A numerical simulation is investigated to enhance the freshwater productivity of the modified solar still desalination system. A phase change material (PCM) is incorporated under the absorber plat as a thermal energy storage medium. Nanoparticles (Al₂O₃) have been added to the saline water and the whole system is covered by a transparent glass. Energy conservation equations were solved numerically by using Comsol software. Calculations have been done for three nanoparticle concentration values (0.02%, 0.1% and 0.2%) and several PCM quantities (10kg, 15kg, 20kg and 25kg). The model considered is validated and the influence of these two last parameters on the freshwater productivity is highlighted. the results indicated that the daily freshwater reached a maximum value of 7.74kg/m² with mpcm=20kg, compared to 6.67kglm² for the modified solar still with nanofluid and without PCM, and to 5.23kglm² for the conventional solar still.