In the present study, dispersion of highly conductive nanoparticles is used as a heat transfer enhancement method to improve the charging time in latent heat thermal energy storage systems. Rubitherm 55 is used as a phase change material (PCM) and is enclosed in a rectangular container. The melting process commences with the application of heat at the right side-wall. The effect of the concentration of nanoparticles on the melting time is studied for various aspect ratios of the container. The results indicate that a lower melting time is obtained for a wider container (i.e larger aspect ratio). For all the considered cases, the addition of the nanoparticles decreases the melting time; however, a more pronounced decrease in melting time was observed for the narrower containers.