An experimental study is conducted in order to investigate the melting and solidification processes of paraffin as a phase change material (PCM) in a tube in shell heat exchanger system. The motivation of this study is to design and construct a novel storage unit responding to the melting/solidification characteristics of the paraffin. The PCM is stored in the vertical annular space between an inner tube through which the heat transfer fluid (HTF), here water, is flowing and a concentrically placed outer shell. This study focuses on the possibility of the heat transfer enhancement in the heat storage geometry. Enhancement is achieved by tilting the outer surface of the storage container, i.e. the surface of the outer shell with a tilting angle of 5 degrees. The paraffin (P1) is used as the PCM because of its low cost, high energy storage density and large scale availability. At first, the thermophysical properties of the paraffin used are determined through differential scanning calorimeter (DSC) analysis. A series of experiments are conducted to investigate the effect of increasing the inlet temperature and the mass flow rate of the HTF both on the charging and discharging processes (i.e. melting and solidification) of the PCM. (c) 2006 Elsevier Ltd. All rights reserved.