The storage system in a concentrated solar plant is considered as an important concern to increase the capacity factor of the plant by producing power during the night or in cloudy days. This paper presents the analysis of a thermocline system, which consists of a single tank that typically works with molten salt and quartzite rock as storage media. A simulation model of heat charging and discharging process is developed with the numerical solution of non-dimensional convection-diffusion equations. These equations describe the heat transfer in the fluid and between the fluid and the filler material. The model is validated with experimental data, and the results are compared with other models. The proposed model is used for the design of an alternative storage system for AndaSol I. Moreover, the model is used to analyze the influence of the ratio between height and diameter of the tank on the energy storage efficiency. Finally, a comparison between three thermal fluids is made in order to find out which heat transfer fluid (HTF) is better for the designed thermal storage tank.