The solid-to-liquid phase change material can be used to convert the thermal energy of low quality into mechanical energy. Its volumetric change rate has a great effect on the converting efficiency. The mathematical model of the volumetric change rate for the phase change processes of hexadecane within a cylindrical container was presented. A special experimental device was designed and constructed to verify the numerical model. The results at different Stefan numbers of the experiments matched well with those from the numerical simulations. The influences of Stefan number, Biot number and radius of cylinder on the volumetric change rate were studied and analyzed. The results showed that the volumetric change rate depends on the mass fraction of liquid phase and the difference between liquid and solid density of the materials. All the factors affecting the phase change rate will influence the volumetric change rate. The volumetric expansion rate is less than theoretical value under an external pressure. While a high-pressure situation is taken into consideration, the numerical model should also be modified by adding a function calculating density varying with pressure to ensure that the model operates properly. The power output can be enhanced by reducing the total time of melting. It can also be improved when the phase change material is partly melted at a volumetric expansion rate less than 100% of the total value. Copyright (C) 2008 John Wiley & Sons, Ltd.