The present work aims to investigate the solidification process of a nanoparticle-enhanced phase change material (NEPCM) filled in a rectangular latent heat storage unit (LHSU). The storage unit consists of several identical slabs filled with n-octadecane as a phase change material (PCM) dispersed with high conductivity nanoparticles (copper). The NEPCM slabs are vertically oriented and separated by rectangular channels in which circulates water as a heat transfer fluid (HTF). A two-dimensional model has been developed and validated by experimental, theoretical and numerical data available in literature. The enthalpy porosity method was adopted for modeling the phase change process. The finite volume approach was used to discretize the resulting equations. The numerical investigations were carried out to investigate the effect of the volumetric fraction of nanoparticles on the heat transfer enhancement during discharging process.