Summary of the hybridGEOTABS Project After more than 4 years working on the project this document is a summary of the context and overall objectives of the project (For the final period, include the conclusions of the action) This includes: What is the problem/issue being addressed? Why is it important for society? What are the overall objectives?   The European Union has set a path towards a decarbonised society in 2050. The European Green Deal aims to reduce the CO2-emissions by at least 55% by 2030. Heating and cooling of buildings constitutes a significant part of the energy use in Europe, and is therefore an important sector in the transition to this low-carbon society. hybridGEOTABS is an HVAC-concept that provides comfort in buildings in a clean and sustainable way. The core of the concept is GEOTABS: a combination of a geothermal system (GEO) and thermally activated building systems (TABS). TABS is a type of radiant heating and cooling emission system that is well-known for providing high thermal comfort. The heating/cooling pipes are embedded in the mass of the building elements (e.g. concrete floor slabs), therefore activating them as thermal storage. By turning entire floor or ceiling surfaces into heating and cooling systems, TABS can provide very low-temperature heating (as low as 22 - 28°C) and high-temperature cooling (as high as 15 - 22°C). These temperatures are close to the temperatures available in the shallow layers of the underground, which allows to operate geothermal (GEO) heat pumps at a very high efficiency. Moreover, in buildings with moderate cooling demands (e.g. in central and northern Europe), the underground temperatures can be directly transferred to the TABS via a heat exchanger, providing passive cooling at negligible energy cost. The geothermal source acts as a seasonal storage, from which heat is extracted in the heating season and injected again in the cooling season. This seasonal energy storage, combined with the short-term thermal energy storage in the TABS, enables an enhanced use of renewable energy. GEOTABS is the comfortable and sustainable core of the hybridGEOTABS system.