A transient phase change thermal storage system of the vertical tube type is studied using a fixed grid numerical model. The solution of the system consists of solving the equations of the heat transfer fluid (HTF), the pipe wall and the phase change material (PCM) as one domain. The control volume finite difference approach is used to solve the equations describing the phase change thermal storage system. The SIMPLE scheme is used to solve for the pressure and velocity fields of the HTF. The radial temperature distribution, the phase change interface position and the latent and sensible heat accumulated along the system axial length are shown for different Reynolds and Stefan numbers, phase change temperature range and time periods. Also shown in this paper is the possibility of using the numerical model to solve problems with a single temperature phase change. The results obtained are important parameters for PCM thermal system performance investigation and design. (C) 2000 Elsevier Science Ltd. All rights reserved.411111651190