The Stored Energy at Sea (StEnSEA) project is a pump storage system designed to store significant quantities of electrical energy offshore. After research and development, it was tested on a model scale in November 2016. It is designed to link in well with offshore wind platforms and their issues caused by electrical production fluctuations. It works by water flowing into a container, at significant pressure, thus driving a turbine. When there is spare electricity the water is pumped out, allowing electricity to be generated at a time of increased need.
Development history In 2011, the physics Prof. Dr Horst Schmidt-Böcking (Goethe University Frankfurt) and Dr. Gerhard Luther (Saarland University) had the idea of a pump storage system that would be placed on the sea bed. This system would use the high water pressure at great water depths to store energy in hollow bodies. Shortly after their idea was published on 1 April 2011 in the newspaper Frankfurter Allgemeine Zeitung, a consortium of the Fraunhofer Institute for Energy Economics and Energy System Technology and the construction company Hochtief AG was set up. In collaboration they conducted a first preliminary sketch, which proved the feasibility of the pump storage concept. Subsequently, the German Federal Ministry for Economic Affairs and Energy supported the development and testing of the new concept.
Physical principle The functionality of a seawater pressure storage power plant is based on usual pumped-hydro storage plants. A hollow concrete sphere with an integrated pump-turbine will be installed on the bottom of the sea. Compared to well known pumped-hydro storage plants, the sea that surrounds the sphere represents the upper water basin. The hollow sphere represents the lower water basin. The StEnSea concept uses the high water pressure difference between the hollow sphere and the surrounding sea, which is about 75 bar (≈1 bar per 10 meters). In case of overproduction of adjacent energy sources such as wind turbines or photovoltaic systems, the pump-turbine will be enabled to pump water from the cavity against the pressure into the surrounding sea. An empty hollow sphere means a fully charged storage system. When electricity is needed, water from the surrounding sea is guided through the turbine into the cavity, generating electricity. The higher the pressure difference between hollow sphere and the surrounding sea, the higher the energy yield during discharging. While discharging the hollow sphere a vacuum will be created inside. To avoid cavitation, the pump turbines and all other electrical components are placed in a centrally mounted cylinder. An auxiliary feed pump in the bottom of the cylinder is required to fill the cylinder with water and produces an inside pressure. "Both pumps require an input pressure above the net positive suction head to avoid cavitation while pumping water from the inner volume into the cylinder or from the cylinder out of the sphere. As the pressure difference for the additional pump is much lower than for the pump turbine the required input pressure is lower as well. The input pressure of both pumps is given by the water column above them. For the additional pump this is the water column in the sphere and for the pump turbine it is the water column in the cylinder." The maximum capacity for the hollow concrete sphere depends on the total pump-turbine efficiency, the installation depth and the inner volume.
C m a x = ρ w a t e r ⋅ η t u r b ⋅ d ⋅ g ⋅ V i n n e r 3 , 69 E 9 {\displaystyle C_{max}={\frac {\rho _{water}\cdot \eta _{turb}\cdot d\cdot g\cdot V_{inner}}{3,69E9}}}
The stored energy is proportional to the ambient pressure in the depths of the sea. Problems considered during the construction of the hollow sphere were choosing a construction-type that withstands the high water-pressure and which is heavy enough to keep the buoyancy force lower than the gravitational force. This resulted in the spherical construction with an inner diameter of 28.6 meter and a 2.72 meter thick wall made of normal watertight concrete.
Pilot test To prove feasibility under real conditions and to acquire measurement data, the Fraunhofer engineers started implementing a pilot project. Hochtief Solutions AG constructed a pilot hollow sphere at a scale of 1:10 out of concrete, with an outer diameter of three meters and an inner volume of eight m3. On 9 November 2016 it was installed in Lake Constance at a depth of 100 meters and tested for four weeks. During the test phase, the engineers were able to successfully store energy and operate the system in different operating modes. The engineers also studied whether a pressure equalization line to the surface is required. In case of application without the compensating cable, a reduction of costs and expense would be possible. The pilot test revealed, that both operation variants work and would be possible to run. In the next step, a possible test location in the sea for the carrying out of a demonstration project is to be scrutinized. Then a sphere with the planned demonstration diameter of 30 meters should be built and installed at a suitable location in the sea. Possible places of installation situated near a coast would be for example the Norwegian trench or some Spanish sea areas. Furthermore, partners from the industry financing half of the project must be found, in order to receive further public funding from the BMWi. Because the total costs for the demonstration project are estimated at a low double-digit million euro amount.
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