Energy is stored in form of gravitational potential energy of water. Round trip energy efficiency varies between 70% to 80%.
Subsea pumped storage is composed of a hollow sphere submerged at great depth. Electricity is stored when water is pumped out of the sphere and created when water is let in via a reversible turbine.
The revolutionary concept brings a range of advantages: no land required, no mechanical structure required other than electric cable, scalability, low impact of natural disasters like earth quakes, no rare raw materials used for construction..
For off-grid islands the concept is a most promising solution for storing renewable energies. A first pilot test was carried out in Lake Constance, see
StEnSea - Fraunhofer Institute
Energiaki Cycladon EPE & Sifnos Island Cooperative currently explore in close cooperation with the German Fraunhofer Institute the opportunity for deploying a large-scale prototype in Greece. For the Cycladic island of Sifnos a feasibility study got successfully accomplished April 2018.