Fabrication and characterization of phase change nanofluid with high thermophysical properties for thermal energy storage

Fabrication and characterization of phase change nanofluid with high thermophysical properties for thermal energy storage

Journal of Molecular Liquids 284 (2019) 23–28 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier...

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Journal of Molecular Liquids 284 (2019) 23–28

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Fabrication and characterization of phase change nanofluid with high thermophysical properties for thermal energy storage Jun Ji a, Yue Chen a, Yinghui Wang a, Xuelai Zhang a,⁎, Zhen Tian a, Sunxi Zhou a,b, Sheng Liu b a b

Institute of Cool Storage Technology, Shanghai Maritime University, Shanghai, 201306, China Beijing Vegetable Research Center, Academy of Agriculture and Forestry Sciences, Beijing, 100097, China

a r t i c l e

i n f o

Article history: Received 8 January 2019 Received in revised form 19 March 2019 Accepted 19 March 2019 Available online 22 March 2019 Keywords: PAAS MWCNT Interfacial Supercooling Cold discharge

a b s t r a c t The thermophysical properties of water were modified by adding MWCNT and the organic PAAS. The effects of MWCNT and PAAS on supercooling degree and phase change characteristics of water were experimentally studied. The results showed that the length of the water phase change platform increases by 41% and decreases by 19.6% when concentration of PAAS is from 0 wt% to 2 wt% and 2 wt% to 5 wt%, respectively; The maximum supercooling degree declines up to 95% by adding MWCNT with the variations of particle size and concentration. The interfacial free energy at MWCNT outer diameter of 8–15 nm is approximately 1.75 and 2.14 larger than particle size of 20–40 nm and N50 nm. The temperature homogeneity of composite material is better than that of DW in application test, and the increased percentage of cold discharge time is less than that of the small dose experiments. The cold discharge time of the optimum PCM at top cold plate and other three positions are 81.8% and 35.3% higher than those of DW. © 2019 Elsevier B.V. All rights reserved.

1. Introduction With increasing concerns over energy demand and environmental problems, energy-efficient use has become one of the world's most noteworthy issues [1]. Phase change materials (PCMs) offer considerable promise for cold chain and food preservation via cold energy storage. The mismatch of energy supply and demand can be solved, and furthermore, the energy efficiency may be improved [2,3]. Water is widely used as the most ideal phase change cold storage material because it is cheap, pollution free and has high latent heat. However, water has a high supercooling degree of about 8 °C, thus it causes a huge waste of energy and reduces the energy efficiency. Methods for thermal conductivity enhancement and supercooling of water have been investigated by many researchers. It is found that the addition of nucleating agent [4,5] or cold fingering [6,7] can optimize the phenomenon of supercooling during water solidification. Spherical metal [8,9], foam metal [10–12], nano-metal [13,14], nano-oxide [15–17], carbon nanotubes [18–20] and etc. have effects on reducing the supercooling degree of water while improving the thermal conductivity. However, the time of heat

⁎ Corresponding author at: No. 1550, Haigang Avenue, Pudong New District, Shanghai, China. E-mail address: [email protected] (X. Zhang).

https://doi.org/10.1016/j.molliq.2019.03.116 0167-7322/© 2019 Elsevier B.V. All rights reserved.

discharge will decrease with the increase of thermal conductivity. Liang et al. found that high polymer materials can extends the phase change time of materials, and slow down the rate of cold discharge [21]. Therefore, it is necessary to find the appropriate concentration of polymer materials to optimize the temperature control capacity of nano-composite materials. Undoubtedly, it is important to maintain the good dispersion stability of nanomaterials in applications, because the nanoparticles can easily form agglomerates due to van der Waals force, high surface energy and surface electrostatic charge. Dispersant can inhibit the agglomeration of nanomaterials, which has become one of the focuses in the field of nanofluid in recent years [22–24]. Jia et al. studied the effect of anionic surfactant SDS with different concentration on the supercooling of TiO2-H2O nanofluid, and found that the supercooling degree was reduced by 30.6% [25]. Srinivas used cationic surfactant CTAB as a stabilizer for three nanofluids [26]. By comparing the stability of TiO 2 nanofluid with three different ionic surfactants, Li found that the anionic surfactant SDBS was the best, and it could reduce the supercooling degree by 37.02% [27]. It can be seen that the research interest is focused on the stability and supercooling of nanofluid with different ionic surfactant instead of organic polymers, and we speculated that the PAAS adsorbed film on the surface of MWCNT will affect the interfacial free energy, which in turn has an impact on the supercooling degree of PCMs.


J. Ji et al. / Journal of Molecular Liquids 284 (2019) 23–28 Table 2 Properties of MWCNT.

Nomenclature Abbreviations MWCNT multi-walled carbon nanotube PAAS polymer polyacrylic acid sodium PCM phase change material SDS sodium dodecyl sulfate CTAB cetyl trimethyl ammonium bromide SDBS sodium dodecyl benzene sulfonate DW distilled water RFID radio frequency identification GSP good supplying practice Symbols ΔT c ΔG θ R σ ρ H

Inner diameter/nm

Outer diameter/nm


Specific surface area/m2·g−1


3–5 5–10 5–15

8–15 20–40 N50

30–50 10–30 10–20

400 200 120

95 95 95

supercooling degree [°C] mass concentration [%] interfacial free energy [mN/m] contact angle [°] radius free surface energy [mN/m] density[kg/m3] latent heat [kJ/kg]

The goals of this work are thus two-fold. First, we wish to determine the influence of organic polymer concentration on the duration of cold discharge. Secondly, we wish to know the combined effects of organic polymers (PAAS) and nanomaterials with different concentration and size. 2. Materials and methods 2.1. Materials and apparatus The experimental apparatus used in this experiment are shown in Table 1. Three different sizes of MWCNT were used as additives. The properties of MWCNT are shown in Table 2. PAAS was used as organic polymer. In order to avoid the influence of ions and substances, DW was used as base fluid. Five samples of 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt% PAAS were added into DW, which were measured by an electronic balance. The material was placed for 12 h to ensure that the PAAS is fully saturated. Nanocomposite materials were prepared by two-step method, and sonicated with an ultrasonic oscillator for 30 min. The nanocomposites with different particle size of MWCNT nanofibers and concentration were prepared respectively. 2.2. Test procedure 2.2.1. Material properties measurements The experimental set up is shown in Fig. 1. In order to eliminate the disturbance of the outside temperature and dust, polyurethane foam was used to seal the beaker. The T-type Table 1 Main experimental apparatus. Name



Low-temperature thermostat bath Thermal constant analyzer Contact angle goniometer (JCY-2) Ultrasonic oscillator Electronic balance Analytical balance Good supplying practice instrument Agilent 34970A DSC (200F3)

−65–100 °C 0.03–100 W/m·K 0–180° 180 W 0–20 g 0–60 g −40-60 °C −200-200 °C −170-600 °C

±0.1 °C b2% 0.1° \ ±0.1 mg ±0. 01 mg ±0.5 °C ±0.01 °C ±1%

Fig. 1. Step cooling test device. 1—Computer; 2—Agilent data logger; 3—DC6515 thermostatic bath; 4—T-type thermocouple; 5—PCM.

thermocouple passed through the polyurethane sealing plug and into the solution leaving N1 cm spacing from the bottom of the beaker. The solidification and melting processes of the PCM were recorded by Agilent with the thermostatic bath at −15 °C and 30 °C, and the data was averaged for 10 times of replicate experiments. Supercooling degree is defined as the difference between the solidification temperature and the onset temperature. Thermal conductivity of the PCM was measured by thermal constant analyzer. The PCM stood for 30 min, and then the probe C5465 of thermal constant analyzer was inserted vertically into the material via the small hole on the top of test chamber. The latent heat of PCM was measured by the differential scanning calorimeter. A sample was placed in the center of the aluminum crucible, which was weighed by analytical balance. Liquid nitrogen was used as the cooling medium at a flow rate of 40 ml/min. The contact angle of nanocomposites was measured with volume of 5 μl by contact angle goniometer.

2.2.2. Application experiment Performance of the optimum PCM was compared with DW. The PCM and DW were filled in the cold storage plate and placed in a cryopreservation tank at −20 °C for 12 h to make them completely frozen. The plates were then transferred to a vacuum incubator. Yogurt was used as the cold object, RFID cards were placed on the bottom corner, side plate, and top plate of the incubator and around the yogurt, which is shown in Fig. 2. GSP instrument was used to measure the process of cold discharge.

Fig. 2. Phase change incubator.

J. Ji et al. / Journal of Molecular Liquids 284 (2019) 23–28


Fig. 3. Effect of PAAS concentration on thermal conductivity. Fig. 5. Effect of particle size and concentration of MWCNT on supercooling degree.

3. Results and discussion 3.1. Effect of PAAS concentration on time of cold discharge Thermal conductivity of PCM is shown in Fig. 3. The effects of the concentration of PAAS from 0% to 5 wt% on the time of cold discharge are shown in Fig. 4. As can be seen from Fig. 3 and Fig. 4, thermal conductivity of PCM is reduced by 2.7% and time of cold discharge is extended by 41% with the PAAS concentration increasing from 0% to 2 wt%. With the continuous increase of PAAS concentration from 2 wt% to 5 wt%, thermal conductivity of PCM is decreased by 2.9% and time of cold discharge is reduced by 19.6%. This can be attributed to the relaxation, gradually stretches, and swelling of polymer chain with the entry of water molecules as the concentration of the PAAS increase from 0 wt% to 2 wt%. The solution changes from free state into gel state, which slows down the movement of the particles, thereby reduces the heat transfer and extends the time of cold discharge. The hydrogel has reached a saturated state with PAAS at a concentration of 2 wt%. With the PAAS concentration increasing from 2 wt% to 5 wt%, the solution arrives to a supersaturated condition. The material presents a solid state and even reunites into pieces due to

the decreasing amount of water molecules which is adsorbed by the polymer chain. It is also found that microporous holes appear inside the material when the concentration of PAAS reaches2 wt%. Meanwhile, with the increase in concentration, it is easier to lock air in the material. Because of the existence of solid state and pores, there is enhanced heat transfer between the liquid and the solid, which shortens the time of cold discharge. 3.2. Effect of particle size and concentration of MWCNT on supercooling degree Due to the hydrophobicity of carbon nanotubes and the agglomeration effect of nanomaterials, the organic polymer PAAS was added as a dispersing agent to the aqueous nano-solution. In this paper, nanocomposites with different particle size and concentrations were prepared respectively while the concentration of PAAS was kept at 2 wt%. The effects of particle size and concentration of MWCNT on supercooling degree are shown in Fig. 5. Outer diameter of MWCNT nanofibers in composite PCMs varied from 8 to 15 nm, 20 to 40 nm and N50 nm. As can be seen from Fig. 5, MWCNT has a significant improvement on supercooling degree. The supercooling degree is decreased with the increase of particle size of MWCNT nanofibers and concentration. With MWCNT concentration increasing from 0.1 wt% to 0.5 wt%, it is found that the supercooling degree of PCM decreases from 1.42 °C to 0.41 °C when the outer diameter is 8–15 nm. Similarly, the supercooling degree of PCM declines from 1.12 °C to 0.38 °C and from 0.89 °C to 0.3 °C for the size of 20–40 nm and N50 nm. And the equations for different outer diameters of the MWCNT nanofibers are shown below, the maximum error of the fitting curves is about 4.7%, 5.1% and 3.3%, respectively. 8–15 nm     C MWCNT C MWCNT ΔT ¼ 2:88153−1:27162 1−e− 0:11006 −1:21082 1−e− 0:12012


20–40 nm     C MWCNT C MWCNT ΔT ¼ 1:78907−0:70514 1−e− 0:16862 −0:74514 1−e− 0:15442


N50 nm

Fig. 4. Effect of PAAS concentration on time of cold discharge.

    C MWCNT C MWCNT ΔT ¼ 1:80573−0:68833 1−e− 0:10518 −0:73533 1−e− 0:09618



J. Ji et al. / Journal of Molecular Liquids 284 (2019) 23–28

group, which is dissociated from the PAAS, attracts the water molecules onto the surface to the adsorbent film. The adsorbed film is similar to a semipermeable membrane, whereby water molecules reach the surface of the nanofibers via adsorbed layer by osmotic action, so that heterogeneous nucleation will occur and reduce the supercooling degree. With the increasing of MWCNT, the supercooling degree is gradually reduced because of the role of MWCNT as nucleating agent. In the cold storage process of PCM, MWCNT is used as nucleation substrate that promotes heterogeneous nucleation of PCM. Therefore, the nucleation interface formed by low-energy nucleus and nanomaterials replaces the nucleation interface of the original pure material. The supercooling degree of PCM reduced as the heterogeneous nucleation interface energy is less than that of pure substances. The number of nucleating agents in the PCM rises with the increasing of MWCNT concentration, which can provide more touch points for the nucleation. Fig. 5 also shows that the supercooling degree decreased with increase of the particle size of MWCNT nanofibers at the same concentration. According to the theory of heterogeneous nucleation, the supercooling degree depends on the interfacial free energy, and the interfacial free energyΔG is mainly determined by contact angle θ and radius R of nucleation substrate, which can be given as follows [30,31]:

Fig. 6. Agglomeration of nanoparticles.

ΔG ¼

8πσ 12

η ¼ cosθ ¼ Fig. 7. Reaction of PAAS to the MWCNT.

f ðη; xÞ


σ 13 −σ 23 σ 12


3ðΔG1 Þ2

"      # 1−ηx 3 x−η x−η 3 þ þ x3 2−3 g g g   2 x−η −1 þ 3ηx g 

Figs. 6 and 7 show the agglomeration of nanofibers and the reaction of PAAS to the MWCNT. In the process of nanocrystallization, a large number of unstable charged particles accumulate on the nascent nano-surface, and MWCNTs are apt to agglomerate under the action of mutual attraction of charged particles. When the distance between the two nanofibers is short, the van der Waals force between the MWCNTs is greater than self-gravity, and promotes the agglomeration of the nanofibers. Hydrogen bond network is changed when the water flows in carbon nanotubes, a layer of water molecules that are composed by the “free” hydroxyl (dangling bond) is formed on the surface of carbon nanotubes [28].Water layer will be combined to form a liquid bridge when the two nanofibers approach, the pressure difference caused by the liquid bridge will induce CNT to attract each other. Nucleated substrate is decreased with the agglomeration of the nanofibers which is caused by these effects, supercooling degree of PCM is then enlarged by nucleation on the new agglomerated substrate. After PAAS is added, a layer of adsorbed film will form on the surface of MWCNTs and produce a spatial repulsion effect [29], so that the MWCNTs can be dispersed in the solution without agglomeration. The (-COO-)

f ðη; xÞ ¼ 1 þ

 1 g ¼ 1 þ x2 −2ηx 2 x¼

R RρHΔT DW ¼ r 2σ 13 T m

ð6Þ ð7Þ ð8Þ

where r is the radius of the ice crystal formed on the nucleation substrate. The parameters σ12, σ13, ρ and H denote the specific interfacial free energy (47.2 mN·m−1) of the solution and the nanoparticles [32], the free surface energy (31.7 mN·m−1) of ice crystals and water [33], the density of ice (910 kg·m−3) [34], and the latent heat of distilled water (335 kJ·kg−1) [35]. X values of three different MWCNT particle sizes at the same concentration are 2.36–4.43, 9.615–15.384 and N24.04, respectively. The contact angle of MWCNT nanofluid is 30.1°. The interfacial free energy ΔG is determined by f(η, x). Fig. 8 shows the change of f(η, x) with different

Fig. 8. With the change of X, function f(η,x).

J. Ji et al. / Journal of Molecular Liquids 284 (2019) 23–28


Fig. 10. Melting curve of the optimum PCM after undergoing 300 freeze-thaw cycles.

Fig. 9. Melting curve of PCM

X and contact angle θ. f(η,x) is decreased as the X increases in the range of 0–46.3.When X is N46.3, f(η, x) approaches a constant of 0.027. When MWCNT size is 8–15 nm, f(η, x) is 0.041–0.066 which is approximately 1.75 and 2.14 larger than particle size of 20–40 nm and N50 nm, respectively. The larger the particle size of the MWCNT nanofibers, the smaller the interfacial free energy of the heterogeneous nucleation on the surface of the nanoparticles, thereby the PCM can be crystallized under low supercooling degree. So, at the same concentration, the supercooling degree of PCM decreases as the particle size of MWCNT nanofibers enlarges. There is no doubt that the agglomeration behavior of nanoparticles is quite complex and directly affects the supercooling degree. Actually, the temperature also has an influence on the nanoparticle agglomeration [36,37], and that's what we're going to focus on.

3.3. Comparison of PCM cold discharge The key to the application of phase-change material in cold chain logistics is the rate of its storage and cold discharge, so it is necessary to ensure that the material can maintain a long cooling time. In order to verify the energy storage performance of the PCM, the composites were compared with the distilled water, and the optimum PCM was determined by DSC test and thermal conductivity test. When the addition amount of three kinds of MWCNT is 0.4 wt%, 0.4 wt% and 0.3 wt%, the supercooling degree of the PCMs reach their respective optimal value. The DSC melting curves of optimum PCMs are shown in Fig. 9, the thermal properties are shown in Table 3. According to the thermal properties of the PCM, the PCM with particle size of 20 - 40 nm is selected as the optimum PCM, which has the highest thermal conductivity, high latent heat value and low supercooling degree, to carry out the comparative experiment. What's more, after the accelerated experiment of 300 freeze-thaw cycles, it demonstrated excellent thermal cycling stability as shown in Fig. 10. The latent heat of fusion is 311.4 J/g, which almost can be negligible.

Next, the performance of the optimum PCM was compared with DW, the experimental results are shown in Fig. 11. The PCM and distilled water in the incubator undergo three stages, which are solid-state cold discharge, phase change cold discharge and liquid-state cold discharge. Fig. 10 shows that the cold maintaining time of the top cold plate is less than that of the other locations because the cold air will sink, especially for distilled water. The phase change cold discharge of the top cold plate is completed in 22 h for DW, and 40 h for optimum PCM, which is extended by 81.8%. The cold discharge of the other three positions have basically the same trend, and that of the optimum PCM is maintained for 46 h, which is 35.3% higher than that of DW, comparing with the previous small doses experimental results, the cold discharge time reduces by 5.7%. The temperature uniformity and cold discharge time of the optimum PCM are better than DW, which is more conducive to the practical transport conditions.

4. Conclusions In this paper, the supercooling degree and time of cold discharge under the condition of adding different concentration of PAAS and MWCNT, and different particle size of MWCNT nanofibers have been investigated. The present study confirms that PAAS can prolong the phase change time of PCM; maximum can be extended by 41%. PAAS can form a layer of adsorbed film on the MWCNT and produces a spatial repulsion effect, which can enhance the nucleating effect of MWCNT and thus decrease the supercooling degree. The particle size and concentration of MWCNT are found to be important factors on the supercooling degree; it is confirmed that the concentration of MWCNT affects the supercooling degree of PCM by changing the number of nucleation substrate. The maximum supercooling degree declines up to 95% by adding MWCNT with the variation of particle size and concentration. The theoretical analysis of heterogeneous nucleation associate with PAAS reveals that the supercooling degree of water decreases by the decline of the heterogeneous nucleation surface free energy when the particle size of MWCNT nanofibers increases. The interfacial free energy at MWCNT size of 8–15 nm is approximately 1.75 and 2.14 larger than particle size of 20–40 nm and N50 nm, respectively. And it shows that the optimum PCM cold discharge time of top cold plate and other three positions are 81.8% and 35.3% higher than those of DW and also have the characteristic of the temperature uniformity.

Table 3 Thermal properties of PCM. Particle size/nm



Latent heat/J·g−1

Thermal conductivity/W·mK−1

8–15 20–40 N50

0.4 0.4 0.3

0.45 0.4 0.39

314.5 313.8 306.9

0.8826 0.8964 0.8633


J. Ji et al. / Journal of Molecular Liquids 284 (2019) 23–28

Fig. 11. Temperature of the incubator (a) distilled water (b) optimum PCM.

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