Multi-megawatt thermo-electric energy storage based on thermodynamic cycles is a promising alternative to PSH (Pumped-Storage Hydroelectricity) and CAES (Compressed Air Energy Storage) systems. The size and cost of the heat storage are the main drawbacks of this technology but using crystalline superficial bedrock as a heat reservoir could be a readily available and cheap solution. SELECO2 research project considers a thermal doublet consisting in a “hot storage” in a bedrock and a cold storage in an ice pool. The complete system includes a heat pump transcritical CO2 cycle as the charging process and a transcritical CO2 cycle of 1 – 10 MWe as the discharging process. Various technical studies are undertaken to assess the performance of such system. Steady-state thermodynamic models have been realized to optimize system efficiency. In addition, unsteady models of geothermal heat exchanger network were developed for the ground heat storage. An experimental device has been designed and built to test the heat-exchange performance and dynamics. The conditions are intended to reproduce real process dynamics at a laboratory scale. The heat exchanger is at 1/10e scale with a 1.6 m height and 40 mm inner diameter. Temperature (40-130°C) and pressure conditions (~8-12MPa) follow the operating conditions of the real process coupled with a granitic bedrock. First results show that energetic and exergetic performances are better if a specific strategy of short charge and discharge cycles is employed rather than longer charge and discharge phases. Moreover experimental results will be used to improve the above-mentioned numerical simulations and to validate CFD simulations.