The transient behavior of a thermal energy storage system was studied numerically. The storage system is composed of a number of rectangular channels for the flowing heat transfer fluid (HTF), separated by a phase change material (PCM). Using an enthalpy method, the melting of the PCM is solved. The forced convective heat transfer inside the channels was analysed by solving the energy equation, which was coupled with the heat conduction equation in the container wall. The velocity profile for the rectangular channel is given by an exact analytical solution. A numerical code based on the finite difference method was developed and validated by comparing numerical predictions with exact analytical solutions available in literature. The parameters that control the thermal behavior were identified. Several numerical simulations were performed to assess the effects of the Reynolds number on the heat transfer process of the system during the melting of a PCM. (C) 2003 Elsevier Ltd. All rights reserved.