Preparation, thermal and rheological properties of hybrid nanocomposite phase change material for thermal energy storage

Applied Energy 115 (2014) 320–330

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Preparation, thermal and rheological properties of hybrid nanocomposite phase change material for thermal energy storage R. Parameshwaran a,b,⇑, K. Deepak a, R. Saravanan c, S. Kalaiselvam a,d a

Department of Mechanical Engineering, Anna University, Chennai 600 025, India Centre for Nanoscience and Technology, Anna University, Chennai 600 025, India c Institute for Energy Studies, Anna University, Chennai 600 025, India d Department of Applied Science and Technology, Anna University, Chennai 600 025, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Hybrid nanocomposite PCM achieved

improved thermal properties and heat storage.  Thermal conductivity of hybrid nanocomposite PCM was improved by 52%.  Hybrid nanocomposite PCM exhibited enhanced thermal stability than pure PCM.  Energy efficiency and savings was obtained with freezing rate reduction of 23.9%.  Increased mass loading of nanocomposite resulted in 4% increased viscosity of PCM.

a r t i c l e

i n f o

Article history: Received 18 May 2013 Received in revised form 16 October 2013 Accepted 4 November 2013

Keywords: Energy efficiency Phase change material Silver–titania hybrid nanocomposite Thermal energy storage Thermal properties Viscosity

a b s t r a c t This paper presents the experimental investigation on the thermal properties and viscosity of the new organic ester phase change material embedded with the silver–titania hybrid nanocomposite (HyNPCM) with the mass proportions ranging from 0.1% to 1.5%. The HyNPCM embedded with the surface functionalized hybrid nanocomposite exhibited improved thermal conductivity from 0.286 W/m K to 0.538 W/ m K, congruent phase change temperature (6.8 °C), high latent heat capacity (90.81 kJ/kg), substantial reduction in the supercooling degree (1.82 °C), thermal stability (191 °C) and chemical stability, while compared to the pure PCM. Experimental results reveal that, the freezing and the melting times of the HyNPCM were reduced by 23.9% and 8.5% respectively, when compared to the pure PCM. The increased mass proportion of HyNC resulted in the increased viscosity up to 3.89%, which suggests the existence of relative dependencies between the thermal properties and the viscosity of the HyNPCM. In total, the improved thermal properties and the heat storage potential of the HyNPCM has facilitated them to be considered as a viable candidate for the cool thermal energy storage applications in buildings without sacrificing energy efficiency. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author at: Department of Mechanical Engineering, Anna University, Chennai 600 025, India; Centre for Nanoscience and Technology, Anna University, Chennai 600 025, India. Tel.: +91 4422359220; fax: +91 4422301656. E-mail addresses: [email protected] (R. Parameshwaran), [email protected] (K. Deepak), [email protected] (R. Saravanan), [email protected] (S. Kalaiselvam). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.11.029

Incessant value-added engineering design and incorporation of the energy efficient thermal interface systems for cooling applications in buildings are greatly necessitated, in recent years. From this perspective, thermal energy storage (TES) systems are primarily intended for enhancing the performance of cooling

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systems in terms of storing and releasing heat energy on shortterm or diurnal or seasonal basis depending on the thermal load requirements. Energy redistribution requirements can be effectively met by using TES systems integrated with the dedicated cooling systems in buildings. In this context, research interests towards developing thermal energy storage (TES) systems incorporating efficient phase change materials (PCM), which would offer energy redistribution requirements in buildings, are increasingly popular [1–6]. Phase change materials are a class of heat storage materials, which would eventually store and release the thermal energy, by undergoing phase transition typically at isothermal conditions. In the category of heat storage materials, organic PCM are much preferred for their smaller temperature swings, good latent heat capacity, congruent phase transition characteristics, low supercooling, non-corrosiveness, and reliability on long term usage [7–11]. The thermophysical properties and the heat storage characteristics of organic PCM being improved by embedding thermally conductive materials into the PCM at its purest form have been reported in [12–16]. Interestingly, the organic PCM under the fatty acid ester classification have been considered as an effectual heat storage material for latent thermal energy storage (LTES) systems. The inherent ester bonding linkages and the carbon chains attributed for accomplishing good latent heat enthalpy, thermal storage and thermal reliability [17,18]. On the other hand, the increasing carbon chains in such kind of ester PCM would increase the fusion temperature noticeably; thereby constricting their usage for cool thermal storage applications. Many research studies performed on conceptualizing the process of incorporating heat enhancement materials produced at nanometer scale into the PCM are increasingly attractive, in recent years [19–25]. The nanomaterials prepared in the range of 1– 100 nm, having high surface to volume ratio, are doped into the PCM in order to acquire fast nucleation as well as thermal conductivity enhancement, during the charging and discharging processes. In addition, the infused nanoparticles would contribute in the formation of the stable nucleus (cluster of small nuclei) or ice-like crystals in the PCM during the freezing process; thereby enabling the nucleation to occur at a faster rate [32]. Furthermore, the effective thermal properties of the nanomaterials would facilitate the PCM to exhibit increased thermal conductivity, good latent heat potential, better heat storage and release characteristics and thermal reliability. In the present work, a new organic ester phase change material embedded with the silver–titania (Ag–TiO2) hybrid nanocomposite (HyNPCM) was prepared and the thermal properties were investigated experimentally for the different mass proportions of the HyNC. The organic ester PCM tested in this study was ethyl cinnamate (EC), which has got a wide range of utilization ranging from the domestic cosmetics to non-cosmetic product applications [26]. Besides, the thermal properties and the heat storage potential of the EC PCM embedded with the HyNC were explored in this work, which stands distinct in its class of organic PCM with reference to the past literatures. The surface functionalized HyNC was analyzed in terms of their morphology, size distribution, surface structure and crystalline nature. The inherent thermal properties of the functional HyNPCM including phase transition temperature, latent heat potential, thermal conductivity, thermal reliability, thermal conductivity, thermal stability and the heat storage performance were investigated experimentally. The significance of the incorporation of the HyNC into the ester PCM was explained and the experimental results are presented. The interdependency between the heat storage performance and the rheological aspects (viscosity) of the

321

HyNPCM due to the increased mass concentration of HyNC are discussed. 2. Experimental 2.1. Materials Ethyl trans-cinnamate (as PCM) was procured from Alfa Aesar, silver nitrate (as precursor) was obtained from Ranbaxy, titania (as precursor) was purchased from Qualigens Fine ChemicalsFischer Scientific, ascorbic acid (as reducing agent) was purchased from SRL Chemicals, and ethanol was used as the dispersant. The chemicals used in the experiment were designated to be of analytical grade and they were used without further purification. The deionized double distilled water (DDW) obtained from the Millipore distiller was used throughout the experiments. 2.2. Preparation of HyNC and HyNPCM The preparation of the HyNC and the HyNPCM was performed using the following processes: In step (1), 1 g of TiO2 powder was dispersed in ethanol using high frequency ultrasonicator (UP200S-Hielscher) for 0.5 h. Step (2): To this white TiO2 precipitate obtained, appropriate quantity of silver nitrate dissolved in DDW was mixed drop wise under continuous stirring for one hour. Step (3): 0.05 g ascorbic acid dissolved in DDW was then subsequently added into the solution obtained above, which produced pale grey color colloidal solution containing Ag–TiO2 hybrid nanocomposite particles. Step (4): The mixture was then washed, filtered and vacuum dried at 90 °C to yield the required Ag–TiO2 hybrid nanocomposite powder. Using the electronic mass balance (Denver Instruments, precision: 0.0001 g) measurements, the Ag–TiO2 HyNC in the requisite mass proportion of 0.1% through 1.5% were obtained. The HyNC was then re-dispersed into the pure PCM using the ultrasonic vibrator (UP200S-Hielscher) to obtain the HyNPCM. The scheme for the preparation of the HyNC and the HyNPCM is represented in Fig. 1. 2.3. Characterisation and experimentation The morphology in terms of the formation, shape and the size of the HyNC were characterized using the high resolution field emission scanning electron microscope (FESEM, SUPRAÒ55, Carl Zeiss, Germany) equipped with the energy dispersive X-ray spectrometry (EDAX) and the transmission electron microscope (TEM, Technai 10 Philips) facilities. The range of the particle size distribution of the HyNC was determined using the particle size analyser (PSA: Malvern ZetaSizer Nano ZS). The PANalytical X’Pert PRO X-ray diffractometer was utilized to identify the crystalline structure of the as-prepared HyNC. The scanning process was performed on the HyNC with a span ranging between 5° and 80° of the 2h° position with scanning step size maintained at 1 s 1. The Fourier Transform Infrared (FTIR) analysis was conducted on the pure PCM and the HyNPCM pelletized samples using the Perkin–Elmer FTIR spectrophotometer in the diffuse reflectance mode of resolution 4 cm 1, and recorded in the transmission mode with the wavenumber ranging from 400 cm 1 to 4000 cm 1. Thermal property analysis in terms of latent heat capacity and phase transition temperature for pure and HyNPCM were determined using differential scanning calorimeter (NETZSCH DSC 200F3, Germany). The samples kept in the sealed aluminum crucible pans were subjected to periodic cooling and heating cycles at a rate of 5 K min 1 in the range of 30 °C to +30 °C and vice versa

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Fig. 1. Scheme for the preparation of the HyNC and HyNPCM.

respectively. The thermal conductivity analysis was performed on the pure PCM and the HyNPCM samples using the NETZSCH LFA 447 NanoFlash equipment. The accuracy of the thermal conductivity analyzer (based on the solid standards) was ±3% with the thermal conductivity measurement ranging from 0.1 W/m K to 2000 W/m K. Thermal reliability of HyNPCM was assessed using Applied Biosystems veriti 96 Well Fast thermal cycler for 1000 cycles from 2 °C to 30 °C. Thermal decomposition and crystallization properties of the pure and the HyNPCM was conducted using the thermogravimetric-differential thermal analyser (NETZSCH STA 449 F3 Jupiter, Germany), wherein the samples were subjected to temperature ranging from 30 °C to 330 °C at a heating rate of 10 K/min under the inert nitrogen atmosphere. The dynamic viscosity of the pure and the HyNPCM were analyzed using the Bohlin CVO rheometer, which essentially comprised of the cone-plate and the spindle assembly. All the measurements using the aforementioned characterization techniques were checked for the repeatability at least three times. The inherent heat storage performance in terms of the freezing (charging) and melting (discharging) characteristics of the pure PCM and the HyNPCM were experimentally determined using the PCM testing facility and methodology as described in the study [27]. Typically, during the freezing process, the heat transfer fluid (DDW) filled inside the TES tank was cooled from the ambient condition of 32 °C to the requisite set temperature of 3.5 °C, by means of the cooling plant. Similarly, during the melting process, the temperature of the heat transfer fluid in the TES tank was maintained at 32 °C, by which the PCM was subjected to undergo the discharging process. The values considered for the range and the error of the sensors/transducers utilized in the experimentation were based on the reported study [27]. It is interesting to note that, the heat storage and release capabilities of the pure and the HyNPCMs would usually be experienced during their phase transition, wherein the latent heat potential exhibited by the PCMs would play a vital role in determining the actual heat storage performance characteristics. 3. Results and discussion Based on the experimental results, the vital aspects of embedding the HyNC into the PCM for improving their thermal properties and the thermal energy storage potential are discussed. 3.1. Morphology of HyNC dispersed in PCM The microscopic images of HyNC and PCM dispersed with HyNC are presented in Fig. 2(A–C). The FESEM results shown in Fig. 2(A–B) infer the evolution of spherical nanocomposite particles of size

ranging between 60 nm and 98 nm being achieved, which essentially comprised of the silver nanoparticles being adsorbed over the surface of chain-like structured titania nanoparticles. This was further confirmed using the TEM result as shown in Fig. 2(C), wherein the size of HyNC ranged from 10 to 95 nm. The arrangement of the silver nanoparticles on the titania surface was distinct in terms of its surface modification and similar in arrangement as [28]. In addition, EDAX spectra strongly supported the formation of the HyNC as depicted in Fig. 2(D). The elemental compositions of silver and titania nanoparticles were obtained to be 27.01 and 72.98 by weight percentage, 7.01% and 92.99% by atomic percentage respectively. The corresponding peaks clearly justified the presence of the highly crystalline silver and titania nanoparticles in the asprepared HyNC. The PSA result as depicted in Fig. 2(E) well supported the formation of the HyNC, wherein the average size distribution of the nanocomposite was measured to be 63 nm. Based on these results, it is inferred that, the HyNPCM embedded with the surface functionalized HyNC can be expected to have improved thermal properties, while compared to the purest form of the PCM. 3.2. Surface structure and chemical stability of HyNPCM The XRD peaks confirmed the formation of silver–titania hybrid nanocomposite particles in terms of intense and sharp Bragg reflections (2h°) exhibited, which were indexed to the corresponding lattice planes as shown in Fig. 3. The weight proportions of titania to silver precursors played an active role in creating potential nucleation sites for the reduction of silver ions to occur on the surface of titania nanoparticles [29]. The sharp peaks obtained due to scattering at interplanar spacing were ascribed primarily to the anatase phase of titania, and the silver nanoparticles thus formed on its surface was highly crystalline with dominant (1 1 1) face centered cubic (FCC) structure as well. The XRD results obtained for the hybrid nanocomposites were in good agreement with the JCPDS File No. 21-1272 and 870720 for titania and silver nanoparticles respectively. Average crystallite sizes obtained using Debye–Scherrer method was found to be 76 nm which is in good agreement with the FESEM, TEM and PSA measurements [30–32]. The surface structure of pure PCM and HyNPCM was characterised using the FTIR analysis as shown in Fig. 4. The vibrational spectra observed at 1704 cm 1, 1636 cm 1, 1576 cm 1 and 1446 cm 1 were primarily ascribed to the stretching frequency of the characteristic carbonyl ester group (C@O), unsaturated hydrocarbon (C@C), aromatic ring (C@CAC) functional groups of the PCM. Similarly, the absorption bands obtained at 3056 cm 1, 1089 cm 1 and 1065 cm 1 were assigned to the aromatic (CAH)

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Fig. 2. Morphology of (A and B) Ag–TiO2 HyNC using FESEM, (C) Ag–TiO2 HyNC dispersed in the organic ester PCM using TEM, (D) elemental compositions of Ag–TiO2 HyNC using EDAX measurement, and (E) size distribution of the HyNC using PSA.

and (CAO) functional groups. The vibrational bands observed between 1205 cm 1 and 1168 cm 1 were attributed to the in-plane bending frequencies of (CAH) group. The absorption peaks observed at 3027 cm 1, 977 cm 1, between 766 cm 1 and 710 cm 1 corresponded to the out-of-plane bending frequencies of (CAH) group in the organic ester PCM.

Interestingly, the non-existence of any new vibrational bands in the FTIR spectra for the HyNPCM, while compared to the pure PCM clearly indicates the physical interaction of the HyNC, which enabled the HyNPCM to be chemically stable and affirmed their reliability on a long run [33,34]. In addition, the absorption peaks observed at 1375 cm 1, 1026 cm 1, 548 cm 1 characteristically

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A (105) A (211)

A

TiO2

R (002)

TiO2

Ag

Ag

Ag (220)

(200)

(b) Ag-TiO2 nanocomposite

TiO2

(311)

A (004)

TiO2

(111)

Intensity (a.u.)

A (101)

TiO2

Ag - Silver nanoparticles TiO2 - Titania nanoparticles A - Anatase R - Rutile

R (301) R (112)

A (200)

TiO2 TiO2

R (310)

324

(a) TiO2 nanoparticles

10

20

30

40

50

60

70

80

2θ (degree) Fig. 3. XRD pattern of silver–titania hybrid nanocomposite.

(f) EC+1.5% Ag-TiO2 HyNC

Transmittance (arb. unit)

(e) EC+1.0% Ag-TiO2 HyNC

1633 1711

(d) EC+0.8% Ag-TiO2 HyNC

1447 1165

(c) EC+0.5% Ag-TiO2 HyNC

548

1375

1026

1572

(b) EC+0.1% Ag-TiO2 HyNC

1070 (a) EC Pure

1449

3054

1221

1091

1576 3027

4000

3600

3200

1704

2800

2400

2000

1636

1600

977

710 766

1168

1200

800

400

Wavenumber (cm-1) Fig. 4. FTIR spectra of (a) pure EC PCM, HyNPCM dispersed with (b) 0.1 wt.% HyNC, (c) 0.5 wt.% HyNC, (d) 0.8 wt.% HyNC, (e) 1.0 wt.% HyNC, and (f) 1.5 wt.% HyNC.

represents the Ti-Ligand bond vibrations. Though, the presence of the coordination complexes of the Ag–TiO2 HyNC in the HyNPCM was characterized from these absorption peaks their intensity was found very small. Thus, the effective dispersion of the HyNC in the pure PCM is thereby confirmed. Furthermore, the minor vibrational shifts observed at 3054 cm 1, 1703 cm 1, 1634 cm 1, 1574 cm 1, 1449 cm 1, 1221 cm 1, 1164 cm 1, 1091 cm 1, 975 cm 1, 767 cm 1, and 709 cm 1 were ascribed to the physical mixing of the 0.1% HyNC with the pure PCM during preparation, and due to any adsorption or surface tension forces being exerted between the PCM and the HyNC as well. Similar shifts in the characteristic peaks were also observed for the mass proportions of HyNC ranging from 0.5% to 1.5%. 3.3. Effect of embedded HyNC on the phase change temperature, latent heat capacity and thermal reliability of the HyNPCM The thermal properties including the phase change (transition) temperature, latent heat capacity and the thermal reliability of the pure PCM and HyNPCM as determined using the DSC facility is depicted in Fig. 5(a and b). The test results pertaining to the phase change temperature, latent heat capacity and the supercooling degree are presented in Table 1. The DSC results suggest that, the

HyNPCM while undergoing cyclic freezing and melting processes has exhibited only single peak, which conformed to the liquid-tosolid phase transition and vice versa respectively. The absence of any solid–solid secondary peak in the results clearly indicates the pure and the HyNPCM to deliver effectively the heat storage and release potential at their maximum ability at one stretch, while subjected to periodic thermal cycles [32,35]. Noticeably, during freezing of the pure PCM, the homogenous nucleation was induced with the supercooling effect being inherently limited to a lesser degree, though the freezing point occurs below the melting temperature as could be seen from the Table 1. Compared to the pure PCM, the HyNPCM (for 0.8%) exhibited better congruent phase transition temperature of 6.7 °C and 6.8 °C with marginal variations in latent heat capacity of 87.15 kJ/kg and 90.81 kJ/kg even after 1000 freezing and melting cycles, respectively. The supercooling degree of HyNPCM was reduced by 1.82 °C (for 0.8%) which signified the active role of embedded nanocomposite in terms of its surface adsorption and effective nucleation. Factually, by embedding the HyNC into the PCM in mass proportions ranging from 0.1% to 1.5%, the heterogeneous nucleation was favoured at the cost of the freezing point depression [36]. The presence of the dispersed HyNC particles in the PCM was effectual that, the cold energy supplied from the heat transfer fluid was

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(a) 2.5

Freezing Potential

exo

Melting Potential

Freezing Effectiveness

Pure EC

Melting Effectiveness

1.00

18

2.0 Latent heat effectiveness

0.98

Heat flow (mW/mg)

1.5 1.0

0.96

8.38 9

0.94 5.41

6

2.97

0.92

7.38

6.21 1.00

3

EC+0.1% Ag-TiO2 HyNC EC+0.5% Ag-TiO2 HyNC EC+0.8% Ag-TiO2 HyNC EC+1.0% Ag-TiO2 HyNC

3.43

2.78

1.74

0.5

12

9.18

Latent heat potential reduction (%)

15

EC+1.5% Ag-TiO2 HyNC

0

0.90 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Heating cycles

Nanocomposite particle concentration (%)

0.0 Cooling cycles

-0.5

Early cycles Freezing

-1.0 -1.5 -2.0 -30

Melting

Onset temperature: 6.67°C Onset temperature: 6.76°C Peak temperature: 5.64°C Peak temperature: 7.65°C Latent heat: 86.98 kJ/kg Latent heat: 90.79 kJ/kg

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

Temperature ( oC)

(b) 2.5

Freezing Potential

exo

Melting Potential

Freezing Effectiveness

Melting Effectiveness

Pure EC

18

1.00

2.0

Heat flow (mW/mg)

1.5 1.0

Latent heat effectiveness

0.98 9.23 0.96

12

8.36 9

0.94 5.40

6

2.98

0.92

7.41

6.19 1.00

3

EC+0.8% Ag-TiO2 HyNC EC+1.0% Ag-TiO2 HyNC EC+1.5% Ag-TiO2 HyNC

0

0.90

0.5

EC+0.5% Ag-TiO2 HyNC

3.42

2.79

1.74

EC+0.1% Ag-TiO2 HyNC

Latent heat potential reduction (%)

15

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Heating cycles

Nanocomposite particle concentration (%)

0.0 Cooling cycles

-0.5

After 1000 cycles Freezing

-1.0 -1.5 -2.0 -30

Melting

Onset temperature: 6.69°C Onset temperature: 6.75°C Peak temperature: 5.65°C Peak temperature: 7.68°C Latent heat: 87.15 kJ/kg Latent heat: 90.81 kJ/kg

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

Temperature ( oC) Fig. 5. DSC graphs obtained for the pure PCM and at different mass proportions of HyNC dispersed into the PCM (a) in early cycles, (b) after 1000 cycles.

transferred to the PCM; thereby the degree of supercooling of HyNPCM was reduced considerably. In addition, the surface functionalized HyNC containing silver nanoparticles on the surface of titania nanoparticles have acted as the extended heat transfer surfaces and potential nucleation sites, which helped to promote the solidification of the HyNPCM at a faster rate. It is interesting to note that, the dispersion of HyNC into PCM in different mass proportions has eventually helped in the formation and growth kinetics of stable PCM crystals (or nucleus) during the freezing (cooling) cycle [32]. The simultaneous rate of formation with the growth of the stable ice-like crystals determined the phase change temperature, latent heat of enthalpy of the pure and the HyNPCM. Similar is the case for the heating cycle of the HyNPCM, wherein the gain in the endothermic internal energy of HyNPCM facilitated for having faster melting processes. The variations observed between the latent heat effectiveness and the latent heat potential reduction of HyNPCM during the freezing and melting cycles are shown in Fig. 5(a and b) (insets). Basically, they were accounted for the reduced mass proportion of the pure PCM by virtue of the increased mass fraction of the hybrid nanocomposite. It is noted from the graphs that, the latent heat effectiveness and the latent heat potential of the HyNPCM were reduced from 0.98 to 0.93 and 1.74–7.38% in freezing, 0.99– 0.92 and 1–9.18% in melting, respectively. This strategy is quite realistic in most of the PCMs embedded with the heat enhancement materials. The DSC results also signify

that, the marginal variations observed in the latent heat enthalpy of the pure PCM in freezing with respect to the melting cycle was attributed to its thermal energy storage potential, phase change characteristics. On the other hand, the minor reduction in the latent heat potential of the HyNPCM was ascribed to the phase transition depression effects, physical amalgamation of the HyNC into the PCM and the viscous effects induced in the HyNPCM due to the addition of the HyNC. Though, some shifts in the phase change temperature and the latent heat potential of HyNPCM were expected, they were almost found equal to that of the pure PCM and make the HyNPCM relatively a suitable candidate for the cool thermal energy storage applications in buildings. Furthermore, embedding of the HyNC in PCM has favoured the thermal energy storage potential and contributed for the elimination of the possibility for any high concentration defects as observed in some organic PCMs [37]. The combination of HyNC and effective intermolecular ester bonding linkages of the PCM has contributed for their improved thermal reliability and free from hysteresis effects [38] on a long run, as evident from the test results obtained after 1000 cycles as shown in Fig. 5(b). Moreover, owing to the high density of HyNPCM (1.052 g/cm3), the storage encapsulation by volume requirement was reduced considerably [39]. This indicates that, the HyNC was effectively dispersed into the HyNPCM matrix; thereby making them thermo-physically stable and reliable on a long term basis [40].

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Table 1 Thermal properties of the pure PCM and the HyNPCMs using DSC measurements. HyNC mass loading in (wt.%)

0 (EC pure) 0.1 0.5 0.8 1.0 1.5

Solid–liquid phase transition on-set temperature (°C)

Solid–liquid phase transition peak temperature (°C)

Latent heat potential (kJ kg 1)

Freezing

Melting

Freezing

Melting

Freezing

Melting

6.75 6.77 6.80 6.83 6.81 6.79

6.80 6.81 6.85 6.91 6.88 6.83

5.67 5.77 5.83 5.88 5.84 5.79

7.74 7.72 7.75 7.70 7.72 7.73

90.70 89.15 88.25 87.69 85.40 84.47

95.60 94.65 92.84 90.69 88.21 87.56

3.4. Thermal conductivity and thermal stability of HyNPCM The variations of latent heat enthalpy and thermal conductivity of the HyNPCM with respect to the HyNC particle concentration is shown in Fig. 6(a). The result reveals the non-linear relationship of the thermal conductivity with increased mass concentration of the HyNC. The thermal conductivity of pure PCM was determined to be 0.257 W/m K [32], whereas for the HyNPCM, the thermal conductivity has increased from 0.286 W/m K to 0.538 W/m K for 0.1% through 1.5% of HyNC mass addition respectively. The test results clearly indicate that, higher the mass proportion of the HyNC in the PCM, higher was the thermal conductivity. This could be ascribed due to the inherent Brownian motion, interparticulate diffusion (or phonon interaction), surface morphology, heat exchange potential and clustering of the HyNC [41,42]. In this context, while compared to the pure PCM, the thermal conductivity of the HyNPCM was improved from 10% to 52% for the aforementioned mass proportions of HyNC. Typically, the dispersion of HyNC into the PCM has actually created the network of densely packed thermal interfaces, which were ascribed to be responsible for acquiring improved thermal conductivity of HyNPCM. Albeit, as titania nanoparticles are considered to be thermal conductivity enhancers for the PCMs, the significance of using the functional HyNC encompassing highly conductive silver nanoparticles on the surface of the titania nanoparticles, could still effect for obtaining higher thermal conductivity in the HyNPCM, significantly. By looking at the molecular level, the increased proportion of HyNC actually improved the thermal conductivity substantially, but it has lead to an undesirable and realistic condition of reduction in the enthalpy of latent heat of the HyNPCM. The reduction of latent heat potential of HyNPCM unlikely can be related to the increased thermal conductivity. Generically, the increase in the thermal conductivity of PCMs solely depends upon the type of the heat transfer enhancement materials and their configurations (for instance, fins or nanomaterials with rod-like, spherical, cubical, tubular or wire shaped structure). The TG-DSC result depicted in Fig. 6(b) infers continuous single step mass loss of the pure PCM and the HyNPCM (0.8% HyNC) from room temperature to 179 °C and 191 °C respectively. The TG curve was almost flat beyond 195 °C. This dominant mass loss was ascribed to the complete decomposition and the evaporation of the organic ester compounds of the pure and the HyNPCM as well. The exothermic peak materialised around 191 °C well confirmed the crystallization nature of the HyNPCM. It was evident from the result that, the HyNPCM was more stable and exhibited increased resistance to the heat than the pure PCM [43]. Besides, the ester bond linkages of the HyNPCM were appreciable in achieving the improved thermal stability. It was observed after the completion of the TG-DSC test measurements that, there remain literally no traces for any erosion of the testing pan material by the PCMs. This eventually confirmed the non-corro-

Degree of supercooling (°C)

2.07 1.95 1.92 1.82 1.88 1.94

siveness of the PCMs being considered and utilized in this study. Thus, the HyNC with mass proportion of 0.8% was considered to be the optimum ratio to be embedded into the pure PCM for accomplishing improved thermal stability on a long term basis. 3.5. Heat storage performance of the HyNPCM The heat storage performance curves in terms of the freezing (charging) and the melting (discharging) of the pure PCM and the HyNPCM are presented in Fig. 7. The observations related to the heat storage performance, which were monitored and recorded during the experimental sequences are included in Table 2. The experimental results presented in Table 2 infer that, for the mass proportions of HyNC varying from 0.1% to 1.5%, the time attained for the commencement of freezing and melting of the HyNPCM were reduced from 5.1% to 23.9% and 1.7–8.5% respectively, while compared to the pure PCM. These trends were expected to be the essential characteristics of such heat enhancement materials which they would exhibited when embedded into the PCM [44,45]. It is noteworthy that, the enhanced phase change aspects and the reduced time for the starting of the phase transition directly reflected the good latent heat storage cum release potential and the minimum primary energy consumption by the HyNPCM. From the molecular level perspective, the presence of the dual surface functional aspects of the HyNC has contributed for achieving effective heterogeneous nucleation, to transfer and distribute the thermal energy within the PCM matrix layers at a faster rate during freezing and melting processes. The inherent Brownian motion, phonon interaction, high surface to volume ratio and the surface modification of the HyNC have also helped in acquiring improved thermal conductivity, faster solidification (crystallization), reduced degree of supercooling and enhanced transfer of heat energy in the HyNPCM. Precisely, the experimental results have justified that, the HyNPCM with the effective and improved thermal properties can be considered as a potential candidate for the energy redistribution requirements for the specialized cool thermal energy storage applications. 3.6. Effect of HyNC concentration on the viscosity of the HyNPCM In reality, by determining the shear induced viscous characteristics of the PCMs (whether they are static or being subjected to flow); the extent to which their latent heat capacity would be influenced due to the incorporation of the heat enhancement materials can be identified. From this perspective, the variation of the instantaneous (dynamic) viscosity and the shear stress measured at various temperatures with the increase in the shear rate of the HyNPCM is depicted in Fig. 8(a). The result infers that, the viscosity of the HyNPCM measured at 30 °C (liquid state) has varied from 0.023 Pa s to 0.0039 Pa s from the commencement up to 74 s 1 of the shear rate.

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(a)

Freezing Enthalpy

Melting Enthalpy

Thermal Conductivity 0.620

97

0.540 93 91

0.460

89 0.380

87 85

0.300

Thermal conductivity (W/mK)

Enthalpy of latent heat (kJ/kg)

95

83 0.220

81 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Nanocomposite particle concentration (%) 0.9

105 Pure PCM TG HyNPCM TG Pure PCM DSC HyNPCM DSC

Crystallization Peak Shift

Td = 179°C

90 Td = 191°C

Td = Decomposition Temperature

TG (%)

75

0.8

0.6

60 0.5 45 0.3

DSC (mW/mg)

(b)

30 15

0.2

Decomposition Temperature Shift

0.0

0 30

60

90

120

150

180

210

240

270

300

330

Temperature (oC) Fig. 6. (a) Variation of latent heat enthalpy and thermal conductivity of the HyNPCM with different mass proportions of the HyNC, (b) TG-DTA curve of pure and the HyNPCM with 0.8% of HyNC.

36

Freezing cycle

Melting cycle

32 EC+0.8% Ag-TiO2 HyNC

o Temperature ( C)

28

EC Pure

24 Liquid PCM discharging sensible heating

Liquid PCM chargingsensible cooling

20

Near isothermal phase change process

16 12

Near isothermal phase change process

Solid PCM dischargingsensible heating

Solid PCM chargingsensible cooling

8 4

0

10

20

30

40

50

60

70

80

90

100

110

120

Time (min) Fig. 7. Freezing and melting curves of the pure PCM and the HyNPCM with 0.8% mass loading of the HyNC.

Beyond this value, the viscosity was found to be almost consistent with the increase in the shear rate up to 1975 s 1. On the other hand, the shear stress was increased from 0.11 Pa to 9.6 Pa for the

corresponding increase in the shear rate. Similar trends were observed in the viscosity and the shear stress of the HyNPCM being tested for the same shear rate and the temperatures ranging from

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Table 2 Heat storage and release characteristics of the pure PCM and the HyNPCMs. Process

Parameter

0 (EC pure)

0.1

0.5

0.8

1.0

1.5

Heat storage (freezing)

Commencement (min) Completion (min) Reduction (%)

19.7 36.7 –

18.7 34.5 5.08

17.3 32.9 12.18

16.7 31.5 15.23

16.0 29.6 18.78

15.0 28.0 23.86

Heat release (melting)

Commencement (min) Completion (min) Reduction (%)

76.3 93.3 –

75.0 90.5 1.70

73.7 88.4 3.41

72.3 85.3 5.24

71.7 83.5 6.03

69.8 81.4 8.52

(a)

HyNC mass loading in (wt.%)

0.025

10.5 30°C 35°C 40°C 45°C 50°C

Temperature @

0.020

9.0

Shear Stress

0.015

6.0

4.5

0.010

Shear stress (Pa)

Viscosity (Pa s)

7.5

3.0 Viscosity

0.005

1.5

0.000 0

200

400

600

800

1000

1200

1400

1600

1800

0.0 2000

Shear rate (s-1) Viscosity

Latent Heat - Melting

Latent Heat - Freezing

4.76

98

4.72

96

Average viscosity (mPa-s)

94 4.68 92 4.64 90 4.60 88 4.56 86 4.52

Latent heat capacity (kJ/kg)

(b)

84 82

4.48 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Nanocomposite particle concentration (%) Fig. 8. (a) Variation of viscosity and shear stress with shear rate of HyNPCM, (b) variation of average viscosity and latent heat capacity of the HyNPCM with the mass loading of the HyNC.

35 °C to 50 °C. It is noteworthy that, the decrease in the viscosity with respect to the increase in the shear rate was attributed to the shear thinning behavior of the HyNPCM [27,46–49]. The steadiness in the viscosity at shear rate greater than 80 s 1 was pronounced for the Newtonian behavior of the HyNPCM. It is obvious that, the shear thinning behavior was ascribed to the well dispersion of the spherical HyNC particles in the PCM [50]. In addition, the viscosity was decreased with the increment in the temperature, which has validated the intrinsic relationship existing between the viscosity and the temperature of the HyNPCM [51].

The variation of average viscosity and the latent heat capacity of the HyNPCM with the HyNC concentration are shown in Fig. 8(b). The test result reveals that, for the nanocomposite particle concentration ranging from 0.1% to 1.5%, the viscosity was increased from 0.35% to 3.89% respectively. This would be the case in real situations, wherein the condition of the suspension stratification can be expected to occur between the PCM and the nanoparticles. The term suspension stratification was coined and reported in this study, in order to anticipate the non-linear viscous diffusion effects of the HyNC (being separated out from the PCM) pertaining to the

R. Parameshwaran et al. / Applied Energy 115 (2014) 320–330

variations observed in the phase transition and enthalpy of latent heat during the freezing and melting cycles. Notwithstanding to the generic perception related to the reduced mass content of the PCM, the cascading effects of the higher concentration and the subsequent suspension stratification of the nanoparticles in the PCM, could also signify the possible reason for the reduction in the latent heat of the PCM. The graphs depicted in Fig. 6(a) reveal the cross variations between the thermal parameters as suggested in [52], but in actuality, the reason could be thought of due to the dominance of the viscous-dissipation effects of the HyNC being embedded at higher mass proportions [53,32]. That is, the thermal conductivity has increased steadily at lower concentrations, whereas the increase in it was less pronounced at high concentration as evident from Fig. 6(a). This could be ascribed to the 4% increased viscous effects of the HyNPCM. The increased viscosity in turn showed both positive aspect and penalty on the thermal conductivity and enthalpy of latent heat at higher concentration of the HyNPCM, respectively. Hence, the nanocomposite concentration of 0.8% was considered to be the peak limiting value in this study as shown in Fig. 8(b), for obtaining the improved thermal properties (including thermal conductivity) and good heat storage characteristics of the HyNPCM dedicated for the cool thermal energy storage applications in buildings. The change in the degree of supercooling beyond 0.8% as observed from the DSC results also supported the effect of viscosity on the thermal properties of HyNPCM. The experimental results emphasize the interrelationship of the thermal and the rheological properties of the HyNPCM with respect to the concentration of HyNC as suggested in [51] and as proposed in [32]. It is thus suggested that, there exists a trade-off between the viscosity and the thermal conductivity of PCMs embedded with nanoparticles, which necessitate for the further scope of evaluation on the parametric interdependencies. Though, the latent thermal energy storage systems incorporating PCMs doped with nanoparticles remains static in nature, the extent to which the incremental viscous effects are concerned would actually decide the heat transfer rate and the operational performance of such system on a long term basis. Interestingly, the concentrations of the HyNC considered in this study with reference to the past literatures were acceptable for achieving the improved thermal properties and heat storage potential of the HyNPCM. This eventually facilitated the HyNPCM to be considered as a potential candidate for the cool thermal energy storage applications in buildings.

4. Conclusions In the present work, the thermal properties and the heat storage characteristics of the new HyNPCM being experimentally investigated has helped in making the following conclusions:  The HyNPCM embedded with the surface functionalized HyNC exhibited improved thermal conductivity up to 52%, congruent phase change temperature (6.8 °C), high latent heat capacity (90.81 kJ/kg), substantial reduction in supercooling degree (1.82 °C), thermal stability (191 °C) and chemical stability, while compared to the pure PCM.  Experimental results on the heat storage performance of the HyNPCM emphasize that, the freezing and melting times of the HyNPCM were reduced by 23.9% and 8.5% respectively, while compared to the pure PCM. For the same mass loading of the HyNC considered, the reduction in the latent heat potential during freezing and melting respectively, was ascertained to the increased viscosity of the HyNPCM from 0.35% to 3.89%.

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 Altogether, the improved thermal properties and the enhanced heat storage characteristics of the HyNPCM have facilitated them to be considered as a potential candidate for the thermal storage cooling applications in buildings.

Acknowledgments The authors gratefully acknowledge DST, New Delhi for providing financial support to carry out this research work under PURSE scheme and UGC Major Research Project (F. No. 42-894/2013 (SR)). One of the authors, Mr. R. Parameshwaran is thankful to DST, New Delhi for the award of DST-PURSE fellowship.

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