CEEGS (CO2 based electrothermal energy and geological storage system) is a cross-sectoral technology for energy transition, with a renewable energy storage system based on the transcritical CO2 cycle, CO2 storage in geological formations and geothermal heat extraction. It is a highly efficient, cost-effective, and scalable (small-to large-scale) concept for large-capacity renewable energy storage. Extended capacity is obtained due to the underground system. It can be integrated into the grid, heating and cooling districts and industries. It also has the capacity for partial CO2 sequestration. The main objective of the project is to provide scientific proof of the techno-economic feasibility of the technology, raising the current low TRL 2 to TRL 4 by addressing gaps in the interface between surface transcritical cycle and the subsurface CO2 storage. CEEGS follows a 3-phase approach: i) From theoretical principles to models, simulations and processes in which advanced numerical simulations integrate reservoir behaviour, wellbore design and surface plant design; ii) From models and simulations to systems/experimental verification addressing CEEGS integration and efficiency in energy systems, with digital functional and laboratory models developed and components validated with results from the CO2 pilot-scale projects and; iii) Social, economic and sustainability assessments where social acceptance studies, LCA and TEA tools evaluate impacts and concept deployment with renewables, hard-to-decarbonise industries, district heating and cooling, or in grid balance. The project is completed with WP1 for coordination and WP7 for results dissemination and exploitation. The project integrates the knowledge and networks for a successful implementation in 3 years with a consortium with partners from 5 EU countries, with multidisciplinary skills on energy systems, energy storage, geology, geothermal systems and CO2 geological storage