In this study, melting of paraffin wax with Al2O3 nanoparticles in a partially heated and cooled square cavity is investigated numerically. The thermally active parts of the enclosure which are facing each other are kept at different constant temperatures while the other parts of the enclosure are insulated. The effect of nanoparticle concentrations (phi = 0 vol%, 1 vol%, 2 vol% and 3 vol%) and orientation of the activated walls together with the temperature of the hot wall on the melting process and stored energy is investigated. Thermophysical properties of NEPCM are considered to be temperature and phase dependent. The computed results showed that considered parameters have a significant effect on the melting rate and stored energy. The results reveal that the highest enhancement is attained for the enclosure filled with phi = 1 vol% of nanoparticle concentration and heated from bottom, and nanoparticle concentration beyond phi = 1 vol% defeats the purpose thus enhancement decreases. (C) 2016 Elsevier Ltd. All rights reserved.