A theoretical model is developed to analyze the characteristics of the thermal storage and heat transfer processes in the packed bed system. Dimensionless formulae are developed using theoretical and numerical analysis. Dimensionless radius of the phase change interface in the spherical capsules, dimensionless axial temperature variation in the heat transfer fluid and the thermal storage in the packed bed system can be obtained through a numerical method, with alternative iteration between temperature and thermal resistance. Simulation results are carefully compared with experimental results. Dimensionless correlations are developed for air and liquid respectively. These simple formulae provide guidelines for the design and optimization of the packed bed thermal system.