Abstract Phase change materials (PCM) play an important role in energy storing and waste heat recovery. A great amount of energy can be stored in PCM in the form of latent heat within a narrow temperature change. However, due to a large difference between the density of solid and liquid phases, a considerable volume change is experienced during a phase change. Consequently, for a constant pressure phase change process, it is necessary to provide a reserved space on top of the PCM surface. This study was devoted to the numerical simulation of RT27 solidification in the spherical capsule filled partially by PCM. The capsule was allowed to breathe freely, so it was necessary to consider the convection in the free space over the PCM surface as well. Moreover, it was shown that filling the capsule partially is a passive method to control the heat release rate. In all, three spheres were considered with 20, 40, and 60 mm diameters and wall temperatures of 20 and 40 K below the PCM mean solidification temperature. Each capsule was assumed to be initially filled with liquid PCM by 25%, 50%, 75%, and 98.5% (approximately full) of the sphere volume. After validating the result, a deep parametric study was performed. It was shown that the PCM volume changed sharply at the beginning and later approached an asymptomatic value. This volume reduction caused a downward velocity field at the beginning of solidification, which was disturbed later by convectional flow and a secondary flow appeared due to air convection. For spheres with the same diameter and different amounts of PCM, except FR=75%, by decreasing the filling ratio, the solidification time decreased considerably. For example, the solidification time of FR=25% was around 60% of FR=98.5%. In return, for spheres with the same amount of PCM but different diameters, on average, FR=98.5% and FR=50% had the highest and the lowest solidification rates respectively. All these observations could be interpreted under the light of the wetted area to PCM volume ratio.