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eutectic salt composite phase change cold storage material
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Preparation and performance of modified expanded graphite/eutectic salt composite phase change cold storage material Ning Xie , Zhongping Li , Xuenong Gao , Yutang Fang , Zhengguo Zhang PII: DOI: Reference:
S0140-7007(19)30430-X https://doi.org/10.1016/j.ijrefrig.2019.10.008 JIJR 4546
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
12 April 2019 17 September 2019 12 October 2019
Please cite this article as: Ning Xie , Zhongping Li , Xuenong Gao , Yutang Fang , Zhengguo Zhang , Preparation and performance of modified expanded graphite/eutectic salt composite phase change cold storage material, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.10.008
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Highlights
The K2HPO4·3H2O-NaH2PO4·2H2O-Na2S2O3·5H2O-H2O eutectic salt has large latent heat.
Na2B4O7·10H2O can effectively reduce the supercooling degree of the eutectic salt.
The adsorption capacity of modified expanded graphite reaches up to 80.71 wt%.
Modified expanded graphite enhances the thermal conductivity of eutectic salt.
Preparation and performance of modified expanded graphite/eutectic salt composite phase change cold storage material
Ning Xiea, Zhongping Lia, Xuenong Gaoa,*, Yutang Fanga, Zhengguo Zhanga
a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation (The Ministry
of Education), School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China.
* Corresponding Author E-mail address: [email protected]
ABSTRACT Cold energy storage technologies using phase change materials (PCM) have received increasing attention these years. In this work, we develop a novel kind of composite PCM using modified expanded graphite (MEG) to adsorb K2HPO4·3H2O– NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt by impregnation method. Within 120 minutes, the adsorption capacity of MEG for eutectic salt is 75.33% higher than that of unmodified expanded graphite (EG). The composite PCM has a phase change temperature of -5.30 ℃, a large latent heat of 161.8 kJ·kg-1, and a low supercooling degree of 1.83 ℃. The thermal conductivity of the composite PCM is 13.3 times as large as that of the eutectic salt. Moreover, the thermal cycle tests demonstrated excellent
thermal
reliability.
These
outstanding
thermal
properties
endow
K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt/MEG composite PCM with broad application prospects in the fields of beer industry, air conditioning refrigeration, refrigerator refrigeration and cold chain logistics. Keywords: Eutectic salt, Modified expanded graphite, Composite PCM, Cold energy storage
Nomenclature PCMs
phase change materials
EG
expanded graphite
MEG
modified expanded graphite
CA
capric acid
PA
palmitic acid
SA
stearic acid
OA
octanoic acid
DSC
Differential scanning calorimeter
FT-IR
Fourier transformation infrared
TBAB
tetra-n-butyl ammonium bromide
PEG
polyethylene glycol
1. Introduction Thermal energy storage technology is an important way to minimize the waste of energy and increase energy utilization efficiency [1, 2]. The core of this technology is the development of phase change materials (PCMs) with excellent performance and low cost[3, 4]. At present, researchers have done a lot of investigations on above normal temperature PCMs. For instance, the mid temperature PCMs with phase change temperatures in the range 15–90 °C can be contributed to solar, medical, textile, electronic and energy-saving applications in building design [5-7]. The high temperature PCMs with a phase change temperature above 90 °C developed mainly for industrial and aerospace applications[8, 9]. However, cooling is also one of the major global energy consuming processes. A suitable cold storage material can accumulate large amount of cold thermal energy into the storage medium during charging process, while retrieve it during discharging process. Therefore, energy conservation can be achieved by reducing the electricity peak load. Compared with sensible cold energy storage system, latent storage system using PCMs attracts more attention due to its higher energy storage capacity and density, thus, only small size energy storage system is needed. Therefore, development of novel kind of PCMs with excellent thermal performance is very important for cold energy conservation including free cooling system and refrigeration fields [10-12]. The refrigeration system can also be optimized by PCMs with suitable thermal properties. For low-temperature PCMs, the current research and application are mainly concentrated in the field of air-conditioning[13, 14]. And few studies pay attention to the development and application of low temperature PCMs with phase transition temperature below 0°C [15]. However, in many industries such as food preservation[16, 17], chemical industry[12], household refrigerators and refrigeration
trucks [18] as well as cold chain logistics[19], PCMs with phase transition temperature below 0 °C have a very large application potential for cold energy saving in freezers for maintaining stable temperatures and increasing energy utilization efficiency. Low-temperature PCMs below 0 °C are mainly divided into three categories: ice, organic PCMs, and inorganic PCMs [12]. Ice storage has the advantages of large latent heat, and low price, but the disadvantages of low evaporation temperature, large supercooling degree and large change in phase change volume limit their practical applications [14]. Organic phase change cold storage materials mainly include paraffin wax, aromatic hydrocarbons, alcohols, etc., which are not easy to encounter phase separation and supercooling problem, but they have the disadvantages of high price, low latent heat of phase change, low thermal conductivity, easy volatilization and flammability[20, 21]. Among the inorganic phase change cold storage materials, salt hydrate PCMs are widely studied, which is a mixture of inorganic salts, water, nucleating agents, stabilizers and other additives[15]. The eutectic salt hydrates are the most important type of low temperature phase change cold storage material, which have the advantages of large latent heat, wide application range, large heat storage density, small change of phase change volume, low toxicity and low price[5, 22]. However, in practical applications, pure eutectic salt hydrates without modification usually have some problems such as large supercooling degree, phase separation, liquid leakage, and low thermal conductivity[23-25]. For solving these problems, combining eutectic salt with porous supporting materials is one of the most effective methods[26]. Expanded graphite (EG) is one kind of functional carbon-based materials with worm-like and porous structures[27]. Due to its low density, high thermal conductivity, loose porous, low cost and good compatibility with organics, it is usually used in
combination with organic PCMs to solve the problems of low thermal conductivity and leakage problems[28]. Zhang et al. [29] prepared a series of capric–palmitic– stearic acid ternary eutectic mixture (CA-PA-SA)/EG composites. It was found that the thermal conductivities of CA–PA–SA/EG composites could be up to 5.225 W·m−1·K−1 when containing 10 wt% EG. Meanwhile, the minimum mass percentage of EG to keep the CA–PA–SA/EG composite form-stable was less than 10%. Li et al. [30] developed a new composite phase change material (PCM) with OA and LA as a base fluid and EG as matrix. The thermal conductivity of OA-LA/EG is 1.275 W·m−1·K−1 which is 2.8 times higher than that of OA-LA. Above researches are proving that EG is a very useful heat transfer enhancer and packaging matrix for property modification of organic PCMs. Considering the benefits of EG in organic PCM/EG composites, it can be expected that if combine inorganic PCMs with EG, their thermal conductivity can be largely improved and leakage problems can be also solved. Xiao et al. [31] investigated the thermal performance of nitrates/EG composites. It was revealed that the thermal conductivities of nitrates could be enhanced greatly with the addition of EG. Huang et al. [32] prepared LiNO3-KCl/EG composite PCM, which was used in solar thermal energy storage applications. The thermal conductivity increases with the compaction density of the composite and the mass fraction of EG. The thermal conductivity of the composite PCM is 1.85~7.56 times higher than the eutectic LiNO3-KCl. However, since the hydrophobic nature of EG leads to its poor compatibility with eutectic salt PCMs, the fabricated inorganic PCM composites always process unstable and nonuniform properties [33]. Therefore, it is of great significance to find a suitable method to increase the compatibility of EG with inorganic PCMs for increasing their thermal performance. For solving the problem of EG when it is incorporated with eutectic salt PCMs, some efforts have
been devoted to improving its compatibility with eutectic salt PCMs. Duan et al. [34] prepared a CaCl2·6H2O/EG composite PCM by vacuum impregnation method. To improving the compatibility between CaCl2·6H2O and EG, CaCl2·6H2O used as the PCM was dispersed by surfactant and then, was absorbed into the porous structure of the EG. The thermal conductivity of the composite PCM was 14 times that of CaCl2·6H2O. Therefore, the obtained composite PCMs are promising for thermal energy storage applications. Xiao et al. [35] pointed out that introducing surfactant such as OP-10 to form a hydrophilic layer on the surface of EG might help improve the binding energy between EG and inorganic PCM. Investigations on Ba(OH)2·8H2O/EG composite showed that no phase separation phenomenon was observed when OP-10 was used and the encapsulation function of EG for Ba(OH)2·8H2O could be enhanced. Zhou et al. [36] used TritionX-100 in the preparation of MgCl2·6H2O-NH4Al(SO4)2·12H2O/EG composite. It was found that the encapsulation function of EG for MgCl2·6H2O-NH4Al(SO4)2·12H2O could be greatly enhanced. Although the compatibility of EG and eutectic salt can be improved by adding a surfactant, it should be noted that, the connections between EG and surfactant are physical connections and the corresponding binding forces are still weak. The EG–surfactant connections in the composite could easily crack after the composite enduring repeated heat storage/release cycles. Given this, it is necessary to find a more suitable method to increase the compatibility of EG with eutectic salt PCMs. In addition, as far as we know, few researchers have improved the cold storage performance of eutectic salts by compounding EG with eutectic salt PCMs whose phase change temperature is lower than 0℃. Therefore, it is of great significance to prepare a novel modified expanded graphite/eutectic salt composite PCM with enhanced cold storage performance.
Based on these investigations, in this work, a novel strategy was designed for developing a form-stable composite PCM for cold energy storage. We firstly fabricated a K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt with a low phase change temperature, a large enthalpy and a low supercooling degree. Secondly, a modified expanded graphite (MEG) with enhanced hydrophilicity was prepared via a heterogeneous nucleation technique and subsequent heat treatment, for increasing its compatibility with salt hydrate PCM. Finally, the eutectic salt was absorbed by MEG for solving leakage problem and thermal conductivity. The adsorption performance and rate of MEG on eutectic salt was investigated. Furthermore, thermophysical properties and thermal reliability as well as the cold storage properties of the prepared composite PCM are also investigated.
2. Experimental description
2.1. Materials
Al(NO3)3·9H2O (AR), K2HPO4·3H2O (AR), NaH2PO4·2H2O (AR) and Na2S2O3·5H2O (AR), Sodium tetraborate (AR) as well as deionized water were used as PCM components. EG (mesh 50) was used as supporting material. Ammonium hydroxide (AR, 25%), Absolute ethyl alcohol (AR) were used for modifying EG. All the chemicals were used as received without further purification.
2.2 Sample preparation The schematic diagram of the preparation process is shown in Fig. 1.
Fig. 1. Schematic diagram of the preparation of the composite PCM.
2.2.1 Preparation of K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt solution
According to a related study by Liang et al. [37], it is known that when the mass fraction of hydrated salts such as K2HPO4·3H2O, NaH2PO4·2H2O, Na2S2O3·5H2O is 6 wt%, a eutectic salt solution can be formed with water. Firstly, 6 wt% K2HPO4·3H2O, 6 wt% NaH2PO4·2H2O, 6 wt% Na2S2O3·5H2O and 82 wt% H2O were placed into a glass bottle, followed by being ultrasonic treatment for 10 minutes until the solution was clear and transparent, which means the K2HPO4·3H2O–NaH2PO4·2H2O– Na2S2O3·5H2O–H2O eutectic salt was obtained. Then, the Na2B4O7·10H2O was used as a nucleating agent to decrease supercooling degree of the eutectic salt. To investigate the effect of the addition of nucleating agent of the eutectic salt solution, a series of eutectic salt PCMs with different mass ratio of Na2B4O7·10H2O were prepared.
2.2.2 Modification of EG
Al2O3 has numerous outstanding characteristics, such as good hydrophilicity, high thermal conductivity, fire resistance, low cost, and nontoxicity [38]. Therefore, Al2O3 layer was chosen as hydrophilic layer and it was coated on the surface of EG by heterogeneous nucleation technique through chemical connections thus the modified EG was prepared. The detailed experimental method for preparing modified EG can be seen in our previous work[39]. Briefly, the mixture of EG and ethanol water solution were heated by water bath for 3 hours. Meanwhile, aluminum nitrate solution and ammonium hydroxide were added into the mixture dropwise. After the Al(OH)3-coated EG was prepared, the residual ammonium hydroxide was washed away by deionized water, followed by calcining at 500 °C in air for 3 h to change Al(OH)3 to Al2O3. As a result, MEG with Al2O3 coating was produced. The stability of such a coating has been demonstrated by various characterizations and chemical interactions between the O atoms in Al2O3 and C atoms on the EG surfaces have been found.
2.2.3 Preparation of eutectic salt solution /MEG Composite blocks
The specific methodology for preparing salt solution /MEG composite blocks is according to the previous study [40]. Firstly, EG and the MEG were respectively compressed into round blocks with identical sizes (20 mm in diameter, 10 mm in thickness) and weighed as m1. The packing densities of EG and MEG blocks were 250 kg·m-3 [41]. Then, the as-fabricated blocks were immersed into the
K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt and kept for 120 min for MEG and 1440 min for EG (until the mass of composite PCM kept stable), respectively. After removing from the eutectic salt solution, excess eutectic salt attaching on the block surface were wiped off. The obtained composite blocks were weighed as m2. The adsorptive capacity of each block for the eutectic salt solution was calculated according to the following formula: (m2 − m1)/m2 × 100% [42, 43]. In addition, the adsorption curves of the blocks for the eutectic salt were recorded by removing the blocks from the eutectic salt every ten minutes, and the adsorptive capacity values after different adsorption times were calculated.
2.3. Characterization
The phase change temperature and phase change enthalpy of the samples were measured using a differential scanning calorimeter instrument (DSC Q20,TA Instrument). For DSC measurements, 5−10 mg of each sample was sealed in an aluminum pan for characterization at a heating rate of 2 °C·min-1 under a constant stream of nitrogen at a flow rate of 50 mL·min-1. A data logger (Agilent34970A, USA) combined with a programmable constant temperature and humidity test chamber (WHTM-80DO, HK WEWON TECHNOLOGY LIMITED) was used to test the heating/cooling curves of the samples [44, 45]. Contact angles of the form-stable composite PCM based on EG and MEG samples were measured by a Contact angle tester (OCA40 Micro, dataphysics, Germany) at room temperature. The samples were compressed into thin slices (12 mm
in diameter and 1 mm in height) under a pressure of 10 Mpa to test its contact angle with melted eutectic salts. Fourier transformation infrared (FT-IR) was used to analyze the adsorption mechanism of Al2O3-coated EG on K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O– H2O eutectic salt. The FT-IR tests were performed at ambient temperature with a spectrometer (Tensor 27, Bruker, Germany) with a frequency range of 4000-400 cm-1. A hot disk thermal constant analyzer (TPS2500, Hot Disk Inc., Sweden) with a type 7577 probe acting as both the heat source and the sensor was applied to measure the thermal conductivities of the pure eutectic salt and the eutectic salt/MEG composite PCM block. A transient plane source method was selected for these measurements. The uncertainty of the measurements was within ±2%. The thermal reliability of the composite PCM was tested as follows: The samples was cooled to -40 °C and kept for 30 min, then heated to 20 °C and kept for another 30 min with the aid of a programmable constant temperature and humidity test chamber. This process was repeated for 400 heating−cooling cycles, and the sample was then characterized by DSC.
3. Results and Discussion
3.1. Thermophysical properties of eutectic salt
Through experiments, it is determined that the phase change temperature of K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt is -5.02 °C, the latent heat is as high as 208.3 kJ·kg-1, and the degree of supercooling is 5.4 °C. In
order to reduce the supercooling degree of the eutectic salt, Na2B4O7·10H2O was used to explore the effect of the mass fraction of nucleating agent on the supercooling degree of the eutectic salt. The cooling curves of eutectic salt with different mass fractions of nucleating agent are shown in Fig. 2, and the corresponding DSC curves are shown in Fig. 3. Table 1 shows the results derived from Fig. 2 and Fig. 3. From the heating/cooling curves in Fig. 2, it can be found that when the mass fraction of nucleating agent is 0.5, 1, 1.5, 2 and 2.5 wt%, the corresponding supercooling degrees are 4.59, 1.84, 1.94, 4.23 and 2.94 °C, respectively. Clearly, when the mass fraction of nucleating agent is 1 wt%, the effect of reducing the supercooling degree of eutectic salt solution is the best, and the supercooling degree is as low as 1.84 oC. From the DSC curve in Fig. 3, it is indicated that mass fraction of nucleating agents selected in the experiment have little effect on the latent heat of the eutectic salt solution, while for the phase change temperature of the eutectic salt solution, it decreases as the mass fraction of the nucleating agent increases, which is consistent with the law of freezing point reduction. Based on the above results, it can be concluded that 1 wt% Na2B4O7·10H2O is the optimum mass ratio for decreasing the supercooling degree of this eutectic salt hydrate. Moreover, such a large latent heat and a suitable phase change temperature below 0 oC make it be a promising candidate for cold energy regulation.
Fig. 2. Cooling curves of the eutectic salts containing different mass fraction nucleating agents.
Fig. 3. DSC curves of the eutectic salt containing nucleating agents with different mass fractions.
Table 1. Phase change parameters of the eutectic salts containing nucleating agents with different mass fractions. Nucleating agent（wt%）
Supercooling degree（℃）
Melting temperature（℃）
Melting enthalpy (kJ·kg-1)
0
5.4
-5.02
208.3
0.5
4.59
-5.14
208.4
1
1.84
-5.26
207.7
1.5
1.94
-5.62
205.6
2
4.23
-5.70
208.6
2.5
2.94
-6.11
209.7
3.2. Adsorption performance and mechanism of MEG for eutectic salt
Fig. 4 is a photograph of EG particles and Al2O3-coated EG particles. It can be seen that the Al2O3-coated EG is slightly smaller than the EG particles. The reason may be that the evaporation of the aqueous ethanol solution destroys the structure of the partially EG during the preparation process. However, the loose worm-like structure is still basically maintained. In order to analyze the compatibility of Al2O3-coated EG with K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt, the contact angles of the EG with the eutectic salt before and after modification are compared. The results are shown in Fig. 5. The contact angle between the EG and the eutectic salt solution is 91.7°, it is revealed that the compatibility between the two components is very poor. By contrast, the contact angle between the Al2O3-coated EG and the eutectic salt solution is reduced to 34.8°, indicating that the compatibility
between the two components has been remarkably enhanced. It can be deduced that MEG has an outstanding wetting property and strong binding force with salt hydrate prepared in this study. This can be very beneficial for preventing leakage problem of salt hydrate and increasing the thermal stability of the EG based salt hydrate composite PCMs.
Fig. 4. (a) the expanded graphite particles, (b) the Al2O3-coated EG particles.
Fig. 5. Visual images of the eutectic salt solution droplets just falling onto (a)EG, (b) Al2O3-coated EG. In order to further analyze the adsorption effect of MEG on K2HPO4·3H2O– NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt, the process of eutectic salt solution absorbed by EG and MEG blocks were also explored. The adsorption curves
of the EG and the Al2O3-coated EG block for the eutectic salt solution are shown in Fig. 6. It can be clearly seen that for the EG block before modification, its adsorption rate for eutectic salt solution is very slow. The adsorption amount is 4.74 wt% at 120 min, and only 9.56 wt% after 24 h. The results show that due to the poor compatibility of unmodified EG with eutectic salt solution, it is difficult to adsorb salt hydrate directly for preparing composite PCM. By contract, for the Al2O3-coated EG block, it can absorb 75.55 wt% eutectic salt solution in 10 minutes at a very fast speed, and the adsorption capacity can reach 79.06 wt% only after 60 minutes. After 120 minutes, the adsorption capacity of composite PCM reaches 80.71 wt%, and then the adsorption capacity is nearly stable. These results indicate that the hydrophilic modification of EG indeed has a magnificent effect for increasing the adsorption ability off salt hydrate PCM. It is very significant for preparing form-stale composite PCM with high thermal storage capacity. And the composite PCM block with 80.71 wt% eutectic salt will be used as following characterization.
Fig. 6. Adsorption curves of the EG and the Al2O3-coated EG block for the eutectic salt solution. In order to explore the adsorption mechanism of Al2O3-coated EG on eutectic salt,
FT-IR
analysis
of
Al2O3-coated
EG,
K2HPO4·3H2O–NaH2PO4·2H2O–
Na2S2O3·5H2O–H2O eutectic salt and composite PCM was carried out. The corresponding FT-IR spectra were shown in Fig. 7. It is not difficult to find that the absorption peaks of infrared spectra of composite PCM are only the combination of Al2O3-coated EG and eutectic salt solution, and there are no new absorption peaks. This indicates that the Al2O3-coated EG did not undergo a chemical reaction while only physical combination during the adsorption of eutectic salt.
Fig. 7. FT-IR spectra of Al2O3-coated EG, the eutectic salt and the composite PCM.
3.3. Thermophysical properties of eutectic salt /MEG composite PCM
The optical images of the Al2O3-coated EG block and the composite PCM block are shown in Fig. 8. The DSC curve of the composite PCM is shown in Fig. 9. It can be seen from the DSC curves that the phase change temperature of the composite PCM is - 5.30 °C, with only one melting peak, and the change of phase change temperature is slight, indicating that the Al2O3-coated EG has little effect on the phase change temperature of eutectic salt. The latent heat of composite PCM is 161.8 kJ·kg-1, which is only 5.9 kJ·kg-1 lower than the theoretical latent heat of 167.7 kJ·kg-1 calculated from the mass fraction 80.71 wt% of eutectic salt solution in composite PCM.
Fig. 8. Digital photos of the Al2O3-coated EG block and the composite PCM block.
Fig. 9. DSC curves of the eutectic salt and the composite PCM. Heating/cooling curves of the eutectic salt solution and the composite PCM are shown in Fig. 10. From Fig. 10, it can be seen that the supercooling degrees of the eutectic salt solution and the composite PCM are very low. In addition, it is noteworthy that the cooling rate and heating rate of composite PCM are faster than that of eutectic salt solution. The reason is that the thermal conductivity of eutectic salt is largely increased by Al2O3-coated EG. In order to explore the effect of Al2O3-coated EG on the thermal conductivity of eutectic salt solution, the thermal conductivities of eutectic salt and composite PCM at room temperature were compared. As shown in Fig. 11, the thermal conductivity of the composite PCM is 8.90 W·m-1·K-1, which is 13.3 times that of the eutectic salt solution. Moreover, the comparison on the thermal properties of form-stable composite PCM prepared in this study with other reported studies is concluded in Table 2. It can be seen that the composite PCM designed in this study has better thermal performance and higher
thermal conductivity, thanks to the higher adsorption capacity of MEG.
Fig. 10. Heating/cooling curves of the eutectic salt and the composite PCM.
Fig. 11. Column chart of thermal conductivity of the eutectic salt and the composite PCM.
Table 2. Comparison of thermal properties of salt hydrate/MEG PCM with that of other form-stable PCMs in the lierature Phase change temperature
Latent heat
Thermal conductivity
Refs
（℃）
(kJ·kg-1)
W·m-1·K-1
PEG/EG
20.18
95.03
——
[46]
MgCl2·6H2O/EG
116.24
116.70
3.588
[47]
MgCl2·6H2O /diatomite
88.89
63.39
1.63
[48]
TBAB hydrate/SiO2
8.33
134
0.41
[49]
CA/PA/EG
22.4
140
0.519
[50]
eutectic salt/MEG
-5.30
161.80
8.90
This study
Form-stable PCMs
3.4. Thermal reliability of the eutectic salt solution /MEG composite PCM
Fig. 12 shows the digital photographs of the pure eutectic salt and the composite PCM before and after recycling. Clearly, the phase separation phenomenon of pure eutectic salt occurs only after one thermal cycling tests, and it becomes more apparent with the increasing cycling times. Compared with the eutectic salt solution, the phase separation phenomenon was not found in the composite PCM even after 400 times cycles, which indicates that the Al2O3-coated EG can effectively solve the phase separation problem of the eutectic salt solution. Additionally, the latent heat and phase change temperature of the composite PCM after thermal cycle tests were also measured, and the corresponding DSC curve is shown in Fig. 13. After 200 cycles and
400 cycles of thermal cycling, the latent heat of the composite PCM were 157.4 and 155.3 kJ·kg-1, respectively and the phase change temperatures were -4.98 and -4.85 °C, respectively. The negligible change confirmed the high thermal reliability of the composite PCM, which is critical for long-time practical use. Furthermore, the heating/cooling curves of the composite PCM after 200 cycles and 400 cycles of thermal cycling were tested. As shown in Fig. 14, it is not difficult to see that heating/cooling curves basically coincide with those before the cycle. All results manifest that the composite PCM has an remarkable cyclic stability and reliability.
Fig. 12. Digital photos of the eutectic salt and the composite PCM at (a) 0th, (b)1th, (c) 200th, (d) 400th thermal cycle.
Fig. 13. DSC curves of the composite PCM at 1th, 200th, 400th thermal cycle.
Fig. 14. Heating/cooling curves of the composite PCM at 1th, 200th, 400th thermal cycle.
5. Conclusion
In this work, a K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt/ modified expanded graphite (MEG) composite PCM was prepared by melt impregnation method. The hydrophilic modification of EG has a very significant effect for solving leakage problem and increasing compatibility with inorganic salt PCM. Within 120 minutes, the adsorption capacity of MEG to eutectic salt reached 80.71 wt%, which was much higher than 4.84 wt% of unmodified EG, indicating that the surface modification of EG greatly enhances its adsorption performance to eutectic salt. The composite PCM has a low phase change temperature of -5.30℃, a large latent heat of 161.8 kJ·kg-1, and a very low supercooling degree of 1.83 ℃. The thermal conductivity of the composite PCM is 8.90 W·m-1·K-1, which is 13.3 times as large as that of the eutectic salt. Moreover, 400 times thermal cycle tests demonstrated excellent thermal reliability of the composite PCM. Additionally, this work can also be a prospect method for preparing form-stable PCMs based salt hydrate PCMs. In summary, compared with traditional cold storage material ，the K2HPO4·3H2O– NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt/MEG composite PCM prepared in this experiment has suitable phase change temperature, high thermal conductivity and good cycle stability, which has practical application value and has broad application prospects in the cold storage fields in frozen food preservation and transportation, liquefaction and petroleum gas transprtation, household freezers and cold chain logistics.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. U1507201)
and Guangdong-Hong
Kong
joint
innovation
projects
(2016A050503020).
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Preparation and performance of modified expanded graphite/eutectic salt composite phase change cold storage material Ning Xie , Zhongping Li , Xuenong Gao , Yutang Fang , Zhengguo Zhang PII: DOI: Reference:
S0140-7007(19)30430-X https://doi.org/10.1016/j.ijrefrig.2019.10.008 JIJR 4546
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
12 April 2019 17 September 2019 12 October 2019
Please cite this article as: Ning Xie , Zhongping Li , Xuenong Gao , Yutang Fang , Zhengguo Zhang , Preparation and performance of modified expanded graphite/eutectic salt composite phase change cold storage material, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.10.008
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Highlights
The K2HPO4·3H2O-NaH2PO4·2H2O-Na2S2O3·5H2O-H2O eutectic salt has large latent heat.
Na2B4O7·10H2O can effectively reduce the supercooling degree of the eutectic salt.
The adsorption capacity of modified expanded graphite reaches up to 80.71 wt%.
Modified expanded graphite enhances the thermal conductivity of eutectic salt.
Preparation and performance of modified expanded graphite/eutectic salt composite phase change cold storage material
Ning Xiea, Zhongping Lia, Xuenong Gaoa,*, Yutang Fanga, Zhengguo Zhanga
a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation (The Ministry
of Education), School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China.
* Corresponding Author E-mail address: [email protected]
ABSTRACT Cold energy storage technologies using phase change materials (PCM) have received increasing attention these years. In this work, we develop a novel kind of composite PCM using modified expanded graphite (MEG) to adsorb K2HPO4·3H2O– NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt by impregnation method. Within 120 minutes, the adsorption capacity of MEG for eutectic salt is 75.33% higher than that of unmodified expanded graphite (EG). The composite PCM has a phase change temperature of -5.30 ℃, a large latent heat of 161.8 kJ·kg-1, and a low supercooling degree of 1.83 ℃. The thermal conductivity of the composite PCM is 13.3 times as large as that of the eutectic salt. Moreover, the thermal cycle tests demonstrated excellent
thermal
reliability.
These
outstanding
thermal
properties
endow
K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt/MEG composite PCM with broad application prospects in the fields of beer industry, air conditioning refrigeration, refrigerator refrigeration and cold chain logistics. Keywords: Eutectic salt, Modified expanded graphite, Composite PCM, Cold energy storage
Nomenclature PCMs
phase change materials
EG
expanded graphite
MEG
modified expanded graphite
CA
capric acid
PA
palmitic acid
SA
stearic acid
OA
octanoic acid
DSC
Differential scanning calorimeter
FT-IR
Fourier transformation infrared
TBAB
tetra-n-butyl ammonium bromide
PEG
polyethylene glycol
1. Introduction Thermal energy storage technology is an important way to minimize the waste of energy and increase energy utilization efficiency [1, 2]. The core of this technology is the development of phase change materials (PCMs) with excellent performance and low cost[3, 4]. At present, researchers have done a lot of investigations on above normal temperature PCMs. For instance, the mid temperature PCMs with phase change temperatures in the range 15–90 °C can be contributed to solar, medical, textile, electronic and energy-saving applications in building design [5-7]. The high temperature PCMs with a phase change temperature above 90 °C developed mainly for industrial and aerospace applications[8, 9]. However, cooling is also one of the major global energy consuming processes. A suitable cold storage material can accumulate large amount of cold thermal energy into the storage medium during charging process, while retrieve it during discharging process. Therefore, energy conservation can be achieved by reducing the electricity peak load. Compared with sensible cold energy storage system, latent storage system using PCMs attracts more attention due to its higher energy storage capacity and density, thus, only small size energy storage system is needed. Therefore, development of novel kind of PCMs with excellent thermal performance is very important for cold energy conservation including free cooling system and refrigeration fields [10-12]. The refrigeration system can also be optimized by PCMs with suitable thermal properties. For low-temperature PCMs, the current research and application are mainly concentrated in the field of air-conditioning[13, 14]. And few studies pay attention to the development and application of low temperature PCMs with phase transition temperature below 0°C [15]. However, in many industries such as food preservation[16, 17], chemical industry[12], household refrigerators and refrigeration
trucks [18] as well as cold chain logistics[19], PCMs with phase transition temperature below 0 °C have a very large application potential for cold energy saving in freezers for maintaining stable temperatures and increasing energy utilization efficiency. Low-temperature PCMs below 0 °C are mainly divided into three categories: ice, organic PCMs, and inorganic PCMs [12]. Ice storage has the advantages of large latent heat, and low price, but the disadvantages of low evaporation temperature, large supercooling degree and large change in phase change volume limit their practical applications [14]. Organic phase change cold storage materials mainly include paraffin wax, aromatic hydrocarbons, alcohols, etc., which are not easy to encounter phase separation and supercooling problem, but they have the disadvantages of high price, low latent heat of phase change, low thermal conductivity, easy volatilization and flammability[20, 21]. Among the inorganic phase change cold storage materials, salt hydrate PCMs are widely studied, which is a mixture of inorganic salts, water, nucleating agents, stabilizers and other additives[15]. The eutectic salt hydrates are the most important type of low temperature phase change cold storage material, which have the advantages of large latent heat, wide application range, large heat storage density, small change of phase change volume, low toxicity and low price[5, 22]. However, in practical applications, pure eutectic salt hydrates without modification usually have some problems such as large supercooling degree, phase separation, liquid leakage, and low thermal conductivity[23-25]. For solving these problems, combining eutectic salt with porous supporting materials is one of the most effective methods[26]. Expanded graphite (EG) is one kind of functional carbon-based materials with worm-like and porous structures[27]. Due to its low density, high thermal conductivity, loose porous, low cost and good compatibility with organics, it is usually used in
combination with organic PCMs to solve the problems of low thermal conductivity and leakage problems[28]. Zhang et al. [29] prepared a series of capric–palmitic– stearic acid ternary eutectic mixture (CA-PA-SA)/EG composites. It was found that the thermal conductivities of CA–PA–SA/EG composites could be up to 5.225 W·m−1·K−1 when containing 10 wt% EG. Meanwhile, the minimum mass percentage of EG to keep the CA–PA–SA/EG composite form-stable was less than 10%. Li et al. [30] developed a new composite phase change material (PCM) with OA and LA as a base fluid and EG as matrix. The thermal conductivity of OA-LA/EG is 1.275 W·m−1·K−1 which is 2.8 times higher than that of OA-LA. Above researches are proving that EG is a very useful heat transfer enhancer and packaging matrix for property modification of organic PCMs. Considering the benefits of EG in organic PCM/EG composites, it can be expected that if combine inorganic PCMs with EG, their thermal conductivity can be largely improved and leakage problems can be also solved. Xiao et al. [31] investigated the thermal performance of nitrates/EG composites. It was revealed that the thermal conductivities of nitrates could be enhanced greatly with the addition of EG. Huang et al. [32] prepared LiNO3-KCl/EG composite PCM, which was used in solar thermal energy storage applications. The thermal conductivity increases with the compaction density of the composite and the mass fraction of EG. The thermal conductivity of the composite PCM is 1.85~7.56 times higher than the eutectic LiNO3-KCl. However, since the hydrophobic nature of EG leads to its poor compatibility with eutectic salt PCMs, the fabricated inorganic PCM composites always process unstable and nonuniform properties [33]. Therefore, it is of great significance to find a suitable method to increase the compatibility of EG with inorganic PCMs for increasing their thermal performance. For solving the problem of EG when it is incorporated with eutectic salt PCMs, some efforts have
been devoted to improving its compatibility with eutectic salt PCMs. Duan et al. [34] prepared a CaCl2·6H2O/EG composite PCM by vacuum impregnation method. To improving the compatibility between CaCl2·6H2O and EG, CaCl2·6H2O used as the PCM was dispersed by surfactant and then, was absorbed into the porous structure of the EG. The thermal conductivity of the composite PCM was 14 times that of CaCl2·6H2O. Therefore, the obtained composite PCMs are promising for thermal energy storage applications. Xiao et al. [35] pointed out that introducing surfactant such as OP-10 to form a hydrophilic layer on the surface of EG might help improve the binding energy between EG and inorganic PCM. Investigations on Ba(OH)2·8H2O/EG composite showed that no phase separation phenomenon was observed when OP-10 was used and the encapsulation function of EG for Ba(OH)2·8H2O could be enhanced. Zhou et al. [36] used TritionX-100 in the preparation of MgCl2·6H2O-NH4Al(SO4)2·12H2O/EG composite. It was found that the encapsulation function of EG for MgCl2·6H2O-NH4Al(SO4)2·12H2O could be greatly enhanced. Although the compatibility of EG and eutectic salt can be improved by adding a surfactant, it should be noted that, the connections between EG and surfactant are physical connections and the corresponding binding forces are still weak. The EG–surfactant connections in the composite could easily crack after the composite enduring repeated heat storage/release cycles. Given this, it is necessary to find a more suitable method to increase the compatibility of EG with eutectic salt PCMs. In addition, as far as we know, few researchers have improved the cold storage performance of eutectic salts by compounding EG with eutectic salt PCMs whose phase change temperature is lower than 0℃. Therefore, it is of great significance to prepare a novel modified expanded graphite/eutectic salt composite PCM with enhanced cold storage performance.
Based on these investigations, in this work, a novel strategy was designed for developing a form-stable composite PCM for cold energy storage. We firstly fabricated a K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt with a low phase change temperature, a large enthalpy and a low supercooling degree. Secondly, a modified expanded graphite (MEG) with enhanced hydrophilicity was prepared via a heterogeneous nucleation technique and subsequent heat treatment, for increasing its compatibility with salt hydrate PCM. Finally, the eutectic salt was absorbed by MEG for solving leakage problem and thermal conductivity. The adsorption performance and rate of MEG on eutectic salt was investigated. Furthermore, thermophysical properties and thermal reliability as well as the cold storage properties of the prepared composite PCM are also investigated.
2. Experimental description
2.1. Materials
Al(NO3)3·9H2O (AR), K2HPO4·3H2O (AR), NaH2PO4·2H2O (AR) and Na2S2O3·5H2O (AR), Sodium tetraborate (AR) as well as deionized water were used as PCM components. EG (mesh 50) was used as supporting material. Ammonium hydroxide (AR, 25%), Absolute ethyl alcohol (AR) were used for modifying EG. All the chemicals were used as received without further purification.
2.2 Sample preparation The schematic diagram of the preparation process is shown in Fig. 1.
Fig. 1. Schematic diagram of the preparation of the composite PCM.
2.2.1 Preparation of K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt solution
According to a related study by Liang et al. [37], it is known that when the mass fraction of hydrated salts such as K2HPO4·3H2O, NaH2PO4·2H2O, Na2S2O3·5H2O is 6 wt%, a eutectic salt solution can be formed with water. Firstly, 6 wt% K2HPO4·3H2O, 6 wt% NaH2PO4·2H2O, 6 wt% Na2S2O3·5H2O and 82 wt% H2O were placed into a glass bottle, followed by being ultrasonic treatment for 10 minutes until the solution was clear and transparent, which means the K2HPO4·3H2O–NaH2PO4·2H2O– Na2S2O3·5H2O–H2O eutectic salt was obtained. Then, the Na2B4O7·10H2O was used as a nucleating agent to decrease supercooling degree of the eutectic salt. To investigate the effect of the addition of nucleating agent of the eutectic salt solution, a series of eutectic salt PCMs with different mass ratio of Na2B4O7·10H2O were prepared.
2.2.2 Modification of EG
Al2O3 has numerous outstanding characteristics, such as good hydrophilicity, high thermal conductivity, fire resistance, low cost, and nontoxicity [38]. Therefore, Al2O3 layer was chosen as hydrophilic layer and it was coated on the surface of EG by heterogeneous nucleation technique through chemical connections thus the modified EG was prepared. The detailed experimental method for preparing modified EG can be seen in our previous work[39]. Briefly, the mixture of EG and ethanol water solution were heated by water bath for 3 hours. Meanwhile, aluminum nitrate solution and ammonium hydroxide were added into the mixture dropwise. After the Al(OH)3-coated EG was prepared, the residual ammonium hydroxide was washed away by deionized water, followed by calcining at 500 °C in air for 3 h to change Al(OH)3 to Al2O3. As a result, MEG with Al2O3 coating was produced. The stability of such a coating has been demonstrated by various characterizations and chemical interactions between the O atoms in Al2O3 and C atoms on the EG surfaces have been found.
2.2.3 Preparation of eutectic salt solution /MEG Composite blocks
The specific methodology for preparing salt solution /MEG composite blocks is according to the previous study [40]. Firstly, EG and the MEG were respectively compressed into round blocks with identical sizes (20 mm in diameter, 10 mm in thickness) and weighed as m1. The packing densities of EG and MEG blocks were 250 kg·m-3 [41]. Then, the as-fabricated blocks were immersed into the
K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt and kept for 120 min for MEG and 1440 min for EG (until the mass of composite PCM kept stable), respectively. After removing from the eutectic salt solution, excess eutectic salt attaching on the block surface were wiped off. The obtained composite blocks were weighed as m2. The adsorptive capacity of each block for the eutectic salt solution was calculated according to the following formula: (m2 − m1)/m2 × 100% [42, 43]. In addition, the adsorption curves of the blocks for the eutectic salt were recorded by removing the blocks from the eutectic salt every ten minutes, and the adsorptive capacity values after different adsorption times were calculated.
2.3. Characterization
The phase change temperature and phase change enthalpy of the samples were measured using a differential scanning calorimeter instrument (DSC Q20,TA Instrument). For DSC measurements, 5−10 mg of each sample was sealed in an aluminum pan for characterization at a heating rate of 2 °C·min-1 under a constant stream of nitrogen at a flow rate of 50 mL·min-1. A data logger (Agilent34970A, USA) combined with a programmable constant temperature and humidity test chamber (WHTM-80DO, HK WEWON TECHNOLOGY LIMITED) was used to test the heating/cooling curves of the samples [44, 45]. Contact angles of the form-stable composite PCM based on EG and MEG samples were measured by a Contact angle tester (OCA40 Micro, dataphysics, Germany) at room temperature. The samples were compressed into thin slices (12 mm
in diameter and 1 mm in height) under a pressure of 10 Mpa to test its contact angle with melted eutectic salts. Fourier transformation infrared (FT-IR) was used to analyze the adsorption mechanism of Al2O3-coated EG on K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O– H2O eutectic salt. The FT-IR tests were performed at ambient temperature with a spectrometer (Tensor 27, Bruker, Germany) with a frequency range of 4000-400 cm-1. A hot disk thermal constant analyzer (TPS2500, Hot Disk Inc., Sweden) with a type 7577 probe acting as both the heat source and the sensor was applied to measure the thermal conductivities of the pure eutectic salt and the eutectic salt/MEG composite PCM block. A transient plane source method was selected for these measurements. The uncertainty of the measurements was within ±2%. The thermal reliability of the composite PCM was tested as follows: The samples was cooled to -40 °C and kept for 30 min, then heated to 20 °C and kept for another 30 min with the aid of a programmable constant temperature and humidity test chamber. This process was repeated for 400 heating−cooling cycles, and the sample was then characterized by DSC.
3. Results and Discussion
3.1. Thermophysical properties of eutectic salt
Through experiments, it is determined that the phase change temperature of K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt is -5.02 °C, the latent heat is as high as 208.3 kJ·kg-1, and the degree of supercooling is 5.4 °C. In
order to reduce the supercooling degree of the eutectic salt, Na2B4O7·10H2O was used to explore the effect of the mass fraction of nucleating agent on the supercooling degree of the eutectic salt. The cooling curves of eutectic salt with different mass fractions of nucleating agent are shown in Fig. 2, and the corresponding DSC curves are shown in Fig. 3. Table 1 shows the results derived from Fig. 2 and Fig. 3. From the heating/cooling curves in Fig. 2, it can be found that when the mass fraction of nucleating agent is 0.5, 1, 1.5, 2 and 2.5 wt%, the corresponding supercooling degrees are 4.59, 1.84, 1.94, 4.23 and 2.94 °C, respectively. Clearly, when the mass fraction of nucleating agent is 1 wt%, the effect of reducing the supercooling degree of eutectic salt solution is the best, and the supercooling degree is as low as 1.84 oC. From the DSC curve in Fig. 3, it is indicated that mass fraction of nucleating agents selected in the experiment have little effect on the latent heat of the eutectic salt solution, while for the phase change temperature of the eutectic salt solution, it decreases as the mass fraction of the nucleating agent increases, which is consistent with the law of freezing point reduction. Based on the above results, it can be concluded that 1 wt% Na2B4O7·10H2O is the optimum mass ratio for decreasing the supercooling degree of this eutectic salt hydrate. Moreover, such a large latent heat and a suitable phase change temperature below 0 oC make it be a promising candidate for cold energy regulation.
Fig. 2. Cooling curves of the eutectic salts containing different mass fraction nucleating agents.
Fig. 3. DSC curves of the eutectic salt containing nucleating agents with different mass fractions.
Table 1. Phase change parameters of the eutectic salts containing nucleating agents with different mass fractions. Nucleating agent（wt%）
Supercooling degree（℃）
Melting temperature（℃）
Melting enthalpy (kJ·kg-1)
0
5.4
-5.02
208.3
0.5
4.59
-5.14
208.4
1
1.84
-5.26
207.7
1.5
1.94
-5.62
205.6
2
4.23
-5.70
208.6
2.5
2.94
-6.11
209.7
3.2. Adsorption performance and mechanism of MEG for eutectic salt
Fig. 4 is a photograph of EG particles and Al2O3-coated EG particles. It can be seen that the Al2O3-coated EG is slightly smaller than the EG particles. The reason may be that the evaporation of the aqueous ethanol solution destroys the structure of the partially EG during the preparation process. However, the loose worm-like structure is still basically maintained. In order to analyze the compatibility of Al2O3-coated EG with K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt, the contact angles of the EG with the eutectic salt before and after modification are compared. The results are shown in Fig. 5. The contact angle between the EG and the eutectic salt solution is 91.7°, it is revealed that the compatibility between the two components is very poor. By contrast, the contact angle between the Al2O3-coated EG and the eutectic salt solution is reduced to 34.8°, indicating that the compatibility
between the two components has been remarkably enhanced. It can be deduced that MEG has an outstanding wetting property and strong binding force with salt hydrate prepared in this study. This can be very beneficial for preventing leakage problem of salt hydrate and increasing the thermal stability of the EG based salt hydrate composite PCMs.
Fig. 4. (a) the expanded graphite particles, (b) the Al2O3-coated EG particles.
Fig. 5. Visual images of the eutectic salt solution droplets just falling onto (a)EG, (b) Al2O3-coated EG. In order to further analyze the adsorption effect of MEG on K2HPO4·3H2O– NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt, the process of eutectic salt solution absorbed by EG and MEG blocks were also explored. The adsorption curves
of the EG and the Al2O3-coated EG block for the eutectic salt solution are shown in Fig. 6. It can be clearly seen that for the EG block before modification, its adsorption rate for eutectic salt solution is very slow. The adsorption amount is 4.74 wt% at 120 min, and only 9.56 wt% after 24 h. The results show that due to the poor compatibility of unmodified EG with eutectic salt solution, it is difficult to adsorb salt hydrate directly for preparing composite PCM. By contract, for the Al2O3-coated EG block, it can absorb 75.55 wt% eutectic salt solution in 10 minutes at a very fast speed, and the adsorption capacity can reach 79.06 wt% only after 60 minutes. After 120 minutes, the adsorption capacity of composite PCM reaches 80.71 wt%, and then the adsorption capacity is nearly stable. These results indicate that the hydrophilic modification of EG indeed has a magnificent effect for increasing the adsorption ability off salt hydrate PCM. It is very significant for preparing form-stale composite PCM with high thermal storage capacity. And the composite PCM block with 80.71 wt% eutectic salt will be used as following characterization.
Fig. 6. Adsorption curves of the EG and the Al2O3-coated EG block for the eutectic salt solution. In order to explore the adsorption mechanism of Al2O3-coated EG on eutectic salt,
FT-IR
analysis
of
Al2O3-coated
EG,
K2HPO4·3H2O–NaH2PO4·2H2O–
Na2S2O3·5H2O–H2O eutectic salt and composite PCM was carried out. The corresponding FT-IR spectra were shown in Fig. 7. It is not difficult to find that the absorption peaks of infrared spectra of composite PCM are only the combination of Al2O3-coated EG and eutectic salt solution, and there are no new absorption peaks. This indicates that the Al2O3-coated EG did not undergo a chemical reaction while only physical combination during the adsorption of eutectic salt.
Fig. 7. FT-IR spectra of Al2O3-coated EG, the eutectic salt and the composite PCM.
3.3. Thermophysical properties of eutectic salt /MEG composite PCM
The optical images of the Al2O3-coated EG block and the composite PCM block are shown in Fig. 8. The DSC curve of the composite PCM is shown in Fig. 9. It can be seen from the DSC curves that the phase change temperature of the composite PCM is - 5.30 °C, with only one melting peak, and the change of phase change temperature is slight, indicating that the Al2O3-coated EG has little effect on the phase change temperature of eutectic salt. The latent heat of composite PCM is 161.8 kJ·kg-1, which is only 5.9 kJ·kg-1 lower than the theoretical latent heat of 167.7 kJ·kg-1 calculated from the mass fraction 80.71 wt% of eutectic salt solution in composite PCM.
Fig. 8. Digital photos of the Al2O3-coated EG block and the composite PCM block.
Fig. 9. DSC curves of the eutectic salt and the composite PCM. Heating/cooling curves of the eutectic salt solution and the composite PCM are shown in Fig. 10. From Fig. 10, it can be seen that the supercooling degrees of the eutectic salt solution and the composite PCM are very low. In addition, it is noteworthy that the cooling rate and heating rate of composite PCM are faster than that of eutectic salt solution. The reason is that the thermal conductivity of eutectic salt is largely increased by Al2O3-coated EG. In order to explore the effect of Al2O3-coated EG on the thermal conductivity of eutectic salt solution, the thermal conductivities of eutectic salt and composite PCM at room temperature were compared. As shown in Fig. 11, the thermal conductivity of the composite PCM is 8.90 W·m-1·K-1, which is 13.3 times that of the eutectic salt solution. Moreover, the comparison on the thermal properties of form-stable composite PCM prepared in this study with other reported studies is concluded in Table 2. It can be seen that the composite PCM designed in this study has better thermal performance and higher
thermal conductivity, thanks to the higher adsorption capacity of MEG.
Fig. 10. Heating/cooling curves of the eutectic salt and the composite PCM.
Fig. 11. Column chart of thermal conductivity of the eutectic salt and the composite PCM.
Table 2. Comparison of thermal properties of salt hydrate/MEG PCM with that of other form-stable PCMs in the lierature Phase change temperature
Latent heat
Thermal conductivity
Refs
（℃）
(kJ·kg-1)
W·m-1·K-1
PEG/EG
20.18
95.03
——
[46]
MgCl2·6H2O/EG
116.24
116.70
3.588
[47]
MgCl2·6H2O /diatomite
88.89
63.39
1.63
[48]
TBAB hydrate/SiO2
8.33
134
0.41
[49]
CA/PA/EG
22.4
140
0.519
[50]
eutectic salt/MEG
-5.30
161.80
8.90
This study
Form-stable PCMs
3.4. Thermal reliability of the eutectic salt solution /MEG composite PCM
Fig. 12 shows the digital photographs of the pure eutectic salt and the composite PCM before and after recycling. Clearly, the phase separation phenomenon of pure eutectic salt occurs only after one thermal cycling tests, and it becomes more apparent with the increasing cycling times. Compared with the eutectic salt solution, the phase separation phenomenon was not found in the composite PCM even after 400 times cycles, which indicates that the Al2O3-coated EG can effectively solve the phase separation problem of the eutectic salt solution. Additionally, the latent heat and phase change temperature of the composite PCM after thermal cycle tests were also measured, and the corresponding DSC curve is shown in Fig. 13. After 200 cycles and
400 cycles of thermal cycling, the latent heat of the composite PCM were 157.4 and 155.3 kJ·kg-1, respectively and the phase change temperatures were -4.98 and -4.85 °C, respectively. The negligible change confirmed the high thermal reliability of the composite PCM, which is critical for long-time practical use. Furthermore, the heating/cooling curves of the composite PCM after 200 cycles and 400 cycles of thermal cycling were tested. As shown in Fig. 14, it is not difficult to see that heating/cooling curves basically coincide with those before the cycle. All results manifest that the composite PCM has an remarkable cyclic stability and reliability.
Fig. 12. Digital photos of the eutectic salt and the composite PCM at (a) 0th, (b)1th, (c) 200th, (d) 400th thermal cycle.
Fig. 13. DSC curves of the composite PCM at 1th, 200th, 400th thermal cycle.
Fig. 14. Heating/cooling curves of the composite PCM at 1th, 200th, 400th thermal cycle.
5. Conclusion
In this work, a K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt/ modified expanded graphite (MEG) composite PCM was prepared by melt impregnation method. The hydrophilic modification of EG has a very significant effect for solving leakage problem and increasing compatibility with inorganic salt PCM. Within 120 minutes, the adsorption capacity of MEG to eutectic salt reached 80.71 wt%, which was much higher than 4.84 wt% of unmodified EG, indicating that the surface modification of EG greatly enhances its adsorption performance to eutectic salt. The composite PCM has a low phase change temperature of -5.30℃, a large latent heat of 161.8 kJ·kg-1, and a very low supercooling degree of 1.83 ℃. The thermal conductivity of the composite PCM is 8.90 W·m-1·K-1, which is 13.3 times as large as that of the eutectic salt. Moreover, 400 times thermal cycle tests demonstrated excellent thermal reliability of the composite PCM. Additionally, this work can also be a prospect method for preparing form-stable PCMs based salt hydrate PCMs. In summary, compared with traditional cold storage material ，the K2HPO4·3H2O– NaH2PO4·2H2O–Na2S2O3·5H2O–H2O eutectic salt/MEG composite PCM prepared in this experiment has suitable phase change temperature, high thermal conductivity and good cycle stability, which has practical application value and has broad application prospects in the cold storage fields in frozen food preservation and transportation, liquefaction and petroleum gas transprtation, household freezers and cold chain logistics.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. U1507201)
and Guangdong-Hong
Kong
joint
innovation
projects
(2016A050503020).
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