| Pumped storage
concrete sphere with a diameter of 9m at a depth of 500-600m. By emptying the sphere, the storage is charged. When water flows in, electricity is generated, and it is discharged. The power of this prototype is 0.5MW with a capacity 0.4MWh. Funding for the project is being provided by the German Federal Ministry for Economic Affairs and Climate Action, who is committing nearly €3.4 million, plus around US$4million from the US Department of Energy.
Sperra will manufacture the concrete sphere in Long Beach using a 3D printing process, possibly in combination with traditional concrete construction. It will have an opening at the top, into which a pump turbine will be integrated in a pipe. When a valve is opened, water flows through the pipe into the sphere. The integrated pump runs in reverse and works as a turbine. The water drives the motor, generating electricity. Thus, the storage is discharged. The idea dates back to 2011 and was developed
by Prof. Dr. Horst Schmidt-Böcking and Dr. Gerhard Luther. An underwater cable connects to the power grid on land or to a floating transformer station of an offshore wind farm. To store energy, the motor pump pumps the water out of the sphere against the pressure of the surrounding water column. The cycle can then begin again. In a field trial with a 3m sphere in Lake Constance, Fraunhofer IEE researchers and their partners say they’ve already proven that this concept works well.
Ideal depth The capacity and performance of the spherical
storage depend primarily on two factors: the volume of the spheres and the water column pressing on them. Fraunhofer IEE experts have calculated that water depths of 600 to 800m are ideal locations from an economic perspective. There, parameters such as pressure, the required sphere weight, and the necessary wall thickness are in optimal proportion
Above: Successful field test in Lake Constance. © Fraunhofer IEE
Below: Image of installation in deep water together with floating wind turbines. ©Sperra
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