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Solidification characteristics of water based graphene nanofluid PCM in a spherical capsule for cool thermal energy storage applications
Accepted Manuscript Title: Solidification characteristics of water based graphene nanofluid PCM in a spherical capsule for cool thermal energy storage applications Author: A. Sathishkumar, V. Kumaresan, R. Velraj PII: DOI: Reference:
S0140-7007(16)00020-7 http://dx.doi.org/doi: 10.1016/j.ijrefrig.2016.01.014 JIJR 3243
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
13-9-2015 24-12-2015 18-1-2016
Please cite this article as: A. Sathishkumar, V. Kumaresan, R. Velraj, Solidification characteristics of water based graphene nanofluid PCM in a spherical capsule for cool thermal energy storage applications, International Journal of Refrigeration (2016), http://dx.doi.org/doi: 10.1016/j.ijrefrig.2016.01.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Solidification characteristics of water based graphene nanofluid PCM in a spherical capsule for cool thermal energy storage applications A. Sathishkumar a, V. Kumaresan a, R. Velraj a,* a
Department of Mechanical Engineering, Anna University, Chennai 600025, India.
*
Corresponding author: R. Velraj ([email protected]).
Highlights
Addition of graphene nanoplatelets greatly reduced degree of subcooling.
Thermal conductivity enhancement of ~ 12 % and 56 % in liquid and solid of NFPCM.
Higher cooling rate was achieved in NFPCM for a given driving potential.
Innermost 9 % of total volume solidified during 22 % of total solidification time.
Abstract
This study aimed to investigate the solidification behavior of water dispersed with graphene nanoplatelets (GNPs) in a spherical container during solidification. The nanofluid phase change materials (NFPCMs) with GNP mass concentration of 0.3 %, 0.6 %, 0.9 % and 1.2 % were prepared and their corresponding thermal conductivity was measured both in liquid and solid state. The thermal conductivity data showed the linear and nonlinear enhancement in liquid and solid state respectively. The addition of GNPs resulted with an appreciable reduction in the subcooling of water from -7 ºC to -2.5 ºC along with reduction of 25 % in solidification time owing to its high thermal conductivity and larger specific surface area. The onset of solidification got advanced in the case of NFPCM due to higher cooling rate in the subcooling
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region for any given driving potential. The accelerated mode of energy charging occurred during 78 % of total solidification time.
Keywords: Subcooling, Solidification, Thermal conductivity, Cool thermal energy storage, Nanofluid phase change material, Graphene nanoplatelets
Nomenclature
k
thermal conductivity (W m-1 K-1)
r
radius of the spherical capsule (mm)
t
time (min)
T
temperature (ºC)
V
volume (m3)
CR
cooling rate (°C min-1)
ΔT
degree of subcooling (ºC)
Abbreviation
DI
deionized
GNP
graphenenanoplatelet
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HTF
heat transfer fluid
PCM
phase change material
RTD
resistance temperature detector
SEM
scanning electron microscope
TEM
transmission electron microscope
TES
thermal energy storage
CTES
cool thermal energy storage
LDPE
low density polyethylene
PTDC
proportionate temperature differential controller
SDBS
sodium dodecyl benzene sulphonate
MWCNT
multiwall carbon nanotubes
NFPCM
nanofluid phase change material
Subscripts
bf
base fluid
nf
nanofluid
sub
subcooling 3 Page 3 of 42
surr
surrounding bath
1. Introduction
The most scientific challenges in many industries such as refrigeration, air-conditioning, transportation, chemical, electronic cooling and manufacturing industries are to achieve an energy efficient cooling for various applications. Among the various energy consuming industries, the building sector consumes nearly 40 % of the world’s energy consumption and there is a persistent interest among the researchers and policy makers on conservation of energy and utilization of alternative energy sources. Moreover, the energy needs for several applications are time dependent as well as in a different pattern and phase, particularly in the building cooling applications. This mismatch between energy supply and demand has made the requirement of the thermal energy storage (TES) systems (Zalba et al., 2003), to store / retrieve the hot / cool thermal energy either in the form of sensible or latent heat by using a suitable storage medium. The integration of cool thermal energy storage (CTES) system in a chiller reduces considerable electric cost by shifting the off-peak times during a day, especially in the large building central air conditioning (Cheralathan et al., 2007), refrigeration in supermarkets (Marimon et al., 2011) and various other places like telecommunication base stations, where the cooling requirement is highly intermittent (Sun et al., 2014).
The CTES system with phase change material (PCM) possesses number of advantages than the sensible storage for the above applications, owing to high storage capacity and phase transition in an isothermal manner. The extensive review (Li et al., 2012; Oro et al., 2012) articles have been reported on the materials, encapsulation geometry and heat transfer in the field of CTES applications. Most of the CTES systems commonly use water as the storage medium, 4 Page 4 of 42
because of its high thermal transport properties and only limited systems use the PCMs other than water like hydrated salts, eutectic mixtures and brine solutions. However, the major drawback of using water as the PCM is its high degree of subcooling that demands the operation of the evaporator at a temperature well below the phase change temperature of water in the chiller system. It is the well-known fact that the specific energy consumption increases by 3 % to 4 % with a reduction in evaporator temperature of 1 °C and the various techniques have already been proposed to reduce the energy consumption by minimizing the subcooling of water in the CTES system.
With the fascinating advancements in the field of nanotechnology, a new concept of dispersing high conductive solid particles at nanometric scale in the phase change material have been developed to enhance the thermal transport properties during charging/discharging process in the TES applications. The literature pertaining to the enhancement in the thermal transport properties of the PCMs dispersed with various nanoparticles are summarized as given below. The phase change characteristics of various PCMs like paraffin with different phase change temperature (Kumaresan et al., 2012; Sari et al., 2007), barium chloride (He et al., 2012), oleic acid (Harikrishnan et al., 2012) and stearic acid (Li et al., 2013) have already been investigated and reported. In general, their results revealed that the addition of nanoparticle leads to enhancement in the thermal conductivity and reduction in solidification/melting time compared to that of base PCM. It is also reported that the effect of addition of nanomaterial on the latent heat of the PCM depends on the concentration of the nanoparticle. In addition, the major problem of subcooling in the PCMs is significantly reduced, particularly in water for CTES applications with the addition of alumina (Wu et al., 2009; Altohamy et al., 2015), copper oxide (Fan et al., 2012), copper (Wu et al., 2010) and multiwall carbon nanotubes (MWCNT) 5 Page 5 of 42
(Kumaresan et al., 2013). Recently, two-dimensional crystalline allotrope of carbon, graphene, receives much attention among the researchers due to excellent thermal and electrical transport properties in number of potential applications (Liu et al., 2015; Baby et al., 2011). Mehrali et al. (2013) prepared the new composite consisting of palmitic acid and graphene oxide through impregnation method. Thermal conductivity of the composite PCM was improved by more than three times due the presence of graphene oxide and it exhibited a good thermal reliability in terms of thermal properties and chemical stability after 2500 melting/freezing cycles. The effects of adding grapheme or exfoliated graphite nanoplatelets with paraffin was investigated by Shi et al. (2013) and their results showed relatively higher thermal conductivity in the case of paraffin with exfoliated graphite nanoplatelets than graphene. A novel shape-stabilized phase change material of stearic acid-graphene oxide was fabricated by imprisoning stearic acid in the interlayer spaces of the multilayer graphene oxide based on the capillary action and interfacial interaction (Li et al., 2013). The composite with the equal mass fraction showed a considerable decrease in melting point and freezing point relative to those of the pristine stearic acid due to the confinement effect of nanoscaled spaces. Zhong et al. (2013) prepared the PCM with threedimensional grapheme aerogel and octadecanoic acid and the thermal conductivity of composite with 20 vol. % augmented by 14 times than the base PCM due to high specific surface area of graphene aerogel. Jeong et al. (2013) reported that thermal conductivity of bio – based PCM with impregnation of exfoliated graphene was enhanced by 3.75 times than the base PCM. However, the latent heat of composite was only 75 % of the base PCM and the presence of exfoliated graphene reduced the subcooling appreciably. It has been observed from the literature that the addition of graphene enhanced the thermal transport properties of the PCM in proportion to particle concentration to make them more suitable for TES applications. Considering the
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excellent thermal transport properties of GNP and the pressing need to develop an energy efficient CTES system, the present research work aims to investigate heat transfer characteristics of water dispersed with GNP in a spherical capsule during solidification at various surrounding temperature conditions of the HTF.
2. Preparation of nanofluid PCM
The preparation of nanofluid phase change material (NFPCM) is more important in the TES applications, due to phase change during charging and discharging process and the continuous thermal cycling increases the flocculation of the nanoparticles in the base PCM. In the present study, a two-step method was used to prepare the water based NFPCM. The materials, deionized water as the base PCM, graphene nanoplatelets (GNPs) as the nanomaterial and sodium dodecyl benzene sulphonate (SDBS) as a surfactant, were used to prepare the NFPCMs. The GNP (Cheep Tubes USA) has 4 – 5 layers with an average thickness of 8 nm and particle size of less than 2 µm with a specific surface area of 600 – 750 m2 g-1. The TEM image of the GNP is shown in Fig. 1 and it is seen from the figure that the GNPs have been entangled with each other. In order to disentangle them, the ultrasonication of the GNPs under dry condition was carried out in water bath for a period of 45 min, before the GNPs were dispersed in the base PCM. The surfactant with a mass fraction of 0.25 % was initially mixed with the PCM and the mixture was stirred in a magnetic stirrer for 15 min. The GNPs were then mixed with the solution and the shear mixing was continued for 15 min and the mixture was transferred to the ultrasonicator. The mixture was then sonicated continuously for a period of 180 min to disperse the GNPs uniformly in the base PCM. Four different NFPCM with the mass 7 Page 7 of 42
concentration of 0.3 %, 0.6 %, 0.9 % and 1.2 % were prepared by using the above procedure. The size of dispersed GNPs in the base PCM was analyzed using the scanning electron microscope (SEM). As shown in Fig. 2, the thickness of GNPs (D1) is about 60 nm and its diameter is in the range of 2.3 µm (D2) – 3.3 µm (D3). The increase in the diameter of the GNPs after dispersion in the base PCM is due to agglomeration during preparation of the NFPCM. After ensuring the thickness of the GNP less than 100 nm, the thermal conductivity of the NFPCMs was measured both in liquid and solid state and reported in the following section.
3. Thermal conductivity measurement It is essential to measure the thermal conductivity of the PCM with respect to temperature, as the variation in its thermal conductivity plays a vital role during charging and retrieval process in the PCM based CTES applications. In the present study, a KD2 Pro analyzer (Decan Devices Inc., USA), which works based on the principal of transient line heat source method, was used to measure the thermal conductivity of the NFPCM both in liquid and solid state. A sensor needle of 60 mm length and 1.2 mm diameter was employed to measure the thermal conductivity of the NFPCM in liquid state in the temperature range of 5 °C – 40 °C with an accuracy of ± 5 %. The solid thermal conductivity was measured in the temperature range from -15 °C to - 5 °C by using another sensor needle of 100 mm long and 2.4 mm diameter with an accuracy of ± 10 %. The desired temperature of the NFPCM was maintained by a refrigerated / heating circulated bath (Juloba, Germany) with a temperature stability of ± 0.01 °C. A known volume of NFPCM measured using a standard volumetric flask of 25 ml (class B) was poured carefully into a cylindrical glass container fitted with a flexible nylon lid, through which the sensor needle was inserted. This sensor was always kept at the center for accurate measurements and a read time of 1 min and 4 min was configured during the measurement in the liquid and 8 Page 8 of 42
solid state respectively. The measurements were made only after the NFPCM attained the desired temperature and five trials were made for each sample to ascertain the repeatability of the experimental data.
4. Experimental setup The schematic arrangement of the experimental setup to conduct solidification characteristic of the NFPCM is illustrated in Fig. 3. It consists of a thermally insulated stainless steel tank of capacity 0.01 m3, fitted with a heating coil of capacity 2000 W and vapor compression refrigeration system consisting of compressor (C), condenser, expansion valve (EV) and evaporator (E). A mixture of water and ethylene glycol (70:30 by volume) was used as the surrounding heat transfer fluid (HTF), normally called as the secondary refrigerant. The desired experimental temperature of the HTF was achieved using a proportionate temperature differential controller (PTDC) that regulates the output of the heating coil based on the temperature of the surrounding HTF and a mechanical stirrer was used to maintain a near uniform temperature throughout the bath. The required quantity of the NFPCM was measured using standard volumetric flask of class ‘B’ and it was transferred to spherical ball made up of low density polyethylene (LDPE) with an outer diameter and wall thickness of 72 mm and 1 mm respectively. The LDPE ball was filled up to its 80 % of total volume with NFPCM, in order to accommodate the increase in volume during the solidification of water based PCM. Three RTDs were located along the radius of the ball, one was at its center (r3) and other two RTDs were at a distance of 15 mm (r2) and 30 mm (r1) from the center of the ball as shown in the Fig.3. The ball filled with the NFPCM was placed in the HTF bath, maintained at a temperature well below the
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solidus temperature of water. The transient temperature variation of the NFPCM was continuously monitored and measured for every 10 s using a data logger (Agilent 34970A), until the temperature of the NFPCM attained the thermal equilibrium with the surrounding HTF. The uncertainties involved in the measured quantities during the experimentation are presented in Table 1.
5. Results and discussions 5.1 Thermal conductivity The thermal conductivity of deionized (DI) water both in liquid and solid state was measured and compared with the ASHRAE standard (ASHRAE, 1997), before measuring the thermal conductivity of the nanofluids. As depicted in Fig. 4, the experimental data showed a good agreement with the standard data with a maximum deviation of ± 2 % and ± 8 % for liquid and solid state respectively. Fig. 5 presents the enhancement in thermal conductivity, defined as k nf k bf k bf
, for various NFPCMs with respect to temperature. It has been observed that the
thermal conductivity of the NFPCM was augmented in proportion with the concentration of GNP at all the temperature conditions. The maximum thermal conductivity enhancement of 56 % (solid state) and 11.7 % (liquid state) was exhibited by the NFPCM containing 1.2 wt. % of GNP at -10 °C and 40 °C respectively. This is due to high specific surface area of the GNPs that in turn enhances the heat transfer rate through its unique two dimensional planer structure and the solid–liquid interfacial layer. The average specific surface area of GNPs is taken as 675 m2 g-1 based on the manufacturer data and its corresponding values in the NFPCM with the GNP concentration of 0.3 %, 0.6 %, 0.9 % and 1.2 % are 324 m2 , 648 m2, 972 m2 and 1296 m2
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respectively. It is prominent to note that the effective thermal conductivity of the NFPCM does not exhibit a proportionate increase with respect to surface area due to increase in interfacial thermal resistance. Similar results of increase in thermal conductivity were reported by Mehrali et al. (2013) in DI water and Lee et al. (2014) in ethylene glycol in liquid state. Further, the thermal conductivity of the NFPCMs showed the temperature independent irrespective of the GNP concentration, as similar to that of the NFPCM with the suspension of CNT. This could be due to the suppression of the Brownian motion of the GNPs in the base fluid due to its larger diameter in micron size. The same temperature independent trend was also reported by Yu et al. (2010) and Huang et al. (2012) for the suspension of graphene oxide nanosheets in ethylene glycol and GNPs in the epoxy composites respectively.
The sensor used to measure the thermal conductivity of ice was calibrated by using the standard solid specimen (Decan Devices Inc., USA) at room temperature (30 °C – 32 ºC). It is evident that the thermal conductivity of the NFPCMs enhanced anomalously in their solid state as shown in Fig. 5. This anomalous enhancement is attributed by the presence of high conductive GNPs that provides negligible interfacial thermal resistance with the solid ice. The thermal conductivity enhancement of about 15 % was noticed in the case of NFPCM containing 0.3 wt. % and this enhancement nonlinearly increased to 42 % with addition of 0.6 wt. % of the GNP. The larger heat transfer rate through the increase in the surface area with further addition of GNP contributed to the nonlinear thermal conductivity enhancement of the NFPCM. However, only 7 – 10 % and ~ 8 % enhancement in thermal conductivity was noticed as the mass concentration of GNP increased from 0.6 % to 0.9 % and 1.2 % respectively. It could be due to two dimensional flake structure of the GNPs that retards the thermal conductivity enhancement factor with respect to increase in concentration. 11 Page 11 of 42
5.2. Transient temperature variation of the NFPCM The effects of surrounding bath temperature and graphene concentration on the solidification behavior of water are discussed in this section. Fig. 6 shows transient temperature variation of DI water at the center of the spherical capsule, when the surrounding bath was maintained at a temperature of -9 °C, -12 °C and -15 °C. It is noticed from the figure that DI water was subcooled to a temperature of - 6.9 °C and -5.8 °C, when the bath temperature was maintained at -12 °C and -9 °C respectively. However, the subcooling of DI water was completely eliminated at the surrounding bath temperature of -15 °C, due to the existence of relatively higher driving potential between the DI water and the surrounding HTF. A significant reduction of 22.5 % and 64.77 % in the solidification time was accomplished by decreasing the surrounding bath temperature from -9 °C to -12 °C and -15 °C respectively. The specific energy consumption by the refrigeration systems increases non-linearly with respect to decrease in evaporator temperature predominantly in the range of -5 °C to -20 °C, depending on the type of primary refrigerant used in the system. Different approaches like operation of the chiller during night time, at or near full load (Rutberget al., 2013) have already been proposed to reduce the energy consumption by the chiller integrated with the CTES system. Also, there is a mammoth potential to reduce the energy consumption through the reduction of subcooling in the PCM, particularly in the case of water as the base PCM.
Fig. 7(a) shows the transient temperature variation of the NFPCM with different concentrations of GNPs at a surrounding bath temperature of -12 °C. Only a negligible effect of the GNPs has been observed on the heat transfer in the region of sensible cooling up to density inversion point (~ 4 °C). In that region, both the DI water and NFPCMs followed the same temperature drop trend with respect to time. This could be due to increase in the viscosity of the 12 Page 12 of 42
NFPCMs with addition of GNPs that suppresses the natural convection effects. It has also been noticed that DI water was subcooled to a temperature -7 °C and the onset of solidification began after 1150 s. There was a small reduction in the degree of subcooling of 1 °C in DI water solution containing the surfactant, compared to that of subcooling in DI water. However, the degree of subcooling was greatly reduced in the NFPCMs in proportion with the concentration of the GNPs and the NFPCM containing 1.2 wt. % showed the subcooling of -2.5 °C as shown in Fig. 7 (b). This clearly indicates the nucleating action of the GNPs in DI water. It is well known that the addition of GNPs in DI water results in the larger contact area due to its unique feature of possessing high specific surface area. This feature is advantageous to initiate the crystallization by increasing the nucleation sites through the reduction of interfacial contact resistance between GNP - DI water. It is also evident that the onset of solidification advanced in proportion with the reduction in the degree of subcooling. For example, the onset of solidification commenced after 1150 s and 730 s respectively for DI water and NFPCM with the mass fraction of 1.2 % at Tsurr = -12 °C. Moreover, the experimental results showed a notable reduction in the solidification duration in the case of all NFPCM compared to that of the base PCM as illustrated in Table. 2 and the maximum reduction in the solidification time of 25 % was noticed (from initial condition to -1 °C) in the NFPCM containing 1.2 wt. % of GNPs. This is well explained by the enhanced thermal conductivity of the NFPCM, owing to the presence of high conductive graphene layers with the large specific surface area, which in turn enhances the heat transfer rate in the NFPCMs. The results of similar experiments conducted with the NFPCMs at the surrounding bath temperature of -9 °C are presented in Fig.8 (a). It is seen from the figure that the sensible cooling of DI water and NFPCM showed the same trend as in the case of Tsurr = -12 °C. DI water and 13 Page 13 of 42
NFPCM with 1.2 wt. % GNP experienced a lower subcooling degree of -5.9 °C and -2.3 °C respectively as shown in Fig. 8 (b), when compared to the results obtained at Tsurr = -12 °C due to relatively lesser cooling rate. The influence of GNP loading on subcooling of the NFPCMs at the surrounding bath temperatures of -12 °C and -9 °C is compared in Table 3. In addition, the solidification duration was also reduced in proportion with the concentration of the GNP at Tsurr = -9 °C and a maximum reduction of 20.6 % was achieved in the NFPCM with 1.2 wt. % as shown in Table. 4. The present results is compared with the reduction in solidification time achieved using conventional techniques such as fins (Stritih et al., 2007; Velraj et al., 1997), insertion of metal beads and porous media (Ettouney et al., 2006; Zhou et al., 2011; Yin et al., 2008), addition of high conductive micro particles (Mettawee et al., 2007) and nanoparticles/nanofibers (Kumaresan et al., 2012; He et al., 2012; Wu et al., 2009; Altohamy et al., 2015; Wu et al., 2010; Kumaresan et al., 2013; Elgafy et al., 2005; Wang et al., 2014; Sanusi et al., 2011) in the base PCM as shown in Fig. 9. A relatively higher reduction in solidification time was reported with addition of metal foams and micro particles in the PCM other than water. Mostly, the researchers used nanoparticles/nanofibers in water for CTES application with the objective of reducing subcooling and enhancing the heat transfer characteristics during solidification. As depicted in Fig. 9, solidification duration of water decreased considerably with addition of metal/metal oxide nanoparticles, but the higher density compared to GNP limits its applications. It is construed from the above discussions that the degree of subcooling decreases with respect to increase in the concentration of GNPs. This reduction in subcooling and enhanced heat transfer characteristics of the NFPCM will play a vital role towards the development of energy efficient chiller based CTES systems suitable for various industrial and building cooling applications.
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5.3. Cooling rate of the NFPCM The cooling rate during subcooling of PCM is an important parameter to be considered in the design of CTES system as it has a direct bearing on the energy consumption of the chiller unit. Considerable research works have been carried out to reduce the subcooling of the PCM through the addition of nucleating agents and maintaining high driving potential between the PCM and the surrounding HTF (Zhang et al., 2010). The cooling rate during the subcooling period is evaluated from the T –t history as shown in Fig. 10. T CR t su b
(1)
where, ΔT and t are the degree of subcooling and total subcooling period respectively. Fig. 11 illustrates the subcooling rate of the NFPCMs with respect to the concentration of GNP at various radial locations in the spherical capsule, when the surrounding bath temperature was maintained at -12 ºC. As seen from the figure, the cooling rate of NFPCM increased in proportion with the concentration of GNP and the cooling rate of DI water augmented from 0.77 ºC min-1 to 1.3 ºC min-1 with addition of 1.2 wt. % of GNP at the center of the capsule. This is due to enhanced thermal conductivity of NEPCM which in turn augments the heat transfer rate during the nucleation process. It is also known that the heterogeneous nucleation rate is inversely proportional to the foreign substrates and hence the presence of GNP with large specific surface area in DI water is very useful to attain higher nucleation rate. However, the cooling of NFPCMs decreased from the surface to the center of the capsule due to existence of the thermal resistance offered by the solidified NFPCM. This difference in the cooling rate caused the NFPCM at the center of the capsule to undergo maximum degree of subcooling for a given driving potential. The same trend of cooling rate exists at various radial location of the capsule when Tsurr = -9 ºC, 15 Page 15 of 42
as depicted in Fig. 12. It is evident from the above results that the addition of GNP increases the cooling rate during subcooling for the given temperature driving potential, which is highly advantageous to facilitate the operation of the CTES system at a relatively higher temperature of the HTF. 5.4. Volume solidified The volume solidified was determined from the T – t history at various radial locations inside the spherical capsule in the case of DI water at the surrounding bath temperature of -12 °C as shown in Fig.13. Time taken for complete solidification was calculated by the difference between the onset of solidification and the time at which the slope of the curve changed from the phase change region in the T – t diagram. The same method has been used to determine the volume solidified for the NFPCMs at Tsurr = -12 °C and Tsurr = -9 °C. Figs. 14–15 compares the percentage of volume solidified between the DI water and the NFPCMs with respect to time during solidification at Tsurr = -12 °C and Tsurr = -9 °C respectively. As shown in Fig. 14, 34 % of volume got solidified during 1840 s (42 % of total solidification time) in the case of DI water and the solidification time was reduced in the case of NFPCMs in proportion with the concentration of the GNPs due to their enhanced heat transfer characteristics. By comparing the results presented in Fig. 7 (a), the time taken to solidify 34 % of total volume by the NFPCM with 0.3 wt. %, 0.6 wt. %, 0.9 wt. % and 1.2 wt. % was respectively 41.8 %, 41.1 %, 36.8 % and 35.2 % of their corresponding total solidification duration. In addition, the reduction in solidification time of 26 % was noticed in the NFPCM containing 1.2 wt. % of GNP, compared to that of the base PCM. Further, the next 57 % of DI water solidified during 34 % of total solidification time, which clearly indicates the existence of accelerated mode of
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energy charging. The same accelerated mode of energy charging prevailed in all the NFPCMs with considerable reduction in solidification time depending on the concentration of the GNP. It is well explained that the increase in thermal conductivity of ice (about 4 times higher than water in liquid state) provides enhanced heat percolation from the outer surface of the capsule to the liquid PCM in the inner core. The experimental results also revealed that the remaining volume of 9 % required 22 % of total solidification time in the case of DI water and the behavior was exhibited by all the NFPCMs. This clearly shows the decelerated mode of energy charging due to the appreciable reduction in the surface area of the spherical capsule. It is construed from the above discussions that there is an immense energy saving potential in the CTES system by operating the chiller for a period of 78 % of total solidification time from the initial condition of the PCM. The same trend of solidification behavior was also observed at Tsurr = -9 °C as shown in Fig. 15. It is seen that 9 % of volume solidified during 21 % of total solidification time in the case of DI water. However, this solidification time was considerably reduced to 10 % in the case of NFPCMs due the predominant effect of nanoplatelets. 6. Conclusions The solidification behavior of water based GNP nanofluid PCM was investigated experimentally at different surrounding bath temperatures and the following conclusions are arrived based on the experimental results.
The maximum thermal conductivity enhancement of 11.7 % and 56 % was observed respectively in the liquid and solid state of the NFPCM containing 1.2 wt. % GNP due to its high thermal conductivity with larger specific surface area.
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The presence of high conductive GNP acted as the nucleating agent to initiate the solidification of the NFPCM in advance to that of the base PCM through considerable reduction in the degree of subcooling. The addition of the GNP also shortened the solidification duration of the NFPCM by 25 % and 20.6 %than that of base PCM at the surrounding bath temperature of -12 ºC and -9 ºC respectively.
The NFPCM exhibited higher cooling rate in the subcooled region for the given driving potential than the base PCM and this facilitates the operation of the CTES system at a relatively higher temperature of the HTF.
It was noticed that only 9 % of volume around the center of the capsule got solidified during 22 % of total solidification time. Hence, there is a significant energy saving potential in the CTES system by operating the chiller for a period of 78 % of the total solidification time at Tsurr = -12 ºC.
The pioneering advancements in the field of nanotechnology will play a vital role to design and develop an energy efficient CTES system suitable for various industrial and building cooling applications.
References Altohamy, A.A., AbdRabbo, M.F., Sakr. R., 2015. Effect of water based Al2O3 nanoparticle PCM on cool storage performance. Appl. Therm. Engineering 84, 331-338
American Society of Heating, Refrigerating and Air-conditioning Engineers, ASHRAE Handbook, 1997: Fundamentals, Atlanta, GA.
Baby,T.T., Ramaprabu, S., 2011. Enhanced conductive heat transfer using graphene dispersed nanofluids. Nano. Res. Lett. 6 (1), 289. 18 Page 18 of 42
Cheralathan, M., Velraj, R., Renganarayanan, S., 2007. Performance analysis on industrial refrigeration system integrated with encapsulated PCM based cool thermal energy storage system. Int. J. Energ. Res. 31 (14), 1398–1413.
Ettouney, H., Alatiqi, I., et al., 2006. Heat transfer enhancement in energy storage in spherical capsules filled with paraffin wax and metal beads. Energ Convers Manage. 47, 211–228
Elgafy, A., Lafdi, K., 2005. Effect of carbon nanofiber additives on thermal behavior of phase change materials. Carbon 43, 3067–74.
Fan, L., Khodadadi, J.M., 2012. A theoretical and experimental investigation of unidirectional freezing of nanoparticle-enhanced phase change materials. ASME J. Heat Trans. 134, 092301-9.
Harikrishnan, S., Kalaiselvam, S., 2012. Preparation and thermal characteristics of CuO– oleic acid nanofluids as a phase change material. Thermochim Acta 533, 46–55.
He, Q., Wang, S., Tong, M., Liu, Y., 2012. Experimental study on thermophysical properties of nanofluids as phase-change material in low temperature cool storage. Energ.Convers. Manage. 64, 199–205. Huang, X., Zhi, C., Jiang, P., 2012. Toward effective synergetic effects from graphene nanoplatelets and carbon nanotubes on thermal conductivity of ultrahigh volume fraction nanocarbon epoxy composites.J. Phys. Chem. 116, 23812−20.
19 Page 19 of 42
Jeong, S.G., Chung, O., Yu, S., Kim, S., Kim, S., 2013. Improvement of the thermal properties of Bio-based PCM using exfoliated graphite nanoplatelets. Sol. Energ. Material. Sol. Cell. 117, 87–92 Kumaresan, V., Velraj, R., Das, S.K., 2012. The effect of carbon nanotubes in enhancing the thermal transport properties of PCM during solidification. Heat Mass Trans. 48 (8), 1345–55. Kumaresan, V., Chandrasekaran, P., Nanda, M., Maini, A.K., Velraj, R., 2013. Role of PCM based nanofluids for energy efficient cool thermal storage system. Int. J. Refrigeration 36, 1641–7. Liu, Y., Li X., Hu, P., Hu, G., 2015. Study on the supercooling degree and nucleation behavior of water-based graphene oxide nanofluids PCM. Int. J. Refrigeration 50, 80-6. Li, G., Hwang, Y., Radermacher, R., 2012. Review of cold storage materials for air conditioning application. Int. J. Refrigeration 35, 2053–77. Li, B., Liu, T., Hua, L., Wang, Y., Nie, S., 2013. Facile preparation and adjustable thermal property of stearic acid–graphene oxide composite as shape-stabilized phase change material. Chemical Engineering Journal 215–216, 819–26. Li, H., Jiang, M., li, Q., li, D., et al., 2013.Aqueous preparation of polyethylene glycol/sulfonatedgraphene
phase
change
composite
with
enhanced
thermal
performance.Energ. Convers. Manage. 75, 482−7. Marimon, M.A., Arias, J., Lundqvist, P., Bruno, J.C., Coronas, A., 2011. Integration of trigeneration in an indirect cascade refrigeration system in supermarkets.Energ.Buildings 43, 1427–34.
20 Page 20 of 42
Mehrali, M., Latibari, S.T., Mehrali, M., Metselaar, H.S., et al., 2013. Shape-stabilized phase change materials with high thermal conductivity based on paraffin/graphene oxide composite. Energ.Convers.Manage.67, 275−82. Mettawee, E.S., Assassa, G.M.R., 2007. Thermal conductivity enhancement in a latent heat storage system.Sol. Energy 81, 839–45. Oro, E., Miro, L., Farid, M.M., Cabeza, L.F., 2012. Thermal analysis of a low temperature storage unit using phase change materials without refrigeration system. Int. J. Refrigeration 35 (6), 1709−14. Rutberg, M., Hastbacka, M., Cooperman, A., Bouza, A., 2013. Thermal energy storage. ASHRAE Journal, 62−6. Sanusi, O., Warzoha, R., Fleischer, A.S., 2011. Energy storage and solidification of paraffin phase change material embedded with graphite nanofibers. Int. J. Heat Mass Trans. 54, 4429–36. Sari, A., Karaipekli, A., 2007. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Appl. Therm. Eng. 27, 1271–7. Shi, J.N., Ger, M.D., Liu, Y.M., Fan, Y.E., et al., 2013.Improving the thermal conductivity and shape-stabilization of phase change materials using nanographite additives.Carbon 51, 365−372. Stritih, U., 2004. An experimental study of enhanced heat transfer in rectangular PCM storage. Int. J. Heat. Mass. Transf. 47, 2841–7.
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Sun, X., Zhang, Q., Medina, M.A., Liu, Y., Liao, S., et al., 2014. A study on the use of phase change materials in combination with a natural cold source for space cooling in telecommunications base stations in China. Appl. Energy 117, 95–103. Velraj, R., Seeniraj, R.V., Hafner, B., Faber, C., Schwarzer, K., 1997. Experimental analysis and numerical modeling of inward solidification on a finned vertical tube for a latent heat storage unit.Sol. Energy 60, 281–90. Wang, X.J., Li, X.F., Xu, Y.H., Zhu, D.S., 2014. Thermal energy storage characteristics of Cu-H2O nanofluids. Energy 78, 212-7. Wu, S., Zhu, D., Li, X., Li, H., Lei, J., 2009.Thermal energy storage behavior of Al 2O3-H2O nanofluids.Thermochim.Acta 483 (1–2), 73–7. Wu, S., Zhu, D., Zhang, D., Huang, J., 2010. Preparation and melting/freezing characteristics of Cu/Paraffin nanofluid as phase change material. Energy Fuels 24, 1894–98. Yin, H., Gao, X., Ding, J., Zhang, Z., 2008. Experimental research on heat transfer mechanism of heat sink with composite phase change materials. Energy Convers. Manage. 49, 1740–6. Yu, W., Xie1, H., Bao, D., 2010.Enhanced thermal conductivities of nanofluids containing graphene oxide nanosheets.Nanotechnology 21, 055705. Zalba, B., Marin, J.M., Cabeza, L.F., Mehling, H., 2003. Review on thermal energy storage with phase change materials, heat transfer analysis and applications. Appl. Therm. Eng. 23, 251–83. Zhang, X.L., Wu, P., Qiu, L.M., Zhang, X.B., Tian, X.J., 2010. Analysis of the nucleation of nanofluids in the ice formation process.Energ. Convers. Manage. 51, 130–4.
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Zhong, Y., Zhou, M., Huang, F., Lin, T., Wana, D., 2013. Effect of graphene aerogel on thermal behavior of phase change materials for thermal management. Sol.Energ. Material. Sol. Cell 113, 195–200 Zhou, D., Zhao, C.Y., 2011. Experimental investigations on heat transfer in phase change materials (PCMs) embedded in porous materials. Appl. Therm. Eng.31, 970–7.
Fig. 1 – TEM image of the graphene nanoplatelets (Source: Cheap Tubes, USA)
23 Page 23 of 42
Fig. 2 – SEM image of the dispersed GNP (0.3 wt. %)
24 Page 24 of 42
Data logger Computer RTDs
PDTC
Stirrer
Heat Transfer Fluid RTD
LDPE spherical ball NFPCM
Heater
RTD RTD1 1(at center) RTD RTD 2 (15 mm from center)
NFPCM centercenterfrocenter RTD 3 (30 mm from center) )
C - Compressor
E EV
E – Evaporator Condenser
C
EV- Expansion Valve
Fig. 3– Schematic diagram of the experimental setup
25 Page 25 of 42
Fig. 4 – Comparison of thermal conductivity of DI water and ice with ASHRAE standard, 1997
26 Page 26 of 42
80
Thermal conductivity increament (%)
0.3 wt. %
Solid state
70
0.6 wt. % 0.9 wt. %
60
1.2 wt. %
50 40
Liquid state
30 20 10 0 -30
-20
-10
0 10 20 Temperature (ºC)
30
40
Fig. 5 – Thermal conductivity enhancement of the NFPCM
27 Page 27 of 42
29 Tbath = - 9 ºC 25
Tbath = - 12 ºC
21
Tbath = - 15 ºC
Temperature (°C)
17 13 9 5 1 -3 -7 -11 -15 0
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
9000
Fig. 6 – Transient temperature variation of DI water at different bath temperatures
28 Page 28 of 42
28
DI Water
24
DI Water + SDBS 0.3 wt. %
20
0.6 wt. % 0.9 wt. %
16 12 8 4 0 -4 -8 -12 0
1000
2000
3000
4000
5000
6000
7000
Time (s)
(a) 28
DI Water DI Water + SDBS
24
0.3 wt. % 20
Temperature (°C)
Temperature (°C)
1.2 wt. %
0.6 wt. % 0.9 wt. %
16
1.2 wt. %
12 8 4 0 -4 -8 -12 0
1000
2000
Time (s)
(b) 29 Page 29 of 42
Fig. 7 – Transient temperature variation of the NFPCM during (a) entire period of experimentation (b) exploded view near subcooling region (Tsurr = -12 ºC).
30 DI Water
26
DI Water + SDBS
22
0.3 wt. % 0.6 wt. %
18 Temperature ( ºC)
0.9 wt. %
14
1.2 wt. %
10 6 2 -2 -6 -10 0
1000 2000 3000 4000 5000 6000 7000 8000 9000 Time (s) (a)
30 Page 30 of 42
30 DI Water
26
DI Water + SDBS 0.3 wt. %
22
0.6 wt. % 0.9 wt. %
Temperature ( ºC)
18
1.2 wt. %
14 10 6 2 -2 -6 -10 0
1000 Time (s)
2000
(b)
Fig. 8–Transient temperature variation of the NFPCM during (a) entire period of experimentation (b) exploded view near subcooling range (Tsurr = -9 ºC)
31 Page 31 of 42
20 He et al.
Sanusi et al.
Kumaresan et al.
Altohamy et al.
Wu et al.
present work (1.2 wt. %)
Elgaty et al.
Wu et al.
Kumaresan et al.
Mettawee et al.
Velraj et al.
100
Wang et al.
60 Stritih et al.
Zhou et al.
80
Yin et al.
Ettouney et al.
40
particles
Micro
Fins
Porous media
Metal beads
Reduction in solidification time (%) 120 High conductive nanoparticles
0
Fig. 9 – Comparison of reduction in solidification time
32
Page 32 of 42
tsub T CR t su b
ΔTsub
Fig. 10 – Cooling rate of DI water in the subcooling region
33 Page 33 of 42
2
Subcooling rate (°C min-1)
1.8
r = 30 mm from the center r = 15 mm from the center At center
1.6
1.4
1.2
1
0.8
0.6 -1.55E-15
0.3
0.6
0.9
1.2
1.5
Mass concentration of the GNPs (%)
Fig. 11 Cooling rate of the NFPCMs in the subcooling region (Tsurr = -12 ºC)
34 Page 34 of 42
Subcooling rate (°C min-1 )
1.6 1.4
r = 30 mm from the center r = 15 mm from the center At center
1.2 1 0.8 0.6 0.4 -1.55E-15
0.3 0.6 0.9 1.2 Mass concentration of the GNPs (%)
1.5
Fig. 12 – Cooling rate of the NFPCMs in the subcooling region (Tsurr = -9 ºC)
35 Page 35 of 42
28
At the center r = 15mm from the center r = 30 mm from the center
24
Temperature (ºC)
20 16 12 8 4 0 -4 -8 -12 0
1000
2000
3000
4000
5000
6000
7000
8000
Time (s)
Fig. 13 – Transient temperature variation of the DI water at various radial locations (Tsurr= 12ºC)
36 Page 36 of 42
6000 r = 30 mm from the center r = 15 mm from the center 5000
At the center
Time (s)
4000
3000
2000
1000
0 0
0.3
0.6
0.9
1.2
1.5
Mass concentration of the GNPs (%)
Fig. 14 – Time to reach complete solidification at various radial locations (Tsurr= -12ºC)
37 Page 37 of 42
7000 r = 30 mm from the center r = 15 mm from the center
6000
At the center
Time (s)
5000
4000
3000
2000
1000
0 0
0.3
0.6
0.9
1.2
1.5
Mass concentration of the GNPs (%)
Fig. 15 – Time to reach complete solidification at various radial locations (Tsurr= -9ºC)
38 Page 38 of 42
Table 1 – Results of uncertainty analysis
Measured quantities
Uncertainty
Diameter of spherical capsule
± 0.02 mm
Mass
± 0.002 g
Temperature
± 0.15 ºC
Volume (25 ml)
± 0.04 ml
Volume (50 ml)
± 0.06 ml
Volume (100 ml)
± 0.1ml
Thermal conductivity ( liquid)
± 5%
Thermal conductivity ( solid)
± 10%
39 Page 39 of 42
Table 2 – Solidification time of the NFPCMs (Tsurr = -12 ºC)
Solidification duration at various radial locations
Concentration of
various radial locations (%)
(s)
the GNP (wt. %) r1* = 30 mm
r2* = 15 mm
0
1820
3410
0.3
1760
0.6
Reduction in solidification duration at
r3 = 0
r3 = 0
r1* = 30 mm
r2* = 15 mm
4450
–
–
–
3320
4260
3.2
2.6
4.2
1680
3180
3840
7.7
6.7
13.7
0.9
1540
3010
3610
15.3
11.7
19
1.2
1450
2890
3320
20.3
15.2
25
*
(at center)
(at center)
from the center
40 Page 40 of 42
Table – 3 Effect of GNPs concentration on subcooling at center of capsule
Concentration
Degree of
Reduction in
of GNP
Subcooling (ºC)
Subcooling (%)
(wt. %)
Tsurr = -12 ºC
Tsurr = -9 ºC
Tsurr = -12 ºC
Tsurr = -9 ºC
0
-6.9
-5.92
–
–
0.3
-5.75
-4.81
17
18
0.6
-4.44
-4.19
36
29
0.9
-3.52
-3.21
49
45
1.2
-2.5
-2.3
64
61
Table 4 – Solidification time of the NFPCMs (Tsurr = -9 ºC)
Solidification duration at various
Concentration of
radial locations
the GNP (wt. %)
Reduction in solidification duration at various radial locations (%)
(s) *
*
r3 = 0
*
r3 = 0
mm
(at center)
r1 = 30 mm
r2 = 15 mm
0
3110
5020
5620
–
–
–
0.3
2990
4890
5010
3.8
2.6
10
0.6
2820
4720
4840
9.4
6
13.5
0.9
2710
4590
4650
12.9
8.6
17.2
1.2
2590
4480
4450
16.8
10.8
20.6
*
(at center)
r1 = 30 mm
r2* = 15
from the center 41 Page 41 of 42
42 Page 42 of 42
S0140-7007(16)00020-7 http://dx.doi.org/doi: 10.1016/j.ijrefrig.2016.01.014 JIJR 3243
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
13-9-2015 24-12-2015 18-1-2016
Please cite this article as: A. Sathishkumar, V. Kumaresan, R. Velraj, Solidification characteristics of water based graphene nanofluid PCM in a spherical capsule for cool thermal energy storage applications, International Journal of Refrigeration (2016), http://dx.doi.org/doi: 10.1016/j.ijrefrig.2016.01.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Solidification characteristics of water based graphene nanofluid PCM in a spherical capsule for cool thermal energy storage applications A. Sathishkumar a, V. Kumaresan a, R. Velraj a,* a
Department of Mechanical Engineering, Anna University, Chennai 600025, India.
*
Corresponding author: R. Velraj ([email protected]).
Highlights
Addition of graphene nanoplatelets greatly reduced degree of subcooling.
Thermal conductivity enhancement of ~ 12 % and 56 % in liquid and solid of NFPCM.
Higher cooling rate was achieved in NFPCM for a given driving potential.
Innermost 9 % of total volume solidified during 22 % of total solidification time.
Abstract
This study aimed to investigate the solidification behavior of water dispersed with graphene nanoplatelets (GNPs) in a spherical container during solidification. The nanofluid phase change materials (NFPCMs) with GNP mass concentration of 0.3 %, 0.6 %, 0.9 % and 1.2 % were prepared and their corresponding thermal conductivity was measured both in liquid and solid state. The thermal conductivity data showed the linear and nonlinear enhancement in liquid and solid state respectively. The addition of GNPs resulted with an appreciable reduction in the subcooling of water from -7 ºC to -2.5 ºC along with reduction of 25 % in solidification time owing to its high thermal conductivity and larger specific surface area. The onset of solidification got advanced in the case of NFPCM due to higher cooling rate in the subcooling
1 Page 1 of 42
region for any given driving potential. The accelerated mode of energy charging occurred during 78 % of total solidification time.
Keywords: Subcooling, Solidification, Thermal conductivity, Cool thermal energy storage, Nanofluid phase change material, Graphene nanoplatelets
Nomenclature
k
thermal conductivity (W m-1 K-1)
r
radius of the spherical capsule (mm)
t
time (min)
T
temperature (ºC)
V
volume (m3)
CR
cooling rate (°C min-1)
ΔT
degree of subcooling (ºC)
Abbreviation
DI
deionized
GNP
graphenenanoplatelet
2 Page 2 of 42
HTF
heat transfer fluid
PCM
phase change material
RTD
resistance temperature detector
SEM
scanning electron microscope
TEM
transmission electron microscope
TES
thermal energy storage
CTES
cool thermal energy storage
LDPE
low density polyethylene
PTDC
proportionate temperature differential controller
SDBS
sodium dodecyl benzene sulphonate
MWCNT
multiwall carbon nanotubes
NFPCM
nanofluid phase change material
Subscripts
bf
base fluid
nf
nanofluid
sub
subcooling 3 Page 3 of 42
surr
surrounding bath
1. Introduction
The most scientific challenges in many industries such as refrigeration, air-conditioning, transportation, chemical, electronic cooling and manufacturing industries are to achieve an energy efficient cooling for various applications. Among the various energy consuming industries, the building sector consumes nearly 40 % of the world’s energy consumption and there is a persistent interest among the researchers and policy makers on conservation of energy and utilization of alternative energy sources. Moreover, the energy needs for several applications are time dependent as well as in a different pattern and phase, particularly in the building cooling applications. This mismatch between energy supply and demand has made the requirement of the thermal energy storage (TES) systems (Zalba et al., 2003), to store / retrieve the hot / cool thermal energy either in the form of sensible or latent heat by using a suitable storage medium. The integration of cool thermal energy storage (CTES) system in a chiller reduces considerable electric cost by shifting the off-peak times during a day, especially in the large building central air conditioning (Cheralathan et al., 2007), refrigeration in supermarkets (Marimon et al., 2011) and various other places like telecommunication base stations, where the cooling requirement is highly intermittent (Sun et al., 2014).
The CTES system with phase change material (PCM) possesses number of advantages than the sensible storage for the above applications, owing to high storage capacity and phase transition in an isothermal manner. The extensive review (Li et al., 2012; Oro et al., 2012) articles have been reported on the materials, encapsulation geometry and heat transfer in the field of CTES applications. Most of the CTES systems commonly use water as the storage medium, 4 Page 4 of 42
because of its high thermal transport properties and only limited systems use the PCMs other than water like hydrated salts, eutectic mixtures and brine solutions. However, the major drawback of using water as the PCM is its high degree of subcooling that demands the operation of the evaporator at a temperature well below the phase change temperature of water in the chiller system. It is the well-known fact that the specific energy consumption increases by 3 % to 4 % with a reduction in evaporator temperature of 1 °C and the various techniques have already been proposed to reduce the energy consumption by minimizing the subcooling of water in the CTES system.
With the fascinating advancements in the field of nanotechnology, a new concept of dispersing high conductive solid particles at nanometric scale in the phase change material have been developed to enhance the thermal transport properties during charging/discharging process in the TES applications. The literature pertaining to the enhancement in the thermal transport properties of the PCMs dispersed with various nanoparticles are summarized as given below. The phase change characteristics of various PCMs like paraffin with different phase change temperature (Kumaresan et al., 2012; Sari et al., 2007), barium chloride (He et al., 2012), oleic acid (Harikrishnan et al., 2012) and stearic acid (Li et al., 2013) have already been investigated and reported. In general, their results revealed that the addition of nanoparticle leads to enhancement in the thermal conductivity and reduction in solidification/melting time compared to that of base PCM. It is also reported that the effect of addition of nanomaterial on the latent heat of the PCM depends on the concentration of the nanoparticle. In addition, the major problem of subcooling in the PCMs is significantly reduced, particularly in water for CTES applications with the addition of alumina (Wu et al., 2009; Altohamy et al., 2015), copper oxide (Fan et al., 2012), copper (Wu et al., 2010) and multiwall carbon nanotubes (MWCNT) 5 Page 5 of 42
(Kumaresan et al., 2013). Recently, two-dimensional crystalline allotrope of carbon, graphene, receives much attention among the researchers due to excellent thermal and electrical transport properties in number of potential applications (Liu et al., 2015; Baby et al., 2011). Mehrali et al. (2013) prepared the new composite consisting of palmitic acid and graphene oxide through impregnation method. Thermal conductivity of the composite PCM was improved by more than three times due the presence of graphene oxide and it exhibited a good thermal reliability in terms of thermal properties and chemical stability after 2500 melting/freezing cycles. The effects of adding grapheme or exfoliated graphite nanoplatelets with paraffin was investigated by Shi et al. (2013) and their results showed relatively higher thermal conductivity in the case of paraffin with exfoliated graphite nanoplatelets than graphene. A novel shape-stabilized phase change material of stearic acid-graphene oxide was fabricated by imprisoning stearic acid in the interlayer spaces of the multilayer graphene oxide based on the capillary action and interfacial interaction (Li et al., 2013). The composite with the equal mass fraction showed a considerable decrease in melting point and freezing point relative to those of the pristine stearic acid due to the confinement effect of nanoscaled spaces. Zhong et al. (2013) prepared the PCM with threedimensional grapheme aerogel and octadecanoic acid and the thermal conductivity of composite with 20 vol. % augmented by 14 times than the base PCM due to high specific surface area of graphene aerogel. Jeong et al. (2013) reported that thermal conductivity of bio – based PCM with impregnation of exfoliated graphene was enhanced by 3.75 times than the base PCM. However, the latent heat of composite was only 75 % of the base PCM and the presence of exfoliated graphene reduced the subcooling appreciably. It has been observed from the literature that the addition of graphene enhanced the thermal transport properties of the PCM in proportion to particle concentration to make them more suitable for TES applications. Considering the
6 Page 6 of 42
excellent thermal transport properties of GNP and the pressing need to develop an energy efficient CTES system, the present research work aims to investigate heat transfer characteristics of water dispersed with GNP in a spherical capsule during solidification at various surrounding temperature conditions of the HTF.
2. Preparation of nanofluid PCM
The preparation of nanofluid phase change material (NFPCM) is more important in the TES applications, due to phase change during charging and discharging process and the continuous thermal cycling increases the flocculation of the nanoparticles in the base PCM. In the present study, a two-step method was used to prepare the water based NFPCM. The materials, deionized water as the base PCM, graphene nanoplatelets (GNPs) as the nanomaterial and sodium dodecyl benzene sulphonate (SDBS) as a surfactant, were used to prepare the NFPCMs. The GNP (Cheep Tubes USA) has 4 – 5 layers with an average thickness of 8 nm and particle size of less than 2 µm with a specific surface area of 600 – 750 m2 g-1. The TEM image of the GNP is shown in Fig. 1 and it is seen from the figure that the GNPs have been entangled with each other. In order to disentangle them, the ultrasonication of the GNPs under dry condition was carried out in water bath for a period of 45 min, before the GNPs were dispersed in the base PCM. The surfactant with a mass fraction of 0.25 % was initially mixed with the PCM and the mixture was stirred in a magnetic stirrer for 15 min. The GNPs were then mixed with the solution and the shear mixing was continued for 15 min and the mixture was transferred to the ultrasonicator. The mixture was then sonicated continuously for a period of 180 min to disperse the GNPs uniformly in the base PCM. Four different NFPCM with the mass 7 Page 7 of 42
concentration of 0.3 %, 0.6 %, 0.9 % and 1.2 % were prepared by using the above procedure. The size of dispersed GNPs in the base PCM was analyzed using the scanning electron microscope (SEM). As shown in Fig. 2, the thickness of GNPs (D1) is about 60 nm and its diameter is in the range of 2.3 µm (D2) – 3.3 µm (D3). The increase in the diameter of the GNPs after dispersion in the base PCM is due to agglomeration during preparation of the NFPCM. After ensuring the thickness of the GNP less than 100 nm, the thermal conductivity of the NFPCMs was measured both in liquid and solid state and reported in the following section.
3. Thermal conductivity measurement It is essential to measure the thermal conductivity of the PCM with respect to temperature, as the variation in its thermal conductivity plays a vital role during charging and retrieval process in the PCM based CTES applications. In the present study, a KD2 Pro analyzer (Decan Devices Inc., USA), which works based on the principal of transient line heat source method, was used to measure the thermal conductivity of the NFPCM both in liquid and solid state. A sensor needle of 60 mm length and 1.2 mm diameter was employed to measure the thermal conductivity of the NFPCM in liquid state in the temperature range of 5 °C – 40 °C with an accuracy of ± 5 %. The solid thermal conductivity was measured in the temperature range from -15 °C to - 5 °C by using another sensor needle of 100 mm long and 2.4 mm diameter with an accuracy of ± 10 %. The desired temperature of the NFPCM was maintained by a refrigerated / heating circulated bath (Juloba, Germany) with a temperature stability of ± 0.01 °C. A known volume of NFPCM measured using a standard volumetric flask of 25 ml (class B) was poured carefully into a cylindrical glass container fitted with a flexible nylon lid, through which the sensor needle was inserted. This sensor was always kept at the center for accurate measurements and a read time of 1 min and 4 min was configured during the measurement in the liquid and 8 Page 8 of 42
solid state respectively. The measurements were made only after the NFPCM attained the desired temperature and five trials were made for each sample to ascertain the repeatability of the experimental data.
4. Experimental setup The schematic arrangement of the experimental setup to conduct solidification characteristic of the NFPCM is illustrated in Fig. 3. It consists of a thermally insulated stainless steel tank of capacity 0.01 m3, fitted with a heating coil of capacity 2000 W and vapor compression refrigeration system consisting of compressor (C), condenser, expansion valve (EV) and evaporator (E). A mixture of water and ethylene glycol (70:30 by volume) was used as the surrounding heat transfer fluid (HTF), normally called as the secondary refrigerant. The desired experimental temperature of the HTF was achieved using a proportionate temperature differential controller (PTDC) that regulates the output of the heating coil based on the temperature of the surrounding HTF and a mechanical stirrer was used to maintain a near uniform temperature throughout the bath. The required quantity of the NFPCM was measured using standard volumetric flask of class ‘B’ and it was transferred to spherical ball made up of low density polyethylene (LDPE) with an outer diameter and wall thickness of 72 mm and 1 mm respectively. The LDPE ball was filled up to its 80 % of total volume with NFPCM, in order to accommodate the increase in volume during the solidification of water based PCM. Three RTDs were located along the radius of the ball, one was at its center (r3) and other two RTDs were at a distance of 15 mm (r2) and 30 mm (r1) from the center of the ball as shown in the Fig.3. The ball filled with the NFPCM was placed in the HTF bath, maintained at a temperature well below the
9 Page 9 of 42
solidus temperature of water. The transient temperature variation of the NFPCM was continuously monitored and measured for every 10 s using a data logger (Agilent 34970A), until the temperature of the NFPCM attained the thermal equilibrium with the surrounding HTF. The uncertainties involved in the measured quantities during the experimentation are presented in Table 1.
5. Results and discussions 5.1 Thermal conductivity The thermal conductivity of deionized (DI) water both in liquid and solid state was measured and compared with the ASHRAE standard (ASHRAE, 1997), before measuring the thermal conductivity of the nanofluids. As depicted in Fig. 4, the experimental data showed a good agreement with the standard data with a maximum deviation of ± 2 % and ± 8 % for liquid and solid state respectively. Fig. 5 presents the enhancement in thermal conductivity, defined as k nf k bf k bf
, for various NFPCMs with respect to temperature. It has been observed that the
thermal conductivity of the NFPCM was augmented in proportion with the concentration of GNP at all the temperature conditions. The maximum thermal conductivity enhancement of 56 % (solid state) and 11.7 % (liquid state) was exhibited by the NFPCM containing 1.2 wt. % of GNP at -10 °C and 40 °C respectively. This is due to high specific surface area of the GNPs that in turn enhances the heat transfer rate through its unique two dimensional planer structure and the solid–liquid interfacial layer. The average specific surface area of GNPs is taken as 675 m2 g-1 based on the manufacturer data and its corresponding values in the NFPCM with the GNP concentration of 0.3 %, 0.6 %, 0.9 % and 1.2 % are 324 m2 , 648 m2, 972 m2 and 1296 m2
10 Page 10 of 42
respectively. It is prominent to note that the effective thermal conductivity of the NFPCM does not exhibit a proportionate increase with respect to surface area due to increase in interfacial thermal resistance. Similar results of increase in thermal conductivity were reported by Mehrali et al. (2013) in DI water and Lee et al. (2014) in ethylene glycol in liquid state. Further, the thermal conductivity of the NFPCMs showed the temperature independent irrespective of the GNP concentration, as similar to that of the NFPCM with the suspension of CNT. This could be due to the suppression of the Brownian motion of the GNPs in the base fluid due to its larger diameter in micron size. The same temperature independent trend was also reported by Yu et al. (2010) and Huang et al. (2012) for the suspension of graphene oxide nanosheets in ethylene glycol and GNPs in the epoxy composites respectively.
The sensor used to measure the thermal conductivity of ice was calibrated by using the standard solid specimen (Decan Devices Inc., USA) at room temperature (30 °C – 32 ºC). It is evident that the thermal conductivity of the NFPCMs enhanced anomalously in their solid state as shown in Fig. 5. This anomalous enhancement is attributed by the presence of high conductive GNPs that provides negligible interfacial thermal resistance with the solid ice. The thermal conductivity enhancement of about 15 % was noticed in the case of NFPCM containing 0.3 wt. % and this enhancement nonlinearly increased to 42 % with addition of 0.6 wt. % of the GNP. The larger heat transfer rate through the increase in the surface area with further addition of GNP contributed to the nonlinear thermal conductivity enhancement of the NFPCM. However, only 7 – 10 % and ~ 8 % enhancement in thermal conductivity was noticed as the mass concentration of GNP increased from 0.6 % to 0.9 % and 1.2 % respectively. It could be due to two dimensional flake structure of the GNPs that retards the thermal conductivity enhancement factor with respect to increase in concentration. 11 Page 11 of 42
5.2. Transient temperature variation of the NFPCM The effects of surrounding bath temperature and graphene concentration on the solidification behavior of water are discussed in this section. Fig. 6 shows transient temperature variation of DI water at the center of the spherical capsule, when the surrounding bath was maintained at a temperature of -9 °C, -12 °C and -15 °C. It is noticed from the figure that DI water was subcooled to a temperature of - 6.9 °C and -5.8 °C, when the bath temperature was maintained at -12 °C and -9 °C respectively. However, the subcooling of DI water was completely eliminated at the surrounding bath temperature of -15 °C, due to the existence of relatively higher driving potential between the DI water and the surrounding HTF. A significant reduction of 22.5 % and 64.77 % in the solidification time was accomplished by decreasing the surrounding bath temperature from -9 °C to -12 °C and -15 °C respectively. The specific energy consumption by the refrigeration systems increases non-linearly with respect to decrease in evaporator temperature predominantly in the range of -5 °C to -20 °C, depending on the type of primary refrigerant used in the system. Different approaches like operation of the chiller during night time, at or near full load (Rutberget al., 2013) have already been proposed to reduce the energy consumption by the chiller integrated with the CTES system. Also, there is a mammoth potential to reduce the energy consumption through the reduction of subcooling in the PCM, particularly in the case of water as the base PCM.
Fig. 7(a) shows the transient temperature variation of the NFPCM with different concentrations of GNPs at a surrounding bath temperature of -12 °C. Only a negligible effect of the GNPs has been observed on the heat transfer in the region of sensible cooling up to density inversion point (~ 4 °C). In that region, both the DI water and NFPCMs followed the same temperature drop trend with respect to time. This could be due to increase in the viscosity of the 12 Page 12 of 42
NFPCMs with addition of GNPs that suppresses the natural convection effects. It has also been noticed that DI water was subcooled to a temperature -7 °C and the onset of solidification began after 1150 s. There was a small reduction in the degree of subcooling of 1 °C in DI water solution containing the surfactant, compared to that of subcooling in DI water. However, the degree of subcooling was greatly reduced in the NFPCMs in proportion with the concentration of the GNPs and the NFPCM containing 1.2 wt. % showed the subcooling of -2.5 °C as shown in Fig. 7 (b). This clearly indicates the nucleating action of the GNPs in DI water. It is well known that the addition of GNPs in DI water results in the larger contact area due to its unique feature of possessing high specific surface area. This feature is advantageous to initiate the crystallization by increasing the nucleation sites through the reduction of interfacial contact resistance between GNP - DI water. It is also evident that the onset of solidification advanced in proportion with the reduction in the degree of subcooling. For example, the onset of solidification commenced after 1150 s and 730 s respectively for DI water and NFPCM with the mass fraction of 1.2 % at Tsurr = -12 °C. Moreover, the experimental results showed a notable reduction in the solidification duration in the case of all NFPCM compared to that of the base PCM as illustrated in Table. 2 and the maximum reduction in the solidification time of 25 % was noticed (from initial condition to -1 °C) in the NFPCM containing 1.2 wt. % of GNPs. This is well explained by the enhanced thermal conductivity of the NFPCM, owing to the presence of high conductive graphene layers with the large specific surface area, which in turn enhances the heat transfer rate in the NFPCMs. The results of similar experiments conducted with the NFPCMs at the surrounding bath temperature of -9 °C are presented in Fig.8 (a). It is seen from the figure that the sensible cooling of DI water and NFPCM showed the same trend as in the case of Tsurr = -12 °C. DI water and 13 Page 13 of 42
NFPCM with 1.2 wt. % GNP experienced a lower subcooling degree of -5.9 °C and -2.3 °C respectively as shown in Fig. 8 (b), when compared to the results obtained at Tsurr = -12 °C due to relatively lesser cooling rate. The influence of GNP loading on subcooling of the NFPCMs at the surrounding bath temperatures of -12 °C and -9 °C is compared in Table 3. In addition, the solidification duration was also reduced in proportion with the concentration of the GNP at Tsurr = -9 °C and a maximum reduction of 20.6 % was achieved in the NFPCM with 1.2 wt. % as shown in Table. 4. The present results is compared with the reduction in solidification time achieved using conventional techniques such as fins (Stritih et al., 2007; Velraj et al., 1997), insertion of metal beads and porous media (Ettouney et al., 2006; Zhou et al., 2011; Yin et al., 2008), addition of high conductive micro particles (Mettawee et al., 2007) and nanoparticles/nanofibers (Kumaresan et al., 2012; He et al., 2012; Wu et al., 2009; Altohamy et al., 2015; Wu et al., 2010; Kumaresan et al., 2013; Elgafy et al., 2005; Wang et al., 2014; Sanusi et al., 2011) in the base PCM as shown in Fig. 9. A relatively higher reduction in solidification time was reported with addition of metal foams and micro particles in the PCM other than water. Mostly, the researchers used nanoparticles/nanofibers in water for CTES application with the objective of reducing subcooling and enhancing the heat transfer characteristics during solidification. As depicted in Fig. 9, solidification duration of water decreased considerably with addition of metal/metal oxide nanoparticles, but the higher density compared to GNP limits its applications. It is construed from the above discussions that the degree of subcooling decreases with respect to increase in the concentration of GNPs. This reduction in subcooling and enhanced heat transfer characteristics of the NFPCM will play a vital role towards the development of energy efficient chiller based CTES systems suitable for various industrial and building cooling applications.
14 Page 14 of 42
5.3. Cooling rate of the NFPCM The cooling rate during subcooling of PCM is an important parameter to be considered in the design of CTES system as it has a direct bearing on the energy consumption of the chiller unit. Considerable research works have been carried out to reduce the subcooling of the PCM through the addition of nucleating agents and maintaining high driving potential between the PCM and the surrounding HTF (Zhang et al., 2010). The cooling rate during the subcooling period is evaluated from the T –t history as shown in Fig. 10. T CR t su b
(1)
where, ΔT and t are the degree of subcooling and total subcooling period respectively. Fig. 11 illustrates the subcooling rate of the NFPCMs with respect to the concentration of GNP at various radial locations in the spherical capsule, when the surrounding bath temperature was maintained at -12 ºC. As seen from the figure, the cooling rate of NFPCM increased in proportion with the concentration of GNP and the cooling rate of DI water augmented from 0.77 ºC min-1 to 1.3 ºC min-1 with addition of 1.2 wt. % of GNP at the center of the capsule. This is due to enhanced thermal conductivity of NEPCM which in turn augments the heat transfer rate during the nucleation process. It is also known that the heterogeneous nucleation rate is inversely proportional to the foreign substrates and hence the presence of GNP with large specific surface area in DI water is very useful to attain higher nucleation rate. However, the cooling of NFPCMs decreased from the surface to the center of the capsule due to existence of the thermal resistance offered by the solidified NFPCM. This difference in the cooling rate caused the NFPCM at the center of the capsule to undergo maximum degree of subcooling for a given driving potential. The same trend of cooling rate exists at various radial location of the capsule when Tsurr = -9 ºC, 15 Page 15 of 42
as depicted in Fig. 12. It is evident from the above results that the addition of GNP increases the cooling rate during subcooling for the given temperature driving potential, which is highly advantageous to facilitate the operation of the CTES system at a relatively higher temperature of the HTF. 5.4. Volume solidified The volume solidified was determined from the T – t history at various radial locations inside the spherical capsule in the case of DI water at the surrounding bath temperature of -12 °C as shown in Fig.13. Time taken for complete solidification was calculated by the difference between the onset of solidification and the time at which the slope of the curve changed from the phase change region in the T – t diagram. The same method has been used to determine the volume solidified for the NFPCMs at Tsurr = -12 °C and Tsurr = -9 °C. Figs. 14–15 compares the percentage of volume solidified between the DI water and the NFPCMs with respect to time during solidification at Tsurr = -12 °C and Tsurr = -9 °C respectively. As shown in Fig. 14, 34 % of volume got solidified during 1840 s (42 % of total solidification time) in the case of DI water and the solidification time was reduced in the case of NFPCMs in proportion with the concentration of the GNPs due to their enhanced heat transfer characteristics. By comparing the results presented in Fig. 7 (a), the time taken to solidify 34 % of total volume by the NFPCM with 0.3 wt. %, 0.6 wt. %, 0.9 wt. % and 1.2 wt. % was respectively 41.8 %, 41.1 %, 36.8 % and 35.2 % of their corresponding total solidification duration. In addition, the reduction in solidification time of 26 % was noticed in the NFPCM containing 1.2 wt. % of GNP, compared to that of the base PCM. Further, the next 57 % of DI water solidified during 34 % of total solidification time, which clearly indicates the existence of accelerated mode of
16 Page 16 of 42
energy charging. The same accelerated mode of energy charging prevailed in all the NFPCMs with considerable reduction in solidification time depending on the concentration of the GNP. It is well explained that the increase in thermal conductivity of ice (about 4 times higher than water in liquid state) provides enhanced heat percolation from the outer surface of the capsule to the liquid PCM in the inner core. The experimental results also revealed that the remaining volume of 9 % required 22 % of total solidification time in the case of DI water and the behavior was exhibited by all the NFPCMs. This clearly shows the decelerated mode of energy charging due to the appreciable reduction in the surface area of the spherical capsule. It is construed from the above discussions that there is an immense energy saving potential in the CTES system by operating the chiller for a period of 78 % of total solidification time from the initial condition of the PCM. The same trend of solidification behavior was also observed at Tsurr = -9 °C as shown in Fig. 15. It is seen that 9 % of volume solidified during 21 % of total solidification time in the case of DI water. However, this solidification time was considerably reduced to 10 % in the case of NFPCMs due the predominant effect of nanoplatelets. 6. Conclusions The solidification behavior of water based GNP nanofluid PCM was investigated experimentally at different surrounding bath temperatures and the following conclusions are arrived based on the experimental results.
The maximum thermal conductivity enhancement of 11.7 % and 56 % was observed respectively in the liquid and solid state of the NFPCM containing 1.2 wt. % GNP due to its high thermal conductivity with larger specific surface area.
17 Page 17 of 42
The presence of high conductive GNP acted as the nucleating agent to initiate the solidification of the NFPCM in advance to that of the base PCM through considerable reduction in the degree of subcooling. The addition of the GNP also shortened the solidification duration of the NFPCM by 25 % and 20.6 %than that of base PCM at the surrounding bath temperature of -12 ºC and -9 ºC respectively.
The NFPCM exhibited higher cooling rate in the subcooled region for the given driving potential than the base PCM and this facilitates the operation of the CTES system at a relatively higher temperature of the HTF.
It was noticed that only 9 % of volume around the center of the capsule got solidified during 22 % of total solidification time. Hence, there is a significant energy saving potential in the CTES system by operating the chiller for a period of 78 % of the total solidification time at Tsurr = -12 ºC.
The pioneering advancements in the field of nanotechnology will play a vital role to design and develop an energy efficient CTES system suitable for various industrial and building cooling applications.
References Altohamy, A.A., AbdRabbo, M.F., Sakr. R., 2015. Effect of water based Al2O3 nanoparticle PCM on cool storage performance. Appl. Therm. Engineering 84, 331-338
American Society of Heating, Refrigerating and Air-conditioning Engineers, ASHRAE Handbook, 1997: Fundamentals, Atlanta, GA.
Baby,T.T., Ramaprabu, S., 2011. Enhanced conductive heat transfer using graphene dispersed nanofluids. Nano. Res. Lett. 6 (1), 289. 18 Page 18 of 42
Cheralathan, M., Velraj, R., Renganarayanan, S., 2007. Performance analysis on industrial refrigeration system integrated with encapsulated PCM based cool thermal energy storage system. Int. J. Energ. Res. 31 (14), 1398–1413.
Ettouney, H., Alatiqi, I., et al., 2006. Heat transfer enhancement in energy storage in spherical capsules filled with paraffin wax and metal beads. Energ Convers Manage. 47, 211–228
Elgafy, A., Lafdi, K., 2005. Effect of carbon nanofiber additives on thermal behavior of phase change materials. Carbon 43, 3067–74.
Fan, L., Khodadadi, J.M., 2012. A theoretical and experimental investigation of unidirectional freezing of nanoparticle-enhanced phase change materials. ASME J. Heat Trans. 134, 092301-9.
Harikrishnan, S., Kalaiselvam, S., 2012. Preparation and thermal characteristics of CuO– oleic acid nanofluids as a phase change material. Thermochim Acta 533, 46–55.
He, Q., Wang, S., Tong, M., Liu, Y., 2012. Experimental study on thermophysical properties of nanofluids as phase-change material in low temperature cool storage. Energ.Convers. Manage. 64, 199–205. Huang, X., Zhi, C., Jiang, P., 2012. Toward effective synergetic effects from graphene nanoplatelets and carbon nanotubes on thermal conductivity of ultrahigh volume fraction nanocarbon epoxy composites.J. Phys. Chem. 116, 23812−20.
19 Page 19 of 42
Jeong, S.G., Chung, O., Yu, S., Kim, S., Kim, S., 2013. Improvement of the thermal properties of Bio-based PCM using exfoliated graphite nanoplatelets. Sol. Energ. Material. Sol. Cell. 117, 87–92 Kumaresan, V., Velraj, R., Das, S.K., 2012. The effect of carbon nanotubes in enhancing the thermal transport properties of PCM during solidification. Heat Mass Trans. 48 (8), 1345–55. Kumaresan, V., Chandrasekaran, P., Nanda, M., Maini, A.K., Velraj, R., 2013. Role of PCM based nanofluids for energy efficient cool thermal storage system. Int. J. Refrigeration 36, 1641–7. Liu, Y., Li X., Hu, P., Hu, G., 2015. Study on the supercooling degree and nucleation behavior of water-based graphene oxide nanofluids PCM. Int. J. Refrigeration 50, 80-6. Li, G., Hwang, Y., Radermacher, R., 2012. Review of cold storage materials for air conditioning application. Int. J. Refrigeration 35, 2053–77. Li, B., Liu, T., Hua, L., Wang, Y., Nie, S., 2013. Facile preparation and adjustable thermal property of stearic acid–graphene oxide composite as shape-stabilized phase change material. Chemical Engineering Journal 215–216, 819–26. Li, H., Jiang, M., li, Q., li, D., et al., 2013.Aqueous preparation of polyethylene glycol/sulfonatedgraphene
phase
change
composite
with
enhanced
thermal
performance.Energ. Convers. Manage. 75, 482−7. Marimon, M.A., Arias, J., Lundqvist, P., Bruno, J.C., Coronas, A., 2011. Integration of trigeneration in an indirect cascade refrigeration system in supermarkets.Energ.Buildings 43, 1427–34.
20 Page 20 of 42
Mehrali, M., Latibari, S.T., Mehrali, M., Metselaar, H.S., et al., 2013. Shape-stabilized phase change materials with high thermal conductivity based on paraffin/graphene oxide composite. Energ.Convers.Manage.67, 275−82. Mettawee, E.S., Assassa, G.M.R., 2007. Thermal conductivity enhancement in a latent heat storage system.Sol. Energy 81, 839–45. Oro, E., Miro, L., Farid, M.M., Cabeza, L.F., 2012. Thermal analysis of a low temperature storage unit using phase change materials without refrigeration system. Int. J. Refrigeration 35 (6), 1709−14. Rutberg, M., Hastbacka, M., Cooperman, A., Bouza, A., 2013. Thermal energy storage. ASHRAE Journal, 62−6. Sanusi, O., Warzoha, R., Fleischer, A.S., 2011. Energy storage and solidification of paraffin phase change material embedded with graphite nanofibers. Int. J. Heat Mass Trans. 54, 4429–36. Sari, A., Karaipekli, A., 2007. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Appl. Therm. Eng. 27, 1271–7. Shi, J.N., Ger, M.D., Liu, Y.M., Fan, Y.E., et al., 2013.Improving the thermal conductivity and shape-stabilization of phase change materials using nanographite additives.Carbon 51, 365−372. Stritih, U., 2004. An experimental study of enhanced heat transfer in rectangular PCM storage. Int. J. Heat. Mass. Transf. 47, 2841–7.
21 Page 21 of 42
Sun, X., Zhang, Q., Medina, M.A., Liu, Y., Liao, S., et al., 2014. A study on the use of phase change materials in combination with a natural cold source for space cooling in telecommunications base stations in China. Appl. Energy 117, 95–103. Velraj, R., Seeniraj, R.V., Hafner, B., Faber, C., Schwarzer, K., 1997. Experimental analysis and numerical modeling of inward solidification on a finned vertical tube for a latent heat storage unit.Sol. Energy 60, 281–90. Wang, X.J., Li, X.F., Xu, Y.H., Zhu, D.S., 2014. Thermal energy storage characteristics of Cu-H2O nanofluids. Energy 78, 212-7. Wu, S., Zhu, D., Li, X., Li, H., Lei, J., 2009.Thermal energy storage behavior of Al 2O3-H2O nanofluids.Thermochim.Acta 483 (1–2), 73–7. Wu, S., Zhu, D., Zhang, D., Huang, J., 2010. Preparation and melting/freezing characteristics of Cu/Paraffin nanofluid as phase change material. Energy Fuels 24, 1894–98. Yin, H., Gao, X., Ding, J., Zhang, Z., 2008. Experimental research on heat transfer mechanism of heat sink with composite phase change materials. Energy Convers. Manage. 49, 1740–6. Yu, W., Xie1, H., Bao, D., 2010.Enhanced thermal conductivities of nanofluids containing graphene oxide nanosheets.Nanotechnology 21, 055705. Zalba, B., Marin, J.M., Cabeza, L.F., Mehling, H., 2003. Review on thermal energy storage with phase change materials, heat transfer analysis and applications. Appl. Therm. Eng. 23, 251–83. Zhang, X.L., Wu, P., Qiu, L.M., Zhang, X.B., Tian, X.J., 2010. Analysis of the nucleation of nanofluids in the ice formation process.Energ. Convers. Manage. 51, 130–4.
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Zhong, Y., Zhou, M., Huang, F., Lin, T., Wana, D., 2013. Effect of graphene aerogel on thermal behavior of phase change materials for thermal management. Sol.Energ. Material. Sol. Cell 113, 195–200 Zhou, D., Zhao, C.Y., 2011. Experimental investigations on heat transfer in phase change materials (PCMs) embedded in porous materials. Appl. Therm. Eng.31, 970–7.
Fig. 1 – TEM image of the graphene nanoplatelets (Source: Cheap Tubes, USA)
23 Page 23 of 42
Fig. 2 – SEM image of the dispersed GNP (0.3 wt. %)
24 Page 24 of 42
Data logger Computer RTDs
PDTC
Stirrer
Heat Transfer Fluid RTD
LDPE spherical ball NFPCM
Heater
RTD RTD1 1(at center) RTD RTD 2 (15 mm from center)
NFPCM centercenterfrocenter RTD 3 (30 mm from center) )
C - Compressor
E EV
E – Evaporator Condenser
C
EV- Expansion Valve
Fig. 3– Schematic diagram of the experimental setup
25 Page 25 of 42
Fig. 4 – Comparison of thermal conductivity of DI water and ice with ASHRAE standard, 1997
26 Page 26 of 42
80
Thermal conductivity increament (%)
0.3 wt. %
Solid state
70
0.6 wt. % 0.9 wt. %
60
1.2 wt. %
50 40
Liquid state
30 20 10 0 -30
-20
-10
0 10 20 Temperature (ºC)
30
40
Fig. 5 – Thermal conductivity enhancement of the NFPCM
27 Page 27 of 42
29 Tbath = - 9 ºC 25
Tbath = - 12 ºC
21
Tbath = - 15 ºC
Temperature (°C)
17 13 9 5 1 -3 -7 -11 -15 0
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
9000
Fig. 6 – Transient temperature variation of DI water at different bath temperatures
28 Page 28 of 42
28
DI Water
24
DI Water + SDBS 0.3 wt. %
20
0.6 wt. % 0.9 wt. %
16 12 8 4 0 -4 -8 -12 0
1000
2000
3000
4000
5000
6000
7000
Time (s)
(a) 28
DI Water DI Water + SDBS
24
0.3 wt. % 20
Temperature (°C)
Temperature (°C)
1.2 wt. %
0.6 wt. % 0.9 wt. %
16
1.2 wt. %
12 8 4 0 -4 -8 -12 0
1000
2000
Time (s)
(b) 29 Page 29 of 42
Fig. 7 – Transient temperature variation of the NFPCM during (a) entire period of experimentation (b) exploded view near subcooling region (Tsurr = -12 ºC).
30 DI Water
26
DI Water + SDBS
22
0.3 wt. % 0.6 wt. %
18 Temperature ( ºC)
0.9 wt. %
14
1.2 wt. %
10 6 2 -2 -6 -10 0
1000 2000 3000 4000 5000 6000 7000 8000 9000 Time (s) (a)
30 Page 30 of 42
30 DI Water
26
DI Water + SDBS 0.3 wt. %
22
0.6 wt. % 0.9 wt. %
Temperature ( ºC)
18
1.2 wt. %
14 10 6 2 -2 -6 -10 0
1000 Time (s)
2000
(b)
Fig. 8–Transient temperature variation of the NFPCM during (a) entire period of experimentation (b) exploded view near subcooling range (Tsurr = -9 ºC)
31 Page 31 of 42
20 He et al.
Sanusi et al.
Kumaresan et al.
Altohamy et al.
Wu et al.
present work (1.2 wt. %)
Elgaty et al.
Wu et al.
Kumaresan et al.
Mettawee et al.
Velraj et al.
100
Wang et al.
60 Stritih et al.
Zhou et al.
80
Yin et al.
Ettouney et al.
40
particles
Micro
Fins
Porous media
Metal beads
Reduction in solidification time (%) 120 High conductive nanoparticles
0
Fig. 9 – Comparison of reduction in solidification time
32
Page 32 of 42
tsub T CR t su b
ΔTsub
Fig. 10 – Cooling rate of DI water in the subcooling region
33 Page 33 of 42
2
Subcooling rate (°C min-1)
1.8
r = 30 mm from the center r = 15 mm from the center At center
1.6
1.4
1.2
1
0.8
0.6 -1.55E-15
0.3
0.6
0.9
1.2
1.5
Mass concentration of the GNPs (%)
Fig. 11 Cooling rate of the NFPCMs in the subcooling region (Tsurr = -12 ºC)
34 Page 34 of 42
Subcooling rate (°C min-1 )
1.6 1.4
r = 30 mm from the center r = 15 mm from the center At center
1.2 1 0.8 0.6 0.4 -1.55E-15
0.3 0.6 0.9 1.2 Mass concentration of the GNPs (%)
1.5
Fig. 12 – Cooling rate of the NFPCMs in the subcooling region (Tsurr = -9 ºC)
35 Page 35 of 42
28
At the center r = 15mm from the center r = 30 mm from the center
24
Temperature (ºC)
20 16 12 8 4 0 -4 -8 -12 0
1000
2000
3000
4000
5000
6000
7000
8000
Time (s)
Fig. 13 – Transient temperature variation of the DI water at various radial locations (Tsurr= 12ºC)
36 Page 36 of 42
6000 r = 30 mm from the center r = 15 mm from the center 5000
At the center
Time (s)
4000
3000
2000
1000
0 0
0.3
0.6
0.9
1.2
1.5
Mass concentration of the GNPs (%)
Fig. 14 – Time to reach complete solidification at various radial locations (Tsurr= -12ºC)
37 Page 37 of 42
7000 r = 30 mm from the center r = 15 mm from the center
6000
At the center
Time (s)
5000
4000
3000
2000
1000
0 0
0.3
0.6
0.9
1.2
1.5
Mass concentration of the GNPs (%)
Fig. 15 – Time to reach complete solidification at various radial locations (Tsurr= -9ºC)
38 Page 38 of 42
Table 1 – Results of uncertainty analysis
Measured quantities
Uncertainty
Diameter of spherical capsule
± 0.02 mm
Mass
± 0.002 g
Temperature
± 0.15 ºC
Volume (25 ml)
± 0.04 ml
Volume (50 ml)
± 0.06 ml
Volume (100 ml)
± 0.1ml
Thermal conductivity ( liquid)
± 5%
Thermal conductivity ( solid)
± 10%
39 Page 39 of 42
Table 2 – Solidification time of the NFPCMs (Tsurr = -12 ºC)
Solidification duration at various radial locations
Concentration of
various radial locations (%)
(s)
the GNP (wt. %) r1* = 30 mm
r2* = 15 mm
0
1820
3410
0.3
1760
0.6
Reduction in solidification duration at
r3 = 0
r3 = 0
r1* = 30 mm
r2* = 15 mm
4450
–
–
–
3320
4260
3.2
2.6
4.2
1680
3180
3840
7.7
6.7
13.7
0.9
1540
3010
3610
15.3
11.7
19
1.2
1450
2890
3320
20.3
15.2
25
*
(at center)
(at center)
from the center
40 Page 40 of 42
Table – 3 Effect of GNPs concentration on subcooling at center of capsule
Concentration
Degree of
Reduction in
of GNP
Subcooling (ºC)
Subcooling (%)
(wt. %)
Tsurr = -12 ºC
Tsurr = -9 ºC
Tsurr = -12 ºC
Tsurr = -9 ºC
0
-6.9
-5.92
–
–
0.3
-5.75
-4.81
17
18
0.6
-4.44
-4.19
36
29
0.9
-3.52
-3.21
49
45
1.2
-2.5
-2.3
64
61
Table 4 – Solidification time of the NFPCMs (Tsurr = -9 ºC)
Solidification duration at various
Concentration of
radial locations
the GNP (wt. %)
Reduction in solidification duration at various radial locations (%)
(s) *
*
r3 = 0
*
r3 = 0
mm
(at center)
r1 = 30 mm
r2 = 15 mm
0
3110
5020
5620
–
–
–
0.3
2990
4890
5010
3.8
2.6
10
0.6
2820
4720
4840
9.4
6
13.5
0.9
2710
4590
4650
12.9
8.6
17.2
1.2
2590
4480
4450
16.8
10.8
20.6
*
(at center)
r1 = 30 mm
r2* = 15
from the center 41 Page 41 of 42
42 Page 42 of 42