This thesis presents the results of a study of the solid-liquid phase change process of water in a rectangular enclosure for application to thermal energy storage systems. The work performed combined experimental results with analytical and numerical methods to develop computer models of the system;Experiments were performed to obtain data to verify the models under various melting and freezing conditions. A connective heat transfer correlation, for melting of ice in a container confined by two horizontal plates, was obtained based on the experimental work. An analytical model based on heat conduction was developed to predict ice formation in an enclosure which was cooled isothermally. A model incorporating the effects of convection, conduction and buoyancy was developed to predict the solid-liquid interface position and melting rate for the melting of unfixed ice in the enclosure which was heated isothermally. Comparisons were made between the models and the experiment data. The results demonstrated satisfactory agreement. Finally, a preliminary model was developed to predict the overall performance of melting in a cool thermal energy storage system. The results of the model were used to produce design criteria for thermal energy storage systems.