This paper presents a simplified two dimensional model for the fusion (solidification) of PCM in a rectangular cavity with internal fins. The formulation of the model follows Voller's approach. The results of the model were compared with available experimental and numerical data and the agreement was found good. Geometric parameters of the cavity such as width, length of fin, thickness of fin and thickness of top and bottom were investigated. Also operational parameters such as the magnitude of the heat flux and its frequency were also analysed. The data obtained seem as a useful guide for engineering calculations of such units.