Natural convection is one of the major factors that affect phase transition processes of solid-liquid phase change materials (PCMs). To optimize PCM-based latent thermal energy storage systems (TESS), a better understanding of the heat transfer interactions during these transitions is needed. In this paper the heat transfer rate enhancement caused by natural convection of PCMs undergoing melting is quantified based on experimental observations. For this, a heat transfer enhancement factor and an effective heat transfer coefficient were developed. Differential scanning calorimetry (DSC) tests were run to measure latent heats of fusion and phase transition temperatures of the PCMs. It was found that the experimentally-obtained temperature ranges required for complete melting exceeded those produced by the DSC tests. The reason for this stemmed from the natural convection of the molten PCM. The effective heat transfer coefficients when natural convection was accounted for were greater than when only heat conduction was considered. The increases in effective heat transfer coefficient were 12% and 30% percent for vertical and horizontal heat transfer paths, respectively. The existence of natural convection reduced the time required for complete melting by approximately 45% for vertical heat transfer. However, the melting process time was longer than the solidification process under the same conditions for a horizontal heat transfer path. The reason for this was attributed to the widening of the temperature range required for melting. (C) 2015 Elsevier Ltd. All rights reserved.