Light from the seabed

An invention with potential: HOCHTIEF engineer Julian Meyer is fascinated when he hears about an idea how to store volatile energy from wind and the sun: a mega hollow concrete body and water pressure at great sea depths. On the basis of the idea published in 2011, Meyer's team set up a project which delivered first results under the management of the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) in 2017. As early as 2018, it was awarded the Innovation Prize of the "Germany - Land of Ideas" initiative.

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Photographers are on the lookout for the best motif. Cameramen are filming the spectacle. When the 20-ton concrete sphere is pulled out of Lake Constance by a truck crane at the beginning of March 2017, it has spent about three months at a water depth of 100 meters. A really big fish on the hook out there. The concrete sphere which was built by HOCHTIEF has met all expectations. It has shown it can store energy, thus also justifying the project's name: StEnSea or "Stored Energy in the Sea.

"StEnSea therefore perfectly qualifies as solution for a key problem of the energy turnaround, viz to keep excess wind power ready for when it is needed," says Stephan Fromknecht, responsible for the project at HOCHTIEF. Fromknecht is one of the engineers whom Julian Meyer has kept enthused for StEnSea since 2011. The team played a major role in the store's success. In particular in the construction of the first model. "Due to its unique material properties, concrete is the suitable building material for StEnSea," says Fromknecht, "and HOCHTIEF therefore also the ideal partner in this project." The model was built in the HOCHTIEF Innovation & Testing Center in Mörfelden-Walldorf, one of the biggest testing laboratories of the German construction industry. It's not for nothing that the staff working at the facility enjoy the reputation of being "concrete whisperers."

Engineering ingenuity.
But how do you build something that has never been built before? With long years of experience and countless simulations of construction stages, transportation and operation statuses that picture reality. The HOCHTIEF engineers divide the production of a storage sphere model with steel fiber concrete into three stages. As formwork, they use a self-carrying system with sandwiched timber boards (see picture 2) and a hexagonal ring for transport and the diving operation in Lake Constance (see picture 3). However, no simulation whatsoever can replace reality. Does the sphere keep what it promises? Yes, it does.

This is how it works:
StEnSea is a hollow body made of concrete and has the shape of a sphere. "Parked" on the seabed, the system can store energy thanks to the high water pressure at great depths. When the store takes in power, water is pumped out of the sphere via an electrical pump. If the stored power is needed elsewhere, water enters into the empty sphere through a turbine and produces electricity via a generator.

Stable conditions.
Wind and sun are intended to help satisfy the growing hunger for energy. The problem is: Wind power and solar facilities do not always deliver electricity when we need it, but when the wind is blowing and the sun shining. In order to function, our power grid has to be supplied permanently with just as much energy as is currently needed. Today, conventional power stations are taken off the grid or are activated when solar and wind power plants deliver too much or too little energy.

If, with Germany's energy turnaround, greater use is to be made of renewable energies, the question arises if and how green power can be stored. This is where StEnSea comes in.

Two professors from Frankfurt and Saarbrücken developed the idea—which sounded like fantasy at the outset—that hollow bodies from concrete on the seabed could function like small pumped-storage power plants and solve this energy turnaround problem. The sphere-shaped stores are meant to act as interim stores of power for periods in which the wind does not blow and the sun does not shine, but in which energy is required nevertheless. On the other hand, they can take in power if wind and solar power production exceeds the demand for energy. A pilot project at Lake Constance was carried out successfully until March 2017. Afterwards, testing greater depths is on the agenda.

For the test in Lake Constance, the HOCHTIEF engineers built a sphere that delivers meaningful data even in relatively low water depths. The next step towards serial production would be a test in the deep sea near the coastline, for example off the Norwegian or Spanish coast. The sphere would then be used in its original size—the size of a real colossus.

Big, bigger, mega.
With a 28-meter diameter—higher than Brandenburg Gate in Berlin—and external walls almost as wide as a highway lane (2.70 meters), a mega concrete sphere is planned to go deep sea diving. On the seabed at a depth of 700 meters, it will have to withstand a pressure of 70 bar and is expected to show in a test then that it can also store power there. When this will happen is still open.

Many spheres. One solution.
HOCHTIEF can build the sphere but what benefits will the structure bring? Reliability and flexibility for the power grid. However, one sphere does not make a summer. A complete "park of spheres", let's say with 80 spheres arranged closely to each other, can supply an entire metropolis with the stored electricity. The researchers of the Fraunhofer Institute have identified suitable locations for using StEnSea off the coasts of Norway, Spain, the USA and Japan. If all locations worldwide were developed, the total electricity storage capacity would be more than 800 terawatt hours according to the researchers' calculations. This is equivalent to five times the power plant output that is fed into the grid in Germany.

The project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) under funding identification numbers 0325584A (HOCHTIEF) and 0325584B (Fraunhofer IWES).

Further information:

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