German Researchers Start Testing Offshore Wind Energy Storage System

German researchers have started trialing a 1:10 scale model of the StEnSea (Stored Energy in the Sea) undersea compressed air energy storage system designed to temporarily store electricity generated by offshore wind turbines.

After several years’ research work, the StEnSea project, funded by Germany’s Federal Ministry for Economic Affairs and Energy, has now entered the test phase.

In the framework of this project, the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) is now developing to application level the “marine egg” invented by Dr. Gerhard Luther from Saarland University and Horst Schmidt-Böcking, emeritus professor at Goethe University Frankfurt.

The scale model with a diameter of about three meters was brought to the ferry terminal in Constance on 8 November and lowered in the Lake Constance to a depth of 100 meters, about 200 meters from the shore in Überlingen, the next day.

“Pumped storage power plants installed on the seabed can use the high water pressure in very deep water to store electrical energy with the aid of hollow spheres,” said Horst Schmidt-Böcking, emeritus professor at Goethe University Frankfurt.

To store the energy, water is pumped out of the sphere using an electric pump; and to generate power, water flows through a turbine into the empty sphere and produces electrical energy via a generator.

The system will be tested for four weeks.

“On the basis of our preliminary study, we carried out a detailed systems analysis with a design, construction and logistics concept for the pressure tank, developed a turbine-pump unit, examined how to connect the sphere to the electricity grid, calculated profitability and drew up a roadmap for the system’s technical implementation,” Project Manager Matthias Puchta from Fraunhofer IWES said.

“The four-week model trials on the scale of 1:10 are starting now in Lake Constance. We will run various tests to check all the details concerning design, installation, configuration of the drivetrain and the electrical system, operation and control, condition monitoring as well as dynamic modeling and simulation of the system as a whole.”

Based on the results of the model trials, the researchers will look for the most suitable areas for a demonstration site in Europe, IWES Head of Division Jochen Bard said.

IWES plans to deploy a sphere with a 30-metre diameter for the demonstration-scale system, which according to Bard, is the most practical size in terms of engineering.

”What’s already certain is that the system can only be used economically in the sea at depths of about 600-800 meters upwards. Storage capacity with the same volume increases linearly with the depth of the water and at 700 meters is about 20 megawatt hours (MWh) for a 30 m sphere,” Bard said.

Bard sees great potential for the use of marine pumped storage systems near highly populated coastal areas.

”With a storage capacity of 20 MWh per sphere and standard technology available today, we can envisage a total electricity storage capacity of 893.000 MWh worldwide. This would make an important and inexpensive contribution to compensating fluctuations in electricity generation from wind and solar power,” Bard said.

Photos: Fraunhofer IWES