The main disadvantage of phase change materials (PCM) is related to their low thermal conductivities. In this study, the melting and solidification of a PCM within three various horizontal annulus configurations including two circular cylinders, one elliptical cylinder in a circular cylinder and one finned cylinder in a circular cylinder are investigated numerically in terms of the aspect ratio and the orientation of the ellipse and the number of fins. Different volume fractions of the copper nanoparticles are added to the base PCM to examine the effect of nano-particles on the heat transfer rate. Results indicate natural convection plays important roles in the melting process where the melting rate at the bottom section of the annulus is lower than that at the top section. By using the vertical-oriented tube instead of the circular one the melting rate increases. Adding nanoparticles to the base PCM enhances the melting and solidification rate as well. However, it does not eliminate the stable heat transfer at the bottom section of the annulus. Inserting fins leads to the significant enhancement of the-melting and solidification rate. It is more efficient during the solidification process due to the suppression of the natural convection effect during the melting process. (C) 2016 Elsevier Ltd. All rights reserved.