Abstract Phase change material (PCM) is applicable in various use cases, such as in a novel hybrid steam/latent heat storage system where containers filled with PCM are placed at the shell surface of a Ruths steam storage (RSS) for retrofitting. The considered approximately rectangular PCM cavity design includes aluminium fins, which is a common choice for heat transfer enhancement. Numerical studies were conducted with two separate numerical models to analyse melting and solidification of PCM in such cavity. Varying aluminium proportions, as well as varying fin spacings were simulated under different orientations of the PCM cavity and their impact on charging/discharging speed was analysed, providing a foundation for design optimization of the considered geometry. Guideline values for optimal aluminium ratio and optimal fin spacing could be obtained. Significant angular dependency on the thermophysical behaviour could be observed during melting, whereas the effect of natural convection during solidification was found to be negligible. The results of this work provide important insight to facilitate the design process of rectangular aluminium finned PCM cavities.