Abstract This article presents the first research effort of our group to formulate, implement and validate a numerical method in order to optimise the design of solar passive walls involving phase change materials (PCMs). The fusion of ice, gallium and the commercially available PCM 27 (hydrated salt), engineered by Cristopia and later embedded within an experiment unit, was studied. Comparisons against other prediction methods and experimental data for the fusion of gallium were carried out with good agreement of the solutions. The proposed enthalpy-based method is found to be excellent to predict the fusion, but still fails to reproduce adequately the exact solidification pattern measured for the PCM 27. Further research is going on to improve the model. Keywords: phase change materialfixed-grid numerical methodenthalpy-porosity formulationfusion