Thermochimica Acta 565 (2013) 137–145 Contents lists available at SciVerse ScienceDirect Thermochimica Acta jo ur nal ho me page: www.elsevier.com/locate/tca Preparation and thermal energy storage behaviour of stearic acid–TiO 2 nanofluids as a phase change material for solar heating systems S. Harikrishnan a,b , S. Magesh b , S. Kalaiselvam b,c,∗ a Centre for Nanoscience and Technology, Anna University, Chennai, India b Department of Mechanical Engineering, Anna University, Chennai, India c Department of Applied Science and Technology, Anna University, Chennai, India a r t i c l e i n f o Article history: Received 23 October 2012 Received in revised form 27 March 2013 Accepted 1 May 2013 Available online 9 May 2013 Keywords: Nanofluids Phase change material Solidification Melting Thermal conductivity a b s t r a c t This paper investigates the phase change behaviour of newly prepared stearic acid–TiO 2 nanofluids as composite phase change materials (PCMs). TiO 2 nanoparticles of 0.05, 0.1, 0.15, 0.2, 0.25, 0.3 wt% were dispersed in stearic acid, individually. The phase change temperatures and latent heats of nanofluids for melting and solidification processes were determined by differential scanning calorimetry (DSC). The complete melting and solidification times of nanofluids were reduced by 7.03, 12.56, 19.59, 28.64, 35.17, 43.72% and 6.62, 13.57, 20.53, 26.82, 34.11, 41.39% for 0.05, 0.1, 0.15, 0.2, 0.25, 0.3 wt% TiO 2 nanoparti- cles, respectively. Time reductions of both the processes proved thermal conductivity enhancement of nanofluids and it was ascertained with laser flash analyzer (LFA) measurements. Based on the results, stearic acid based composites could be recommended as potential candidate for low temperature solar thermal energy storage applications due to their better thermal reliability, chemical stability and heat transfer characteristics. © 2013 Elsevier B.V. All rights reserved. 1. Introduction On account of energy crisis, scientists and engineers are urged to find new energy sources, which could bridge the gap between energy demand and energy supply. Before finding an efficient energy source, it is essential to store the heat energy from sun and also energy available during off-peak periods. Since, it would help to cater the energy needs to the desired level [1]. Phase change mate- rials (PCMs) employing in latent thermal energy storage (LTES) system is the most popular technique due to its distinctive features like high storage density for given volume and storing either cool or heat energy almost at constant temperature [2–5]. But, PCMs as latent heat storage materials in thermal energy storage have low thermal conductivity, which limits their utility for large scale applications as low thermal conductivity of PCMs decelerates the energy storage and release rates [6]. Various new methods such as introducing metal fins, metal screen and other metal structures into PCM, multiple PCMs technique, impregnating porous material with ∗ Corresponding author at: Department of Mechanical Engineering, Anna Univer- sity, Chennai, India. Tel.: +91 4422359220; fax: +91 4422301656. E-mail addresses: ramhkn@yahoo.co.in (S. Harikrishnan), mageshsankar92@yahoo.com (S. Magesh), kalai@annauniv.edu, nanokalai@gmail.com (S. Kalaiselvam). PCM and dispersing microparticles into PCM were suggested for improving the thermal conductivity of PCMs [7–14]. Despite these new developed methods prevailing in practices, thermal conduc- tivity of PCMs is able to achieve minimum enhancement only and not as expected level. This is due to the fact that these new methods have certain limitations like dimension of metal fin, number of fins, arrangement of fins, sedimentation of microparticles, etc. Development of nanotechnology has introduced the new method known as “nanofluids”. Nanofluids comprise of solid nanoparticles and melted PCM as composite materials, which would enhance the thermal and heat transfer characteristics of the PCM [15,16]. As far as thermal conductivity enhancement is concerned metal and metal oxide nanoparticles, graphene and car- bon nanotubes (CNTs) are preferred. Size and shape of particles make significant effect on the thermal conductivity enhancement of the nanofluids [17]. Based on the past literatures, it is reported that as particle size reduces the thermal conductivity of nanoflu- ids increases and vice versa. On the contrary, there are also some experimental studies from the literatures reported that if size of the particles is reduced then, thermal conductivity enhance- ment of nanofluids will also be decreased. Mostly, shapes of the particles dispersed into base fluid are cylindrical and spherical par- ticles. Cylindrical nanoparticles suspended into base fluid would be expected to have higher thermal conductivity than spherical par- ticles dispersed into base fluid because, cylindrical particles have 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.05.001