StEnSea

Operating principle of StEnSea
Marine Pumped Storage Power Plants

Figure: Discharging: Water flows through the turbine into the empty sphere and generates electricity via a generator; Charging: Water is pumped out of the sphere by the electrically driven pump.

Marine pumped storage power plants are a novel approach to transferring the well-established concept of pumped storage systems to deep-sea environments. These offshore pumped storage systems are to be used in water depths between 600 m and 800 m and utilize the pressure in deep water to store energy. In contrast to conventional pumped storage power plants, the surrounding water serves as the upper reservoir, eliminating the need for complex piping. The storage capacity of the system is proportional to the volume and depth of the system.

The StEnSea system consists of a hollow concrete sphere that functions as the storage reservoir, and an inserted technical unit containing the pump turbine, a controllable valve, and the components for measurement, control, and regulation (MCR). The technical unit can be removed from the concrete sphere installed on the seabed, maintained or repaired on land, and then reinserted.

An empty sphere corresponds to a fully charged storage unit. To discharge the storage unit, the valve is opened and water can flow into the sphere through the pump turbine. The inflowing water drives the generator via the turbine, which feeds electricity into the grid. Charging is achieved by pumping the water out of the sphere against the surrounding water pressure using surplus energy.

StEnSea - Stored Energy in the Sea

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Techno-Economic Assessment

The techno-economic assessment has shown that the costs of the StEnSea system are similar to those of conventional pumped storage plants. The construction of conventional pumped storage plants is hindered in many places by conflicts of use and the resulting impact on the landscape and local water balance. The conflict of use and the impact of the StEnSea technology on the environment are considered to be significantly lower, meaning that no major restrictions are expected in the selection of new locations.

Another advantage of the technology is its modular design. In a StEnSea park, any number of units can be interconnected to achieve different outputs and capacities. This increases the flexibility and scalability of the technology and thus the range of potential applications.

Global Potential

Using a Geographic Information System (GIS), potential sites were identified worldwide. The following parameters and thresholds were used to determine the global potential of the StEnSea technology:

Water depth: 600 m - 800 m
Slope: ≤ 1°
Distance to the power grid: ≤ 100 km
Distance to ports/locations from which maintenance can be carried out: ≤ 100 km
Distance to ports/locations from which installation can be carried out: ≤ 500 km

Locations with unsuitable geomorphology, such as trenches, ridges, valleys, gorges, and steep slopes, were excluded.

Country	ARea in km²	Energy in TWh
Sum of all areas	111,659	817
TOP 10	64,965	475
USA	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

Development

The StEnSea Development

In 2011, Prof. Horst Schmidt-Böcking and Dr. Gerhard Luther came up with the idea for this new pumped storage technology. Their initiative led to the Fraunhofer IEE's StEnSea research project conducted from 2013 to 2017. During this project, a 1:10 scale prototype was built and successfully tested in Lake Constance in 2016. Additional simulations and analyses of the full-scale system advanced the technology from TRL 2 to TRL 5 (Technology Readiness Level). The investigations showed that technical implementation at a 1:1 scale (see table below) is feasible.

In a follow-up project StEnSea 2.0, a 1:3 scale prototype is planned to be installed at a depth of approximately 650 m off the California coast. The aim is to investigate offshore logistics, installation, and long-term operation. The planned long-term operation will allow to analyze and evaluate the long-term effects on the concrete sphere and the pump turbine. The planned activities could raise the technology to TRL 6, paving the way for the implementation of large-scale commercial projects. If the promising results of the first research project can be confirmed, the StEnSea technology has great potential to become an important part of the future energy storage portfolio.

Parameters	StEnSea System 1:10 (Lake Constance)	StEnSea System 1:3 (California)	StEnSea-System 1:1
Outer diameter / m	3	10	30
Weight / t	20	1000	20,000
Water depth / m	100	500 - 700	600 - 800
Capacity / MWh	0.001 – 0.003	0.5 - 1	20
Power / MW	0.002 - 0.004	0.5 - 1	5 - 7
Efficiency	0.40	0.60	0.80

Awards

The successful test in Lake Constance met with widespread interest worldwide. In addition, the work has been recognized by multiple 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

Video (in German)

Finalist of the Hessian State Prize for innovative energy solutions 2018

more info (in German)

Deutschland Land der Ideen Ausgezeichneter Ort 2018

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

Publications

Bard, J., Dick, C., Puchta, M., StEnSea - ein neuartiges Offshore-Pumpspeicherkonzept. WASSERWIRTSCHAFT 107 (2017), 69–72. doi: https://doi.org/10.1007/s35147-017-0161-x.
Dick, C., Ernst, B., Puchta, M., Bard, J., Encyclopedia of Energy Storage – Deep-sea Pumped Hydro Storage. Elsevier (2022), 88–99. doi: https://doi.org/10.1016/B978-0-12-819723-3.00089-5.
Dick, C., Puchta, M., Bard, J., StEnSea – Results from the pilot test at Lake Constance. Journal of Energy Storage 42 (2021), 103083. doi: https://doi.org/10.1016/j.est.2021.103083.
Dick, C., Sprengelmeyer, J., Falzone, G., Stored Energy in the Sea – Combining 3D Printed Pumped Hydro Energy Storage Systems with Floating Offshore Wind in California. Journal of Ocean Technology 18 (2023). url: https://www.thejot.net/article-preview/?show_article_preview=1415&jot_download_article=1415
Hahn, H., Hau, D., Dick, C., Puchta, P., Techno-economic assessment of a subsea energy storage technology for power balancing services. Energy 133 (2017), 121–127. doi: https://doi.org/10.1016/j.energy.2017.05.116.
Puchta, M., Bard, J., Dick, C., Hau, D., Krautkremer, B., Thalemann, F., Hahn, H., Development and testing of a novel offshore pumped storage concept for storing energy at sea – Stensea. Journal of Energy Storage 14 (2017), 271–275. doi: https://doi.org/10.1016/j.est.2017.06.004.
Schmidt-Böcking, H. W., Luther, G., Düren, M., Puchta, M., Bender, T., Garg, A., Ernst, B., Frobeen, H., Renewable Electric Energy Storage Systems by Storage Spheres on the Seabed of Deep Lakes or Oceans. Energies 17 (2024). doi: https://doi.org/10.3390/en17010073

Master's Theses

Jänisch, V.,Techno-Economic Analysis Of The Application Of The Stensea Concept In The Hambach Opencast Mining Lake. Master thesis (2020), University of Karlsruhe, Karlsruher Institut für Technologie (KIT).
Sprengelmeyer, J., Feasibility study for the application of an offshore pumped storage technology in California. Master thesis (2023), Clausthal University of Technology.

Project Pages

The operating principle of marine pumped storage power plants is being investigated in model tests at several locations in the StEnSea and StEnSea 2.0 projects. As part of StEnSea, a 1:10 scale prototype was successfully tested in Lake Constance, while StEnSea 2.0 plans to install a 1:3 scale prototype off the coast of California.

Research Project