This research systematically studies the impacts of thermal conductivity and density of phase change materials (PCM) on the characteristics of PCM-based thermal energy storage systems (TES). We show that the eutectic PCM, owing to its high thermal conductivity, has more stable temperature evolution than that of paraffin wax during both heat charging and discharging processes. The paraffin wax has an enhanced charging rate, when the heat is applied at the bottom, as a result of convection driven by the obvious temperature-dependent density of liquid paraffin wax. The convection and orientation effects are significant in the charging process of paraffin wax, but insignificant for eutectic PCM, which are further confirmed by visualization and numerical studies. Specifically, a gap forms between the covered plate wall and the solid paraffin wax after discharging process. This gap significantly inhibits the charging process in a heat charging and discharging cycle. We also observed that the charging efficiency of the paraffin wax is substantially reduced by its high contact thermal resistance, which is 2-3 orders of magnitude higher than that of eutectic PCM. This shows that rapid thermal energy charging/discharging rates, a highly desirable stable working temperature, and orientation-insensitivity of TES can be achieved using PCM with a high thermal conductivity and a temperature-independent density. (C) 2019 Elsevier Ltd. All rights reserved.