Frontiers in Heat and Mass Transfer (FHMT), 2, 043005 (2011) DOI: 10.5098/hmt.v2.4.3005 Global Digital Central ISSN: 2151-8629 1 THERMAL PERFORMANCE ENHANCEMENT OF PARAFFIN WAX WITH AL 2 O 3 AND CuO NANOPARTICLES – A NUMERICAL STUDY A. Valan Arasu a,* Agus P. Sasmito b,† , Arun S. Mujumdar b a Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai-625015, Tamilnadu, India b Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore ABSTRACT The heat transfer enhancement of paraffin wax, a cheap and widely used latent heat thermal energy storage material, using nanoparticles is investigated. The effects of nanoparticle volume fraction on both the melting and solidification rates of paraffin wax are analysed and compared for Al 2 O 3 and CuO nanoparticles. Present results show that dispersing nanoparticles in smaller volumetric fractions increase the heat transfer rate. The enhancement in thermal performance of paraffin wax is greater for Al 2 O 3 compared with that for CuO nanoparticles. . Keywords: thermal storage; phase change material; paraffin wax; melting; solidification; nanoparticle. * Corresponding author, Email: a_valanarasu@yahoo.com † Currently associated with Minerals Metals Materials Technology Centre (M3TC), National University of Singapore. Email: ap.sasmito@gmail.com 1. INTRODUCTION Well designed energy storage systems not only reduce the mismatch between supply and demand but also improve the performance and reliability of energy systems and can play an important role in conserving energy. Thermal energy can be stored in the form of sensible heat and/or latent heat. Of the two, the latent heat thermal energy storage (LHTS) technique has proved to be a better engineering option due to its various advantages such as large energy storage for a given volume, uniform energy storage/supply, compactness, etc. LHTS units employ phase change materials (PCMs), which undergo phase change (solid-to-liquid and vice versa) during the energy transfer process. Numerous PCMs with their properties, advantages and limitations have been comprehensively reported in Refs. (Sharma and Sagara, 2005; Zalba et al., 2003; Kenisarin and Mahkamov, 2007). LHTS units have the potential to serve as promising energy storage devices due to high thermal energy density and isothermal heat transfer process. Nevertheless, the low heat flux achieved due to the low thermal conductivity of most phase change materials, which drastically affects the melting and solidification performance of the system, widespread use of latent heat stores has not yet been realised. A larger heat flux can be achieved by enhancing the effective thermal conductivity. Different approaches have been proposed to overcome this problem: use of metal thin strips (Hoogendoorn and Bart, 1992), thin walled rings (Velraj et al. 1999), porous metals (Weaver and Viskanta, 1986) porous graphite (Tayeb, 1996), metal foam matrix (Calmidi and Mahajan, 1999) and carbon fibers (Fukai et al. 2000 and 2002) are among the common techniques used to enhance the effective thermal conductivity of PCMs. The presence of the nanoparticles in the PCM increases significantly the effective thermal conductivity of the fluid and consequently enhances the heat transfer characteristics (Cabeza et al. , 2002; Mettawee, and Assassa, 2007; Khodadadi, and Hosseinizadeh, 2007; Zeng et al. 2007; Pincemin et al., 2008; Kim and Drzal, 2009; Ho and Gao, 2009). In the present work, a numerical investigation is carried out to estimate the effect on thermal performance of paraffin wax due to the enhancement in thermal conductivity using alumina(Al 2 O 3 ) and copper oxide (CuO) nanoparticles. The effect of volumetric concentration of the nanoparticles on the melting and solidification performance is examined. 2. MODEL FORMULATION The physical model consists of a rectangular channel (60 cm x 1 cm) carrying water as the heat transfer fluid (HTF), surrounded by a PCM channel (60 cm x 1 cm) both at the top and the bottom. Top half of the symmetrical physical model is shown in Fig.1. Water, as it flows though the channel, exchanges heat with the upper and lower PCM along its flow path. The upper half can be considered as a rectangular enclosure heated from below, in which case the heat transfer is controlled by convection and conduction. But, there can be only conduction heat transfer in the case of the lower PCM as it being heated from above. In order to study the natural convection effect, in the present work, only the upper half is modeled for numerical analysis. During charging process, hot water flows from the left end to the right end of the inner tube and during the discharge mode the cold water flows from the left end to the right end (same as hot water flow direction). The outer walls of the PCM channel are well insulated. In the present work, paraffin wax and paraffin wax with Alumina (Al 2 O 3 ) and with Copper oxide (CuO) in different compositions varying from 1- 5% (by volume) are used as PCMs and water is used as HTF respectively. Assumptions made in the present study are: (a) top half of the 2-D model with natural convection is analysed, (b) flow of HTF through the channel is laminar, (c) thermal losses through the outer walls of the PCM channel is negligible, (d) heat transfer in the PCM is both conduction and convection controlled, (e) thermophysical properties of the PCM are different for the solid and liquid phases, (f) Frontiers in Heat and Mass Transfer Available at www.ThermalFluidsCentral.org