This project is about decarbonising energy, climate-change mitigation and facilitating growth in new renewable energy installations. HDHydro: An energy-storage solution that uses an innovative environmentally benign, high-technology fluid in a pumped-energy-storage system. Pumped-energy-storage is conceptually very simple: at times of energy abundance, low-cost electricity is used to pump massive quantities of water, up mountains, from a lower-reservoir to an upper-reservoir. As energy prices rise, the water is released back down the mountain, through pipes and a turbine, which regenerates electricity back to the electricity-grid, typically round-trip efficiencies for pumped energy storage using water is about 80%. However Instead of using water, RheEnergise use their proprietary high-density fluid (2.5x the density of water), in a buried, closed-loop system. All else being equal, this increases the power and energy available by a factor of 2.5\. The fluid is an environmentally benign suspended-solid in water. Round-trip efficiencies are ~83%, without parasitic cooling loads, found in battery systems. Because of the high-density fluid it also means that projects can be installed on low hills rather than mountains, as HDHydro projects achieve the same power, compared to water, on sites that are 2.5x lower. HDHydro projects are located on small hills rather than mountains. The-Committee-on-Climate-Change, The-Energy-Systems-Catapult, Aurora-Energy-Research, BloombergNEF and others all agree that energy-storage is an essential technology to balance the supply and demand of future energy-systems dominated by large volumes of intermittent renewable-energy and base-load nuclear generation. Both generation types will rely on energy-storage to match variable supply with variable demand. This project is Phase1: A technical feasibility study with some initial enabling works that would facilitate the delivery of a phase2 project (if successful in the application process). The Phase2 project would be a physical exhibition quality demonstration of an operational reversible-pump-turbine specifically designed for use with the RheEnergise high-technology fluid. The turbine would be mounted on portable base (ISO container sizing) be connected to pressurised store tanks (to simulate the elevation of a hill) use a manifold and valves to control the flow. It would include a bespoke power-system, operational control system and customer display interface. The project has been scoped to ensure deliverability against COP-26 timescales. This project fulfils the UKs aims of 'build-back-better' following Covid-19\. It is * Highly innovative across several engineering disciplines * Provides large UK business opportunities * Protects the climate and environment * Shows customer interest and the opportunity for volume sales. * The ability to construct a demonstration by COP26\.