International Journal of Refrigeration 89 (2018) 40–50 Contents lists available at ScienceDirect International Journal of Refrigeration journal homepage: www.elsevier.com/locate/ijrefrig Experimental study on the effect of Zirconia nanoparticles on solidification heat transfer characteristics: A comparison with Titania nanoparticles Hoda Aslani, Mohammad Moghiman ∗ Department of Mechanical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran a r t i c l e i n f o Article history: Received 25 August 2017 Revised 11 December 2017 Accepted 14 January 2018 Available online 31 January 2018 Keywords: Solidification Nucleation Supercooling degree Nanofluid Surfactant Phase change material a b s t r a c t In this study, the influence of Zirconia (ZrO 2 ) and Titania (TiO 2 ) nanopaticles on liquid–solid phase tran- sition of aqueous nanofluids with/without Poly vinyl pyrrolidone as surfactant are experimentally com- pared. A cooling generation apparatus based on the compression refrigeration cycle has been used to ex- plore the solidification behavior of nanofluids as phase change materials. The experimental results show that ZrO 2 and TiO 2 nanoparticles considerably reduce the solidification supercooling degree of deionized water (as basefluid). Only adding 0.04 wt% ZrO 2 and TiO 2 nanoparticles to base fluid, the percentage of reduction in supercooling degree attained 81% and 65%, respectively. The results reveal that although the presence of surfactant in nanofluids reduces the supercooling degree and slightly solidification time of both ZrO 2 and TiO 2 nanofluids; but it has no influence on onset nucleation time. Comparison of ZrO 2 and TiO 2 nanofluids with/without surfactant presents that ZrO 2 provides faster solid layers formation and has more energy saving potential in storage systems due to its lower supercooling degree. © 2018 Elsevier Ltd and IIR. All rights reserved. Étude expérimentale de l’effet de nanoparticules de zirconium sur les caractéristiques de transfert de chaleur par solidification: comparaison avec les nanoparticules de titane Mots-clés: Solidification; Nucléation; Degré de surfusion; Nanofluide; Tensioactif; Matériau à changement de phase 1. Introduction Application of Phase Change Material (PCM) to store and re- lease latent heat in energy storage systems as an efficient method, allows high energy storage capacity and massive charge/discharge rate (Nomura et al., 2016; Lei et al., 2016; Elmozughi et al., 2014; Abdollahzadeh and Esmaeilpour, 2015). Various techniques have been proposed and investigated to improve operation of energy Abbreviations: COP, coefficient of performance; DNSD, dimensionless number of supercoiling degree; DNST, dimensionless number of solidification time; DW, deion- ized water; ONT, onset nucleation time, S; PCM, phase change material; PVP, poly vinyl pyrrolidone; SD, supercoiling degree, °C; ST, solidification time, S. ∗ Corresponding author. E-mail addresses: aslani_hoda@yahoo.co.in (H. Aslani), moghiman@um.ac.ir (M. Moghiman). storage system by enhancing thermal conductivity of PCMs such as introducing metal structures into PCM, dispersing micro parti- cles into PCM and using double layer network for Phase change composites (Wang et al., 2016a; Wang et al., 2016b; Golestaneh et al., 2016). Nowadays, due to rapid development of nanotechnology, the thermophysical properties of PCMs in the presence of nanopar- ticles (Yiamsawasd et al., 2012; Raja et al., 2016; Mahbubul et al., 2013) and likewise phase changing process of nanofluids (Kim et al., 2011; Moghiman and Aslani, 2013; Altohamy et al., 2015) have attracted significant research attention. Preliminary ev- idences indicated that the aqueous nanofluid could be an effec- tive material to modify the performance of cooling energy storage system (Chandrasekaran et al., 2014b; Mo et al., 2012); because nanoparticles act as nucleating agent to promote solidification rate by improving heterogeneous nucleation (which takes place in https://doi.org/10.1016/j.ijrefrig.2018.01.009 0140-7007/© 2018 Elsevier Ltd and IIR. All rights reserved.