One year ago, scientists sank a concrete sphere one hundred metres below the surface of Lake Constance. The three-metre-diameter hollow body provided a functional model for an offshore pumped storage power plant. Now they have received the “German Renewables Award 2017” for their research work. The developers at Fraunhofer IWES are already planning the next step.

How can surplus wind power from offshore wind turbines be stored right on the spot? Horst Schmidt-Böcking from Frankfurt University and Gerhard Luther from Saarland University in Saarbrücken came up with the idea of constructing a pumped storage power plant on the seabed. The basic principle is simple: hollow concrete spheres are sunk into deep water. When the wind blows strongly, electric pumps pump water out of the spheres. The stored energy can then be accessed when needed. Water then flows under high pressure through a turbine into the empty sphere and produces electricity by means of a generator.

With a diameter of 30 metres and a water depth of 700 metres, a concrete sphere can store around 20 MWh.

The Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) in Kassel showed that the concept works during a four-week trial. For the “Stored Energy in the Sea (StEnSea)” project, they sunk a functional, 1:10 scale model into Lake Constance in November 2016, lowering it to a depth of about 100 metres.

The test results have been highly promising. “We actually managed to successfully store energy and drive a diverse range of different cycles,” said Project Manager Dr Matthias Puchta in an interview.

German Renewables Award

The project, which was funded by the German Federal Ministry for Economic Affairs and Energy as part of its Energy Storage research initiative, received the German Renewables Award in the “Project of the Year” category. The eight-member jury from science and industry conferred the award at a gala held in the International Maritime Museum in Hamburg's HafenCity.

Matthias Puchta: “In the next stage we're aiming to test a three times larger concrete sphere in the sea, which can then store about 50 to 100 times more energy. Such storage systems can be economically applied at sea depths of 600 to 800 metres. Possible locations lie in particular off the coasts of Europe, Japan and the USA. We have identified a potential around 1,000 times the pumped storage capacity currently installed worldwide – which will make an important contribution to the international energy transition.”

The project forms part of the Electricity Storage research initiative and is documented on the initiative's website. In an interview, Matthias Puchta explained how the pumped storage power plant works, which problems have to be solved and what potential exists worldwide.

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