Storing energy at sea
The development and research project “StEnSEA” (Stored Energy in the Sea) is investigating the installation of large storage facilities on the sea floor, in combination with offshore wind farms. The physical principle on which the energy storage facility operates is similar to that of conventional pumped storage power plants, but based not on two reservoirs, but a hollow sphere. The inflowing water drives a turbine to generate electricity. When there is a surplus of electricity in the grid, part or all of the water is pumped out of the sphere. The commercial target size per sphere is currently at about 20 MWh per storage unit.
|Project status||Implementation of a pilot project (3-meter sphere, 100 meters water depth)|
|Project duration||January 2013 until June 2017|
The concept StEnSEA (Stored Energy in the SEA) of the pumped storage in the sea uses the sea as upper reservoir where the pressure gradient roughly corresponds to the depth of water. An artificial cavity with an integrated reversible pump turbine is placed on the sea bed as the lower reservoir. When power is needed, water flows into the sphere and drives the turbine thus generating power. If surplus power is available, water can be pumped out of the sphere again, thus effectively charging the storage system. This StEnSEA concept allows the installation of large storage capacities in the immediate vicinity of future offshore wind parks.
Functional principle of the sea storage
The functional principle is similar to ordinary pumpedstorage plants: when power is needed, water flows into the sphere and drives the turbine thus generating power. If surplus power is available (usually during the night), water can be pumped out of the sphere again, thus effectively charging the storage system. This innovative concept uses the sea itself as upper reservoir.
A hollow sphere with an inner diameter of 30m will be submerged to a water depth of about 700m, so the hydrostatic water pressure creates an energy potential. Due to this applied pressure, electrical energy can be generated with the help of turbines and generators as the water flows into the sphere. A cable connection to the transformer station and from there to the mainland makes the transport of electrical energy possible. The other way around, exess energy, for example from renewable sources, can be used to pump out water from the hollow sphere. The commercial target size per sphere is currently at about 20 MWh (4 hours discharge time for a 5 MW pump turbine) per storage unit.
For the time being, the current approaches of the feasibility study are based on a maximum water depth of 700 m. This is based on the fact, that there are state-of-the-art turbines that can function in such water depths. Construction and installation would be possible in even greater water depths without problems. However, in such case a separated pump and turbine system must be used and/or the turbine technology should be further developed.
The focus is on the development of a competitive storage system. The pumpturbine technology is that is aimed to use is generally available, only a few project-specific adaptations are required. Crucial are the construction costs of the storage unit and the production of the hollow sphere on the seabed.
The commercial target size per sphere is currently at about 20 MWh (4 hours discharge time for a 5 MW pump turbine) per storage unit. Larger storage capacity can be reached, when multiple spheres are connected to a so called energy park.
Larger storage capacity can be reached, when multiple spheres are connected to a so called energy park. For this size, the storage costs per storage unit are around a few euro cents / Kwh and the construction and equipment costs are in the range of 1.200-1.400 €/kW. For comparison, the costs for a state-of-the-art PSH are between 1.000-1.500 €/kW. The underlying studies and calculations concerning the storage capacity assume a charge-discharge efficiency of 80-85 %. For the commercial use it is planned to connect a large number (> 80) of the storage units to achieve a relevant overall performance/capacity for the energy market.