Fraunhofer Institute for Energy Economics and Energy System Technology
Fraunhofer Institute for Energy Economics and Energy System Technology

StEnSea - Stored Energy in the Sea

Deep sea pumped hydro storage

Future application of a StEnSea plant
© Hochtief
Future application of a StEnSea plant

Deep sea pumped hydro storage is a novel approach towards the realization of an offshore pumped hydro energy storage system (PHES), which uses the pressure in deep water to store energy in hollow concrete spheres. The spheres are installed at the bottom of the sea in water depths of 600 m to 800 m. This technology is also known as the »StEnSea«-system (Stored Energy in the Sea) after the first research project that was carried out to proof the technology in a field test.

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Working Principle

Working Principle

Stensea Working principle
© Fraunhofer IEE

The StEnSea system consists of two main components. One is a hollow concrete sphere representing the storage tank and the other is the cylindrical technical unit holding the pump turbine, a controllable valve and the components of the Supervisory Control and Data Acquisition (SCADA) system. The technical unit is removable and can be recovered separately, which facilitates maintenance and repairs.

An empty sphere corresponds to a fully charged storage unit. Opening the controllable valve enables water to flow through the technical unit into the sphere. The inflowing water drives a turbine and a generator that feeds electricity into the grid. This represents the discharging phase of the storage system. Recharging is achieved by pumping the water out of the sphere against the surrounding water pressure using energy from the grid.

 

Techno-economic Assessment

The techno-economic assessment shows that the StEnSea system is cost competitive with conventional pumped hydro energy storage (PHES). While the exploitation of PHES often raises environmental issues due to land demand and its impact on the water regime, there are no major restrictions expected for the StEnSea technology. Another advantage is the modular set up that is achieved by combining several StEnSea units to a plant. This increases the flexibility of the plant and therefore the range of possible applications.