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expanded graphite composite phase change material
Solar Energy Materials and Solar Cells 169 (2017) 215–221
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Preparation and thermal energy storage properties of LiNO3-KCl-NaNO3/ expanded graphite composite phase change material
MARK
⁎
Tao Xua, Yantong Lia, Jiayu Chena, , Junwan Liub a
Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 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
b
A R T I C L E I N F O
A B S T R A C T
Keywords: LiNO3-KCl-NaNO3 PCM Thermal energy storage Thermal conductivity
Recent studies suggest expanded graphite (EG) can significantly enhance the thermal conductivity, stability, efficiency of phase change material (PCM). This study aims to investigate the thermal-physical properties of the LiNO3-KCl-NaNO3/expanded graphite composite PCM. The testing PCM samples were prepared using the Capillary Method with 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt% EG. When EG mass fraction is 25 wt%, no leakage was observed in the leakage test. The SEM image shows that the EG had intense absorption ability to the LiNO3-KCl-NaNO3 composites. The X-ray powder diffraction (XRD) comparison between the pure salt and the composite PCM suggests that no new substance produced after adding the EG into the eutectic LiNO3-KClNaNO3. Differential scanning calorimetry (DSC) curves of the composite PCM indicate that the latent heat was reduced dramatically when the EG and the eutectic LiNO3-KCl-NaNO3 were blended together. In the experiment, thermal conductivity and the compress density of the composite PCM show a strongly linear relationship. Also, the thermal conductivity of the PCM can be enhanced after adding the EG into the pure salt. Finally, the thermal performance of the latent heat system using the eutectic LiNO3-KCl-NaNO3 and the composite PCM were compared. Due to the high thermal conductivity, the charging rate of the system utilizing the composite PCM is much faster than the pure salt.
1. Introduction Comparing to regular materials, PCM has large energy storage capacity and able to maintain constant temperature during the phase change process [1–4]. Therefore, PCM has been investigated in many research related to building energy efficiency, such as heating [5,6], natural ventilation [7–10], solar domestic water system [11–13], and building envelop design, such as wallboard [14–16]. Organic phase change material (OPCM) is one of the most common PCM in the energy storage system. However, OPCM yield low conductivity [17] and result in low heat transfer capacity and thermal utilization efficiency in the energy storage system. Low efficiency in energy storage and release hinders the widely application of OPCM. Inorganic PCM also subject to the same constraint as OPCM [18]. For example, the inorganic eutectic LiNO3-KCl-NaNO3 PCM has a high latent heat but low thermal conductivity [19]. To enhance the heat transfer ability of PCM, many researchers propose various approaches to intensify the heat transfer area between the heat transfer fluid (HTF) and PCM, such as the bundled tube structures [20], the wavy surface [21], the multiple PCM configuration ⁎
Corresponding author. E-mail address: [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.solmat.2017.05.035 Received 11 January 2017; Received in revised form 29 April 2017; Accepted 15 May 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
[22], additional fins [23–25] and etc. However, these approaches often have large material waste, higher maintenance cost, and corrosion problems [26]. Given the high thermal conductivity and rich network of micro-porous structures [27,28], expanded graphite (EG) shows a great potential in resolving these problems for PCM. For example, Gao et al. [29] added four mass fraction EG into the erythritol and achieved a melt temperature of nearly 119 °C. In their experiment, they also found that the thermal conductivity could be increased by approximately 2.5 times when mixing the erythritol with 4 wt% EG. In the study of Xiao et al. [30], the EG with the mass fraction of 5 wt%, 10 wt%, 20 wt% was examined to investigate the thermal conductivity improvement with the eutectic NaNO3-KNO3. Wang et al. [31] blended the polyethylene glycol with the EG to improve the thermal conductivity of the pure salt. Alrashdan et al. [32] found that the thermal conductivity of the paraffin/EG composite PCM could reach up to 14.5 W/(m·K), which is much higher than the pure paraffin. Aktay et al. [33] also reported that the thermal conductivity of PCM could be boosted near 3–5 times through adding EG into eutectic NaNO3-KNO3. Extending previous studies, this paper intends to develop an effective PCM encapsulation that adding EG into the eutectic LiNO3-
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precision resistance selection to deal with the signal generated by the computer. Two cylindrical samples of the test materials with similar thickness were made by the mould and the tablet machine. The probe of the Hot-Disk was placed between these two samples to detect its resistance. The recorded resistance was used to construct the quantitative correlation between the temperature and time as the following equation:
KCl-NaNO3 to enhance the thermal conductivity of PCM. The Capillary Method was utilized to prepare the composite PCM. Scanning Electron Microscopy (SEM), XRD, DSC and other thermal conductivity measurement apparatus were employed to explore the thermo-physical properties of the composite PCM, including the phase change temperature, latent heat, and thermal conductivity. For the investigation of practical composite PCM implementation in the heat storage system, an experiment was conducted to compare the thermal performance the pure salt and the proposed composite PCM.
R (t ) = R0 {1 + β [ΔTi + ΔTave (τ )]}
(1)
where t is time; τ is the dimensionless constant; R0 is the resistance of the nickel plate when t=0; β is the temperature coefficient of the nickel resistance; △Ti is the temperature difference between two sides in the film of protective layer; △Tave is the raising temperature of the sample contacting with the probe. Each sample test was replicated at least three times and the average value of measurements was considered as the final testing results. Each new test should wait at least 30 min after the previous test to ensure the heat was completely dissipated. Also, such operation is able to avoid evident convection and vibration, and enable better internal temperature distribution. As a comparison of thermal conductivity, the eutectic salt, which was compressed into a cylinder with the size of Φ40*14.72 mm and the apparent density of 1.482 g/cm3, was also tested.
2. Experimental description 2.1. Preparation of materials In previous study, Gasanaliev et al. [19] mixed lithium nitrate, potassium chloride and sodium nitrate with the mass ratio of 55.4%: 40.1%: 4.5%. Their study obtained composite PCM with a melting temperature of 160 °C and a latent heat of 266 J/g. Extending Gasanaliev et al.’s work, this study adopted the Capillary Adsorption Method to blend lithium nitrate, potassium chloride, and sodium nitrate with the same mass ratio. Mortar was utilized to mix these materials and grinded the mixture into fine powder. Then expanded graphite was yielded through microwave irradiation treatment of the raw expandable graphite (expansion ratio: 280 ml/g) in a microwave oven (Type: EG720FFI-NS, Midea, China) with a power of 1.15 kW. Then the materials were mixed with the fine powder uniformly. The mixture was placed into a crucible, which was placed and heated inside a muffle (Type: SX2-12-12, Hongye Electrical Appliance Factory, Xinghua City, China) with a temperature ranged between 0 to 1200 °C and a power of 12 kW. Once the mixture was heated to 300 °C, it will stay heat insulation for 4 h.
2.4. Testing device of thermal energy storage performance Fig. 1 shows the schematic diagram of the energy storage experimental system for the composite PCM. A heat exchange tube was placed in the center of the heat storage unit and allowed the heat transfer fluid (HTF) flowed through. The space between the heat exchange tube and the heat storage unit was filled with the composite PCM. During the experiment, the oil in two fuel tanks was heated to 220 °C by the electrical heating rods firstly. Then the oil in one fuel tank was driven into the heat exchange tube by the oil pump to heat up the composite PCM. After that, the oil left the tube and flowed into the other fuel tank. When the temperature of the PCM reached the preset temperature threshold, the heater strip and oil pump should be closed. The shell of the heat storage unit has an outer shell with a diameter of 100 mm, a length of 500 mm, and a thickness of 2 mm. The heat exchange tube has an outer shell with a diameter of 25 mm and a thickness of 2 mm. Thermal insulations were applied in the heat storage unit shell, fuel tanks, and the pipeline. SERIOLA-K3120 was selected as experiment oil with a specific heat capacity of 2.8 kJ/(kg K), a density of 876 kg/m3, and a thermal conductivity of 0.1016 W/(m ·K). The target temperature of the oil in the experiment operation is 260 °C. A multifunctional digital data acquisition system (Type: 34970A, Agilent Inc.) was connected to a computer to monitor and record the real-time
2.2. Characterization of materials The microstructure of the EG and the composite PCM was observed through a scanning electron microscope (Type: S-3700N, Hitachi, Japan). Before scanning, it is necessary to spray metal on the surface of the sample for 30 min in order to intensify the surface electrical conductivity. The sake of this operation is to ensure uniform contrasts so that the electron beam emitted by the SEM well distributed on the sample surface. This is also helpful in preventing the effect of the assembling electron beam on the image formation. The differential scanning calorimeter (Type: DSCQ20, TA Instrument Inc., USA) was used to investigate the materials’ thermalphysics characteristics, including melting temperature and latent heat. Before the test, the samples were sealed by an aluminum crucible. In order to ensure the uniform heat transfer between the crucible and the samples, the variation range of samples mass and the temperature should be controlled within 3–10 mg and −10–60 °C, respectively. Both the heating and cooling rate were set to 5 °C/min. The testing environment used the nitrogen for the operation of melting and solidifying. The PCM melting temperature was selected as the extrapolated onset temperature on the DSC curve. The temperature can be determined by the intersection between the extension cord of the baseline and the tangent line at the initial linear part of the peak. The PCM latent heat was calculated by integrating the area under the DSC curve peak. 2.3. Thermal conductivity measurement In the experiment, the thermal conductivity of the composite PCM was measured by the Hot-Disk thermal constant analyzer (Hot Disk Inc., Sweden). Based on the Transient Plane Heat Source technique proposed by Gustafsson [34], the analyzer is able to measure the thermal conductivity in the range of 0.005–500 W/(m K) with error within ± 3%. The measurement requires stable power supply and accurate
Fig. 1. The schematic diagram of the energy storage experimental system.
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Fig. 2. The leakage results of the composite PCM in different mass fractions of EG.
spaces between the EG molecules. Otherwise, the extra eutectic LiNO3KCl-NaNO3 will appear outside the pores of EG, resulting in the leakage. Then the filter papers were wetted. Fig. 2 shows outcomes of the leakage test. It can be observed that the leak amount reduces as the mass fraction of EG increases. The 25 wt% EG with the eutectic LiNO3KCl-NaNO3 had no leakage. Based on such observation, the best EG mass fraction in this experiment is 25 wt%. In this case, the EG is fully saturated and ideal for further packaging. Contrasting to study of Sari [35], in which only 10 wt% EG could fully encapsulate the paraffin, this study found the compatibility between the EG with the eutectic salts was worse than the organic paraffin. Since EG is non-hydrophilic, to prevent leakage, eutectic salt should be absorbed into the pores of EG, while paraffin is adhered to the surface of EG by the high capillary force [18].
temperature dynamic of the oil and the PCM at different positions. 3. Results and discussion 3.1. Adsorption saturation of composite PCM The study in this section compares the adsorption saturation of the composite PCM in different EG mass fraction. Firstly, five samples of the composite PCM with 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt% of EG were placed in five filter papers. They were heated under the constant temperature of 220 °C for 1 h in a drying oven. The test allows the composite PCM that have a phase change temperature of approximately 160 °C to melt. If the EG adsorption is not saturated, no eutectic LiNO3-KCl-NaNO3 is leaked due to the adsorptive action of the porous
Fig. 3. Photographs of the EG (a), and the composite PCM (b).
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Fig. 4. SEM images: (a) EG (×50); (b) EG (×1000) and (c) the composite PCM (×1000).
3.2. Structures of composite PCM Fig. 3 shows photographs of the EG and the composite PCM. Fig. 4 shows the microstructures of the EG and the LiNO3-KCl-NaNO3/EGPCM obtained from SEM. In Fig. 4a, it is found that the EG contains lots of worm-like particles. When zoom in, the amplified SEM image of the EG (Fig. 4b) shows the internal structure of the worm-like particle is made of a large volume of irregular honeycomb networks. Similar microstructures can be observed in the LiNO3-KCl-NaNO3/EG-PCM as well (Fig. 4c). The LiNO3-KCl-NaNO3 particles are distributed inside the pores of the honeycomb network, which suggests that the EG has strong absorption ability for the LiNO3-KCl-NaNO3. 3.3. Thermal properties of composite PCM Fig. 5 shows DSC curves of the composite PCM with different EG mass fractions. The sample with 0% EG mass fraction serves as the reference group. The tested phase change temperature and latent heat of the reference sample are 153.97 °C and 226.63 J/g, respectively. These observed properties are lower than the phase change temperature of 160 °C and the latent heat of 266 J/g presented in Gasanaliev et al.’s review [19]. It is worth to mention that the purity and absorbency of eutectic LiNO3-KCl-NaNO3 could result in different experiment results. Although all cases have a similar curve shape, it was found that adding EG into the eutectic LiNO3-KCl-NaNO3 moderated the heat flow curves and changed their thermal-physics. Both the endothermic peak and exothermic peak of the samples with EG are broader than that with the eutectic LiNO3-KCl-NaNO3. This suggests the improvement of the thermal conductivity with the assistance of EG when using the same heating and cooling rate [36–38]. Table 1 shows the tested phase change temperature and latent heat
Fig. 5. DSC curves of the composite PCM in different EG mass fractions.
of the composite PCM with different EG mass fractions. w is the EG mass fraction; Tm and Ts are the melting and solidification temperature. Hm,exp and Hm,the are the experimental and theoretical values of the melting enthalpy. em is the relative error between the experimental and theoretical values of the melting enthalpy. Hs,exp and Hs,the are the experimental and theoretical values of the solidification enthalpy. es is the relative error between the experimental and theoretical values of the solidification enthalpy. The theoretical value of the latent heat Hthe then can be calculated by the following equation [39]: 218
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Table 1 The tested phase change temperature and latent heat of the composite PCM. w (wt %)
Tm (°C)
Ts (°C)
Hm,exp (kJ/kg)
Hm,the (kJ/kg)
em
Hs,exp (kJ/kg)
Hs,the (kJ/kg)
es
0 10 15 20 25
153.97 149.25 144.03 145.22 132.74
144.76 149.56 152.04 147.23 140.75
226.63 186.35 179.35 164.55 149.70
– 203.97 192.64 181.31 169.98
– 8.6% 6.9% 9.2% 11.9%
226.13 190.35 178.15 161.85 149.40
– 203.52 192.21 180.91 169.60
– 6.5% 7.3% 10.5% 11.9%
Hthe = (1 − w%) Hexp, w =0
(2)
where Hexp,w=0 is the experimental value of latent heat when w=0. The results suggest that the experimental value of latent heat is consistent with the theoretical values of latent heat with relative errors less than 12%. Also, the tested heat latent of the composite PCM Hexp in different EG mass fractions is less than the theoretical heat latent. This experimental error could be caused by good moisture absorption behavior of eutectic LiNO3-KCl-NaNO3. From the table, it also can be observed that the melting temperature of the composite PCM is less than the eutectic LiNO3-KCl-NaNO3. The blending of LiNO3, KCl and NaNO3 formed an eutectic mixture and had stronger acting forces. When the EG is added into the eutectic mixture molecules, the acting force then is destroyed and results in the reduction of the surface vapor pressure [36,37]. Thus, the melting temperature of the composite PCM is less than the eutectic LiNO3-KCl-NaNO3. Fig. 6 shows XRD patterns of LiNO3-KCl-NaNO3/25 wt%EG-PCM, LiNO3-KCl-NaNO3, and EG. Comparing the diffraction peaks of the LiNO3-KCl-NaNO3 with the standard PDF cards, it can be found that the primary concentration in this sample is KCl and LiNO3, and the secondary is KCl and LiNO3. The least concentration is NaNO3, whose diffraction peak was not evidently examined in the XRD test. The diffraction peaks of other elements in the LiNO3-KCl-NaNO3/25 wt% EG-PCM sample are almost same with the LiNO3-KCl-NaNO3. This indicates only physical compound occurred between LiNO3-KCl-NaNO3 and EG, and no chemical reaction happened. The reason is that the LiNO3-KCl-NaNO3 highly disperses into the EG after blending these two materials. The porous structure of the EG and the low diffraction intensity cause the reduction in diffraction peaks. They are also the reasons why no change occurred in the relative intensity after mixing. Fig. 7 shows the thermal conductivity variation of the composite PCM with the compress density. When tested under a compress density of 1.482 g/cm3, the eutectic salt's thermal conductivity is 1.608 W/ (m k). The thermal conductivity varies from 18.57 to 31.53 W/(m k) when the composite PCM has a compress density range from 0.9 to 2 g/
Fig. 7. The thermal conductivity variation of the composite PCM with the compress density.
cm3, which is 11.5–19.6 times to that of the eutectic salt. It also can be observed that the thermal conductivity increases as the compress density increases. The relationship between the thermal conductivity and the compress density could be fitted with a linear correlation as:
y = 11.781x + 7.967
(R2 = 0.9963)
(3)
3.4. Thermal energy storage performance in a LTES system Fig. 8 shows the temperature variations over time during the melting process in the heat storage unit using the eutectic LiNO3-KClNaNO3 and the composite PCM. The flow velocity of the HTF is set to 0.5 m/s. From Fig. 8a, it can be seen that the pure eutectic salt melting starts in the region close to the flow fluid inlet because its temperature is higher than others. The PCM temperature increases rapidly with time proceeds until reaching the melting temperature. Then the pure eutectic begins to melt. The ascending trend of the temperature reduces with the increase of the time due to the occurrence of the phase change process. In Fig. 8b, the temperature variation of the composite PCM is similar to the eutectic salt. The temperature of the heat storage unit (T3) in Fig. 8b is also higher than that of the heat storage unit (T1) in Fig. 8a. This suggests the heat transfer mechanism of the eutectic LiNO3-KCl-NaNO3 is same as the composite PCM. Also, the temperature of the eutectic LiNO3-KCl-NaNO3 reaches 490 K at near 48,000 s as shown in Fig. 8a, while the temperature of the composite PCM at three different positions reaches 490 K at approximately 6500 s in Fig. 8b. This suggests that the charging rate of the composite PCM is much faster than the eutectic salt; in other word, adding the EG into the eutectic effectively enhances its thermal conductivity. 4. Conclusion The thermal-physical properties of the LiNO3-KCl-NaNO3/EG composite PCM with different mass fraction EG were investigated in this study. The leakage results indicated that 25 wt% was the optimal mass fraction where no leakage occurred. It was also found that no chemical reactions happened after blending the EG and the eutectic LiNO3-KClNaNO3. Although the latent heat of the PCM decreased after adding the EG into the pure salt, the thermal conductivity was significantly improved. In addition, a linear correlation has been observed between the thermal conductivity and the compress density of the composite PCM. After comparing the thermal performance of the latent heat and the composite PCM, the results suggest that the charging rate of the
Fig. 6. XRD patterns of LiNO3-KCl-NaNO3/25 wt%EG-PCM (a), LiNO3-KCl-NaNO3 (b), and EG (c).
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Preparation and thermal energy storage properties of LiNO3-KCl-NaNO3/ expanded graphite composite phase change material
MARK
⁎
Tao Xua, Yantong Lia, Jiayu Chena, , Junwan Liub a
Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 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
b
A R T I C L E I N F O
A B S T R A C T
Keywords: LiNO3-KCl-NaNO3 PCM Thermal energy storage Thermal conductivity
Recent studies suggest expanded graphite (EG) can significantly enhance the thermal conductivity, stability, efficiency of phase change material (PCM). This study aims to investigate the thermal-physical properties of the LiNO3-KCl-NaNO3/expanded graphite composite PCM. The testing PCM samples were prepared using the Capillary Method with 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt% EG. When EG mass fraction is 25 wt%, no leakage was observed in the leakage test. The SEM image shows that the EG had intense absorption ability to the LiNO3-KCl-NaNO3 composites. The X-ray powder diffraction (XRD) comparison between the pure salt and the composite PCM suggests that no new substance produced after adding the EG into the eutectic LiNO3-KClNaNO3. Differential scanning calorimetry (DSC) curves of the composite PCM indicate that the latent heat was reduced dramatically when the EG and the eutectic LiNO3-KCl-NaNO3 were blended together. In the experiment, thermal conductivity and the compress density of the composite PCM show a strongly linear relationship. Also, the thermal conductivity of the PCM can be enhanced after adding the EG into the pure salt. Finally, the thermal performance of the latent heat system using the eutectic LiNO3-KCl-NaNO3 and the composite PCM were compared. Due to the high thermal conductivity, the charging rate of the system utilizing the composite PCM is much faster than the pure salt.
1. Introduction Comparing to regular materials, PCM has large energy storage capacity and able to maintain constant temperature during the phase change process [1–4]. Therefore, PCM has been investigated in many research related to building energy efficiency, such as heating [5,6], natural ventilation [7–10], solar domestic water system [11–13], and building envelop design, such as wallboard [14–16]. Organic phase change material (OPCM) is one of the most common PCM in the energy storage system. However, OPCM yield low conductivity [17] and result in low heat transfer capacity and thermal utilization efficiency in the energy storage system. Low efficiency in energy storage and release hinders the widely application of OPCM. Inorganic PCM also subject to the same constraint as OPCM [18]. For example, the inorganic eutectic LiNO3-KCl-NaNO3 PCM has a high latent heat but low thermal conductivity [19]. To enhance the heat transfer ability of PCM, many researchers propose various approaches to intensify the heat transfer area between the heat transfer fluid (HTF) and PCM, such as the bundled tube structures [20], the wavy surface [21], the multiple PCM configuration ⁎
Corresponding author. E-mail address: [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.solmat.2017.05.035 Received 11 January 2017; Received in revised form 29 April 2017; Accepted 15 May 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
[22], additional fins [23–25] and etc. However, these approaches often have large material waste, higher maintenance cost, and corrosion problems [26]. Given the high thermal conductivity and rich network of micro-porous structures [27,28], expanded graphite (EG) shows a great potential in resolving these problems for PCM. For example, Gao et al. [29] added four mass fraction EG into the erythritol and achieved a melt temperature of nearly 119 °C. In their experiment, they also found that the thermal conductivity could be increased by approximately 2.5 times when mixing the erythritol with 4 wt% EG. In the study of Xiao et al. [30], the EG with the mass fraction of 5 wt%, 10 wt%, 20 wt% was examined to investigate the thermal conductivity improvement with the eutectic NaNO3-KNO3. Wang et al. [31] blended the polyethylene glycol with the EG to improve the thermal conductivity of the pure salt. Alrashdan et al. [32] found that the thermal conductivity of the paraffin/EG composite PCM could reach up to 14.5 W/(m·K), which is much higher than the pure paraffin. Aktay et al. [33] also reported that the thermal conductivity of PCM could be boosted near 3–5 times through adding EG into eutectic NaNO3-KNO3. Extending previous studies, this paper intends to develop an effective PCM encapsulation that adding EG into the eutectic LiNO3-
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precision resistance selection to deal with the signal generated by the computer. Two cylindrical samples of the test materials with similar thickness were made by the mould and the tablet machine. The probe of the Hot-Disk was placed between these two samples to detect its resistance. The recorded resistance was used to construct the quantitative correlation between the temperature and time as the following equation:
KCl-NaNO3 to enhance the thermal conductivity of PCM. The Capillary Method was utilized to prepare the composite PCM. Scanning Electron Microscopy (SEM), XRD, DSC and other thermal conductivity measurement apparatus were employed to explore the thermo-physical properties of the composite PCM, including the phase change temperature, latent heat, and thermal conductivity. For the investigation of practical composite PCM implementation in the heat storage system, an experiment was conducted to compare the thermal performance the pure salt and the proposed composite PCM.
R (t ) = R0 {1 + β [ΔTi + ΔTave (τ )]}
(1)
where t is time; τ is the dimensionless constant; R0 is the resistance of the nickel plate when t=0; β is the temperature coefficient of the nickel resistance; △Ti is the temperature difference between two sides in the film of protective layer; △Tave is the raising temperature of the sample contacting with the probe. Each sample test was replicated at least three times and the average value of measurements was considered as the final testing results. Each new test should wait at least 30 min after the previous test to ensure the heat was completely dissipated. Also, such operation is able to avoid evident convection and vibration, and enable better internal temperature distribution. As a comparison of thermal conductivity, the eutectic salt, which was compressed into a cylinder with the size of Φ40*14.72 mm and the apparent density of 1.482 g/cm3, was also tested.
2. Experimental description 2.1. Preparation of materials In previous study, Gasanaliev et al. [19] mixed lithium nitrate, potassium chloride and sodium nitrate with the mass ratio of 55.4%: 40.1%: 4.5%. Their study obtained composite PCM with a melting temperature of 160 °C and a latent heat of 266 J/g. Extending Gasanaliev et al.’s work, this study adopted the Capillary Adsorption Method to blend lithium nitrate, potassium chloride, and sodium nitrate with the same mass ratio. Mortar was utilized to mix these materials and grinded the mixture into fine powder. Then expanded graphite was yielded through microwave irradiation treatment of the raw expandable graphite (expansion ratio: 280 ml/g) in a microwave oven (Type: EG720FFI-NS, Midea, China) with a power of 1.15 kW. Then the materials were mixed with the fine powder uniformly. The mixture was placed into a crucible, which was placed and heated inside a muffle (Type: SX2-12-12, Hongye Electrical Appliance Factory, Xinghua City, China) with a temperature ranged between 0 to 1200 °C and a power of 12 kW. Once the mixture was heated to 300 °C, it will stay heat insulation for 4 h.
2.4. Testing device of thermal energy storage performance Fig. 1 shows the schematic diagram of the energy storage experimental system for the composite PCM. A heat exchange tube was placed in the center of the heat storage unit and allowed the heat transfer fluid (HTF) flowed through. The space between the heat exchange tube and the heat storage unit was filled with the composite PCM. During the experiment, the oil in two fuel tanks was heated to 220 °C by the electrical heating rods firstly. Then the oil in one fuel tank was driven into the heat exchange tube by the oil pump to heat up the composite PCM. After that, the oil left the tube and flowed into the other fuel tank. When the temperature of the PCM reached the preset temperature threshold, the heater strip and oil pump should be closed. The shell of the heat storage unit has an outer shell with a diameter of 100 mm, a length of 500 mm, and a thickness of 2 mm. The heat exchange tube has an outer shell with a diameter of 25 mm and a thickness of 2 mm. Thermal insulations were applied in the heat storage unit shell, fuel tanks, and the pipeline. SERIOLA-K3120 was selected as experiment oil with a specific heat capacity of 2.8 kJ/(kg K), a density of 876 kg/m3, and a thermal conductivity of 0.1016 W/(m ·K). The target temperature of the oil in the experiment operation is 260 °C. A multifunctional digital data acquisition system (Type: 34970A, Agilent Inc.) was connected to a computer to monitor and record the real-time
2.2. Characterization of materials The microstructure of the EG and the composite PCM was observed through a scanning electron microscope (Type: S-3700N, Hitachi, Japan). Before scanning, it is necessary to spray metal on the surface of the sample for 30 min in order to intensify the surface electrical conductivity. The sake of this operation is to ensure uniform contrasts so that the electron beam emitted by the SEM well distributed on the sample surface. This is also helpful in preventing the effect of the assembling electron beam on the image formation. The differential scanning calorimeter (Type: DSCQ20, TA Instrument Inc., USA) was used to investigate the materials’ thermalphysics characteristics, including melting temperature and latent heat. Before the test, the samples were sealed by an aluminum crucible. In order to ensure the uniform heat transfer between the crucible and the samples, the variation range of samples mass and the temperature should be controlled within 3–10 mg and −10–60 °C, respectively. Both the heating and cooling rate were set to 5 °C/min. The testing environment used the nitrogen for the operation of melting and solidifying. The PCM melting temperature was selected as the extrapolated onset temperature on the DSC curve. The temperature can be determined by the intersection between the extension cord of the baseline and the tangent line at the initial linear part of the peak. The PCM latent heat was calculated by integrating the area under the DSC curve peak. 2.3. Thermal conductivity measurement In the experiment, the thermal conductivity of the composite PCM was measured by the Hot-Disk thermal constant analyzer (Hot Disk Inc., Sweden). Based on the Transient Plane Heat Source technique proposed by Gustafsson [34], the analyzer is able to measure the thermal conductivity in the range of 0.005–500 W/(m K) with error within ± 3%. The measurement requires stable power supply and accurate
Fig. 1. The schematic diagram of the energy storage experimental system.
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Fig. 2. The leakage results of the composite PCM in different mass fractions of EG.
spaces between the EG molecules. Otherwise, the extra eutectic LiNO3KCl-NaNO3 will appear outside the pores of EG, resulting in the leakage. Then the filter papers were wetted. Fig. 2 shows outcomes of the leakage test. It can be observed that the leak amount reduces as the mass fraction of EG increases. The 25 wt% EG with the eutectic LiNO3KCl-NaNO3 had no leakage. Based on such observation, the best EG mass fraction in this experiment is 25 wt%. In this case, the EG is fully saturated and ideal for further packaging. Contrasting to study of Sari [35], in which only 10 wt% EG could fully encapsulate the paraffin, this study found the compatibility between the EG with the eutectic salts was worse than the organic paraffin. Since EG is non-hydrophilic, to prevent leakage, eutectic salt should be absorbed into the pores of EG, while paraffin is adhered to the surface of EG by the high capillary force [18].
temperature dynamic of the oil and the PCM at different positions. 3. Results and discussion 3.1. Adsorption saturation of composite PCM The study in this section compares the adsorption saturation of the composite PCM in different EG mass fraction. Firstly, five samples of the composite PCM with 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt% of EG were placed in five filter papers. They were heated under the constant temperature of 220 °C for 1 h in a drying oven. The test allows the composite PCM that have a phase change temperature of approximately 160 °C to melt. If the EG adsorption is not saturated, no eutectic LiNO3-KCl-NaNO3 is leaked due to the adsorptive action of the porous
Fig. 3. Photographs of the EG (a), and the composite PCM (b).
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Fig. 4. SEM images: (a) EG (×50); (b) EG (×1000) and (c) the composite PCM (×1000).
3.2. Structures of composite PCM Fig. 3 shows photographs of the EG and the composite PCM. Fig. 4 shows the microstructures of the EG and the LiNO3-KCl-NaNO3/EGPCM obtained from SEM. In Fig. 4a, it is found that the EG contains lots of worm-like particles. When zoom in, the amplified SEM image of the EG (Fig. 4b) shows the internal structure of the worm-like particle is made of a large volume of irregular honeycomb networks. Similar microstructures can be observed in the LiNO3-KCl-NaNO3/EG-PCM as well (Fig. 4c). The LiNO3-KCl-NaNO3 particles are distributed inside the pores of the honeycomb network, which suggests that the EG has strong absorption ability for the LiNO3-KCl-NaNO3. 3.3. Thermal properties of composite PCM Fig. 5 shows DSC curves of the composite PCM with different EG mass fractions. The sample with 0% EG mass fraction serves as the reference group. The tested phase change temperature and latent heat of the reference sample are 153.97 °C and 226.63 J/g, respectively. These observed properties are lower than the phase change temperature of 160 °C and the latent heat of 266 J/g presented in Gasanaliev et al.’s review [19]. It is worth to mention that the purity and absorbency of eutectic LiNO3-KCl-NaNO3 could result in different experiment results. Although all cases have a similar curve shape, it was found that adding EG into the eutectic LiNO3-KCl-NaNO3 moderated the heat flow curves and changed their thermal-physics. Both the endothermic peak and exothermic peak of the samples with EG are broader than that with the eutectic LiNO3-KCl-NaNO3. This suggests the improvement of the thermal conductivity with the assistance of EG when using the same heating and cooling rate [36–38]. Table 1 shows the tested phase change temperature and latent heat
Fig. 5. DSC curves of the composite PCM in different EG mass fractions.
of the composite PCM with different EG mass fractions. w is the EG mass fraction; Tm and Ts are the melting and solidification temperature. Hm,exp and Hm,the are the experimental and theoretical values of the melting enthalpy. em is the relative error between the experimental and theoretical values of the melting enthalpy. Hs,exp and Hs,the are the experimental and theoretical values of the solidification enthalpy. es is the relative error between the experimental and theoretical values of the solidification enthalpy. The theoretical value of the latent heat Hthe then can be calculated by the following equation [39]: 218
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Table 1 The tested phase change temperature and latent heat of the composite PCM. w (wt %)
Tm (°C)
Ts (°C)
Hm,exp (kJ/kg)
Hm,the (kJ/kg)
em
Hs,exp (kJ/kg)
Hs,the (kJ/kg)
es
0 10 15 20 25
153.97 149.25 144.03 145.22 132.74
144.76 149.56 152.04 147.23 140.75
226.63 186.35 179.35 164.55 149.70
– 203.97 192.64 181.31 169.98
– 8.6% 6.9% 9.2% 11.9%
226.13 190.35 178.15 161.85 149.40
– 203.52 192.21 180.91 169.60
– 6.5% 7.3% 10.5% 11.9%
Hthe = (1 − w%) Hexp, w =0
(2)
where Hexp,w=0 is the experimental value of latent heat when w=0. The results suggest that the experimental value of latent heat is consistent with the theoretical values of latent heat with relative errors less than 12%. Also, the tested heat latent of the composite PCM Hexp in different EG mass fractions is less than the theoretical heat latent. This experimental error could be caused by good moisture absorption behavior of eutectic LiNO3-KCl-NaNO3. From the table, it also can be observed that the melting temperature of the composite PCM is less than the eutectic LiNO3-KCl-NaNO3. The blending of LiNO3, KCl and NaNO3 formed an eutectic mixture and had stronger acting forces. When the EG is added into the eutectic mixture molecules, the acting force then is destroyed and results in the reduction of the surface vapor pressure [36,37]. Thus, the melting temperature of the composite PCM is less than the eutectic LiNO3-KCl-NaNO3. Fig. 6 shows XRD patterns of LiNO3-KCl-NaNO3/25 wt%EG-PCM, LiNO3-KCl-NaNO3, and EG. Comparing the diffraction peaks of the LiNO3-KCl-NaNO3 with the standard PDF cards, it can be found that the primary concentration in this sample is KCl and LiNO3, and the secondary is KCl and LiNO3. The least concentration is NaNO3, whose diffraction peak was not evidently examined in the XRD test. The diffraction peaks of other elements in the LiNO3-KCl-NaNO3/25 wt% EG-PCM sample are almost same with the LiNO3-KCl-NaNO3. This indicates only physical compound occurred between LiNO3-KCl-NaNO3 and EG, and no chemical reaction happened. The reason is that the LiNO3-KCl-NaNO3 highly disperses into the EG after blending these two materials. The porous structure of the EG and the low diffraction intensity cause the reduction in diffraction peaks. They are also the reasons why no change occurred in the relative intensity after mixing. Fig. 7 shows the thermal conductivity variation of the composite PCM with the compress density. When tested under a compress density of 1.482 g/cm3, the eutectic salt's thermal conductivity is 1.608 W/ (m k). The thermal conductivity varies from 18.57 to 31.53 W/(m k) when the composite PCM has a compress density range from 0.9 to 2 g/
Fig. 7. The thermal conductivity variation of the composite PCM with the compress density.
cm3, which is 11.5–19.6 times to that of the eutectic salt. It also can be observed that the thermal conductivity increases as the compress density increases. The relationship between the thermal conductivity and the compress density could be fitted with a linear correlation as:
y = 11.781x + 7.967
(R2 = 0.9963)
(3)
3.4. Thermal energy storage performance in a LTES system Fig. 8 shows the temperature variations over time during the melting process in the heat storage unit using the eutectic LiNO3-KClNaNO3 and the composite PCM. The flow velocity of the HTF is set to 0.5 m/s. From Fig. 8a, it can be seen that the pure eutectic salt melting starts in the region close to the flow fluid inlet because its temperature is higher than others. The PCM temperature increases rapidly with time proceeds until reaching the melting temperature. Then the pure eutectic begins to melt. The ascending trend of the temperature reduces with the increase of the time due to the occurrence of the phase change process. In Fig. 8b, the temperature variation of the composite PCM is similar to the eutectic salt. The temperature of the heat storage unit (T3) in Fig. 8b is also higher than that of the heat storage unit (T1) in Fig. 8a. This suggests the heat transfer mechanism of the eutectic LiNO3-KCl-NaNO3 is same as the composite PCM. Also, the temperature of the eutectic LiNO3-KCl-NaNO3 reaches 490 K at near 48,000 s as shown in Fig. 8a, while the temperature of the composite PCM at three different positions reaches 490 K at approximately 6500 s in Fig. 8b. This suggests that the charging rate of the composite PCM is much faster than the eutectic salt; in other word, adding the EG into the eutectic effectively enhances its thermal conductivity. 4. Conclusion The thermal-physical properties of the LiNO3-KCl-NaNO3/EG composite PCM with different mass fraction EG were investigated in this study. The leakage results indicated that 25 wt% was the optimal mass fraction where no leakage occurred. It was also found that no chemical reactions happened after blending the EG and the eutectic LiNO3-KClNaNO3. Although the latent heat of the PCM decreased after adding the EG into the pure salt, the thermal conductivity was significantly improved. In addition, a linear correlation has been observed between the thermal conductivity and the compress density of the composite PCM. After comparing the thermal performance of the latent heat and the composite PCM, the results suggest that the charging rate of the
Fig. 6. XRD patterns of LiNO3-KCl-NaNO3/25 wt%EG-PCM (a), LiNO3-KCl-NaNO3 (b), and EG (c).
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