Air conditioning and refrigeration have an increasing technological importance and the use of storage process is economically a good solution. The presented cool thermal storage process consists in the use of the solid-liquid transformation of Phase Change Materials (PCM) encapsulated in spherical nodules. These nodules fill a tank placed in the refrigeration loop. Laboratory experiments allow to know exactly the crystallization process of one nodule or of a small number of nodules and to specify the nature and the importance of the undercooling phenomenon (delay of crystallization). The erratic character of the crystallizations is particularly in evidence. A model, based on a combination of the energy equation and nucleation theories, is validated by the experiments and gives information on the kinetics of phase transformations.