This study focuses on the phase change behavior of a material subjected to an external heat transfer fluid. The work is based on a dimensionless study of heat flux involved during phase change, and a time-dependent boundary Robin condition is proposed to model the external heat transfer fluid. The charge/discharge cycles model relies on a time dependance of the external reference temperature, to represent successively the periods of phase change material melting (charge) and solidification (discharge). We chose an abrupt change in the external temperature to represent the action of an operator that switches between load and discharging periods. As the final shape of the tank will have an influence on the performances of charge and discharge, the simulations results, in terms of temperature, solid fraction and released heat flux, are presented for various Biot (Bi) and Stefan (Ste) numbers that give information on time and magnitude of charged and discharged energies. The study shows that an increase of Bi implies melting in a deeper thickness of PCM, higher values of temperature on the convective boundary condition and higher charged and discharged energies. An increase of Ste leads to higher temperatures on the convective boundary condition during charging periods, lower values during discharge and exhibits deeper melting during charge. Moreover, this energy varies during the cycles, and tends toward a constant value, this trend being fitted by a time function. The dimensionless study shows advantages concerning the material selection, but also drawbacks concerning the difficulty to highlight the specific temperatures (melting, external) when plotting the dimensionless form.< Réduire