http://forschung-energiespeicher.info/en/projektschau/gesamtliste/projekt-einzelansicht/95/Kugelpumpspeicher_unter_Wasser/The development and research project “StEnSEA” (Stored Energy in the Sea) is investigating the installation of large storage facilities on the sea floor, in combination with offshore wind farms. The physical principle on which the energy storage facility operates is similar to that of conventional pumped storage power plants, but based not on two reservoirs, but a hollow sphere. The inflowing water drives a turbine to generate electricity. When there is a surplus of electricity in the grid, part or all of the water is pumped out of the sphere. The commercial target size per sphere is currently at about 20 MWh per storage unit.
he functional principle is similar to ordinary pumpedstorage plants: when power is needed, water flows into the sphere and drives the turbine thus generating power. If surplus power is available (usually during the night), water can be pumped out of the sphere again, thus effectively charging the storage system. This innovative concept uses the sea itself as upper reservoir.
A hollow sphere with an inner diameter of 30m will be submerged to a water depth of about 700m, so the hydrostatic water pressure creates an energy potential. Due to this applied pressure, electrical energy can be generated with the help of turbines and generators as the water flows into the sphere. A cable connection to the transformer station and from there to the mainland makes the transport of electrical energy possible. The other way around, exess energy, for example from renewable sources, can be used to pump out water from the hollow sphere. The commercial target size per sphere is currently at about 20 MWh (4 hours discharge time for a 5 MW pump turbine) per storage unit.
For the time being, the current approaches of the feasibility study are based on a maximum water depth of 700 m. This is based on the fact, that there are state-of-the-art turbines that can function in such water depths. Construction and installation would be possible in even greater water depths without problems. However, in such case a separated pump and turbine system must be used and/or the turbine technology should be further developed.