We consider an electricity storage concept based on CO2 transcritical cycles and geothermal heat exchangers. Developing a reliable and possibly simple model of the ground heat storage is fundamental for assessing the global performance of the system and for its optimization. Nevertheless this is not a trivial task due to the significant size of the storage and the different time-scales involved in the process. A physically-based 1D/2D model for the ground heat storage, that simulates both flow and energy transfer inside the heat exchangers and the heat transfer in the ground will be presented. The model will be then used for a preliminary optimization of process and storage parameters.