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.

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.


Worldwide Potential

Field testing of the 1:10 scaled prototype in Lake Constance
© Fraunhofer IEE
Field testing of the 1:10 scaled prototype in Lake Constance

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
Country Area in km² Capacity TWh
Total area 111,659 817
TOP 10 64,965 475
United States 10,226 75
Japan 9,511 70
Saudi Arabia 8,535 62
Indonesia 8,002 59
Bahamas 6,201 45
Libya 5,836 43
Italy 5,572 41
Spain 4,299 31
Greece 3,476 25
Kenya 3,307 24

Next Steps

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.


Parameter Value
Inner diameter 28.6 m
Inner volume 12,249 m³
Wall thickness 2.86 m
Concrete quantity 9,260 m³
Concrete density 2,500 kg/m³
Weight of the concrete sphere 23,150 t
Installation depth 750 m
Charge capacity 31.1 MWh
Discharge capacity 22.4 MWh
Power 5 MW
Discharge time 4.5 h
Full cycle efficiency
ηcycle=ηpump∙ ηturbine

The StEnSea History

In 2011, Prof. Horst Schmidt-Böcking and Dr. Gerhard Luther promoted the idea for this novel PHES. Their initiative led to the research project »StEnSea« of the Fraunhofer IEE (former IWES) which was conducted from 2013 to 2017. During this project a 1:10 scaled prototype was built and successfully tested in Lake Constance in 2016. Additional simulations and analysis regarding the full-scale system moved the StEnSea technology from Technology Readiness Level TRL 2 to TRL 5.

The investigations displayed that the technical implementation in the targeted scale (see table on the right for full scale design) is possible. The two main components, the concrete sphere and the pump turbine, were analyzed in cooperation with experts from the corresponding fields, which significantly increased the quality of the results. The field test proved that the concept works properly in an underwater environment and helped to identify challenges in installation and operation.


The successful test in Lake Constance met with widespread interest worldwide. In addition, the work has been recognized by a number of awards. This includes: German Renewables Award 2017 in the category “Project of the year”; Finalist of the Hessian State Prize for innovative energy solutions 2018; Deutschland Land der Ideen Ausgezeichneter Ort 2018; Green Awards 2019 Top 3 in the category “Innovation of the year”


Projekt of the year - German Renewables Award 2017


Finalist of the Hessian State Prize for innovative energy solutions 2018

Deutschland Land der Ideen Ausgezeichneter Ort 2018

Green Awards 2019 Top 3 in the category “Innovation of the year”

EXPO in Dubai: Offshore pumped storage for the global energy transition

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.

3D miniatures of the StEnSea facilities as an exhibit at Expo 2020 in Dubai.
© German Pavilion Expo 2020 Dubai Bjoern Lauen
3D miniatures of the StEnSea facilities as an exhibit at Expo 2020 in Dubai.