- Email: [email protected]
graphite nano-particles
Journal of Energy Storage 27 (2020) 101166
Contents lists available at ScienceDirect
Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Thermal conductivity enhancement of sodium thiosulfate pentahydrate by adding carbon nano-tubes/graphite nano-particles
T
⁎
Rabi Ibrahim Rabady , Dua'a S Malkawi Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
A B S T R A C T
Phase change materials are great thermal energy storage medium, while their low thermal conductivity presents the main obstacle for their potential applications. Improvement of the thermal conductivity and specific heat of Sodium Thiosulfate Pentahydrate as a base phase change material with joining Carbon Nano tubes CNT and Graphite Nano particles GNP as Nano-fillers was investigated in this work. CNT and GNP Nano-fillers added in a mass fraction of 1, 3, 5 and 7% in Sodium Thiosulfate Pentahydrate. Thermal conductivity and specific heat were measured by Hot Disk Thermal Constants Analyzer Instrument. Results have shown that increasing the mass fraction of Nano-fillers increases the composite's thermal conductivity. Thermal conductivity of the composite containing: 7% GNP was 2.944 W/ m.k with 155.33% enhancement, in the other hand, thermal conductivity of the composite containing: 7% CNT was 4.031 W/m.k with 249.61% enhancement. Moreover, the charging/discharging rates have been enhanced by adding Nano-fillers to Sodium Thiosulfate Pentahydrate.
1. Introduction Using Phase Change Material PCM as storage media for latent thermal energy is an effective way for using thermal energy. PCMs may involve only sensible heat, latent heat or a combination of both. PCMs give a high energy storage capacity by absorbing or releasing latent heat during phase change process during isotherm conditions. PCMs classified into two types: Organic (Paraffins and nonparaffin) and Inorganic (Salt hydrate and metallics). The main characteristics that choosing organic and nonorganic PCM depends on them are: Suitable phase-transformation temperature, high latent heat, high specific heat and high thermal conductivity [1–5]. Recently, inorganic type of PCMs such as salt hydrates have been investigated as thermal energy storage and found with favorable thermal properties [6]. They have a higher thermal conductivity, a higher latent heat, and lower cost relative to organic phase change materials [7]. On the other hand, inorganic PCMs (salts and salt hydrates) have phase segregation and supercooling problems, which will reversibly affect the energy storage capacity [8]. Another key property of PCMs is the thermal conductivity, since it directly affects the charging/discharging rates of the PCM with heat energy. Higher thermal conductivities are favorable in order to facilitate convenient use of stored heat energy. However, most PCMs suffer low thermal conductivities that negatively affect their effectiveness. Different suggestions were proposed and investigated in order to overcome the low thermal conductivity issue by incorporating a metallic mesh into the PCM volume or by small size spherical capsulation of the PCM, which effectively increases the interacting surface area between the
⁎
PCM and the surrounding medium. Recently, induced by the extraordinary interest and advances in the technology, researchers proposed enhancing the PCMs’ thermal conductivity by adding nanoparticles [9–15]. Comparison for the findings of previous research studies which investigated adding nanoparticles to PCM to improve the thermal conductivity is of interest. Nevertheless, it was difficult to make comprehensive and solid comparison due to various PCM and nanoparticles sizes and thicknesses used in these investigations. Zeng et al. present results of an experimental investigation of melting of nanoparticle enhanced phase change materials. Multi-walled carbon nanotubes (CNT) were dispersed in 1-dodeconal to prepare samples with various weight percentages (0, 1 and 2 wt.%). The thermal conductivity increases with increasing the weight percentages of the CNTs; for instance, the relative enhancement is 4.6% for the 1 wt.% [14]. He et al. present the preparation and thermal characterization of Myristic acid as PCM enhanced by multi-walled carbon nanotubes (MWCNTs) and nano-graphite (NG). It was found that the thermal conductivity of the PCM composites can be improved by 47.30% and 44.01%,% respectively, by adding MWCNTs and NG with a concentration of 3 wt.% for the MWCNT [15]. In this paper, further experimental investigation is performed in order to examine the performance improvement due to adding nano-fillers such as: Carbon Nanotubes or Graphite Nano-Particles to Sodium Thiosulfate Pentahydrate at different mass fraction. The purpose of this paper is to evaluate and compare the heat transfer enhancement of the phase change nanocomposites using different nanofillers and to develop nano-PCM composites with desired characteristics, especially with high thermal conductivity, to facilitate for
Corresponding author. E-mail address: [email protected] (R.I. Rabady).
https://doi.org/10.1016/j.est.2019.101166 Received 25 September 2019; Received in revised form 2 December 2019; Accepted 17 December 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
2. Experimental
temperature is maintained at 50–60 °C. Sodium thiosulfate pentahydrate should be melted until is converted to liquid phase. The measured amount of nano-fillers is added to liquid Sodium thiosulfate pentahydrate, and then the composites were prepared by shear mixing with a magnetic stirrer for 20 min, followed by ultrasonic shaking for 50 min. To the above solution, Carboxymethyl cellulose (CMC) and sodium dodecyl sulfate (SDS) were mixed and thoroughly stirred. The CMC used as a stabilizing or nucleating agent, which helps in preventing phase segregation and supercooling problems, a main problem encountered in salt hydrates. SDS used as surfactant solution to help nano-fillers to dispersed in salt hydrate. During this step, the temperature was kept above the melting temperature of Sodium thiosulfate pentahydrate. (4) The molten composite is poured gently into a rectangular mold then put it in hydraulic press to take the mold shape without bubbles and allowed to solidify at room temperature.
2.1. Materials
2.3. Instruments
Table 1 Specifications of nano-fillers.
Diameter Purity Ash Color
MWCNT
GNP
50–80 nm > 90% < 0.8% Black
40 nm > 95% < 0.5% Black
effective thermal energy storage. The thermal conductivities and specific heat capacity of these nanocomposites are measured by Hot Disk thermal constants analyzer. Finally, the charging and discharging period performances of these different phase change nanocomposites are also investigated.
The used instruments for the experimental work were as follow:
Sodium thiosulfate pentahydrate as PCM with melting point of about 48 °C was purchased from US Research Nanomaterials. Inc. (Houston, USA). Because of its melting point and charging/ discharging cycling durability, such salt hydrate can be suitable in solar hydronic systems which find broad application in domestic use as clean source of hot water. Multi-walled carbon nanotube MWCNT and Graphite nano-powder are the two types of nano-fillers were used, with specification listed in Table 1. Fig. 1 presents the SEM graphs of these nano-fillers. All materials were utilized as supplied without further treatment or purification.
• Hot plate was used to raise the temperature of the samples until they melt with magnetic stirrer. • Supersonic shaker was used to mix the nanoparticles with Sodium thiosulfate pentahydrate homogeneously. • The samples were prepared by casting molding then pressed by •
2.2. Preparation of composite PCM Various samples were prepared with weight fractions of 1, 3, 5, and 7% of MWCNT and GNP nanoparticles. The preparation of composites has been characterized by five steps: (1) Pre-melting of pure PCMs, to homogenize the pure PCM. Sodium thiosulfate pentahydrate has been put in suitable Vials to assist the handling of the preparation steps of composites. (2) Weighing of a solid PCM with and nano-fillers to prepare the samples. For each type of the nano-fillers, the mass fractions in samples are 1, 3, 5, and 7%. (3) The Vial is placed in hot plate within a water beaker with
hydraulic press, the average thickness of the sample is 3 mm, and can be immediately used for thermal transport measurements. Hot Disk Thermal Constants Analyzer shown in Fig. 2 was used to measure the thermal conductivity, specific heat and diffusivity of the samples. A Hot Disk thermal analyzer uses the Transient Plane Source (TPS) method based on a transient technique. The sensor of this device includes an electrically conducting model in the shape of a double spiral, which is engraved on a very thin Nickel foil. This spiral is surrounding by two thin insulating sheets. This sensor is placed between two sample pieces to perform a thermal transport measurement.
3. Results and discussion 3.1. Thermal conductivity analysis The temperature variations of phase change materials are used for thermal energy storage. The energy storage rate depends mainly on the
Fig. 1. SEM micrographs of: (a) MWCNT, (b) GNP. (Source: US Research Nanomaterials. Inc.). 2
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
Fig. 2. Hot disk thermal constants analyzer that was used to measure the thermal conductivity.
thermal conductivity of materials; therefore, the thermal conductivity of PCMs is an important property that must be investigated in order to be improved by proper method. The thermal conductivity of Sodium thiosulfate pentahydrate was measured as reference value as 1.153 W/m.K. The experimental result for the thermal conductivity of the composite PCMs with 1%, 3%, 5% and 7% nano-fillers are listed in Tables 2 and 3; which are also fitted as shown in Fig. 3. It is clear that the thermal conductivity of the composite PCM increased remarkably with the increasing of the nano-fillers content. Relative thermal conductivity enhancement factor defined as: (k-k0)/k0 is used in order to describe the quantitatively enhancement of nano-filler in increasing of thermal conductivity. Adding Carbon Nano tubes to Sodium thiosulfate pentahydrate yields better thermal conductivities than adding the GNP. Moreover, increasing the CNT mass fraction maintain thermal conductivity enhancement of the composite, but it becomes almost steady with the GNP addition as depicted from Fig. 3. These two aspects propose that incorporating CNT to Sodium thiosulfate pentahydrate would be more advantageous and promising for useful application. Based on the above experimental results of the two types of used nanoparticles, it can be concluded that the enhancement of thermal conductivity is directly related to the nanomaterial microstructure shown in Fig. 1. For instance, PCM with CNT can form columnar structure layer on the nanoparticle surface, which helps in producing effective heat conduction paths, hence enhanced thermal conductivity. Whereas, for PCM with GNP case, the spherical sheet structure helps to form nano-layer that facilitates better heat conduction since the thermal conduction in the transversal direction is much lower than that in the longitudinal direction.
Table 3 Thermal conductivity of Sodium thiosulfate pentahydrate /CNT composites. Thermal parameters
Thermal conductivity (W/m K) The relative thermal conductivity enhancement (%)
Thermal conductivity (W/m K) The relative thermal conductivity enhancement (%)
Mass fraction 1%
3%
5%
7%
2.208 91.5
2.578 123.59
2.611 126.45
2.944 155.33
3%
5%
7%
2.824 144.93
3.107 169.47
3.603 212.49
4.031 249.61
Fig. 3. Thermal conductivity of the composite PCMs versus the mass fraction of the nanoparticles.
Table 2 Thermal conductivity of Sodium thiosulfate pentahydrate/GNP composites. Thermal parameters
Mass fraction 1%
Enhancement of thermal conductivity consequently affects directly from the charging time that is required to reach certain temperature, which is critical thermal energy storage applications, as depicted in Figs. 4 and 5 presents. Apparently, increasing the CNT mass fraction enhances both the thermal conductivity as well as the charging time which was decreased from 30 min to 7 min only with 7% mass fraction. Whereas, adding 7% of GNP decreased the charging time from 30 min to about 11 min. 3
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
Fig. 7. STP with GNP composites behavior during discharging time. Fig. 4. STP with CNT composites behavior.during charging time.
Fig. 8. Comparison between studies added GNP.
Fig. 5. STP with GNP composites behavior during charging time.
Fig. 6. STP with CNT composites behavior during discharging time.
Fig. 9. Comparison between studies added GNP.
4
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
Fig. 10. DSC analysis for PCM with 7%GNP: (a) Melting curve, (b) Solidified curve.
additive wt.% of the nano filers.
Figs. 6 and 7 present the effect of adding CNT and GNP to Sodium Thiosulfate Pentahydrate on discharging time when the samples cooled at room temperature (18 °C). For CNT case the discharging time was decrease from 40 min for Sodium Thiosulfate Pentahydrate to 15 min with 7% CNT. Whereas, for mass fraction of 7% GNP the discharging time was decrease from 40 min to 18 min. Figs. 8 and 9 present a comparison results between our recent study and that of M. He et al [15]. Obviously, not only that our findings in enhancing the relative thermal conductivity by nano filters addition are far better than that of He et al, but also it covers wider range for the
3.2. DSC analysis Melting and solidified temperature of the Sodium thiosulfate pentahydrate was investigated using DSC for maximum mass percent of two types of nanoparticles, 7% GNP and 7% CNT. The melting temperature and enthalpy of Sodium thiosulfate pentahydrate is 48 °C and 210 kJ/kg, respectively. The DSC curves of Sodium thiosulfate pentahydrate with 7% of GNP 5
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
Fig. 11. DSC analysis for PCM with 7%CNT: (a) Melting curve, (b) Solidified curve.
slight increase in the melting temperature was observed as depicted from the DSC curves of the CNT/ sodium thiosulfate pentahydrate composite compared to that melting temperature of the pure sodium thiosulfate pentahydrate as observed in Fig. 11. Moreover, the melting temperature of the 7% CNT/ sodium thiosulfate pentahydrate composite was 55.8 °C. The adding of CNT also shows positive influence on easing the effect of the supercooling phenomenon where the solidified temperature was 20.1 °C. The supercooling effect was improved
are shown in Fig.10, which shows that the melting temperature of the 7% GNP composite is about 53.1 °C. It can be concluded that the melting temperature is almost not affected by the concentration of the added GNPs since slight decrease in the melting temperature was observed when compared with that of pure sodium thiosulfate pentahydrate. The solidified DSC curves show that the composite exhibits supercooling phenomenon with solidified temperature as 25.6 °C. Similar to the GNP/ sodium thiosulfate pentahydrate composite,
6
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
References
compared to that of pure Sodium thiosulfate pentahydrate due to the nucleating effect of the additives which served as nucleating agents that facilitate the crystallization of the composite.
[1] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sustain. Energy Rev. 13 (2) (2009) 318–345. [2] D.R. Rousse, N. Ben Salah, S. Lassue, An overview of phase change materials and their implication on power demand,” 2009, IEEE Electr. Power Energy Conf. EPEC 2009 (2009) 1–6. [3] S. Khare, M. Dell'Amico, C. Knight, S. McGarry, Selection of materials for high temperature latent heat energy storage, Sol. Energy Mater. Sol. Cells 107 (2012) 20–27. [4] B. Cárdenas, N. León, High temperature latent heat thermal energy storage: phase change materials, design considerations and performance enhancement techniques, Renew. Sustain. Energy Rev. 27 (2013) 724–737. [5] A. Abhat, Low temperature latent heat thermal energy storage: heat storage materials, Sol. Energy 30 (4) (1983) 313–332. [6] J. Wang, H. Xie, Z. Xin, Y. Li, C. Yin, Investigation on thermal properties of heat storage composites containing carbon fibers, J. Appl. Phys. 110 (9) (2011) pages:094302-094302-5. [7] M. Liu, W. Saman, F. Bruno, Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems, Renew. Sustain. Energy Rev. 16 (4) (2012) 2118–2132. [8] H. Liu, H.B. Awbi, Performance of phase change material boards under natural convection, Build. Environ. 44 (9) (2009) 1788–1793. [9] L.W. Fan, X. Fang, X. Wang, Y. Zeng, Y.-Q. Xiao, Z.-T. Yu, et al., Effects of various carbon nano-fillers on the thermal conductivity and energy storage properties of paraffin-based nanocomposite phase change materials, Appl. Energy 110 (2013) 163–172. [10] N. Zhang, Y. Yuan, X. Cao, Y. Du, Z. Zhang, Y. Gui, Latent heat thermal energy storage systems with solid–liquid phase change materials: a review, Adv. Eng. Mater. 20 (6) (2018) 1–30. [11] Y. Lin, Y. Jia, G. Alva, G. Fang, Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage, Renew. Sustain. Energy Rev. 82 (May 2017) (2018) 2730–2742. [12] R.J. Warzoha, A.S. Fleischer, Effect of carbon nanotube interfacial geometry on thermal transport in solid-liquid phase change materials, Appl. Energy 154 (2015) 271–276. [13] W.G. Alshaer, E. Palomo del Barrio, M.A. Rady, O.E. Abdellatif, S.A. Nada, Analysis of the anomalous thermal properties of phase change materials based on paraffin wax and multi walls carbon nanotubes, Int. J. Heat Mass Transf. Appl. 1 (5) (2013) 297–307. [14] Y. Zeng, L. Fan, Y. Xiao, Z. Yu, K. Cen, An experimental investigation of melting of nanoparticle-enhanced phase change materials (NePCMs) in a bottom-heated vertical cylindrical cavity, Int. J. Heat Mass Transf. 66 (2013) 111–117. [15] M. He, L. Yang, W. Lin, J. Chen, X. Mao, Z. Ma, Preparation, thermal characterization and examination of phase change materials (PCMs) enhanced by carbonbased nanoparticles for solar thermal energy storage, J. Energy Storage 25 (2019) 100874July.
4. Conclusion It can be concluded from the results of this work that the nano-fillers (CNT or GNP) can significantly enhance the thermal conductivity of Sodium Thiosulfate Pentahydrate as PCM. The enhancements in thermal conductivity were observed to increase by increasing the nanoparticles mass fraction. For example, the overall relative enhancements were 155.33% and 249.61% in thermal conductivity of 7% mass fraction of GNP- Sodium Thiosulfate Pentahydrate and CNT- Sodium Thiosulfate Pentahydrate, respectively. Moreover, adding nano-fillers to Sodium Thiosulfate Pentahydrate decreases the charging/discharging time at the melting point in more convenient manner; hence boost their broad application. The supercooling phenomenon of sodium thiosulfate pentahydrate has been greatly improved after adding nanoparticles, stabilizing and nucleating agents. We believe the experimental investigations and findings of this work are of great interest since they contribute to the urge of better use and storage purposes of clean energy. Declaration of Competing Interest Confirm that there is no any kind of conflict of interest upon publishing this work with any one. Acknowledgment Authors would like to acknowledge the financial support provided by Deanship of Research and research facilities labs by Chemical Engineering Department at Jordan University of Science and Technology. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2019.101166.
7
Contents lists available at ScienceDirect
Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Thermal conductivity enhancement of sodium thiosulfate pentahydrate by adding carbon nano-tubes/graphite nano-particles
T
⁎
Rabi Ibrahim Rabady , Dua'a S Malkawi Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
A B S T R A C T
Phase change materials are great thermal energy storage medium, while their low thermal conductivity presents the main obstacle for their potential applications. Improvement of the thermal conductivity and specific heat of Sodium Thiosulfate Pentahydrate as a base phase change material with joining Carbon Nano tubes CNT and Graphite Nano particles GNP as Nano-fillers was investigated in this work. CNT and GNP Nano-fillers added in a mass fraction of 1, 3, 5 and 7% in Sodium Thiosulfate Pentahydrate. Thermal conductivity and specific heat were measured by Hot Disk Thermal Constants Analyzer Instrument. Results have shown that increasing the mass fraction of Nano-fillers increases the composite's thermal conductivity. Thermal conductivity of the composite containing: 7% GNP was 2.944 W/ m.k with 155.33% enhancement, in the other hand, thermal conductivity of the composite containing: 7% CNT was 4.031 W/m.k with 249.61% enhancement. Moreover, the charging/discharging rates have been enhanced by adding Nano-fillers to Sodium Thiosulfate Pentahydrate.
1. Introduction Using Phase Change Material PCM as storage media for latent thermal energy is an effective way for using thermal energy. PCMs may involve only sensible heat, latent heat or a combination of both. PCMs give a high energy storage capacity by absorbing or releasing latent heat during phase change process during isotherm conditions. PCMs classified into two types: Organic (Paraffins and nonparaffin) and Inorganic (Salt hydrate and metallics). The main characteristics that choosing organic and nonorganic PCM depends on them are: Suitable phase-transformation temperature, high latent heat, high specific heat and high thermal conductivity [1–5]. Recently, inorganic type of PCMs such as salt hydrates have been investigated as thermal energy storage and found with favorable thermal properties [6]. They have a higher thermal conductivity, a higher latent heat, and lower cost relative to organic phase change materials [7]. On the other hand, inorganic PCMs (salts and salt hydrates) have phase segregation and supercooling problems, which will reversibly affect the energy storage capacity [8]. Another key property of PCMs is the thermal conductivity, since it directly affects the charging/discharging rates of the PCM with heat energy. Higher thermal conductivities are favorable in order to facilitate convenient use of stored heat energy. However, most PCMs suffer low thermal conductivities that negatively affect their effectiveness. Different suggestions were proposed and investigated in order to overcome the low thermal conductivity issue by incorporating a metallic mesh into the PCM volume or by small size spherical capsulation of the PCM, which effectively increases the interacting surface area between the
⁎
PCM and the surrounding medium. Recently, induced by the extraordinary interest and advances in the technology, researchers proposed enhancing the PCMs’ thermal conductivity by adding nanoparticles [9–15]. Comparison for the findings of previous research studies which investigated adding nanoparticles to PCM to improve the thermal conductivity is of interest. Nevertheless, it was difficult to make comprehensive and solid comparison due to various PCM and nanoparticles sizes and thicknesses used in these investigations. Zeng et al. present results of an experimental investigation of melting of nanoparticle enhanced phase change materials. Multi-walled carbon nanotubes (CNT) were dispersed in 1-dodeconal to prepare samples with various weight percentages (0, 1 and 2 wt.%). The thermal conductivity increases with increasing the weight percentages of the CNTs; for instance, the relative enhancement is 4.6% for the 1 wt.% [14]. He et al. present the preparation and thermal characterization of Myristic acid as PCM enhanced by multi-walled carbon nanotubes (MWCNTs) and nano-graphite (NG). It was found that the thermal conductivity of the PCM composites can be improved by 47.30% and 44.01%,% respectively, by adding MWCNTs and NG with a concentration of 3 wt.% for the MWCNT [15]. In this paper, further experimental investigation is performed in order to examine the performance improvement due to adding nano-fillers such as: Carbon Nanotubes or Graphite Nano-Particles to Sodium Thiosulfate Pentahydrate at different mass fraction. The purpose of this paper is to evaluate and compare the heat transfer enhancement of the phase change nanocomposites using different nanofillers and to develop nano-PCM composites with desired characteristics, especially with high thermal conductivity, to facilitate for
Corresponding author. E-mail address: [email protected] (R.I. Rabady).
https://doi.org/10.1016/j.est.2019.101166 Received 25 September 2019; Received in revised form 2 December 2019; Accepted 17 December 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
2. Experimental
temperature is maintained at 50–60 °C. Sodium thiosulfate pentahydrate should be melted until is converted to liquid phase. The measured amount of nano-fillers is added to liquid Sodium thiosulfate pentahydrate, and then the composites were prepared by shear mixing with a magnetic stirrer for 20 min, followed by ultrasonic shaking for 50 min. To the above solution, Carboxymethyl cellulose (CMC) and sodium dodecyl sulfate (SDS) were mixed and thoroughly stirred. The CMC used as a stabilizing or nucleating agent, which helps in preventing phase segregation and supercooling problems, a main problem encountered in salt hydrates. SDS used as surfactant solution to help nano-fillers to dispersed in salt hydrate. During this step, the temperature was kept above the melting temperature of Sodium thiosulfate pentahydrate. (4) The molten composite is poured gently into a rectangular mold then put it in hydraulic press to take the mold shape without bubbles and allowed to solidify at room temperature.
2.1. Materials
2.3. Instruments
Table 1 Specifications of nano-fillers.
Diameter Purity Ash Color
MWCNT
GNP
50–80 nm > 90% < 0.8% Black
40 nm > 95% < 0.5% Black
effective thermal energy storage. The thermal conductivities and specific heat capacity of these nanocomposites are measured by Hot Disk thermal constants analyzer. Finally, the charging and discharging period performances of these different phase change nanocomposites are also investigated.
The used instruments for the experimental work were as follow:
Sodium thiosulfate pentahydrate as PCM with melting point of about 48 °C was purchased from US Research Nanomaterials. Inc. (Houston, USA). Because of its melting point and charging/ discharging cycling durability, such salt hydrate can be suitable in solar hydronic systems which find broad application in domestic use as clean source of hot water. Multi-walled carbon nanotube MWCNT and Graphite nano-powder are the two types of nano-fillers were used, with specification listed in Table 1. Fig. 1 presents the SEM graphs of these nano-fillers. All materials were utilized as supplied without further treatment or purification.
• Hot plate was used to raise the temperature of the samples until they melt with magnetic stirrer. • Supersonic shaker was used to mix the nanoparticles with Sodium thiosulfate pentahydrate homogeneously. • The samples were prepared by casting molding then pressed by •
2.2. Preparation of composite PCM Various samples were prepared with weight fractions of 1, 3, 5, and 7% of MWCNT and GNP nanoparticles. The preparation of composites has been characterized by five steps: (1) Pre-melting of pure PCMs, to homogenize the pure PCM. Sodium thiosulfate pentahydrate has been put in suitable Vials to assist the handling of the preparation steps of composites. (2) Weighing of a solid PCM with and nano-fillers to prepare the samples. For each type of the nano-fillers, the mass fractions in samples are 1, 3, 5, and 7%. (3) The Vial is placed in hot plate within a water beaker with
hydraulic press, the average thickness of the sample is 3 mm, and can be immediately used for thermal transport measurements. Hot Disk Thermal Constants Analyzer shown in Fig. 2 was used to measure the thermal conductivity, specific heat and diffusivity of the samples. A Hot Disk thermal analyzer uses the Transient Plane Source (TPS) method based on a transient technique. The sensor of this device includes an electrically conducting model in the shape of a double spiral, which is engraved on a very thin Nickel foil. This spiral is surrounding by two thin insulating sheets. This sensor is placed between two sample pieces to perform a thermal transport measurement.
3. Results and discussion 3.1. Thermal conductivity analysis The temperature variations of phase change materials are used for thermal energy storage. The energy storage rate depends mainly on the
Fig. 1. SEM micrographs of: (a) MWCNT, (b) GNP. (Source: US Research Nanomaterials. Inc.). 2
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
Fig. 2. Hot disk thermal constants analyzer that was used to measure the thermal conductivity.
thermal conductivity of materials; therefore, the thermal conductivity of PCMs is an important property that must be investigated in order to be improved by proper method. The thermal conductivity of Sodium thiosulfate pentahydrate was measured as reference value as 1.153 W/m.K. The experimental result for the thermal conductivity of the composite PCMs with 1%, 3%, 5% and 7% nano-fillers are listed in Tables 2 and 3; which are also fitted as shown in Fig. 3. It is clear that the thermal conductivity of the composite PCM increased remarkably with the increasing of the nano-fillers content. Relative thermal conductivity enhancement factor defined as: (k-k0)/k0 is used in order to describe the quantitatively enhancement of nano-filler in increasing of thermal conductivity. Adding Carbon Nano tubes to Sodium thiosulfate pentahydrate yields better thermal conductivities than adding the GNP. Moreover, increasing the CNT mass fraction maintain thermal conductivity enhancement of the composite, but it becomes almost steady with the GNP addition as depicted from Fig. 3. These two aspects propose that incorporating CNT to Sodium thiosulfate pentahydrate would be more advantageous and promising for useful application. Based on the above experimental results of the two types of used nanoparticles, it can be concluded that the enhancement of thermal conductivity is directly related to the nanomaterial microstructure shown in Fig. 1. For instance, PCM with CNT can form columnar structure layer on the nanoparticle surface, which helps in producing effective heat conduction paths, hence enhanced thermal conductivity. Whereas, for PCM with GNP case, the spherical sheet structure helps to form nano-layer that facilitates better heat conduction since the thermal conduction in the transversal direction is much lower than that in the longitudinal direction.
Table 3 Thermal conductivity of Sodium thiosulfate pentahydrate /CNT composites. Thermal parameters
Thermal conductivity (W/m K) The relative thermal conductivity enhancement (%)
Thermal conductivity (W/m K) The relative thermal conductivity enhancement (%)
Mass fraction 1%
3%
5%
7%
2.208 91.5
2.578 123.59
2.611 126.45
2.944 155.33
3%
5%
7%
2.824 144.93
3.107 169.47
3.603 212.49
4.031 249.61
Fig. 3. Thermal conductivity of the composite PCMs versus the mass fraction of the nanoparticles.
Table 2 Thermal conductivity of Sodium thiosulfate pentahydrate/GNP composites. Thermal parameters
Mass fraction 1%
Enhancement of thermal conductivity consequently affects directly from the charging time that is required to reach certain temperature, which is critical thermal energy storage applications, as depicted in Figs. 4 and 5 presents. Apparently, increasing the CNT mass fraction enhances both the thermal conductivity as well as the charging time which was decreased from 30 min to 7 min only with 7% mass fraction. Whereas, adding 7% of GNP decreased the charging time from 30 min to about 11 min. 3
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
Fig. 7. STP with GNP composites behavior during discharging time. Fig. 4. STP with CNT composites behavior.during charging time.
Fig. 8. Comparison between studies added GNP.
Fig. 5. STP with GNP composites behavior during charging time.
Fig. 6. STP with CNT composites behavior during discharging time.
Fig. 9. Comparison between studies added GNP.
4
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
Fig. 10. DSC analysis for PCM with 7%GNP: (a) Melting curve, (b) Solidified curve.
additive wt.% of the nano filers.
Figs. 6 and 7 present the effect of adding CNT and GNP to Sodium Thiosulfate Pentahydrate on discharging time when the samples cooled at room temperature (18 °C). For CNT case the discharging time was decrease from 40 min for Sodium Thiosulfate Pentahydrate to 15 min with 7% CNT. Whereas, for mass fraction of 7% GNP the discharging time was decrease from 40 min to 18 min. Figs. 8 and 9 present a comparison results between our recent study and that of M. He et al [15]. Obviously, not only that our findings in enhancing the relative thermal conductivity by nano filters addition are far better than that of He et al, but also it covers wider range for the
3.2. DSC analysis Melting and solidified temperature of the Sodium thiosulfate pentahydrate was investigated using DSC for maximum mass percent of two types of nanoparticles, 7% GNP and 7% CNT. The melting temperature and enthalpy of Sodium thiosulfate pentahydrate is 48 °C and 210 kJ/kg, respectively. The DSC curves of Sodium thiosulfate pentahydrate with 7% of GNP 5
Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
Fig. 11. DSC analysis for PCM with 7%CNT: (a) Melting curve, (b) Solidified curve.
slight increase in the melting temperature was observed as depicted from the DSC curves of the CNT/ sodium thiosulfate pentahydrate composite compared to that melting temperature of the pure sodium thiosulfate pentahydrate as observed in Fig. 11. Moreover, the melting temperature of the 7% CNT/ sodium thiosulfate pentahydrate composite was 55.8 °C. The adding of CNT also shows positive influence on easing the effect of the supercooling phenomenon where the solidified temperature was 20.1 °C. The supercooling effect was improved
are shown in Fig.10, which shows that the melting temperature of the 7% GNP composite is about 53.1 °C. It can be concluded that the melting temperature is almost not affected by the concentration of the added GNPs since slight decrease in the melting temperature was observed when compared with that of pure sodium thiosulfate pentahydrate. The solidified DSC curves show that the composite exhibits supercooling phenomenon with solidified temperature as 25.6 °C. Similar to the GNP/ sodium thiosulfate pentahydrate composite,
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Journal of Energy Storage 27 (2020) 101166
R.I. Rabady and D.S. Malkawi
References
compared to that of pure Sodium thiosulfate pentahydrate due to the nucleating effect of the additives which served as nucleating agents that facilitate the crystallization of the composite.
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4. Conclusion It can be concluded from the results of this work that the nano-fillers (CNT or GNP) can significantly enhance the thermal conductivity of Sodium Thiosulfate Pentahydrate as PCM. The enhancements in thermal conductivity were observed to increase by increasing the nanoparticles mass fraction. For example, the overall relative enhancements were 155.33% and 249.61% in thermal conductivity of 7% mass fraction of GNP- Sodium Thiosulfate Pentahydrate and CNT- Sodium Thiosulfate Pentahydrate, respectively. Moreover, adding nano-fillers to Sodium Thiosulfate Pentahydrate decreases the charging/discharging time at the melting point in more convenient manner; hence boost their broad application. The supercooling phenomenon of sodium thiosulfate pentahydrate has been greatly improved after adding nanoparticles, stabilizing and nucleating agents. We believe the experimental investigations and findings of this work are of great interest since they contribute to the urge of better use and storage purposes of clean energy. Declaration of Competing Interest Confirm that there is no any kind of conflict of interest upon publishing this work with any one. Acknowledgment Authors would like to acknowledge the financial support provided by Deanship of Research and research facilities labs by Chemical Engineering Department at Jordan University of Science and Technology. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2019.101166.
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