Elsevier

Journal of Energy Storage

Volume 43, November 2021, 103181
Journal of Energy Storage

Dominant roles of eccentricity, fin design, and nanoparticles in performance enhancement of latent thermal energy storage unit

https://doi.org/10.1016/j.est.2021.103181Get rights and content

Highlights

  • The provision of eccentricity significantly enhanced the melting rate of PCM.

  • Energy storage rate was enhanced via eccentricity and optimized fin design.

  • Correlations of melting time and Nusselt number were developed for optimum configuration.

  • Al2O3 & CuO nanoparticles further reduced melting time of PCM for optimum configuration.

  • Al2O3 nanoparticles manifested higher energy storage rates of PCM as compared CuO nanoparticles.

Abstract

In this paper, numerical and experimental investigations of thermal performance enhancement of horizontal longitudinal shell and tube latent heat thermal energy storage unit (LHTESU) via eccentricity (e), fin design optimization, and nanoparticles in the phase change material (PCM) of stearic acid are presented. An enthalpy-porosity-based two-dimensional, transient numerical methodology is used after validating against the experimental results and the literature. Five different eccentric positions, e = 0.14, 0.28, 0.42, 0.56, and 0.63 are investigated by modifying the design of Y-finned tube. The eccentricity and fin design modifications improved natural convection effects as compared to concentric Y-finned tube. Thermal performances of LHTESU are analyzed on the basis of melting time, heat storage capacity, heat storage rate, and performance enhancement ratio. Based on the performance analysis, an optimum eccentric configuration of LHTESU is proposed. For optimized eccentric unit (e=0.42), a reduction of 34.14% in melting time along with 30.7% improvement in thermal energy storage rate is achieved as compared to concentric (e=0) LHTESU. The effect of temperature of the tube on PCM's melting time and heat transfer is also analyzed and two important correlations are proposed. The thermal performance of the optimized eccentric unit is further enhanced by adding nanoparticles of Al2O3 and CuO ranging from 0.5% to 10% by volume in the pure PCM. The nano-enhanced PCM with 1% of Al2O3 further improves melting and energy storage rates of optimum LHTESU by 10%.

Introduction

In recent years, shell and tube latent heat thermal energy storage units (LHTESU) are being used for energy storage and applications concerning clean and sustainable power. Consideration of LHTESU for energy storage is due to the irreplaceability and limited availability of fossil fuels in nature. High thermal storage capability with relatively lower volumes, along with the storage of latent heat at nearly constant temperatures are among the advantages of LHTESU. Moreover, phase change materials (PCM) used in these systems are chemically stable, non-corrosive, and have low vapor pressure at their corresponding operating temperatures. Therefore, LHTESU contributes to various engineering applications which include waste heat recovery [1], refrigeration and air conditioning systems [2,3], emission reduction in the cold start-up of vehicles [4], and solar energy systems [5].

Despite numerous advantages, the storage process is challenging due to the lower thermal conductivity of PCM, which limits the heat transfer performance of LHTESU. Therefore, heat transfer augmentation methods are employed to improve the energy storage rate by introducing extended surfaces such as longitudinal, circular, and pin fins [6], [7], [8], [9], [10], [11] and by using nano-particles into the PCM [12], [13], [14], [15]. Recently, the fractal-tree-shaped fins in the LHTESU designs have become a popular choice due to their high rate of heat transfer in the PCM as compared to the conventional fins [16]. Wu et al., [17] numerically studied the effect of tree and wheel shaped fins for shell and tube LHTESU. Both fin configurations resulted in a smooth temperature distribution during the melting of the PCM. Moreover, a significant improvement in the convection effects during melting and solidification was reported which ultimately enhanced the thermal performance of the LHTESU. Hasnain et al., [18] investigated thermal performance augmentation of a PCM by using branched fins and in shell and tube LHTESU. It was observed that single and double branched fin configurations reduced the melting time of PCM by 35.4% and 45.9%, respectively, as compared to the Y-fin configuration. More recently, metal foams have been introduced in LHTESU to improve heat transfer in PCM. The role of metal foam in thermal performance improvement of multitube arrangements of shell and tube LHTESU has been studied in [19]. It was observed that metal foam enhanced the melting and solidification rates of LHTESU by up to 92% and 94%, respectively. Deng et al., [20] experimentally investigated the effect of metal form dispersed in the PCM by focusing on the pore size distribution of the metal foam described by the fractal Brownian motion. The decrease in porosity and fractal distance lead to an increase in heat transfer and reduction in melting time of the PCM.

Although several methods of heat transfer improvement in PCM exist but one of the simplest methods of thermal performance enhancement through structural change in horizontal shell and tube LHTESU is the adjustment of the relative position of the tube with respect to the shell center. Dutta et al. [21] numerically investigated and experimentally validated the melting of paraffin placed between two co-axial horizontal cylinders for different eccentric positions and angles of the inner heat cylinder. They found that net heat transfer to the PCM increased by shifting the inner heated cylinder downwards in the outer cylinder due to enhanced recirculations and natural convection effects. However, the overall heat transfer in the PCM decreased significantly by moving the inner heated cylinder upwards due to the small convection zone. Yusuf et al. [22] experimentally investigated the effect of eccentricity on the thermal performance of a horizontally configured shell and tube LHTESU. The eccentricity was changed by moving the inner tube downwards at three different positions in the direction of gravity. They reported that moving the tube downwards augmented the melting performance of LHTESU considerably. Melting time for the maximum eccentric case was reduced by 67% compared to the concentric configuration. In a numerical and experimental study, Dhaidan et al. [23] investigated the melting of PCM inside an annular container by providing a constant heat flux and examining the effect of eccentricity. For the maximum eccentricity of the tube, the charging time was reduced by 18.7%. In a similar study to Yusuf et al. [22], Pahamli et al. [24] numerically analyzed the effects of eccentricity and operational parameters on the charging performance of a single pass shell and tube LHTESU containing RT-50 as the PCM. In their work, the tube was moved at three different positions in the direction of gravity to produce eccentricity. They reported a reduction of 67% in the charging time for the case with maximum downward eccentricity. Zheng et al. [25] optimized the performance of an LHTESU numerically for melting and melting-solidification by the implementation of eccentricity in the geometry. They defined an optimal eccentricity by plotting the Rayleigh number and concluded a 59-73% reduction in melting time of the PCM with the generation of eccentricity comparing to the concentric LHTESU. Darzi et al. [26] analyzed the melting behavior of n-eicosane acting as the PCM in eccentric and concentric horizontal annuli. The melting performance of the LHTESU for three different positions of the tube was investigated by plotting the temporal melting fraction. A noticeable reduction of 36% and 69% in melting time was observed by moving the tube in two different positions, respectively, from a concentric to eccentric system. Eslamnezhad et al. [27] selected a triplex-tube (horizontal) LHTESU for numerically investigating the melting characteristics of a PCM. For the sake of performance enhancement of the LHTESU, they used a combination of fins and eccentricity in their system. They compared the melting process with fins and eccentricity and concluded that combining both enhancement techniques reduced the melting time by 17.9%. Cao et al. [28] experimentally and numerically studied the effect of the eccentricity of the tube in a concentric shell and an LHTESU. They concluded that for a maximum eccentricity of 30mm, there was a 57% reduction in melting time of the PCM. Based on their results, they proposed an average heat transfer coefficient which was shown to be higher for the eccentric LHTESU. Using an eccentric configuration of the LHTESU, Yusuf et al. [29] experimentally studied the effect that eccentricity has on the solidification of the PCM. To achieve an eccentric system, they moved the tube both upwards and downwards with respect to gravity. Their work showed that producing eccentricity either in the upward or downward direction led to a longer energy extraction time of the PCM.

The thermal conductivity of popular PCM is known to be small, which hinders the thermal performance of heat storage systems. One method to improve the thermal characteristics of PCM is through the dispersion of nanoparticles in the phase change media. Ammar et al. [12] enhanced the thermal performance of a triplex-tube type LHTESU by introducing nanoparticles into virgin paraffin wax (RT-82). Dispersing Al2O3 between 1-10% by volume in the PCM, resulted in an improvement of 32.5% in thermal conductivity, originally at 0.2 W/mK. The enhanced thermal characteristics improved the performance of the LHTESU by 17% in terms of the PCM melting time. In an experimental study, Nada et al. [30] experimentally investigated the performance of a nano-enhanced PCM (RT-55) based photovoltaic (PV) module in terms of its thermal regulation and efficiency using aluminium oxidenanoparticles. They concluded that the nano-enhanced PV system decreased the system's temperature by 10.6°C which corresponds to a 13.2% increase in its efficiency. Sciacovelli et al. [31] performed an experimental-numerical investigation on shell and tube type LHTESU with and without the inclusion of nanoparticles in the PCM. The system performance was gauged via the complete melting time for both nano-enhanced and virgin PCM. The study reported a 15% reduction in the complete melting time of the PCM through the addition of copper nanoparticles. Zakir et al. [13] performed a comparative study on the performance of LHTESU by enhancing the thermal conductivity of paraffin RT44-HC (PCM) in a shell and tube heat exchanger. Nanoparticles of Aluminium oxide, Aluminium nitride, and graphene were used in their study to enrich the PCM. The effect of the inclusion of nanoparticles on thermal performance was investigated by adding nanoparticles of 1%, 3%, and 5% by volume in the PCM. The experimental data showed that with an inlet temperature of 47 °C, the melting time of the PCM was reduced by 33.75%, 35.90%, and 62.56% for 1% of nanoparticles of aluminium oxide, aluminium nitride, and graphene, respectively. Saw et al. [32] experimentally evaluated the performance improvement of a solar LHTESU through the addition of copper nanoparticles in a paraffin wax PCM. In their study, a 2% addition of copper nanoparticles into the PCM improved thermal conductivity and thermal diffusivity of the PCM by 46.3% and 44.9%, respectively. In terms of efficiency, an improvement of 1.7% was reported for the solar LHTESU with the nano-enhanced PCM. Sadegh et al. [33] investigated heat transfer characteristics of an LHTESU after dispersing TiO2 nanoparticles into n-octadecane acting as the PCM. In addition, they performed experiments with different Stefan numbers for gauging the effect of the provided temperature of a heat transfer fluid on the melting of the PCM. They reported that the inclusion of only 4% nanoparticles in the PCM, resulted in a significant enhancement in heat transfer characteristics of the system. Recently, micro and nano enhanced PCM have also been comprehensively reviewed by Mohamed et al., [34]. Different PCM enhancement methods were discussed with the perspective of their applications in different thermal energy storage devices.

As noted in the literature, valuable contributions have been made to eccentric LHTESU in terms of performance enhancement. However, most of the eccentricity-based literature work centers around non-finned LHTESU. However, the experimental and numerical study of Khan and Khan [35] manifested significant thermal performance improvement of stearic acid based LHTESU by using the different orientations of longitudinal fins of the heat transfer tube. They reported that with a Y-fin configuration, the melting time of the PCM is reduced by up to 50.7% with a 10% enhancement in the total energy storage capacity. Therefore, simultaneous provision of eccentricity and fins to heat transfer tube in LHTESU has enormous potential in thermal performance improvement of LHTESU. Moreover, the addition of nanoparticles in PCM can further accelerate the charging process by enhancing the thermal conductivity of the PCM. However, nanoparticles can significantly alter the energy storage capacity of the LHTESU. Most of the studies in the literature focus on the optimization of the melting time of PCM but ignore the overall energy storage performance of the system. Therefore, quantitative analysis of an eccentric and finned thermal energy storage unit with nano-enhanced PCM requires simultaneous gauging of multiple performance parameters which fulfill its applicability to real-world systems.

The present study is focused on thermal performance enhancement of stearic acid based LHTESU through eccentricity and fin design of heat transfer tube. A concentric Y-type longitudinally finned LHTESU is used as a base case. The performance-indicating parameters such as PCM's melting time, heat storage rate, and total thermal energy storage are analyzed. Based on the obtained results, an optimum eccentric LHTESU with a corresponding optimal fin configuration is proposed. A detailed study of flow physics involved in the melting process of PCM is performed. Moreover, a temporal energy storage enhancement metric is defined for assessing the performance of the LHTESU. The thermal performance of the optimum configuration of the LHTESU is further augmented by adding Aluminium oxide (Al2O3) and Copper oxide (CuO) nanoparticles in PCM. The heat transfer and thermal energy storage capacities of the nano-enhanced PCM are investigated in detail. The suitable type and percentage of nanoparticles are highlighted which improve both charging and energy storage performance of optimum LHTESU configuration.

Section snippets

Domain configuration and thermo-physical properties

A three-dimensional model of the concentric Y-fin LHTESU used in the experiments is shown in Fig. 1(a). The computational domain used for the numerical analysis is represented by a two-dimensional cross-section of a shell and tube type heat exchanger with concentric and eccentric tube positions as presented in Fig. 1(b) and (c), respectively. The concentric cross-section consisting of an inner copper tube of radius Rt=16.05mm and a thickness of tt=3mm is placed at the center of a steel shell

Experimental setup

The experimental setup used in this study consists of a shell and tube latent heat storage unit connected to hot and cold-water reservoirs as shown in Fig. 5. A schematic diagram of the setup is presented in Fig. 5(a), demonstrating the general layout of the complete experimental setup. The actual experimental setup is shown in Fig. 5(b). The setup consists of a well-insulated 1m long horizontal heat storage unit containing steel shell and copper tube comprising three longitudinal 120o apart

Temporal variation of melting faction and temperature distribution of PCM

Performance of the LHTESU with varying eccentricities is determined by investigating some performance indicating parameters such as melting time, temperature distribution, energy storage capacity, and energy storage rate. Fig. 9 shows the line plots of space averaged liquid fraction(δ¯) and temperature of the PCM as a function of melting Fourier number (FoM) for varying levels of eccentricities. The temporal variation of liquid fraction, shown in Fig. 9(a), is almost identical for all the cases

Conclusion

In this paper, the thermal performance of horizontally configured longitudinally finned concentric LHTESU is enhanced and numerically analyzed through eccentricity, fin optimization, and the addition of nanoparticles. HTF tube is moved vertically in the downward direction with fin modifications keeping the volume of PCM constant. Important performance indicators include melting and energy performance enhancement, energy storage rate, and total energy stored. Based on the critical

CRediT authorship contribution statement

Lehar Asip Khan: Conceptualization, Methodology, Validation, Investigation, Formal analysis, Data curation, Writing – original draft, Writing – review & editing. Muhammad Mahabat Khan: Conceptualization, Methodology, Validation, Investigation, Formal analysis, Data curation, Writing – original draft, Writing – review & editing, Supervision, Project administration. Hassan Farooq Ahmed: Methodology, Validation, Data curation, Writing – original draft, Writing – review & editing. Muhammad Irfan:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

All persons who have made substantial contributions to the work reported in the manuscript (e.g., technical help, writing and editing assistance, general support), but who do not meet the criteria for authorship, are named in the Acknowledgements and have given us their written permission to be named. If we have not included an Acknowledgements, then that indicates that we have not received substantial contributions from non-authors.

References (47)

  • A. Arshad et al.

    Preparation and characteristics evaluation of mono and hybrid nano- enhanced phase change materials (NePCMs) for thermal management of microelectronics

    Energy Convers. Manage.

    (2020)
  • T. Xiong et al.

    Nano-enhanced phase change materials (NePCMs): a review of numerical simulations

    Appl. Therm. Eng.

    (2020)
  • A. Pourakabar et al.

    Enhancement of phase change rate of PCM in cylindrical thermal energy storage

    Appl. Therm. Eng.

    (2019)
  • Z. Deng et al.

    Melting behaviors of PCM in porous metal foam characterized by fractal geometry

    Int. J. Heat Mass Transf.

    (2017)
  • M. Yusuf Yazici et al.

    Effect of eccentricity on melting behavior of paraffin in a horizontal tube-in-shell storage unit: an experimental study

    Sol. Energy

    (2014)
  • N.S. Dhaidan et al.

    Experimental and numerical investigation of melting of NePCM inside an annular container under a constant heat flux including the effect of eccentricity

    Int. J. Heat Mass Transf.

    (2013)
  • Y. Pahamli et al.

    Analysis of the effect of eccentricity and operational parameters in PCM-filled single-pass shell and tube heat exchangers

    Renew. Energy

    (2016)
  • Z.J. Zheng et al.

    Eccentricity optimization of a horizontal shell-and-tube latent-heat thermal energy storage unit based on melting and melting-solidifying performance

    Appl. Energy

    (2018)
  • A.A.R. Darzi et al.

    Numerical study of melting inside concentric and eccentric horizontal annulus

    Appl. Math. Model.

    (2012)
  • H. Eslamnezhad et al.

    Enhance heat transfer for phase-change materials in triplex tube heat exchanger with selected arrangements of fins

    Appl. Therm. Eng.

    (2017)
  • X. Cao et al.

    Effect of natural convection on melting performance of eccentric horizontal shell and tube latent heat storage unit

    Sustain. Cities Soc.

    (2018)
  • M.Y. Yazici et al.

    On the effect of eccentricity of a horizontal tube-in-shell storage unit on solidification of a PCM

    Appl. Therm. Eng.

    (2014)
  • S.A. Nada et al.

    Improving the thermal regulation and efficiency enhancement of PCM- Integrated PV modules using nano particles

    Energy Convers. Manage.

    (2018)
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