Prototype for testing: The concrete hollow sphere was manufactured at
a 1:10 scale and has a three-metre diameter.
© Fraunhofer IWES / Energiesystemtechnik
StEnSea pumped storage power plant
09.01.2017
Concrete sphere for the sea successfully stores electricity
How can energy be stored near offshore wind farms in the future?
Researchers have developed a new storage system that works with
hollow spheres on the seabed. The principle is similar to conventional
pumped storage power plants. At the end of 2016, the researchers at
Fraunhofer Institute for Wind Energy and Energy System Technology
IWES tested a three-metre concrete prototype in Lake Constance. The
test results are highly promising.
The schematic shows the principle behind the
StEnSea sphere-based pumped storage
system.
© Hochtief Solutions
How can the enormous amounts of electricity generated by offshore wind
power be temporarily stored on site? Researchers from the Fraunhofer Institute for Wind Energy and Energy
System Technology IWES have now developed a new concept to achieve this. In the "Stored Energy in the Sea"
(StEnSea) project, the scientists have constructed a pumped storage power plant that uses the sea itself as the
upper storage reservoir. Hollow spheres on the bottom of the ocean serve as the lower reservoir. If more wind
power is generated than required, it is used to empty the spheres by pumping. When there is a demand for
electricity, high-pressure water flows back into the sphere via a turbine. The turbine drives a generator that feeds
the energy stored in the sphere into the grid as electricity. This principle will enable large storage capacities to be
installed in future in the vicinity of offshore wind farms.
After several years of research, the first prototype went into the testing phase at the end of November 2016. The
researchers tested the concrete hollow sphere in Lake Constance for one month. The 1:10 scale model with a
diameter of about three metres was lowered to a depth of 100 metres about 200 metres offshore. During the
four-week test, the researchers investigated detailed aspects concerning the construction, installation, design of
the powertrain and electrical system, the operation management and control, the status monitoring as well as the
dynamic modelling and simulation of the overall system. The results are consistently positive. "We actually
managed to successfully save energy and run a variety of different cycles. It worked exactly as we had imagined,"
says StEnSea project manager Matthias Puchta.
The deeper, the better
In large water depths the hollow spheres can utilise the high water pressure. The storage capacity for a 30-metre
sphere at 700 metres is about 20 megawatt-hours. It increases linearly with the water depth for a given volume.
But the ratio of the wall thickness of the concrete sphere to the water pressure also plays a role, explains the
project manager: "In about 750 metres of water depth, the external pressure means that the wall thickness has to
sphere at 700 metres is about 20 megawatt-hours. It increases linearly with the water depth for a given volume.
But the ratio of the wall thickness of the concrete sphere to the water pressure also plays a role, explains the
project manager: "In about 750 metres of water depth, the external pressure means that the wall thickness has to
exactly correspond to how much the sphere has to weigh anyway so that it stands securely on the seabed without
anchoring."
With the results of the pilot experiment, the research team wants to more closely investigate suitable locations for
a demonstration project in Europe more closely. They are aiming for a sphere diameter of 30 metres for the
system's demonstration scale. This is the currently sensible target size in technical and economic terms, given the
underlying engineering conditions. There is considerable potential for using sea-based pumped storage systems
at offshore sites, especially off the coasts of densely populated regions such as in the Norwegian Trench. The EU
has the greatest potential, followed by the USA and Japan.
Project manager Matthias Puchta in interview
In the interview, project manager Matthias Puchta from Fraunhofer IWES talks in detail about the successful test
in Lake Constance and about the cost estimates. He is optimistic about all the places where the new storage
system can be deployed. The full interview is available at forschung-energiespeicher.info.
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