits from financial support of the Key Program of Shanghai Municipal Science and Technology Commission (Grant No. 16040501600) and National Key R & D Project (Grant No. 2018YFD0401305). References [1] G. Fang, F. Tang, L. Cao, Preparation, thermal properties and applications of shapestabilized thermal energy storage materials, Renew. Sust. Energ. Rev. 40 (C) (2014) 237–259. [2] A. Arora, G. Sant, N. Neithalath, Numerical simulations to quantify the influence of phase change materials (PCMs) on the early- and later-age thermal response of concrete pavements, Cem. Concr. Compos. 81 (2017) 11–24. [3] L.F. Cabeza, A. Castell, C. Barreneche, et al., Materials used as PCM in thermal energy storage in buildings: a review, Renew. Sust. Energ. Rev. 15 (3) (2011) 1675–1695. [4] A.D. Gracia, L.F. Cabeza, Phase change materials and thermal energy storage for buildings[J], Energ. Buildings 103 (2015) 414–419. [5] Z. Zhou, Z. Zhang, J. Zuo, et al., Phase change materials for solar thermal energy storage in residential buildings in cold climate, Renew. Sust. Energ. Rev. 48 (2015) 692–703. [6] F. Souayfane, F. Fardoun, P.H. Biwole, Phase change materials (PCMs) for cooling applications in buildings: a review, Energ. Buildings 129 (2016) 396–431. [7] B. Xu, P. Li, C. Chan, Application of phase change materials for thermal energy storage in concentrated solar thermal power plants: a review to recent developments, Appl. Energy 160 (2015) 286–307. [8] G. Ma, S. Liu, S. Xie, et al., Binary eutectic mixtures of stearic acid- n -butyramide/n -octanamide as phase change materials for low temperature solar heat storage, Appl. Therm. Eng. 111 (2017) 1052–1059. [9] S. Kahwaji, M.B. Johnson, A.C. Kheirabadi, et al., Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures, Sol. Energy Mater. Sol. Cells 167 (2017) 109–120. [10] J.K. Carson, A.R. East, The cold chain in New Zealand-a review, Int. J. Refrig. 87 (2018) 185–192. [11] Q. Yan, Research on fresh produce food cold chain logistics tracking system based on RFID, Adv. J. Food Sci. Technol. 7 (3) (2015) 191–194. [12] D.C. Murillo, Refrigerated container versus bulk: evidence from the banana cold chain, Marit. Policy Manag. 42 (3) (2015) 228–245. [13] X. Huang, Y.D. Cui, G.Q. Yi, et al., Preparation and properties of composite phase change material of lauric acid and expanded graphite, J. Chem. Eng. S1 (2015) 70–374. [14] Y. Taguchi, H. Yokoyama, H. Kado, et al., Preparation of PCM-microcapsules by using oil absorbable polymer particles, Colloids Surf. A Physicochem. Eng. Asp. 301 (7) (2007) 5–41. [15] T. Ying, S.U. Dang, J. Bai, Organic Phase Change Compound Materials for Nonfreezing Cold Chain. Transactions of the Chinese Society for Agricultural Machinery, 2017. [16] Y.P. Zhang, J.H. Ding, X. Wang, et al., Influence of additives on thermal conductivity of shape-stabilized phase change material, Sol. Energy Mater. Sol. Cells 90 (11) (2006) 1692–1702.
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583 [17] A. Siahpush, J. O'Brien, J. Crepeau, Phase change heat transfer enhancement using copper porous foam, J. Heat Transf. 130 (8) (2008) 318–323. [18] R.K. Sharma, P. Ganesan, V.V. Tyagi, et al., Thermal properties and heat storage analysis of palmitic acid-TiO2, composite as nano-enhanced organic phase change material (NEOPCM), Appl. Therm. Eng. 99 (2016) 1254–1262. [19] C.H. Y B Cui, S. Hu Liu, et al., The experimental exploration of carbon nanofiber and carbon nanotube additives on thermal behavior of phase change materials, Sol. Energy Mater. Sol. Cells 955 (4) (2011) 1208–1212. [20] Y. Huang, X. Zhang, Heat transfer property of lauryl alcohol-capric acid-nanoparticle composite phase change materials, CIESC J. 67 (6) (2016) 2271–2276. [21] J.L. Zeng, Z. Cao, D.W. Yang, et al., Effects of MWNTs on phase change enthalpy and thermal conductivity of a solid-liquid organic PCM, J. Therm. Anal. Calorim. 95 (2) (2009) 507–512. [22] H. Zhang, X. Gao, C. Chen, et al., A capric–palmitic–stearic acid ternary eutectic mixture/expanded graphite composite phase change material for thermal energy storage, Compos. A: Appl. Sci. Manuf. 87 (2016) 138–145. [23] N. Zhang, Y.P. Yuan, Y.X. Du, et al., Preparation and properties of palmitic-stearic acid eutectic mixture/expanded graphite composite as phase change material for energy storage, Energy 78 (2014) 950–956. [24] Y.G. Yuan, Y.P. Yuan, N. Zhang, Preparation and properties of lauric-palmitic-stearic acid eutectic mixture/expanded graphite composite phase change material for energy storage, J. Chem. Eng. 65 (S2) (2014) 286–292.
583
[25] N. Zhang, Y.P. Yuan, X. Wang, et al., Preparation and characterization of lauricmyristic-palmitic acid ternary eutectic mixtures/expanded graphite composite phase change material for thermal energy storage, Chem. Eng. J. 231 (2013) 214–219. [26] Y.G. Yuan, Y.P. Yuan, N. Zhang, Preparation and thermal characterization of capricmyristic-palmitic acid/expanded graphite composite as phase change material for energy storage, Mater. Lett. 125 (2014) 154–157. [27] C. Liu, Y.P. Yuan, N. Zhang, et al., A novel PCM of lauric-myristic acid/expanded graphite composite for thermal energy storage, Mater. Lett. 120 (2014) 43–46. [28] W. Zhou, K. Li, J. Zhu, et al., Preparation and thermal cycling of expanded graphite/ adipic acid composite phase change materials, J. Therm. Anal. Calorim. 129 (3) (2017) 1–7. [29] H. Yu, J. Gao, Y. Chen, et al., Preparation and properties of stearic acid/expanded graphite composite phase change material for low-temperature solar thermal application, J. Therm. Anal. Calorim. 124 (1) (2016) 87–92. [30] S.A. Mohamed, F.A. Al-Sulaiman, N.I. Ibrahim, et al., A review on current status and challenges of inorganic phase change materials for thermal energy storage systems, Renew. Sust. Energ. Rev. 70 (2017) 1072–1089.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Preparation and thermophysical properties of low temperature composite phase change material octanoic-lauric acid/ expanded graphite Yuyang Li, Xuelai Zhang ⁎, Jotham Muthoka Munyalo, Zhen Tian, Jun Ji Institute of Thermal Storage Technology, Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China
a r t i c l e
i n f o
Article history: Received 3 September 2018 Received in revised form 10 November 2018 Accepted 20 December 2018 Available online 21 December 2018 Keywords: Expanded graphite Composite phase change materials Thermodynamic properties Stability
a b s t r a c t A new composite phase change material (PCM) was developed with octanoic acid and lauric acid as a base fluid and expanded graphite (OA-LA/EG) as matrix. The OA-LA/EG base fluid was determined with mass ratio 81:19 and the optimal ratio of EG was 7 wt%. The transition temperature and latent heat of OA-LA/EG were measured by differential scanning calorimetry (DSC). The thermal conductivity of OA-LA/EG was measured by Hot Disk thermal constant analyzer. In addition, the cool storage and discharge cycle tests were carried out for 100 times to validate the stability of OA-LA/EG. The experimental results show that, the phase transition temperature of OA-LA/EG is 3.6 °C and the latent heat of phase change is 132.8 J·g−1. The thermal conductivity of OA-LA/EG is 1.275 W·(m·K)−1 which is 2.8 times higher than that of OA-LA. The phase change temperature, latent heat and thermal conductivity demonstrated good reliability. The composite PCM developed has great prospect in the practical application to medical refrigeration transportation system and air conditioning cold storage system with temperature range of 2–8 °C. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Energy crisis is getting more and more serious in recent years. LHTES (Latent heat thermal energy storage) with PCMs utilize the latent heat during phase change process to realize energy storage and utilization, which can achieve “shift peak fill valley” and consequently alleviate the pressure on power grid [1,2]. At present, PCMs have been widely used in LHTES systems, such as energy saving in buildings [3–6], solar thermal storage [7–9], food and medicine cooling [10–12]. In particular, the mobility of PCMs can solve the application limitation of energy in space and time. Therefore, it is of great significance to develop efficient PCMs to meet the requirements of engineering applications. Organic PCMs have shown advantages in low super-cooling degree, non-toxicity, low corrosiveness and good thermal reliability in practical applications [13]. Taguchi et al. [14] prepared pentadecane methyl methacrylic acid micro-capsule with a phase transition temperature of 9.5 °C and latent heat of 97 J·g−1. Ying et al. [15] studied lauric acidtetradecane binary composite PCM with a phase transition temperature of 4.03 °C and latent heat of 207.05 J·g−1. The latent heat of these organic PCMs is promising while the thermal conductivity is generally low. Zhang et al. [16] found that the thermal conductivity of capricpalmitic-stearic acid was 0.3407 W·(m·K)−1. Siahpush et al. [17] ⁎ Corresponding author. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.molliq.2018.12.111 0167-7322/© 2018 Elsevier B.V. All rights reserved.
studied organic PCM eicosane and found that the thermal conductivity was 0.423 W·(m·K)−1. With respect to the enhancement of thermal conductivity, many researchers have done a lot of work. A series of modified techniques have been proposed to improve thermal conductivity, such as high thermal conductivity materials being added into the PCMs. The additives mainly include metal particles and their oxides, carbon nanofiber, carbon nanotubes and expanded graphite. Sharma et al. [18] found that the thermal conductivity of palmitic acid was increased by 80% after adding TiO2 with a ratio of 5 wt%. Cui et al. [19] found that if the thermal conductivity of pure soy wax was 0.324 W·(m·K)−1, the thermal conductivities of 10% carbon nanofiber/soy wax and carbon nanotubes/soy wax were 0.469 and 0.403 W·(m·K)−1. Huang and Zhang [20] reported that the thermal conductivity of lauryl alcoholcapric acid was increased by 20.5% after adding multiwalled carbon nanotubes (MWNTs) with a ratio of 0.1 wt%. Zeng et al. [21] found that when the loading of MWNTs increased to 5% and no surfactant was added, the thermal conductivity of the composite PCM was enhanced to be 26% higher than that of PA. Zhang et al. [22] selected nine kinds of high thermal conductivity mediums such as kieselguhr, organic bentonite and EG to improve the thermal conductivity of the shape-stabilized PCM. They found that EG demonstrated the best performance in thermal conductivity enhancement. Yuan task group [23–27] studied the thermal conductivity of compound PCMs of stearic-palmitic acid, lauric-palmitic-stearic acid, lauric-myristicpalmitic acid, capric-myristic-palmitic acid, lauric-myristic-stearic acid
578
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
Fig. 1. SEM images of: (a) EG without expansion; (b) EG; (c) OA-LA/EG; (d) OA-LA/EG after circulation.
and so on with EG. The experimental results show that the thermal conductivities of the materials all obviously improved by the addition of EG. Zhou et al. [28] obtained adipic acid with a thermal conductivity of 4.35 W·(m·K) −1 by adding of 10 wt% EG, the thermal conductivity of which is 8 times higher than pure adipic acid. Yu et al. [29] found that when the composite PCM was prepared with
10% EG, the time required for heat storage/retrieval with the stearic acid decreased by 34.8 and 57.3%, respectively. However, the temperature of PCMs in the above study is not suitable for medical refrigerated transportation and air conditioning cold storage. Furthermore, they did not carry out a complete cool storage and discharge experiment after the cycle.
Fig. 2. OA-LA/EG images with different EG mass ratio.
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
579
The aim of this paper is to study the new developed binary organic PCM OA-LA/EG, which can be used in medical refrigeration transport system and cold-storage air-conditioning system at 2–8 °C. In order to enhance the thermal conductivity of OA-LA, EG with micrometer-scale grid structure was used as additive. EG is worm-like and its surface has micrometer-scale grid structure, which can adsorb OA-LA. The composite phase change material OA-LA/EG improves the thermal conductivity through the overlap of EG. The thermodynamic properties and cyclic stability of OA-LA/EG were investigated. 2. Experiments 2.1. Experimental technique 2.1.1. Reagents and instruments The ideal PCMs should have the following characteristics [30]: high latent heat of phase transformation, high crystallization rate, appropriate phase transition temperature, chemical stability, non-toxic, nonexplosive, non-corrosive, less costly and easily available. After comprehensive consideration, OA and LA were selected as the main raw materials for composite PCM in this study. OA and LA are analytical reagents and the expansion volume of EG is 250 mL·g−1. Experimental instruments mainly include box-type resistance furnace (SX2-4-10A), scanning electron microscope (KYKY-EM6000), SBC12 ion sputter, differential scanning calorimetry DSC (Model 200F3, temperature accuracy of 0.1 °C, enthalpy accuracy 0.1%), Hot Disk thermal constant analyzer (TPS2500s type, accuracy of 2%), magnetic stirrer, low-temperature thermostat bath (DC-6515), thermocouple (accuracy 0.01 °C), electrothermal blowing dry box, Agilent temperature-time recorder (34970A, recording time interval of 3 s), electronic analytical balance (FA2004, accuracy 0.1 mg), high and low temperature alternating test box (YSGJW-100C). 2.1.2. Preparation of OA-LA OA and LA were weighed, and placed them in the beaker. Notably, the mass ratio of OA and LA was determined as 81:19 in this study. Then, the beaker was heated to 40 °C and mixed by magnetic stirring apparatus for 30 min to guarantee the uniformity. The solutions with the mass of 29.1 g, 28.8 g, 28.5 g, 28.2 g, 27.9 g, 27.6 g, 27.3 g, and 27 g were prepared for the experiment. 2.1.3. Preparation and SEM scanning experiment of EG EG has a flake structure with a smooth surface and no voids and is necessary to be expanded before it can be used in experiments. EG was expanded in an electric box-type resistance furnace, which was set at 800 °C. The duration of expansion process was set 10 min. The SEM micrographs of EG was demonstrated in Fig. 1. Comparing Fig. 1 (a) with Fig. 1(b), it is clear that the surface of EG is relatively smooth before being expanded, while the surface of EG is worm-like with a clear porous structure after the high temperature expansion. The porous structure in expanded EG contributes to adsorbing more OA-LA base solution, which can achieve the purpose of enhancing the thermal conductivity. 2.1.4. Preparation of OA-LA/EG EG with porous structure was used as the matrix of OA-LA solutions in order to enhance the thermal conductivity of OA-LA. Eight groups of 30 g OA-LA/EG were prepared. The mass of OA-LA base solutions were 29.1 g, 28.8 g, 28.5 g, 28.2 g, 27.9 g, 27.6 g, 27.3 g, and 27 g. The mass of EG were 0.9 g, 1.2 g, 1.5 g, 1.8 g, 2.1 g, 2.4 g, 2.7 g, and 3 g, respectively. Thus, the mass proportions of EG in the eight groups were 3%, 4%, 5%, 6%, 7%, 8%, 9% and 10%, respectively. The prepared OA-LA/EG was stirred every half an hour for six times so that EG and OA-LA mixed uniformly. Eight groups of OA-LA/EG images with different EG mass ratio are shown in Fig. 2. As the proportion of EG increases, it can be seen from the images that the volume of OA-LA/EG are 32.1 mL, 38.6 mL, 48.7 mL, 56.3 mL,
Fig. 3. Schematic diagram of experimental setup for cool storage and discharge test 1polyurethane sealing cover; 2-beaker; 3-PCM; 4-thermocouple; 5-low temperature thermostat bath; 6-Agilent temperature-time recorder; 7-data display.
67.4 mL, 76.6 mL, 90.1 mL and 98.5 mL, respectively. However, all the mass is 30 g. The addition of EG makes the liquid OA-LA change into a solid form.
2.2. Experimental methods 2.2.1. The optimal ratio of EG Since EG does not participate phase transition at the phase change temperature, the higher the content of OA-LA is, the greater the latent heat is. Nevertheless, the content of EG can affect the thermal conductivity of the material and the speed of energy storage and release in practical applications. In this paper, the maximum adsorption ratio of EG to OA-LA mixed solution is defined as the optimal ratio of OA-LA to EG. The OA-LA/EG samples were placed in an electrothermal blowing dry box at 50 °C for 60 min. By comparing the mass before and after drying, the optimal proportion of EG was qualitatively determined by the smallest mass loss.
2.2.2. Cool storage and discharge experiment The performance of the material generally was validated by the cool storage and discharge experiments, which were carried out with two low-temperature thermostat baths. Experimental setup for cool storage and discharge of PCMs is shown in Fig. 3. One of the low-temperature thermostat baths was maintained at −15 °C and the other lowtemperature thermostat bath was kept at 25 °C. The configured 30 g OA-LA and OA-LA/EG were placed in a low-temperature thermostat bath with the temperature of −15 °C to cool the materials. The temperature of the material was measured by thermocouple and recorded by an Agilent temperature-time recorder. When the temperature is near −15 °C, the beaker was quickly removed and placed in another lowtemperature thermostat bath with the temperature of 25 °C. The cool storage and discharge curve of the materials were plotted by experimental data.
Table 1 Weight changes of OA-LA/EG before and after drying. Mass ratio of EG/% 3 4 5 6 7 8 9 10
Weight of material (before drying)/g
Weight of material (After drying)/g
Mass loss/g
Mass loss rate/%
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
0.1408 0.1846 0.2216 0.2591 0.2846 0.2848 0.2852 0.2849
0.1592 0.1154 0.0784 0.0409 0.0154 0.0152 0.0148 0.0151
53.1 38.5 26.1 13.6 5.1 5.1 4.9 5.0
580
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
2.2.5. Testing of material stability Low temperature composite PCM (30 g) of OA-LA/EG was weighed, and put it in the high and low temperature alternating test box. The temperature range of high and low temperature alternating test box was set at −40–60 °C. The period of stability test was 60 min and totally 100 cycles were conducted. The melting temperature, latent heat and mass loss of materials were measured. To validate the stability of OALA/EG, the results were compared with those of the original OA-LA/EG which were not treated in high and low temperature alternating test box. 3. Results and discussion 3.1. The optimal proportional of EG
Fig. 4. The fitting curve of OA-LA/EG mass loss.
2.2.3. Measurement of phase transition temperature and latent heat The phase transition temperature of material determines the range of application. The latent heat of material plays a key role in determining whether it is suitable for practical application. Therefore, experiments for testing phase transition temperature and latent heat were performed by DSC. Liquid nitrogen was used to cool the PCM and nitrogen vapor was used as protective gas and purge gas. Indium was performed as a reference material for calibration. The temperature was in the range of −20 to 30 °C and the purge gas flow rate was 20 mL·min−1. The protective gas flow rate was 60 mL·min−1 and heating and cooling rate was 5 K·min−1.
2.2.4. Measurement of thermal conductivity Thermal conductivity of PCMs has a great influence in the practical application. TPS2500s thermal constant analyzer was used to test the thermal conductivity of the material. Before testing, the machine was turn on and preheated for 30 min. The C5465 probe was inserted into the middle of the mixed solution and the vertical distances from the probe center to the liquid level and the wall of the beaker were recorded. In order to avoid the influence of indoor air flow and reduce the measurement error, the test probe and beaker was closed inside a container. The test temperature is 18 °C, which is the ambient temperature.
The comparison of weight changes of the composite PCM before and after electrothermal blowing dry box treatment is shown in Table 1. As can be seen from Table 1, the mass loss of the composite PCM with EG mass ratio of 3%, 4%, 5% and 6% decreased linearly. The mass loss rates were 53.07%, 38.47%, 26.13% and 13.63%, respectively. The insufficient addition of expanded graphite leading to some OA-LA cannot be adsorbed, which results in a higher mass loss rate. The mass loss rates of composite PCMs with EG mass ratios of 7%, 8%, 9% and 10% are between 4.9% and 5.1%, respectively. The mass loss rates of these four groups of materials are almost equal and stable. Therefore, the optimal mass ratio of EG is determined at 7%. As can be seen from Fig. 4, when the proportion of EG is less than the optimal addition ratio of 7%, the mass loss rate of OA-LA/EG has its regularity, which is approximately fitting to logarithmic function: y ¼ –57:26 ln ðxÞ–146:73 R2 ¼ 0:9971
3.2. Phase transition temperature and latent heat DSC results of OA-LA and OA-LA/EG are shown in Figs. 5 and 6. The phase transition temperature of OA-LA/EG is 3.6, which is reduced by 0.2 °C compared to OA-LA. The latent heat of OA-LA is 132.8 J·g−1, which is reduced 8.9 J·g−1 compared to OA-LA. The reason is that EG just acts as a skeleton without phase transformation in the composite PCM and does not contribute to its latent heat. The proportion of EG (7 wt%) matches the decrease of the latent heat of 6.3%.
Fig. 5. DSC curve of OA-LA.
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
581
Fig. 6. DSC curve of OA-LA/EG.
3.3. Thermal conductivity of OA-LA/EG The thermal conductivity of common organic PCMs is very low, which is only 0.2–0.4 W·(m·K)−1. Thermal conductivity is an important factor affecting the use of PCMs. Higher thermal conductivity can speed up the cool storage and discharge of PCMs, which can ensure uniform and stable temperature of materials in application and improve the utilization of materials. The step-cooling curves of OA-LA and OALA/EG are shown in Fig. 8. During sensible heat release, OA-LA/EG takes 2.40 min and OA-LA/EG takes 1.45 min for the temperature dropped from 20 to 10 °C. The addition of EG increased the cooling rate of sensible heat by 65.5%. By adding OA-LA into EG matrix, the perfect heat conduction grid structure can be formed due to the overlap joint of EG, which can improve the way of heat conduction. Hence, heat transfer is given a priority to heat conduction, whose transmission speed is high. Which makes the thermal conductivity of OA-LA increased significantly. During the absorption of latent heat of fusion, the time of OA-LA is 8.45 min and the time of OA-LA/EG is 4.95 min when the temperature increased from 5 °C to 7 °C. In the period of latent heat release, the content of OA-LA was 69.25 min and OA-LA/EG was 9.40 min when the temperature of material declined from 5 to 3 °C. The addition of EG improves the release rate of OA-LA latent heat by 636.7%. After adding EG, OA-LA is divided into several small units to
increase the contact area of the material. EG can not only increase the thermal conductivity, but also greatly enhance the coagulation and nucleation rate of the material. The thermal conductivity of OA-LA and OALA/EG are 0.3357 W·(m·K)−1 and 1.275 W·(m·K)−1, respectively. The temperature of the test environment is 18 °C at room temperature. The addition of EG increased the thermal conductivity by 2.8 times, so material can achieve rapid storage and discharge. Therefore, OA-LA/EG has a broader application space in medical refrigeration transport system and cold-storage air-conditioning system. 3.4. Thermal reliability of OA-LA/EG Stability is an important performance indicator of the material. The results of 100 cycles of cool storage and discharge experiments on OA-LA/ EG before and after the storage and discharge are shown in Fig. 9. DSC results of OA-LA/EG and OA-LA/EG after 100 cycles are shown in Fig. 7. The phase transition temperature of OA-LA/EG after 100 cycles is 3.7, which is increased by 0.1 °C compared to that before the cycle. The latent heat of OA-LA/EG after 100 cycles is 132.3 J·g−1, which was 0.6 J·g−1 lower than that before cycling. The latent heat and phase transition temperature of OA-LA/EG did not change obviously before and after the cycle. The results of 100 cycles of cool storage and discharge experiments on OA-LA/EG before and after the storage and discharge are shown in
Fig. 7. DSC curve of OA-LA/EG after 100 cycles.
582
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583
4. Conclusion
Fig. 8. Step cooling curve of OA-LA and OA-LA/EG.
Fig. 9. In the phase of sensible heat release, when the temperature of material is decreased from 20 to 10 °C, the original OA-LA/EG takes 1.45 min and the OA-LA/EG after 100 cycles takes 1.64 min. After 100 cycles, the rate of sensible cooling of OA-LA/EG is decreased 13.1%. During the stage of latent heat release, when the temperature of the material is decreased from 5 to 3 °C, the original OA-LA/EG takes 9.4 min and the OA-LA/EG after 100 cycles takes 11.50 min. After 100 cycles, the release rate of latent heat of OA-LA/EG is reduced by 22.3%. During the phase of latent heat absorption, the temperature of the material increased from 5 to 7 °C, the time for OA-LA/EG was 4.95 min and the time for OA-LA/EG after 100 cycles was 5.82 min. After circulation, the absorption rate of melting latent heat of OA-LA/ EG decreased by 17.6%. The thermal conductivity of OA-LA/EG after 100 cycles is 1.132 W·(m·K)−1, which decreased by 11.2% compared with that before cycling. After circulation, the mass loss rate was 6.5% which was greater than the mass loss rate range 4.9 to 5.1%. The reason the sensible heat, latent heat and the thermal conductivity of OA-LA/EG are less than those before the cycling of 100 times, is that a small part of the OA-LA spill from the EG, which makes the grid structure cut off by it, and then lead to the slowdown of heat transfer. After 100 cycles of OA-LA/EG, the phase change temperature, latent heat and thermal conductivity did not change obviously.
Fig. 9. Cold storage and discharge curves before and after OA-LA/EG 100 cycles.
(1) A novel low-temperature composite PCM OA-LA/EG used in medical refrigeration transport system and cold-storage airconditioning system with temperature range of 2–8 °C has been developed. The optimal mass ratio of EG was determined as 7% for OA-LA by electrothermal blowing dry box. The phase transition temperature of OA-LA/EG is 3.8 °C and the latent heat is 141.7 J·g−1. (2) The thermal conductivity of OA-LA was increased 2.8 times after addition of EG. Compared to the base OA-LA solution, the cooling rate of sensible heat of OA-LA/EG was increased by 65.5%, the heat absorption rate of latent heat was increased by 70.7% and the release rate of latent heat was increased by 636.7%. (3) Both the phase change temperature and latent heat of the OA-LA/ EG were not changed obviously after 100 cycles of cool storage and discharge contrast experiment. Due to the stable thermal storage and discharge properties of the material, the newly developed OA-LA/EG can maintain stable thermodynamic properties in practical application. In further work, the experiments with vacuum insulation incubator to simulate the properties of the material in application will be conducted.
Acknowledgements This study benefits from financial support of the Key Program of Shanghai Municipal Science and Technology Commission (Grant No. 16040501600) and National Key R & D Project (Grant No. 2018YFD0401305). References [1] G. Fang, F. Tang, L. Cao, Preparation, thermal properties and applications of shapestabilized thermal energy storage materials, Renew. Sust. Energ. Rev. 40 (C) (2014) 237–259. [2] A. Arora, G. Sant, N. Neithalath, Numerical simulations to quantify the influence of phase change materials (PCMs) on the early- and later-age thermal response of concrete pavements, Cem. Concr. Compos. 81 (2017) 11–24. [3] L.F. Cabeza, A. Castell, C. Barreneche, et al., Materials used as PCM in thermal energy storage in buildings: a review, Renew. Sust. Energ. Rev. 15 (3) (2011) 1675–1695. [4] A.D. Gracia, L.F. Cabeza, Phase change materials and thermal energy storage for buildings[J], Energ. Buildings 103 (2015) 414–419. [5] Z. Zhou, Z. Zhang, J. Zuo, et al., Phase change materials for solar thermal energy storage in residential buildings in cold climate, Renew. Sust. Energ. Rev. 48 (2015) 692–703. [6] F. Souayfane, F. Fardoun, P.H. Biwole, Phase change materials (PCMs) for cooling applications in buildings: a review, Energ. Buildings 129 (2016) 396–431. [7] B. Xu, P. Li, C. Chan, Application of phase change materials for thermal energy storage in concentrated solar thermal power plants: a review to recent developments, Appl. Energy 160 (2015) 286–307. [8] G. Ma, S. Liu, S. Xie, et al., Binary eutectic mixtures of stearic acid- n -butyramide/n -octanamide as phase change materials for low temperature solar heat storage, Appl. Therm. Eng. 111 (2017) 1052–1059. [9] S. Kahwaji, M.B. Johnson, A.C. Kheirabadi, et al., Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures, Sol. Energy Mater. Sol. Cells 167 (2017) 109–120. [10] J.K. Carson, A.R. East, The cold chain in New Zealand-a review, Int. J. Refrig. 87 (2018) 185–192. [11] Q. Yan, Research on fresh produce food cold chain logistics tracking system based on RFID, Adv. J. Food Sci. Technol. 7 (3) (2015) 191–194. [12] D.C. Murillo, Refrigerated container versus bulk: evidence from the banana cold chain, Marit. Policy Manag. 42 (3) (2015) 228–245. [13] X. Huang, Y.D. Cui, G.Q. Yi, et al., Preparation and properties of composite phase change material of lauric acid and expanded graphite, J. Chem. Eng. S1 (2015) 70–374. [14] Y. Taguchi, H. Yokoyama, H. Kado, et al., Preparation of PCM-microcapsules by using oil absorbable polymer particles, Colloids Surf. A Physicochem. Eng. Asp. 301 (7) (2007) 5–41. [15] T. Ying, S.U. Dang, J. Bai, Organic Phase Change Compound Materials for Nonfreezing Cold Chain. Transactions of the Chinese Society for Agricultural Machinery, 2017. [16] Y.P. Zhang, J.H. Ding, X. Wang, et al., Influence of additives on thermal conductivity of shape-stabilized phase change material, Sol. Energy Mater. Sol. Cells 90 (11) (2006) 1692–1702.
Y. Li et al. / Journal of Molecular Liquids 277 (2019) 577–583 [17] A. Siahpush, J. O'Brien, J. Crepeau, Phase change heat transfer enhancement using copper porous foam, J. Heat Transf. 130 (8) (2008) 318–323. [18] R.K. Sharma, P. Ganesan, V.V. Tyagi, et al., Thermal properties and heat storage analysis of palmitic acid-TiO2, composite as nano-enhanced organic phase change material (NEOPCM), Appl. Therm. Eng. 99 (2016) 1254–1262. [19] C.H. Y B Cui, S. Hu Liu, et al., The experimental exploration of carbon nanofiber and carbon nanotube additives on thermal behavior of phase change materials, Sol. Energy Mater. Sol. Cells 955 (4) (2011) 1208–1212. [20] Y. Huang, X. Zhang, Heat transfer property of lauryl alcohol-capric acid-nanoparticle composite phase change materials, CIESC J. 67 (6) (2016) 2271–2276. [21] J.L. Zeng, Z. Cao, D.W. Yang, et al., Effects of MWNTs on phase change enthalpy and thermal conductivity of a solid-liquid organic PCM, J. Therm. Anal. Calorim. 95 (2) (2009) 507–512. [22] H. Zhang, X. Gao, C. Chen, et al., A capric–palmitic–stearic acid ternary eutectic mixture/expanded graphite composite phase change material for thermal energy storage, Compos. A: Appl. Sci. Manuf. 87 (2016) 138–145. [23] N. Zhang, Y.P. Yuan, Y.X. Du, et al., Preparation and properties of palmitic-stearic acid eutectic mixture/expanded graphite composite as phase change material for energy storage, Energy 78 (2014) 950–956. [24] Y.G. Yuan, Y.P. Yuan, N. Zhang, Preparation and properties of lauric-palmitic-stearic acid eutectic mixture/expanded graphite composite phase change material for energy storage, J. Chem. Eng. 65 (S2) (2014) 286–292.
583
[25] N. Zhang, Y.P. Yuan, X. Wang, et al., Preparation and characterization of lauricmyristic-palmitic acid ternary eutectic mixtures/expanded graphite composite phase change material for thermal energy storage, Chem. Eng. J. 231 (2013) 214–219. [26] Y.G. Yuan, Y.P. Yuan, N. Zhang, Preparation and thermal characterization of capricmyristic-palmitic acid/expanded graphite composite as phase change material for energy storage, Mater. Lett. 125 (2014) 154–157. [27] C. Liu, Y.P. Yuan, N. Zhang, et al., A novel PCM of lauric-myristic acid/expanded graphite composite for thermal energy storage, Mater. Lett. 120 (2014) 43–46. [28] W. Zhou, K. Li, J. Zhu, et al., Preparation and thermal cycling of expanded graphite/ adipic acid composite phase change materials, J. Therm. Anal. Calorim. 129 (3) (2017) 1–7. [29] H. Yu, J. Gao, Y. Chen, et al., Preparation and properties of stearic acid/expanded graphite composite phase change material for low-temperature solar thermal application, J. Therm. Anal. Calorim. 124 (1) (2016) 87–92. [30] S.A. Mohamed, F.A. Al-Sulaiman, N.I. Ibrahim, et al., A review on current status and challenges of inorganic phase change materials for thermal energy storage systems, Renew. Sust. Energ. Rev. 70 (2017) 1072–1089.