The outlet fluid temperature of a latent heat thermal energy storage unit was numerically estimated using a simplified three-dimensional heat transfer model. In this model, thermal conductivity of liquid phase assumed to have a high value, while free convection heat transfer in the liquid phase is neglected. Comparison of calculated and experimental values showed that the simplified model was able to predict the outlet fluid temperature, which was independent of the thermal conductivity of the liquid phase when the thermal conductivity exceeded a critical value. Moreover, fluid dynamics and heat transfer around heat transfer tubes were numerically predicted using a two-dimensional model that takes into account the free convection heat transfer. Comparison of the numerical results given by the two models showed that the simplified model is applicable to energy thermal storage in which free convection dominates the heat transfer rate.