FLUWS aims to develop and validate a more flexible, reliable, environmentally friendly and cost-effective thermal energy storage (TES) system futureproofed for next-generation concentrating solar power plants operating at higher temperatures and hybridized with PV, two of the main paths for reaching cost-efficiency of CSP. FLUWS validates up to TRL 5 a novel TES concept that ensures elevated thermal efficiency with minimum environmental impact thanks to on the one hand the upcycling of waste and residual materials from the ceramic industry and the use of air as heat transfer fluid and on the other thanks to building on previous consortium know-how in the development of new cost-effective radial packed-bed TES and materials for high-temperature applications. The new FLUWS TES will enable more flexible and modular CSP systems as it will have embedded electric heaters driven by renewable electricity and will be designed for easier integration with compact gas Brayton cycles, thus facilitating the provision of additional services from CSP to the grid and widening the applications of CSP as a competitive technology for combined heat and power (CHP) in the industrial sector. FLUWS addresses key technological challenges: development of high-temperature solid TES media materials based on upcycling of waste and residual material streams; Production of bricks-shaped TES materials via low energy demanding extrusion processes; Development of high temperature (≥800°C) packed bed TES with embedded electric heaters with enhanced performance and reduced structural challenges facilitating upscaling and commercial uptake; Development of high-temperature TES with minimal environmental impact and maximized circularity along the full value chain; Deployment of comprehensive modelling suites for industry and grid operators to maximize the dispatchability of CSP plants, improving their role in the energy sector and the variety of provided services.