Recognising that current storage solutions are unable to stabilize enough the intermittent renewable energy production, new long term energy storage solutions are becoming mandatory. Current long-term energy storage is mainly provided by Pumped-Storage Hydroelectricity (PSH). Compressed Air Energy Storage (CAES) has appeared for decades as a credible alternative but its poor energy efficiency, the need of fossil fuels and the use of existing underground cavities as storage reservoirs have limited its development. Variations to CAES have shown low efficiency, losing a big percentage of energy as heat and mechanical losses. Since the 2010s, there is a strong revival of scientific and industrial interest on CAES, led by China and the European Union (EU). For the EU, leading the new generation of high-efficient, low climate-impact and long-term energy storage research, is key to increase its energy independency. In this context, the main objective of Air4NRG is the development of an innovative, efficient (over 70% RTE), long- term, and sustainable CAES prototype, which can enhance renewable energy availability and offers robustness and safety while increasing cost effectiveness and improving the environmental footprint. At the same time, it will promote innovation and competitiveness in the European energy storage industry, while prioritizing the principles of circular economy and environmental sustainability. Another key factor of the solution is the integrability to the electrical grid system and their intelligent EMS, which will be proven by the end of the project through end user integration activities (TRL5). The project will result in two prototypes: one of them will be a plug and play system fitting into a standard 40ft container with an over ten- hours storage duration, while the other will be a larger-scale system of 200kW with the same duration of storage. The developed system is a rare material-free solution with simple industrial infrastructure needs, allowing its full development within the EU, strengthening Europes position in the energy storage system sector.