Hybrid energy storage systems (HESS) with redox flow batteries and supercapacitors working as a team are uniquely suited to specific applications. Modern energy grids rely on renewable energies (e.g. solar power) and are characterised by higher fluctuations in both power generation and energy consumption. To absorb resulting power peaks and cope with the increased demand for renewable energies, modern grids need more dynamic storage systems. The EU-funded HyFlow project focuses on technological and ecological improvements of the HEES components, their management systems and their interaction through the complete supply chain. By bringing the best of both worlds, this solution can unlock different applications in the grid, boosting stability while decreasing dependency on fossil fuels.