Experimental charging and discharging performance of alumina enhanced pentaerythritol using a shell and tube TES system

Sustainable Cities and Society 51 (2019) 101767

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Experimental charging and discharging performance of alumina enhanced pentaerythritol using a shell and tube TES system

T

⁎

K.P. Venkitaraja,b, S. Suresha, , B. Praveena a b

Department of Mechanical Engineering, NIT Tiruchirappalli, Tiruchirappalli 620015, India Department of Mechanical Engineering, College of Engineering, Adoor 691551, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Pentaerythritol Solid–solid PCM Al2O3 nanoparticles Charging and discharging Energy efficiency Exergy efficiency

The work presented in this paper is the results of an experimental study conducted on the thermal energy storage (TES) performance of pentaerythritol (PE) added with alumina (Al2O3) nanoparticles. The charging and discharging experiments are performed using PE added with 0.5 and 1.0 wt.% of Al2O3 in a shell and tube type thermal energy storage system. The experimental data are analyzed to obtain the charging/discharging efficiencies and the overall efficiency of the TES system at different flow rates 2, 4, 6 LPM of the heat transfer fluid (HTF, Therminol-55). Exergy analysis based on the 2nd law of thermodynamics is also carried out in this experimental work. The results indicated that charging and discharging time is reduced due to the addition of Al2O3 with PE at all volume flow rates of HTF. The maximum charging and discharging powers of 289.3 W and 230.8 W respectively are observed in the case of PE + 1.0% Al2O3 corresponding to the flow rate of 6 LPM. The efficiencies of charging and discharging showed maximum values of 86.8% and 75.0% respectively at 6 LPM. The overall energy efficiency of the thermal energy storage system found increased from 38.3% obtained in the case of PE to 50.5% and 58.5% obtained for PE added with 0.5 wt.% and 1.0 wt.% of Al2O3 nanoparticles respectively.

1. Introduction The latent heat storage method using phase change materials (PCM) have been employed for various applications such as solar, medical, textile, electronic, energy efficient buildings, air conditioning, industrial and aerospace, etc. (Nkwetta & Haghighat, 2014). Extensive work is going on the application of PCM for thermal energy storage in heat exchangers (Anish, Mariappan, & Suresh, 2019; Seddegh, Joybari, Wang, & Haghighat, 2017) and energy efficient buildings (Arcuri, Spataru, & Barrett, 2017; Mourid, El Alami, & Kuznik, 2018). Solid–liquid PCM which are mostly used in the field of thermal energy storage and heat transfer applications suffer from serious limitations like expansion and leakage in their liquid phase. An easy way to avoid these limitations is to use a PCM that undergoes a solid–solid phase transition. Solid–solid PCM undergo a solid to solid phase transition which is associated with absorption and release of a large amount of heat. They change their crystalline structure from one lattice type to another at a definite temperature (Cao, Ye, & Yang, 2013) and this

transformation involves latent heats comparable to the most effective solid/liquid PCM. Several materials show the solid–solid reversible phase transition behavior, but only a handful has sufficient latent heat to be a possible latent heat storage material. These solid–solid PCM are categorized into two types, namely, organic solid–solid PCM and inorganic solid–solid PCM (Busico, Carfagna, Salerno, Vacatello, & Fittipaldi, 1980; Gu, Xi, Cheng, & Niu, 2010; Jiang, Ding, & Li, 2002; Landi & Vacatello, 1975; Li et al., 1999; Li & Ding, 2007; Pielichowska & Pielichowski, 2010; Qi & Liu, 2006; Ruan, Li, & Hu, 1995; Ruiyun, Dejun, Xian, & Jing, 1990; Xi, Gu, Cheng, & Wang, 2009). Organic solid–solid PCM mainly comprises a particular group of hydrocarbon compounds, polyhydric alcohols, and polymers. Polyalcohols or polyols are hydrocarbon compounds that have bodycentered tetrahedral molecular structure at low temperatures. These polyalcohols and their amine derivatives show a solid-state crystalline transition because of the presence of metastable, vibrant hydrogen bonds between molecules in their crystal lattice. When the temperature rises, these bonds break and the crystal structure changes from an

Abbreviations: PCM, phase change material; HTF, heat transfer fluid; PE, pentaerythritol; DSC, Differential Scanning Calorimetry; LPM, liter per minute; TES, thermal energy storage; SSPCM, solid–solid PCM; TEM, transmission electron microscope; SEM, scanning electron microscope; XRD, X-ray diffraction; EDX, Energy Dispersive X-ray; FESEM, field emission scanning electron microscope ⁎ Corresponding author at: Department of Mechanical Engineering, National Institute of Technology Tiruchirappalli, India. E-mail addresses: [email protected] (K.P. Venkitaraj), [email protected] (S. Suresh), [email protected] (B. Praveen). https://doi.org/10.1016/j.scs.2019.101767 Received 6 April 2019; Received in revised form 7 July 2019; Accepted 5 August 2019 Available online 12 August 2019 2210-6707/ © 2019 Elsevier Ltd. All rights reserved.

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Notations and symbols T cp,p mp Qsp Qst Qre Qsen Qtrs m˙ h m˙ c cp,h cp,c

Pc Pd tc td ηc ηd η Th,i Th,o T¯p,i T¯p,f Tc,o Tc,i

temperature (°C) specific heat of the PCM (kJ/kg K) mass of the PCM (kg) heat supplied (kJ) heat stored (kJ) heat rejected (kJ) sensible heat (kJ) solid–solid transition heat (kJ) mass flow rate hot fluid (kg/s) mass flow rate of cold fluid (kg/s) specific heat of hot fluid (kJ/kg K) specific heat of cold fluid (kJ/kg K)

average charging power (W) average discharging power (W) time for charging (min/s) time for discharging (min/s) charging efficiency (%) discharging efficiency (%) overall energy efficiency (%) inlet temperature of hot HTF (°C) outlet temperature of hot HTF mean initial temperature of PCM (°C) mean final temperature of PCM (°C) outlet temperature of cold HTF (°C) inlet temperature of cold HTF (°C)

tested composite PCM comprising carbonate-salt for medium and hightemperature energy storage applications. Li et al. (2017) investigated calcium chloride hexahydrate with aluminum oxide nanoparticles for thermal energy storage. Singh et al. (2017) experimentally studied the thermal energy storage performance of Myo-Inositol based nano PCM. They used Myo-Inositol added with 1%, 2% and 3% mass fractions of CuO and Al2O3 nanoparticles. Based on the results of Differential Scanning Calorimetry (DSC), Thermogravimetric analysis (TGA), and Fourier Infrared Transforms (FTIR) analysis, they commented that the nano-enhanced myo-inositol is a potential PCM for thermal energy storage. Though there are numerous investigations reported on solid–liquid PCM, the papers on thermal energy storage performance of SSPCM with heat transfer enhancement additives are decidedly less in number. Hu, Zhao, Jin, and Chen (2014) studied the solid-state phase transition of PE added with nano-aluminum nitride. In our recent work (Venkitaraj & Suresh, 2019) we studied the thermal and chemical stability of PE added with 0.1 wt.% of metal oxide nanoparticles. We found that the nanoparticles decreased the subcooling found during the cooling cycle. We have earlier investigated the energy storage characteristics and crystallization kinetics of PE added with indium (Venkitaraj & Suresh, 2018). The experimental heat transfer analysis of PCM involves conducting charging and discharging performances of the PCM for evaluating the energy efficiency of the thermal energy storage system. Numerous experimental and numerical studies on the thermal performance of latent heat storage (LHS) system have been reported in the literature. Among the various works reported, a sizeable number of research works discusses the various improvements in the thermal performances of the LHS system. This includes geometrical design (Khan, Khan, & Ghafoor, 2016; Khodadadi & Zhang, 2001; Medrano et al., 2009), using metal fins (Liu, Sun, & Ma, 2005), dispersion of thermally conductive particles (Liu, Su, Tang, & Fang, 2016), use of form stabled and encapsulated PCM (Giro-Paloma, Martinez, Cabeza, & Fernandez, 2016; Liu, Rao, Zhao, Huo, & Li, 2015). Researchers have been trying different geometric configurations of the LHS system for performing the charging and discharging processes. Khodadadi and Zhang (2001) numerically investigated the effect of convection on the melting process using a spherical container. They observed a higher rate of heat transfer at the upper side of the sphere than at the lower side. Medrano et al. (2009) conducted experimental heat transfer study of the PCM embedded in a graphite matrix using five different types of heat exchangers and they observed greater heat transfer in the case of pipe in pipe type heat exchanger. Liu et al. (2005) studied the thermal energy storage analysis of stearic acid using vertical annulus storage system. They investigated the dependence of Reynolds number on the heat transfer performance of stearic acid. As mentioned in the discussion above, the geometric configuration of the heat exchanger in TES is a very important matter. Among the

ordered tetrahedral phase to a disordered cubic phase accompanied with the absorption of a significant amount of energy, more significant the disorder, more substantial is the amount of energy absorbed. Now, when the temperature is lowered, at the transition temperature, the hydrogen bonds reform and the materials regain its original tetrahedral structure which causes the stored thermal energy to get released (Singh et al., 2015). Polyalcohols also called as “plastic crystals,” which display crystalline disorder at elevated temperatures accompanied by the absorption of thermal energy (Timmermans, 1961). Among polyalcohols, the most exciting materials are pentaerythritol (PE), pentaglycerine (PG), Neopentyl glycol (NPG) and Neopentyl alcohol (NPA). Certain amine derivatives of polyalcohols such as Tris (hydroxymethyl) aminomethane (TAM) and Aminoglycol (AMPL) were also very tested and proved by various researchers as potential solid–solid PCM for thermal energy storage applications (Benson, Burrows, & Webb, 1986; Murrill & Breed, 1970). Most organic and some inorganic PCM have an inherently low thermal conductivity which leads to poor charging and discharging performances. Improving the thermal conductivity of PCM by dispersing thermally conductive materials is more widely used for enhancing the effectiveness of the PCM-based TES systems. The majority of PCM, in general, has a very poor thermal conductivity which results in lower charging and discharging rates. The literature reported the use of thermally conductive particles like nanoparticles as additives to enhance the thermal energy storage performance of organic and inorganic PCM. The research findings also indicated that the enhancement of thermal conductivity accompanied by a slight decrease in the phase change properties of PCM (Adorno & Silva, 2006). Siegel (1977) investigated the use of high conductivity particles to enhance the rate of solidification molten salt. He conducted an analysis to determine how the solidification rate is influenced by the introduction of high conductivity particles into a solidifying low conductivity material. He concluded that for reasonable concentrations such as 20% particles by volume, the heat removal rate for a plane geometry can be increased by 10–20% depending on the ratio of particle to matrix conductivity. Cabeza, Mehling, Hiebler, and Ziegler (2002) and Py, Olives, and Mauran (2001) studied PCM/graphite enclosed in waterlogged metal modules. They observed a heat barrier between the metal modules and water. Yin, Gao, Ding, and Zhang (2008) experimented paraffin/expanded graphite composite and reported that the thermal conductivity enhanced about 17 fold compared to the thermal conductivity of pure paraffin. Kim and Drzal (2009) reported that the presence of conductive graphite affects the phase change properties of PCM. Elgafy and Lafdi (2005) studied the performance enhancement of paraffin/carbon nanofibers (CNF) with the mass fraction of CNF varied between 1% and 4%. Teng and Yu (2012) prepared composite PCM comprising paraffin and 1.0, 2.0, and 3.0 wt.% of Al2O3, TiO2, and SiO2 nanoparticles. Their study revealed that TiO2 enhances the thermal performance of paraffin more efficiently compared to other nanoparticles. Ge et al. (2014) 2

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a very little effect on the thermal performance due to the conduction dominant heat transfer during the charging and discharging process. The result of varying the number and orientation of HTF tubes in using a horizontal shell and tube heat exchanger on thermal performance was numerically studied by Luo, Yao, Yi, and Tan (2015). They observed better thermal performance in the case of multi-tube arranged symmetrically with respect to the center. The results of the experimental investigations using shell and tube heat exchangers can be further improved by the use of extended surfaces (fins) or by enhancing the thermal conductivity of PCM by adding thermally conductive particles. Darzi, Jourabian, and Farhadi (2016) performed a numerical study of thermal performance of n-eicosane using a longitudinally finned horizontal shell and tube heat exchanger. They reported an increase in the solidification when fins were employed. The results of the experimental and numerical study conducted by Meng and Zhang (2017) for testing the thermal performance of paraffin-copper foam composite using a rectangular shaped shell and tube heat exchanger was used. They indicated the dependence of solidification time on the temperature gradient between the HTF and PCM. The experimental paper published by Pandiyarajan, Pandian, Malan, Velraj, and Seenira (2011) discussed the thermal energy storage performance of a finned shell and tube heat exchanger for IC engine

different types of heat exchangers, shell and tube type is very much used by various researchers due to its simple installation requirements and better heat transfer performance (Agyenim, Hewitt, Eames, & Smyth, 2010). The numerical work reported by Wang, Zhang, Wang, and He (2013) involved a shell and tube heat exchanger to study the thermal energy storage performance of n-octadecane. Their results revealed that the charging time reduces significantly with the increase in the inlet temperature of the HTF. They also reported that the charging time reduces with the increase in the mass flow rate of the fluid. The experimental study conducted by Ezan, Ozdogan, and Erek (2011) discussed the thermal storage performance of a shell and tube heat exchanger using water as the HTF. They reported the effects varying the inlet temperature, flow rate, shell diameter and shell material on the thermal performance of the system. They observed that the charging and discharging processes are dominated by free convection. The effect varying the inlet temperature on the thermal performance is more compared to the effect of varying the flow rate of the HTF. The numerical study conducted by Seddegh, Wang, and Henderson (2016) reported the thermal storage performance of paraffin using a shell and tube system. They tested the storage performance of the LHS system corresponding to the horizontal and vertical orientation of the shell and tube heat exchanger and concluded that the geometrical orientation has

Fig. 1. (a) Molecular structure of PE molecule, (b) XRD pattern of PE, (c) SEM image of PE, and (d) photograph of PE. 3

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using pentaerythritol as the PCM. The objective of the work was to determine the influence of the PCM to fins volume ratio on the performance of the LHS unit for cooking. They concluded that the charging and discharging performances decrease with PE to fin volume ratio. In one of our recent work (Venkitaraj, Suresh, & Venugopal, 2018), we experimentally investigated the thermal performance of pentaerythritol added with Al2O3 nanoparticles for IC engine exhaust heat recovery applications. The experimental results showed that the addition of 0.1% and 0.5% weight fractions of Al2O3 results in 36.36% and 73.39% increase in the amount of heat recovered compared to pure PE. The present work reports the energy and exergy analysis of charging and discharging processes using PE added with 0.5 and 1.0 wt.% of Al2O3 using Therminol-55 as the heat transfer fluid for charging and discharging of PE. A shell and tube heat exchanger consisting of three horizontal circular tubes was used as the TES for conducting the energy storage and discharge experiments. As far as the knowledge of the authors concerned, there is no research work that performed the energy and exergy analysis of a solid–solid PCM based TES using therminol as the HTF is reported in the literature.

exhaust heat recovery. Their study indicated about 10–15% of waste heat recovery using the finned shell and tube TES system. They suggested a cascade LHS system using multiple PCM for increasing the % heat recovery. Khan and Khan (2017) investigated the charging and discharging of paraffin using a longitudinally finned shell and tube heat exchanger for domestic and industrial applications. They investigated the effect of inlet temperature and volume flow rate of HTF on phase transition rate and mean power. by regulating the inlet temperature or volume flow rate of HTF, the influence of overall thermal resistance was minimized. Mean discharge power is enhanced by 36.05% as the inlet temperature is reduced from 15 °C to 5 °C. Likewise, the mean discharge power is improved by 49.75% as the volume flow rate was increased from 1.5 LPM to 3 LPM. They also observed that with an increase in volume flow rate, the discharge time of the equal amount of thermal energy 12.09 MJ was reduced by 24%. It as, therefore, concluded that by adjusting operating conditions, the required demand for output temperature and mean discharge power can be attained. Agarwal and Sarviya (2016) experimentally evaluated the thermal and heat transfer characteristics of paraffin using a shell and tube type LHS system during charging and discharging processes. They used air as heat transfer fluid (HTF) and determined the effectiveness of the LHS for drying of food products. Majority of energy storage applications involve the use of solid–liquid phase change materials for thermal energy storage. However, the solid–liquid PCM have disadvantages of volume expansion and leakage issues in their liquid phase. These limitations do not exist in the case of solid–solid PCM since these type of PCM exhibit structural changes involving enthalpy change in their solid state. Polyalcohols are a class of organic solid–solid PCM having very high enthalpy change of transition. But, being organic materials, the thermal conductivity of polyalcohols is low causing poor charging and discharging performances. Therefore, to improve the heat transfer in organic PCM, thermally conductive particles are employed. Nanoparticles are one type of heat transfer enhancement additives that are widely used by various researchers. Most of the experimental investigations reported in the literature discussed the charging and discharging performances of solid–liquid PCM. Again, to improve the thermal performance of the system, the researchers made use of finned type shell and tube based TES. The papers published on the charging and discharging performances of solid–solid PCM are limited. Some of the papers available in literature reports study on the characterization, charging and discharging performances of pure solid–solid PCM. But, there is very little research work that studied the effect of heat transfer enhancement additives on charging and discharging performances of polyalcohols is reported in the literature. The work presented in this thesis is aimed at filling this gap found in the literature. NKhonjera et al. (2016) experimentally studied the charging and discharging performance of rectangular type TES provided with fins

2. Material preparation Pentaerythritol [PE] is a polyalcohol having the chemical formula C5H12O4. PE is an organic solid–solid PCM that exhibits a structural transition from tetragonal to cubic at a fixed temperature in the range 187–189 °C. At ordinary temperature, PE has the body-centered tetragonal (BCT) molecular structure consisting of four C–CH2–OH fragments (Fig. 1(a)). In a PE molecule, the middle carbon atom is tetragonally surrounded by four other carbon atoms. The PE molecules are arranged flat in a sheet parallel to the crystallographic c-plane. The transition to the face-centered cubic (FCC) crystalline structure is accompanied by the absorption of the energy required for the breaking of hydrogen bonds. During the phase transition, the hydrogen bonds break resulting in a change in the crystal structure. The separated molecules are held in their lattice sites by dispersion forces. During the discharging period, the hydrogen bonds reform by rejecting the stored energy. Fig. 1(b) shows the X-ray diffraction (XRD) pattern of PE obtained using Bruker AXS D8 Advance X-ray diffractometer having Cu-kα1 radiation in the range of 0–90°. The reflections in the XRD pattern obtained were recognized as matching to the tetragonal phase of PE using JCPDS (Joint Committee on Powder Diffraction Standards). The Scanning Electron Microscope (SEM) analysis was performed using Jeol JSM 6390LV SEM to study the microstructure of PE. The obtained SEM image showed that PE has a loose microstructure with a large number of individual lamellae on the surface (Fig. 1(c)). Pentaerythritol (analytical reagent grade, purity 98.0%) in white crystalline powder form was supplied by Alfa Aesar, USA (Fig. 1(d)). The alumina nanoparticles supplied by Alfa Aesar, USA were used as

Fig. 2. (a) XRD pattern and (b) SEM image of Al2O3. 4

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the heterogeneous mixing of pentaerythritol and alumina was the fact that the dispersion of Al2O3 nanoparticles was purely physical using a ball-mill. The region circled in red color indicates the particle aggregation and the area shown by blue arrow represent the region that is deprived of nanoparticles. Fig. 4(b) and (c) shows the EDX spectra of the PE added with Al2O3 nanoparticles before and after thermal cycling. The elemental mapping is shown in Fig. 4(b) also indicated the heterogeneous mixing of Al2O3 nanoparticles with PE. The EDX spectrum of the thermally cycled sample shown in Fig. 4(c) confirms that the dispersion of alumina nanoparticles in PE was not much disturbed even after several thermal cycles. Fig. 5 shows the Fourier Transform Infrared (FTIR) spectrum of the PE + 0.5 wt.% Al2O3 before and after thermal cycling. Since the spectra of the uncycled and cycled samples are identical, it can be confirmed that there involved no chemical reaction between the PCM and the nanoparticles.

the additive to pentaerythritol in this experimental study. The specifications of the nanoparticles are as follows: alumina (Al2O3), NanoDur 99.5%, APS: 40–50 nm, SA: 32–40 m2/g, MW: 101.96. The XRD pattern of Al2O3 nanoparticles confirmed its crystalline nature. The diffraction peaks in the XRD pattern of Al2O3 nanoparticles (Fig. 2(a)) were identified as those present in the case of hexagonal α-Al2O3. Fig. 2(b) exhibits the microstructure of alumina obtained using Jeol JSM 6360 Scanning Electron Microscope. The dimension, profile, growth, and distribution of the Al2O3 nanoparticles were studied by transmission electron microscopic (TEM) images. The images were obtained using Jeol/JEM 2100 having point and lattice resolution of 0.23 nm, and 0.14 nm respectively, and magnification ranging from 2000× to 1,500,000×. The TEM photographs of the Al2O3 nanoparticles are shown in Fig. 3. The obtained images indicated the spherically shaped alumina nanoparticles have particle sizes ranging from 14 to 58 nm. The alumina nanoparticles were added to pentaerythritol in 0.5% and 1% weight fractions. The dispersion of nanoparticles in PCM was ensured with the help of the low energy ball mill run at 200 rpm for 1.5 h. The dispersion stability of nanoparticles and particle aggregation in the prepared PCM samples were analyzed by a field emission scanning electron microscope (FESEM) system to obtain the topographical and elemental information of the PE/Al2O3 composite PCM. Elemental identification with mapping to study the dispersion of nanoparticles in PE was carried out by Energy Dispersive X-ray (EDX) mapping. Fig. 4(a) shows the FESEM image of PE added with Al2O3 nanoparticles. The image shows nanoparticle agglomeration at some places with some areas without any presence of nanoparticles. The reason for

3. Thermal conductivity measurement The thermal conductivity of the PCM samples was determined by Thistory method (Venkitaraj & Suresh, 2018). The T-history analysis is based on lumped capacitance theory in which a uniform temperature profile is maintained. In order to ensure that the lumped heat capacity theory is applicable, the test conditions used in T-history analysis should have of Biot number (Bi) value less than unity. In T-history method, the variation of temperature of the test samples and the reference sample with time during the cooling process is recorded. The thermal conductivity values of the test samples are then determined by

Fig. 3. TEM images of Al2O3 nanoparticles. 5

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Fig. 4. (a) FESEM image, (b) EDX spectrum of PE with Al2O3 before thermal cycling, and (c) EDX spectrum of PE with Al2O3 after thermal cycling.

instrument had a temperature range from −130 °C to max. 450 °C and a measurement range ± 350 mW at RT with a measurement resolution of 0.04 mW at room temperature, temperature accuracy of ± 0.2 °C and temperature reproducibility of ± 0.1 °C. The PCM samples were subjected to heating and cooling between the temperature range 30–280 °C at 10 °C/min.

5. Charging and discharging performance 5.1. Experimental setup The performance of pentaerythritol added with Al2O3 nanoparticles in energy storage application was tested by conducting charging and discharging experiments using a thermal energy storage system. A schematic diagram of the experimental setup is shown in Fig. 6. The experimental setup comprises a shell and tube type heat exchanger filled with the PCM. The shell side of the heat exchanger filled the PCM. The fluid flowing through the copper tubes exchanges heat with the PCM during the charging and discharging process. The hot thermal fluid from the hot fluid container is pumped through the heat exchanger using a gear pump. The flow rate of the hot thermal fluid is controlled using a bye pass valve provided in the delivery side of the gear pump. A rotameter is provided to monitor the flow rate of hot therminol flowing through the heat exchanger. The hot fluid after flowing through the copper tubes flows back to the hot reservoir. During the flow through the heat exchanger, the hot fluid loses its heat to the PCM surrounding the copper tubes. This is the charging cycle of the experiment. The cold line of the experimental setup consists of a cold fluid container centrifugal pump; rotameter, and an air-cooled heat exchanger. During the discharge cycle, the cold thermal fluid is pumped

Fig. 5. FTIR spectrum of PE added with Al2O3 nanoparticles.

comparing their cooling curves with the cooling curves of the reference samples. The detailed experimental requirements and the procedure of T-history method are given in our previously published article (Venkitaraj & Suresh, 2018). 4. Determination of enthalpy of transition Thermal property analysis in terms of latent heat capacity and phase transition temperature for pure and nano-enhanced PE were determined using DSC (Mettler Toledo DSC 822e, Hong Kong). The 6

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Fig. 6. Schematic diagram of the experimental setup for heat transfer study.

through the heat exchanger using the centrifugal pump. The flow rate of the cold fluid is monitored and controlled by using a rotameter and a valve provided in the delivery side of the centrifugal pump. The cold fluid flowing through the copper tube receives heat stored by the PCM. The cold fluid leaving from the other end of the heat exchanger flows back to the cold reservoir via an air-cooled heat exchanger. During the flow through the radiator, it loses heat. The temperature of the PCM inside the shell at six different locations is also monitored using the Ktype thermocouples. The temperature of the PCM inside the shell at the leading end, middle and, trailing ends are also monitored using the Ktype thermocouples. A multi-channel data logger records the PCM and the fluid temperatures during the charging and discharging process. The photograph of the actual experimental setup fabricated included as Fig. 7. Fig. 8 shows the photographs of the shell and tube heat exchanger before and after insulating. Fig. 9 shows the constructional features of the shell and tube heat

exchanger. The heat exchanger comprises three tubes of 20 mm inner diameter, 2 mm thickness, and 45 cm effective length enclosed within a shell of 65 mm inner diameter, and 5 mm thickness. The tubes are made of copper and the shell is made of mild steel. The shell insulated using glass wool and asbestos in order to minimize the heat loss to the surroundings. The quantity of PCM required for filling in space inside the shell estimated to be 1.22 kg. The heat transfer fluid used in the hot and cold circuits is Therminol-55. Some important properties of Therminol55 are shown in Table 1. The temperature of therminol oil maintained at 200 °C by using two hand immersion heaters of 1500 W each. The gear pump (Make: HMP pumps, power: 0.5 HP, size: ½″ × ½″, rpm: 1440, maximum discharge: 30 LPM) was used to pump the hot therminol oil through the heat exchanger. The cold fluid pumped by using a monoblock centrifugal pump (Make: Lakshmi pumps, power: 0.5 HP, size: 1″ × 1″, rpm: 1400, max. discharge: 45 LPM). The flow rate of hot therminol oil is measured using a metal tube magnetic rotameter

Fig. 7. The photograph of the actual experimental setup. 7

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Fig. 8. Shell and tube heat exchanger.

(Make: Eureka, model: SSVS-MTS-4, range: 1.1–11 LPM). The flow rate in the cold side of the experimental setup measured by using an acrylic body rotameter (Make: flow point, range: 1–10 LPM). Two metal containers, each 20-l capacity, were used as hot and cold fluid reservoirs. The entire hot circuit, part of the cold circuit between the heat exchanger exit and air-cooled heat exchanger inlet are well insulated to minimize the heat loss to the surroundings.

Table 1 Properties of Therminol-55. Property

Value

Appearance Composition Average molecular weight Density

Clear, yellow liquid Synthetic hydrocarbon 320 864 kg/m3 @33 °C 815 kg/m3 @220 °C 19 mm2/s @ 40 °C 0.876 1.95 kJ/kg K @33 °C 2.21 kJ/kg K @220 °C 351 °C 315 °C

Kinematic viscosity Specific gravity Specific heat

5.2. Procedure The experiments performed using pure PE and PE added with 0.5 and 1.0 wt.% of Al2O3. The detailed experimental procedure is given below. The heat exchanger filled with pure PE on the shell side mounted in the experimental setup. The heaters in the hot fluid reservoir are switched on to heat the therminol oil above 200 °C. The hot therminol oil pumped through the heat exchanger by switching on the gear pump. The flow rate of the oil set at 2 LPM by regulating the bye pass valve. The temperature of the hot therminol entering and leaving the heat exchanger and the PCM temperatures at different locations recorded. The gear pump switched off when the PCM temperature recorded crosses the solid–solid transition temperature of the PCM. This completes the charging process. The cold therminol oil pumped through

Normal boiling point Maximum use temperature

the heat exchanger by starting the centrifugal pump. The flow rate of the cold therminol adjusted to 2 LPM by operating the valve. The temperature of the cold therminol entering and leaving the heat exchanger and the PCM temperatures at different locations recorded. The centrifugal pump switched off when the PCM temperature restored to its ambient temperature. This completes the discharging process. The experiment repeated for hot and cold therminol flow rates

Fig. 9. Constructional details of the heat exchanger. 8

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part of the exergy is stored in the PCM. The rate of exergy supplied by HTF during the charging period is computed using the following equation.

of 4 LPM and 6 LPM. The experiment is repeated for PE/LMA composite PCM of 0.5 and 1.0 wt.% Al2O3. 5.3. Data reduction

T ˙ input = m˙ h cp,h ⎡ (Th,i − Th,o) − To ln ⎛⎜ h,i ⎞⎟ ⎤ Ex ⎢ ⎥ ⎝ Th,o ⎠ ⎦ ⎣

5.3.1. Energy analysis The heat supplied to the PCM is given by,

Qsp = m˙ h c p,h (Th,i − Th,o) tc, kJ

where To is the ambient temperature. The rate of exergy stored in the PCM is obtained as

(1)

where m˙ h is the mass flow rate (kg/s) of hot HTF, cp,h is the specific heat of hot HTF, Th,i and Th,oare the inlet and the exit temperature of the hot HTF, and tc is the charging time. The heat stored in the TES is calculated as Heat stored = sensible heat + solid–solid transition heat.

Qst = Qsen + Qtrs = m p c p,p (T¯p,f − T¯p,i ) + m p htrs , kJ

˙ stored = m p htrs ⎡1 − To ⎤ + m p c p,p ⎡ (Ttrs − T¯p,i ) − To ln ⎛⎜ Ttrs ⎞⎟ ⎤ Ex ⎢ ¯ ⎥ ⎢ Ttrs ⎥ ⎦ ⎣ ⎝ Tp,i ⎠ ⎦ ⎣ ¯ ⎛ Tp,f ⎞ ⎤ ¯ + m p c p,p ⎡ ⎢ (Tp,f − Ttrs) − To ln ⎜ Ttrs ⎟ ⎥ ⎝ ⎠⎦ ⎣

(2)

where Tp,i and Tp,f are the initial and final PCM temperatures and Ttrs is the PCM transition temperature. The rate of exergy extracted from the PCM by the HTF during the discharging period is computed using the equation,

mp – the mass of the PCM (pentaerythritol), T¯p,f – mean final PCM temperature, T¯p,i – mean initial temperature of the PCM, htrs – enthalpy of transition, cp,p is the specific heat of PCM. The average charging power (Pc) is obtained as the ratio of the heat stored (Qst) to the time for charging (tc).

i.e. Pc =

Qst tc

T ˙ output = m˙ c cp,c ⎡ (Tc,o − Tc,i ) − To ln ⎛⎜ c,o ⎞⎟ ⎤ Ex ⎢ ⎥ ⎝ Tc,i ⎠ ⎦ ⎣

(3)

The charging exergy efficiency (ψchar) is computed as the ratio of rate of exergy stored by the PCM to the exergy supplied by the HTF during the charging period. Similarly, discharging exergy efficiency (ψdischar) is estimated as the ratio of the rate of exergy extracted from the PCM to the exergy stored by the PCM. The overall exergy efficiency (ψ) of the thermal energy storage process is computed from the exergy efficiencies of charging and discharging processes as ψ = ψchar × ψdischar.

Now, charging efficiency is found as the ratio of the heat stored (Qst) by the PCM to the heat supplied (Qsp) by the HTF during the charging cycle. i.e.,

Charging efficiency (%),

ηc =

Qst × 100 Qsp

(4)

Heat rejected to the cold HTF,

Qre = m˙ c c p,c (Tc,o − Tc,i) td, kJ

(5)

6. Results and discussion

where m˙ c – mass flow rate (kg/s) of cold HTF, cp,c – specific heat of cold HTF, Tc,i and Tc,o – the inlet and the outlet temperature of the cold HTF, and td – time taken for the discharge. The average discharging power (Pd) is calculated as the ratio of the heat rejected (Qre) to the cold HTF to the discharging time (td).

i.e.,

Pd =

Qre td

6.1. Thermal properties of nano-enhanced PE The thermal conductivity of the PE with and without the addition of Al2O3 nanoparticles calculated using the temperature data obtained from the T-history plots. In T-history method, the thermal properties of the PCM samples were estimated by comparing their temperature plot with that of the glycerin which was taken as the reference medium. The obtained thermal conductivity value of pure PE was 0.106 W/m K. The values obtained for the PE samples added with Al2O3 nanoparticles showed very significant changes in the thermal conductivity values. The T-history results showed that the thermal conductivity of PE enhanced by 33.0% and 51.9% respectively corresponding to 0.5 and 1.0 wt.% of Al2O3 nanoparticles. The improved thermal conductivities of PE + 0.5 wt.% Al2O3 and PE + 1.0 wt.% Al2O3 obtained as 0.141 W/ m K and 0.161 W/mK respectively. The variation of thermal conductivity with temperature is also investigated using the T-history data recorded in the application range 40–205 °C of pentaerythritol. The whole range is divided into three smaller temperature ranges: (i) from 40 to 95 °C with 77.5 °C as the mean temperature, (ii) from 95 to 150 °C with 122.5 °C as the mean temperature, and (iii) from 150 to 205 °C with 177.5 °C as the mean temperature, the range in which the solid–solid transition occurs in PE (Fig. 10). Table 2 shows the thermal conductivity measurements obtained for pure PE and PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles in the above temperature ranges. It can be understood from the tabulated results that the thermal conductivity of all the tested samples decreases with temperature. Table 3 displays the thermal conductivity values of the PCM samples before and after thermal cycling. It can be seen from the table that the thermal conductivity of the PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles were significantly greater than that of pure PE even after 1000 thermal cycles. This meant that Al2O3 nanoparticles did not

(6)

The discharging efficiency determined as the ratio of the amount of heat rejected to the cold HTF (Qre) to the heat stored (Qst) by the PCM during the discharging cycle, i.e.,

Discharging efficiency (%),

ηd =

Qre × 100 Qst

(7)

The overall energy efficiency of the thermal energy storage (TES) system used in this experimental study calculated by combining the efficiencies computed separately for the charging and discharging processes (Jagadheeswaran, Pohekar, & Kousksou, 2010; Rezaei et al., 2013).

The overall energy efficiency of the TES system,

η = ηc × ηd

(8)

5.3.2. Exergy analysis The energy analysis explained in the previous section is based on the first law of thermodynamics. Even though it provides an estimation of the performance of the thermal energy storage system, it does not reflect the quality of energy stored/recovered. This shortfall can be overcome by performing the exergy analysis based on the second law of thermodynamics. Exergy is the quality or usefulness of energy. It is the maximum amount of work that can be generated by a system as it comes to equilibrium with surrounding. During charging mode, the HTF transfers the exergy to PCM and 9

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6.3. Charging performance analysis 6.3.1. Charging time The charging performance experiments were conducted to study the effect of heat transfer enhancement additive (Al2O3 nanoparticles) on the thermal energy storage performance of PE. For this, experiments were conducted using pure PE and PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles. During the charging process, the HTF (therminol oil) heated to about 225 °C and circulated through the heat exchanger. The PCM temperature variation from the inlet to the exit of the heat exchanger in the axial direction is recorded for evaluating the charging performance of the PCM. Fig. 12 shows the temperature variation in the case of pure PE for HTF flow rates of 2, 4, and 6 LPM. The variation of the PCM temperature at six locations in the heat exchanger is plotted in this figure. During the charging process, the PCM in the shell side of the heat exchanger absorbs the heat of the hot HTF. The charging process continued until all the temperatures recorded by the thermocouples exceeded the solid–solid transition temperature of pure PE. The charging process ended in 152 min when the HTF flow rate of 2 LPM maintained through the heat exchanger. The charging period found decreased to 103 min and 68 min when the therminol flow rate changed to 4 LPM and 6 LPM respectively. The increase in charging time observed was because of the increased energy supplied at a higher flow rate. The charging experiments repeated for PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles. Fig. 13 shows the temperature distribution in the PCM during the charging process of PE added with 0.5 wt.% of Al2O3 nanoparticles. The charging of PE + 0.5 wt.% Al2O3 ended in 127 min when the HTF flow rate of 2 LPM maintained through the heat exchanger. The charging period decreased to 89 min and 56 min when the therminol flow rates changed to 4 LPM and 6 LPM respectively. Fig. 14 shows the charging process in the case of PE + 1.0 wt.% Al2O3 at different flow rates of the HTF. The charging of PE + 1.0 wt.% Al2O3 ended in 112 min when the HTF flow rate of 2 LPM maintained through the heat exchanger. The charging period decreased to 78 min and 50 min when the flow rates changed to 4 LPM and 6 LPM respectively. Fig. 15 shows the % reduction in the charging time of PE due to the addition of Al2O3 nanoparticles at different flow rates of the HTF. It can be understood from the graph that the charging time of PE decreases due to the addition of 0.5 and 1.0 wt.% of Al2O3 nanoparticles at all volume flow rates of the heat transfer fluid.

Fig. 10. Variation thermal conductivity of PE + Al2O3 with temperature. Table 2 Thermal conductivity variation with temperature. Temperature range (°C)

40–95 95–150 150–205

Average Temp (°C)

67.5 122.5 177.5

Thermal conductivity (W/m K) PE

PE + 0.5% Al2O3

PE + 1.0% Al2O3

0.112 0.108 0.104

0.148 0.139 0.132

0.165 0.157 0.149

Table 3 The thermal conductivity of PE with Al2O3 nanoparticles before and after thermal cycling. PCM samples

Thermal conductivity (k) (W/m K) Before thermal cycling

After 1000 thermal cycles

PE

0.106

PE + 0.5 wt.% Al2O3

0.141 (+33.0%) 0.161 (+51.9%)

0.096 (−9.4%) 0.128 (+20.8%) 0.140 (+32.1%)

PE + 1.0 wt.% Al2O3

undergo any deterioration in thermal property and caused very significant enhancement thermal conductivity of PE even after several thermal cycles.

6.2. Enthalpy of transition The enthalpy of solid–solid transition is an important thermal characteristic parameter of pentaerythritol. Differential Scanning Calorimetry (DSC) analysis of the PCM samples was performed using Mettler Toledo DSC 822e, Hong Kong. The heat capacity and enthalpy of the solid–solid transition of the PCM samples obtained using the DSC data. Fig. 11 illustrates the DSC plot obtained for pure PE sample. The two peaks seen in the DSC curve represent the phase change process in PE. The first peak corresponds to the solid to solid phase change and the second peak corresponds to the solid–liquid phase change. Table 4 summarizes the temperature and enthalpy of the solid–solid transition of pure PE and PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles during the charging and discharging processes. Fig. 11. DSC curve of pure PE. 10

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Table 4 Enthalpy of the transition of the PCM samples. Samples

PE PE + 0.5% Al2O3 PE + 1.0% Al2O3

Heating Solid–solid transition

Cooling Solid–solid transition

Onset (°C)

Peak (°C)

End set (°C)

Enthalpy change (kJ/kg)

Onset (°C)

Peak (°C)

End set (°C)

Enthalpy change (kJ/kg)

181.3 180.8 180.9

187.8 188.4 188.4

193.6 190.0 200.6

263.9 255.0 251.7

165.6 169.5 169.8

163.5 168.9 169.6

163.0 162.9 162.6

238.9 229.5 226.5

Fig. 12. Charging process of pure PE at different flow rates of HTF.

This accounts for a charging efficiency of 68.9%. When the flow rate further increased to 6 LPM, the heat supplied and heat stored estimated as 907.8 kJ and 1256.8 kJ showing a charging efficiency of 72.2%. The charging efficiency estimated for PE + 0.5 wt.% Al2O3 and PE + 1.0 wt. % Al2O3 is also shown in Fig. 17. The results of PE + 0.5 wt.% Al2O3 indicated charging efficiencies of 72.4%, 76.7% and 80.7% at HTF flow rates of 2, 4 and 6 LPM respectively. The heat supplied and heat stored at these flow rates obtained as 1235.2 kJ, 1154.2 kJ, 1089.3 kJ, and 894.4 kJ, 884.7 kJ, 878.7 kJ respectively. The results of PE + 1.0 wt.% Al2O3 indicated charging efficiencies of 80.3%, 83.0% and 86.8% at HTF flow rates of 2, 4 and 6 LPM respectively. The heat supplied and heat stored at these flow rates obtained as 1093.4 kJ, 1050.2 kJ, 1089.3 kJ, and 877.5 kJ, 872.1 kJ, 868.0 kJ respectively.

6.3.2. Charging power and charging efficiency The average energy stored in unit time is estimated as the charging power. Fig. 16 shows the average charging power estimated in the case of pure PE and PE added with 0.5 wt.% and 1.0 wt.% of Al2O3 nanoparticles. It can be understood that the charging power of PE increases with the addition of Al2O3 nanoparticles for all the heating rates considered in this study. The average energy storage power of pure PE corresponding to 2 LPM, 4 LPM, and 6 LPM flow rates estimated as 100.5 W, 149.0 W, and 222.5 W respectively. The average energy storage power corresponding to 2 LPM, 4 LPM, and 6 LPM flow rates increased to 117.45 W, 165.7 W, and 261.5 W respectively when 0.5 wt.% of Al2O3 nanoparticles were added to PE. When the weight fraction of Al2O3 nanoparticles increased to 1.0%, the average energy storage power corresponding to 2 LPM, 4 LPM, and 6 LPM flow rates estimated as 130.5 W, 186.3 W and 289.3 W respectively. Fig. 17 shows the charging efficiency obtained in the case of pure PE and PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles at different HTF flow rates. In the case of pure PE, an amount of 916.4 kJ energy stored out of the 1418.2 kJ heat supplied at an HTF flow rate of 2 LPM, indicating a charging efficiency of 64.6%. When the flow rate changed to 4 LPM, 911.6 kJ of heat was stored out of 1322.8 kJ of heat supplied.

6.4. Discharging performance analysis 6.4.1. Discharging time To study the discharge performance of the PCM, the PCM is first charged to a temperature above the solid–solid transition point and then allowed to cool back to the ambient conditions. During the discharging process, the HTF at room temperature circulated through the heat exchanger. The PCM rejects the heat stored to the circulating HTF. 11

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Fig. 13. Charging process of PE + 0.5 wt.% Al2O3 at different flow rates of HTF.

Fig. 14. Charging process of PE + 1.0 wt.% Al2O3 at different flow rates of HTF. 12

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Fig. 18 shows the temperature variation in the case of pure PE for HTF flow rates of 2, 4, and 6 LPM. The variation of the PCM temperature at six locations in the heat exchanger is plotted in this figure. During the discharging, the PCM in the shell side of the heat exchanger rejects the heat to the HTF. The discharging process continued until all the temperatures recorded by the thermocouples recorded temperature equal to the ambient condition. The charging process ended in 106 min when a flow rate of 2 LPM maintained through the heat exchanger. The charging period found decreased to 86 min and 65 min when the HTF flow rate changed to 4 LPM and 6 LPM respectively. The decrease in charging time observed was because of the increased heat absorption at the higher flow rates. The discharging performances of PE added with 0.5 and 1.0 wt.% of heat transfer enhancement additives Al2O3 nanoparticles, LMA and LMM are also analyzed in this section. Fig. 19 shows the temperature distribution in the PCM during the discharging process of PE added with 0.5 wt.% of Al2O3 nanoparticles. The results showed that the discharging time corresponding to the HTF flow rate of 2 LPM decreased from 106 min to 90 min. This indicated that the discharging time reduced by 15% due to the increased heat transfer rated resulted from the addition of 0.5 wt.% of Al2O3 nanoparticles. The discharging time found decreased from 86 min to 72 min when the flow rate changed to 4 LPM indicating a 16.3% decrease. The discharge time further decreased to 54 min indicating a 16.9% decrease when the flow rate adjusted to 6 LPM. Fig. 20 shows the temperature distribution in the PCM during the discharging process of PE added with 1.0 wt.% of Al2O3 nanoparticles. The results showed that the discharging time corresponding to the HTF rate of 2 LPM decreased from 106 min to 78 min. This indicated that the discharging time reduced by 26.4% due to the increased heat transfer rated resulted from the addition of 1.0 wt.% of Al2O3 nanoparticles. The discharging time found decreased from 86 min to 65 min when the flow rate changed to 4 LPM indicating a 24.4% decrease. The discharge time further decreased to 57 min indicating a 26.2% decrease when the flow rate adjusted to 6 LPM. Fig. 21 summarizes the % reduction in the discharging time of PE due to the addition of 0.5 and 1.0 wt.% of Al2O3 nanoparticles.

Fig. 15. Reduction in charging time of PE due to Al2O3.

6.4.2. Discharging power and efficiency The average energy released per unit time to the circulating HTF is estimated as the discharging power. Fig. 22 shows the average discharging power estimated in the case of pure PE and PE added with 0.5 wt.% and 1.0 wt.% of Al2O3 nanoparticles at different flow rates of the HTF. The experimental results showed that the discharging power of PE increases with the addition of additives for all the heating rates considered in this study. The average energy discharging power of pure PE corresponding to 2 LPM, 4 LPM, and 6 LPM flow rates estimated as 75.8 W, 98.8 W, and 136.5 W respectively. The average discharging power corresponding to 2 LPM, 4 LPM, and 6 LPM flow rates increased to 101.1 W, 134.8 W, and 187.0 W respectively when 0.5 wt.% of Al2O3 nanoparticles were added to PE. When the weight fraction of Al2O3 nanoparticles increased to 1.0%, the average energy discharging power corresponding to 2 LPM, 4 LPM, and 6 LPM flow rates estimated as 124.1 W, 153.9 W, and 230.8 W respectively. Fig. 23 shows the discharging efficiency obtained in the case of pure PE and PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles at different HTF flow rates. In the case of pure PE, an amount of 482.2 kJ of energy discharged, out of the 916.4 kJ heat stored, at an HTF flow rate of 2 LPM. This indicated a discharging efficiency of 52.6%. When the flow rate changed to 4 LPM, 510.0 kJ of heat was released to the HTF, out of 911.6 kJ of heat available. This accounts for a discharging efficiency of 55.9%. When the flow rate further increased to 6 LPM, the heat rejected to the HTF estimated as 532.2 kJ, out of 907.8 kJ heat stored by the PCM during the charging cycle. This gave a discharging

Fig. 16. Average charging power at different HTF flow rates.

Fig. 17. Charging efficiency at different flow rates.

The PCM temperature variation from the inlet to the exit of the heat exchanger in the axial direction is recorded for evaluating the discharging performance of the PCM.

13

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Fig. 18. Discharging process of PE at different flow rates of HTF.

Fig. 19. Discharging process of PE + 0.5 wt.% Al2O3 at different flow rates of HTF.

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Fig. 20. Discharging process of PE + 1.0 wt.% Al2O3 at different flow rates of HTF.

Fig. 22. Average discharging power at different HTF flow rates. Fig. 21. % reduction in discharging time of PE due to Al2O3 nanoparticles at different HTF flow rates.

6 LPM calculated as 66.2%, 68.8%, and 75.0% respectively. The heat rejected to the HTF at these flow rates obtained as 580.9 kJ, 600.1 kJ, and 650.9 kJ respectively. The heat stored in the previous heating cycle was 877.5 kJ, 872.1 kJ, and 868.0 kJ corresponding to the flow rates 2, 4, and 6 LPM respectively.

efficiency of 58.6%. The discharging efficiencies obtained for PE + 0.5 wt.% Al2O3 and PE + 1.0 wt.% of Al2O3 are also displayed in Fig. 23. The results of PE + 0.5 wt.% Al2O3 indicated discharging efficiencies of 61.0%, 65.8% and 70.2% at HTF flow rates of 2, 4 and 6 LPM respectively. During the discharging cycle, 545.9 kJ of heat rejected, out of 894.4 kJ of heat available, to the HTF circulated at 2 LPM. The heat released at the flow rate of 4 LPM estimated to be 582.3 kJ, out of 884.7 kJ heat stored by the PCM during the charging period. The heat discharged at 6 LPM flow rate estimated to be 617 kJ, out of 878.7 kJ heat that was stored during the heating cycle. The discharging efficiency of PE + 1.0 wt.% Al2O3 corresponding to the HTF flow rates of 2, 4 and

6.5. Overall energy efficiency The overall energy efficiency of the thermal energy storage (TES) system used in this experimental study is calculated by combining the energy efficiencies computed separately for the charging and discharging processes. Fig. 24 displays the overall efficiencies obtained using the different PCM samples for energy storage and release. The experimental results showed that the TES system using pure PE gave overall 15

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to the improved thermal conductivity of PE due to the presence of conductive Al2O3 nanoparticles as reported in the section on thermal property measurement. The maximum value of the overall efficiency obtained corresponding to 1.0 wt.% of Al2O3 nanoparticles at HTF flow rate of 6 LPM. This indicated that the charging and discharging occurs more efficiently at higher weight % of the heat transfer enhancement additives and at higher flow rates of the HTF. Higher the weight % of Al2O3 nanoparticles, greater is the enhancement in the thermal conductivity which resulted in the heat transfer at an enhanced rate. During the charging process, the hot HTF transfers heat to the PCM filled in the shell side of the heat exchanger. Initially, heat is transferred from the circulating HTF to the inner surface of the tubes through convection. The heat is then conducted through the tube material by conduction. The PCM layer in contact with the outer surfaces of the tube receives this heat by conduction and then flows outwards into the bulk of the PCM. The continuous heating of the PCM due to the transfer of the heat from the HTF causes the temperature to increase linearly. When the temperature of PE reaches its solid–solid transition temperature, the added heat causes the breaking of metastable hydrogen bonds in the PE molecules. The breaking of hydrogen bonds is an endothermic process involving a large enthalpy change. Thus, a considerable amount of heat is absorbed by the PCM during the breaking of bonds. At ordinary temperature, P PE has the body-centered tetragonal (BCT) crystal structure. At the transition temperature, the crystal structure changes to face-centered cubic (FCC). If the PCM is further heated above this transition temperature, the PCM receives heat again by sensible heating. During the discharging process, the heat stored by the PCM is rejected to the circulating cold HTF. The heat from the outer layer of the PCM in contact with the shell is conducted toward the center resulting in the sensible cooling of the PCM. When the temperature of the PCM reaches the solid–solid transition temperature, the hydrogen bonds which were broken earlier get reforms and the PE molecule regains its original BCT structure. This reformation process is accompanied by the release of heat. When the temperature of the PCM reaches below the transition temperature, further release of heat is because of the sensible cooling of the PCM. The outside surface of the metal tube wall receives heat by conduction from the PCM layer in contact with the tubes. Heat is conducted through the material of the tubes and then transferred to the cold HTF by convection from the inner surface of the tubes. The addition of Al2O3 nanoparticles has resulted in increasing the thermal conductivity of PE. This caused an enhanced rate of heat transfer by conduction through the PCM. Subcooling in solid–solid PCM refers to the cooling of material below its transition temperature but without the solid to solid transition actually taking place. The large

Fig. 23. Discharging efficiency of PE with additives at different flow rates.

Fig. 24. Overall energy efficiency of the thermal energy storage system.

energy efficiencies of 34%, 38.5% and 42.3% corresponding to HTF flow rates of 2 LPM, 4 LPM, and 6 LPM respectively. The average efficiency of the system in the flow rate range 2–6 LPM was estimated to be 38.3%. The addition of Al2O3 nanoparticles to PE resulted in an increase in the overall efficiency of the system. The overall efficiency of the TES system employing PE + 0.5 wt.% Al2O3 showed overall energy efficiencies of 44.2%, 50.5%, and 56.7% respectively corresponding to HTF flow rates of 2 LPM, 4 LPM, and 6 LPM. The average efficiency of the system in the flow rate range 2–6 LPM found to be 50.5%. When the Al2O3 wt.% increased to 1.0, the TES system exhibited overall efficiencies of 53.2%, 59.1% and 65.1% when the HTF flow rates were 2 LPM, 4 LPM, and 6 LPM respectively. The average efficiency of PE + 1.0 wt.% Al2O3 in the flow rate range 2–6 LPM found to be 58.5%. Fig. 25 shows the % enhancement in the overall energy efficiency of the TES system at different flow rates. The addition of 0.5 wt.% of Al2O3 nanoparticles to PE at HTF flow rates of 2, 4 and 6 LPM gave enhancement of 10.2%, 12%, and 14.4% respectively compared to pure PE. When the weight fraction of Al2O3 nanoparticles was increased to 1.0%, the overall energy efficiency showed enhancement of 19.2, 20.6 and 22.8 at HTF flow rates of 2, 4 and 6 LPM respectively compared to PE. The results of the experimental investigation discussed in the preceding sections revealed that the incorporation of Al2O3 nanoparticles enhanced the thermal energy storage performance of pentaerythritol. The enhanced energy storage and release performance can be attributed

Fig. 25. % enhancement in overall efficiency. 16

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degree of sub-cooling may affect the performance of phase change materials and limit their application. During subcooling, the storage material behaves similar to a sensible storage material, and the storage capacity is reduced. This undesired subcooling in PE was currently reduced by using Al2O3 nanoparticles as nucleating agents, also called nucleators. These additives remain solid at all times and act as centers of crystal growth for the material that undergoes the phase change. The reduction in subcooling was attributed to being heterogeneous nucleation which occurred along with the Al2O3 particles in the crystallization process. 6.6. Exergy analysis The data obtained from the charging and discharging experiments are used to carry out the exergy analysis of the TES system. Exergybased performance evaluation of TES system gives more perspective measure than energy based one as it reveals the true potential of the system and the economic assessment of the storage/discharge process. The exergy input, exergy stored and the exergy output estimated in the case of PE, PE + 0.5 wt.% Al2O3 PE + 1.0 wt.% Al2O3 for flow rates of 2, 4 and 6 LPM of the HTF are summarized in Table 5. The energy efficiency of a TES under ideal condition will be 100% as there is no energy loss in the system. But in the actual case, there are always irreversibilities in the system which results in energy loss. Therefore, energy efficiencies calculated do not reflect the useful energy since the lost energy is not. In the case of exergy evaluation, both the useful and lost energies are taken into consideration. Due to this reason, the exergy efficiency is much less than the energy efficiency for all the test conditions mentioned above. The results show that addition of the alumina nanoparticles has resulted in enhancing the exergy efficiency of the TES system. For each PCM tested, the exergy efficiency is found increasing with the increase in the flow rate of the HTF. The reason for the enhanced heat transfer occurred at a higher flow rate was because of the increased turbulence in the flow. The increased turbulence in the region near to the tube wall caused very efficient fluid mixing and efficient redevelopment of the thermal/hydrodynamic boundary layer leading to the improvement in the convective heat transfer (Chandrasekar, Suresh, & Chandra Bose, 2010; Suresh, Venkitaraj, Selvakumar, & Chandrasekar, 2012). The experimental results summarized in Table 3 indicate that the addition of Al2O3 nanoparticles to PE has resulted in an increase in the charging and discharging exergy efficiency of the TES for all the flow rates of the HTF tested. Table 5 also presents the overall exergy efficiency of the TES using pure PE and PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles. In the case of pure PE, overall exergy efficiencies of 21.5%, 25.2%, and 29.1% have obtained corresponding to the HTF flow rates of 2, 4 and 6 LPM respectively. The addition of 0.5 wt.% of Al2O3 to PE resulted in an increase in the exergy efficiency by 5.6%, 5.8% and 7.5% corresponding to the HTF flow rates of 2, 4 and 6 LPM respectively. When the weight fraction of Al2O3 was increased to 1.0%, the test

Fig. 26. Lost and useful exergy.

results showed 12.6%, 15.1% and 18% increase in the overall exergy efficiency corresponding to the flow rates 2, 4 and 6 LPM respectively. Amount of exergy lost at each test condition is estimated from the input and output exergy values and are presented in Fig. 23. The quantification of lost exergy is very important in the life cycle cost evaluation of PCM. It can be understood from Fig. 26 that the fraction of useful exergy increases due to the incorporation of Al2O3 nanoparticles for all the flow rates of HTF. In other words, the fraction of lost exergy decreases due to the addition of Al2O3 nanoparticles.

6.7. Experimental uncertainties The main parameter monitored during T-history, and thermal storage/release performance tests were the PCM and HTF temperatures. In this experimental study, K type thermocouples connected to a multichannel data acquisition system (KEYSIGHT 34972A LXI) were used to record the various temperatures. The uncertainties in the energy measurements were calculated using the temperature and flow rate uncertainties. The uncertainties in the flow meter reading are specified by the manufacturer as ± 1% of full flow. The calibrated temperature sensors showed an uncertainty of ± 0.1 °C. Now the uncertainty in the heat supplied/recovered (δQ) was calculated using the know uncertainty values of temperature (δT) and mass flow rate of the HTF (δm˙ ). Therefore,

Table 5 Input, stored and output exergy. Flow rate (LPM)

(kJ/day)

˙ stored Ex (kJ/day)

(kJ/day)

Charging exergy efficiency (%) ψchar

Discharging exergy efficiency (%) ψdischar

Overall exergy efficiency (%) ψ

PE

2 4 6

6655 9277 12,759

3182 4693 6979

1440 2337 3719

47.8 50.6 54.7

45.2 49.8 53.3

21.6 25.2 29.1

PE + 0.5 wt.% Al2O3

2 4 6

7086 9312 13,339

3740 5223 8191

1930 2889 4879

52.8 56.1 61.4

51.6 55.3 59.6

27.2 31 36.6

PE + 1.0 wt.% Al2O3

2 4 6

6897 9201 13,065

4132 5903 9059

2356 3710 6153

59.9 64.2 69.3

57 62.9 67.9

34.2 40.3 47.1

PCM

˙ input Ex

˙ output Ex

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˙ ˙ T )2 δQ = tcp,htf (ΔT δm˙ )2 + 2(mδ

References

where m˙ – the mass flow rate of the HTF, cp,htf – specific heat of the HTF, ΔT – the difference between the inlet and exit fluid temperatures, t – time of charging/discharging. The uncertainty in the heat stored was estimated as δQstored = Mcp,pcm (ΔT × δT) + M(δh), where δh is the uncertainty in the enthalpy change given by the DSC equipment and M is the mass of the PCM and cp,pcm is the specific heat of the PCM. Using the above relations, the average uncertainties in heat supplied, the heat stored and heat recovered in the HTF flow rate range considered in this work were estimated as 8.1%, 7.1%, and 8.5% respectively.

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7. Summary and conclusion The effect of adding Al2O3 nanoparticles on the thermal energy storage performance of PE was studied by conducting the experiments using pure PE and PE added with 0.5 and 1.0 wt.% of Al2O3 nanoparticles. During the charging process, the hot HTF (therminol oil) at about 225 °C, and circulated through the heat exchanger at different flow rates of 2 LPM, 4 LPM, and 6 LPM. During the discharging process, cold HTF (therminol oil) at about 30 °C, and circulated through the heat exchanger at different flow rates of 2 LPM, 4 LPM, and 6 LPM. The main findings of the experimental study are given below

• The results of the charging and discharging experiments indicated a • •

•

• •

significant reduction in the charging and discharging time of PE due to the addition of 0.5 and 1.0 wt.% of the additives Al2O3 nanoparticles. The charging and discharging power and efficiency of PE with Al2O3 nanoparticles found increased at all flow rates of the HTF. The maximum enhancement in the efficiency of charging and discharging observed at the highest flow rate of 6 LPM. The charging efficiency found increased to 72.4%, 76.7% and 80.7% at HTF flow rates of 2, 4 and 6 LPM respectively when 0.5 wt.% of Al2O3 nanoparticles was added to PE. The results of PE + 1.0 wt.% Al2O3 indicated a further increase in the charging efficiencies to 80.3%, 83.0% and 86.8% corresponding to the HTF flow rates of 2, 4 and 6 LPM respectively. The discharging efficiency found increased to 61.0%, 65.8% and 70.2% at HTF flow rates of 2, 4 and 6 LPM respectively when 0.5 wt. % of Al2O3 nanoparticles was added to PE. The results of PE + 1.0 wt.% Al2O3 indicated a further increase in the discharging efficiency to 66.2%, 68.8% and 75.0% corresponding to the HTF flow rates of 2, 4 and 6 LPM respectively. The mean value of the overall energy efficiency of the thermal energy storage system using pure PE estimated to be 38.3%. The addition of 0.5 and 1.0 wt.% of Al2O3 nanoparticles caused the mean value of overall efficiency to increase to 50.5% and 58.5% respectively. The overall efficiency of a thermal energy storage system employing 0.5 and 1.0 wt.% of Al2O3 nanoparticles showed very significant improvement at all flow rates of HTF considered in this study. Therefore, it can be concluded that the use of 0.5 and 1.0 wt.% Al2O3 nanoparticles for heat transfer enhancement results in very significant improvement in the thermal performance of PE for long term energy storage applications.

Acknowledgments Authors express their sincere gratitude to the Centre for Engineering Research and Development (KTU/RESEARCH 3/1199/2017 dated 19.04.2017) for the financial assistance given for the execution of the work reported in this paper. 18

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