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. In that context, the aim of this work is i) to assess the performance of a massive electricity storage concept based on CO2 transcritical cycles and ground heat exchangers, and ii) to carry out the preliminary design of the whole thermal doublet system including the reservoir using ice for latent cold storage. This later includes a transcritical heat pump as the charging process (~1-10 MWe). Various technical studies are undertaken to assess the performance of such system. Steady-state thermodynamic models have been realized to optimize system efficiency, including the investigation of regenerative or multi-stage cycles. In addition, unsteady models of geothermal heat exchanger network were developed for the ground heat storage. Coupling between different models has also been achieved. Finally an experimental device has been designed and built to test the heat-exchange dynamics with conditions are intended to reproduce real process dynamics at a laboratory scale (heat exchanger 1/10e scale ~ 1.6 m high, real temperature ~130°C and pressure conditions ~12MPa).