StEnSea - Stored Energy in the Sea

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.

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.

A Geographical Information System (GIS) was used to identify potential installation sites worldwide. For the identification, different parameters and thresholds were derived from the analysis. The following values were used to derive the worldwide potential of the StEnSea technology:

• Water depth: 600 m - 800 m
  
• Slope: ≤ 1°
  
• Distance to the electrical grid: ≤ 100 km
  
• Distance to maintenance bases: ≤ 100 km
  
• Distance to installation bases: ≤ 500 km
  
• Unsuitable geomorphology like trenches,
  spreading ridges, rift valleys, canyons,
  seamounts, escarpments, fans

A follow up research project, which is under negotiation with the funding agencies, aims towards proving the possibility to install the system in the intended water depth. It will help to gain new insights into the offshore installation and operation. Through the continuous operation, it will be possible to analyze and assess long-term effects on the concrete sphere and pump turbine. The planned work would move the technology to TRL 6 and therefore prepare the realization of commercial full-scale systems. If the promising results of the first research project can be continued, the StEnSea technology could become an important component of the future energy storage portfolio.

As one of about 40 partners, Fraunhofer IEE is represented in the German pavilion with the research vision StEnSea - Stored Energy in the Sea.

The BMWi-funded StEnSea - Stored Energy in the Sea project of the Fraunhofer Institute for Energy Economics and Energy System Technology IEE in Kassel was one of the 100 innovative winners of the "Landmarks in the Land of Ideas" competition in Germany in 2018. Now it has made the leap to the international stage and shows how the joy of experimentation, curiosity and courage to rethink can lead to forward-looking innovations in the field of energy storage.