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Energy Procedía 14 (2012) 643 - 648

Energy

Procedía

2011 2nd International Conference on Advances in Energy Engineering (ICAEE2011)

Conductivity particles dispersed organic and inorganic phase change materials for solar energy storage-an exergy based

comparative evaluation

S.Jegadheeswarana*, S.D.Pohekara, T.Kousksoub

"Mechanical Engineering Area, Tolani Maritime Institute, Induri, Talegaon-Chakan Road, Pune 410 507, India bLaboratoire de Thermique Énergétique et Procédés, Avenue de l'Université, BP 1155, 64013 Pau Cedex, France

Abstract

The performance enhancement of a shell and tube latent heat thermal storage (LHTS) system due to dispersion of conductivity nano-particles is investigated. Two phase change materials (PCM) are considered, one is organic PCM (paraffin wax) and the other is inorganic (hydrated salt). The numerical study involves both charging and discharging modes. The performance enhancements of the two PCMs are compared in terms of exergy stored/recovered and exergy efficiency.

© 2011 PPiblished by Elsevier Ltd. Selection ajnd/or peer-review under responsibility oof the organizing committee of 2nd International Conference on Advances in Energy Engineering (ICAEE).

Keywords: Exergy ;Latent heat storage ;Numerical modelling ;Particle dispersion ;Solar energy

1. Introduction

Phase change material (PCM) based latent heat thermal storage (LHTS) systems are expected to be critical in managing renewable energy sources like solar thermal energy because of the mismatch between

availability of source and demand. In order to make best use of LHTS systems, the performance

enhancement is emphasized as the available PCMs are of low thermal conductivity [1]. The thermal

conductivity can be improved through composite PCMs, which are generally obtained by adding either

graphite [2, 3] or high conductivity metal particles [4, 5].

* Corresponding author. Tel.: +91-2114-242044; fax: +91-2114-241517.

E-mail address: jdees.2002@gmail.com.

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of 2nd International

Conference on Advances in Energy Engineering (ICAEE).

doi:10.1016/j.egypro.2011.12.887

Nomenclature

cP specific heat, J/kg.K

e particle volume fraction

Ex exergy rate, W

g gravitational acceleration, m/s

H enthalpy, J/kg

k thermal conductivity, W/m.K

□ mass flow rate, kg/s

P pressure, Pa

Q heat transfer rate, W

t time, s

T temperature, K

V velocity, m/s

Subscripts

in inlet

out outlet

ref reference or ambient

Although there are reports on particle based composites, the thermal analysis of systems utilizing composite PCMs is generally carried out based on first law of thermodynamics. However, only second law analysis takes into account the quality of energy along with quantity. The present authors have already analyzed melting and solidification behaviours of nano-copper particles dispersed paraffin wax applying exergy principles [6, 7].

— ——^

Fig. 1. Schematic of LHTS system (a) physical system; (b) computational model

S.Jegadheeswaran etal\/Energy Procedia 14- (2012) 64-3 - 648

As an extension of these works, the main objective of present work is to compare exergy performances of two different PCMs, one from organic group (paraffin wax) and another from inorganic group (hydrated salt), under the influence of copper particles.

2. Materials and methods

2.1. Physical system and operating conditions

The LHTS module is a shell and tube heat exchanger as shown in Fig.1 (a). The shell serves as PCM compartment. Since the LHTS unit is for solar water heater, water acts as heat transfer fluid (HTF), which flows through the tube. Paraffin wax and hydrated salt are used as PCMs whereas copper particles are conductivity materials. The properties of the selected PCMs are given in Table 1. In the simulations, the inlet temperature of HTF is assumed constant and is more than the melting point of PCM and less than that during charging and discharging respectively. To ensure the axissymetric phase change around the

tube, the unit is kept vertical and therefore, the two dimensional analysis is carried out for one half of the unit with axis and all walls of shell as adiabatic boundaries (Fig.2 (b)).

Table 1. Properties of PCMs

PCM Melting Latent heat of Thermal conductivity Density Specific heat Dynamic

temperature (oC) fusion (J/kg) (W/m.K) (kg/m3) (J/kg.K) viscosity (Pa.s)

Paraffin wax 55-59 178000 0.2 910 (solid), 2100 0.0273

777 (liquid)

Hydrated salt 57-58 250000 0.65 (solid) 1400 (solid), 2500 0.031

0.40 (liquid) 1290 (liquid)

2.2. Numerical model

The model described here is based on the authors' previous works [6, 7]. To describe the heat transfer characteristics of PCM, we use the following energy equation.

d(pH) = V.(kVT) 1

In Eq. (1), phase change of PCM is accounted for by computing enthalpy as the sum of the sensible enthalpy and the enthalpy change due to phase change with the latter is a product of latent heat and liquid fraction. It should be mentioned that the natural convection in the liquid PCM has very little effect on the solidification whereas melting is dominated by natural convection. Hence, no momentum equation is necessary for solidification. Even for melting, the natural convection can be modeled without momentum equation, by introducing effective thermal conductivity for liquid PCM (kf = k 0.18 Ra 025).Through Rayleigh number (Ra), the buoyancy induced convection is automatically taken into account.

For HTF, the conventional continuity, momentum and energy equations are employed.

The properties of composites should be evaluated as a function of particle volume fraction in the mixture. The corresponding expressions and their validity are described in Refs [6, 7].

The exergy efficiency of charging can be defined as the ratio of rate of exergy stored in PCM and rate of exergy transferred by HTF. The expressions for exergy rates are given in Eq. (2).

Exm-F = rn c

:p„„ \(in Tr\f ] Tr\f ln

To investigate the consequence of reduced latent heat as a result of particles presence, a ratio of total exergy stored with and without particles after complete melting is also computed.

For discharging process, the exergy efficiency is the ratio of rate of exergy recovered by HTF and rate of exergy transferred by PCM. Eq. (3) presents the relevant exergy rates.

(T„, -Tn )~Tr\f MT^

The effect of reduced latent heat value on the discharging performance can be gauged through the ratio of total exergy recovered with and without particles after complete solidification. The total exergy stored/recovered are obtained by Simpson's 1/3 rd rule based numerical integration of exergy rates.

2.3. Solution procedure

Finite volume method based computational fluid dynamics (CFD) code was employed for solving governing equations. Following our previous works [6, 7], a grid size of 0.0015 x 0.0015 was used for meshing and a time step size of 0.1s was chosen for unsteady analysis. To define the effective thermal conductivity only for liquid PCM, a used defined function (UDF) was complied in the solver.

3. Results and discussions

Since the validation of the numerical model has been extensively reported in our previous publications [6, 7], the same is not repeated here. Thus, only the new results are presented.

Fig.2. Effect of particle volume fraction on discharging exergy efficiency (a) paraffin wax as PCM; (b) hydrated salt as PCM

3.1. Exergy efficiency

The results of discharging exergy efficiency reveal that hydrated salt exhibits higher exergy efficiency than paraffin wax (Fig.2). The system with hydrated salt not only displays higher heat transfer rate, but

S.Jegadheeswaran etal.\/Energy Procedia 14- (2012) (¡4-3 - 648

also produces higher temperature HTF. This is the reason for higher exergy efficiency in case of hydrated salt system. The higher exergy efficiency is also displayed by all hydrated salt composites in comparison with paraffin wax composites. As a matter of fact, the exergy efficiency of any hydrated salt composite is roughly twice that of its paraffin counterpart. The same is observed throughout the solidification.

Tmmsi Tlnw(H)

Fig.3. Effect of particle volume fraction on charging exergy efficiency (a) paraffin wax as PCM; (b) hydrated salt as PCM

Contrary to solidification, exergy efficiency is found to be increasing for some period initially and same starts decreasing thereafter (Fig.3). The decrease in exergy efficiency at later stages is attributed to the fact that the natural convection becomes weaker as a result of particles. Although this trend is shown by both paraffin wax and hydrated salt, from quantitative perspective, the hydrated salt systems appear to be relatively better one. The results also indicate that the difference between the melting exergy efficiencies of paraffin and salt is not as high as that in case of solidification. This clears that hydrated salt PCMs can be preferred over paraffin wax straightaway, if solidification alone is the matter of concern. On the other hand, when it comes to melting, since exergy performance of hydrated salt is not well far ahead of paraffin, it is not a clear choice.

Fig. 4. Effect of particle volume fraction on exergy performance (a) total exergy recovered; (b) total exergy stored

3.2. Total exergy recovered/stored

As it can be seen from Fig.4 (a), the addition of particles considerably increases the total exergy recovered from PCM. Hence, particles addition enhances the quality of recovered energy despite reducing the storage capacity of the unit. This outcome clears that the reduced latent heat has no role in

determining the quality of energy recovered. This could be due to the enhanced heat transfer rate as a result of presence of particles. The higher heat transfer rates minimize the exergy destruction. The reduction in solidification time can also be stated as a reason for better exergy performance, as composite PCMs take lesser time to solidify. It can also be noticed that the addition of particles is more pronounced in terms of enhancement of exergy recovered when the PCM is paraffin wax, especially when the particle fraction is above 0.5. Although both the PCMs enjoy the benefit of particle addition in terms of reduced solidification time, the reduction in solidification time is relatively higher in case of paraffin wax. Hence, the increase of exergy recovered is relatively more when paraffin wax is employed.

Since composite PCMs posses high heat transfer rate and could complete melting process faster, higher exergy storage capacity may be expected as in the case of discharging. However, the results shown in Fig.4 (b) display a different behavior altogether, i.e. particles addition reduces the quality of energy stored. Unlike solidification, during melting, motion of liquid PCM is inevitable. One of the sources for exergy destruction is the viscous force. It is evident that the presence of particles increases the viscosity of liquid PCM, which in turn leads to more exergy destruction. In this perspective, it can be stated that the effect of viscosity on exergy destruction is more dominant than that of heat transfer rate and charging time. The comparison of exergy performance of paraffin wax and hydrated salt during melting also reveals the same. For any particle fraction, since the viscosity of hydrated salt is higher than that of paraffin wax, the former encounters more exergy destruction than latter. Therefore, hydrated salt systems store less exergy than paraffin wax when particles are added.

4. Conclusions

In the present work, the exergy performances of particles dispersed paraffin wax and hydrated salt in a shell and tube LHTS unit were investigated and compared. The transient numerical calculations were performed using CFD code FLUENT. The results indicate that hydrated salt composites exhibit better exergy efficiency than paraffin wax composites due to their higher thermal conductivity. However, the former cannot be stated as better choice because of its inability to store /recover more exergy. Hence, the choice of PCM should not be based on its high thermal conductivity alone.

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[3] Wang N, Zhang XR, Zhu DS, Gao JW. The investigation of thermal conductivity and energy storage properties of graphite/paraffin composites. J Therm Anal Calorim 2011. doi: 10.1007/s 10973-011-1467-z.

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