This study investigates the inward solidification process of a phase change material (PCM) encapsulated in a spherical and cylindrical container with a third kind of boundary condition as well as the complete solidification and complete charging time of a cool storage tank which consists of spherical containers filled with a PCM.The phase change problem inside the containers is modeled in spherical and cylindrical coordinates and solved numerically by using enthalpy method with control volume approach. Computer programs are written in Basic language to determine the dimensionless total solidification time of the PCM held in a spherical and cylindrical container. These programs are run many times by varying the affecting parameters to obtain data sets of the dimensionless total solidification time of PCM. These data sets are then used to derive correlations which express the dimensionless total solidification time of the PCM in terms of the affecting parameters by applying multiple regression analysis.Another program is written to simulate a cool storage tank with spherical containers. The complete solidification and complete charging time of the tank are calculated by using this program. A parametric study is realized for the cool storage tank by varying the design parameters and the influence of these variations on the complete solidification and complete charging time is investigated.