Concrete spheres store electricity under water more efficiently than batteries

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Electricity can not only be stored in batteries, but also in concrete spheres. The Fraunhofer Institute for Energy Economics and Energy System Technology (IEE) demonstrated this years ago in Lake Constance. Now it wants to test the system on a larger scale off the coast of California.

The hollow concrete spheres are located underwater and make use of the pressure of the water column. If there is excess electricity in the grid, they are sucked dry by pump turbines, creating a vacuum. To recover the energy, water flows back into the spheres at high pressure, driving the pump turbines and generating electricity. An underwater cable transmits it to land.

The first pilot test on Lake Constance as part of the "StEnSea" project took place back in 2016. At that time, the IEE sank a three-metre sphere at a depth of 100 meters. In California, off the coast of Long Beach, a nine-metre sphere is now to be sunk at a depth of 500 to 600 meters. Such a spherical storage facility has an output of 0.5 megawatts and can store 0.4 megawatt hours of electricity.

Pump manufacturer Pleuger Industries and start-up Sperra are involved in the project, which will produce the concrete sphere "using a 3D printing process, possibly in combination with traditional concrete construction", according to an IEE press release.

The advantage of spherical storage lies in costs and capacity

According to the IEE, the system's efficiency of 75 to 80 percent is slightly lower than that of conventional pumped storage plants. The advantage of concrete spheres over batteries and pumped storage: lower costs and high capacity. For a storage park with six spheres, a total output of 30 megawatts and 520 cycles per year, the storage costs should be around 4.6 cents per kilowatt hour, according to the IEE.

The depth of the water is decisive for the economic efficiency. According to calculations by the Fraunhofer experts, 600 to 800 meters are ideal: "This is where parameters such as pressure, the required sphere weight and the required wall thickness are in optimal proportion to each other."

The IEE sees suitable locations near the coast in Norway, Portugal, Brazil, Japan or the US west and east coasts, as well as in deep lakes or flooded open-cast mines. "The seabed should be reasonably flat and level, but does not need to be prepared before the sphere is installed," says project manager Bernhard Ernst to the MIT Technology Review.

According to the IEE, the ten best European locations alone offer a potential of 166,000 gigawatt hours. By way of comparison, all German pumped storage power plants have a capacity of around 40 GWh, while the battery storage systems currently installed have a capacity of just under 17 GWh.

Two business models

There are two main business models for the spherical storage units: arbitrage trading of exchange electricity and the provision of balancing energy to stabilize the electricity grids.

The researchers estimate the service life of the concrete sphere at 50 to 60 years. The pump turbine and generator would have to be replaced after every 20 years. "The storage system consists of the outer concrete sphere and the inner technical unit with pump, measurement and control technology," explains Bernhard Ernst. "We can bring this technical unit to the surface for maintenance and repairs. The concrete sphere remains on the seabed."

Apart from the dimensions of the spheres, little has changed in terms of design. "During the test in Lake Constance, we laid a venting hose on land and connected it to the sphere via a valve," says Ernst. "As there was no difference between an open and closed valve during operation, we are dispensing with the hose in the current project."

Goal: 30-meter spheres

Before California, the main aim now is to "test the technology at great depths and try out different logistics and installation. This time we won't be using a crane and the sphere will be manufactured directly in the port."

The plant should be in operation by the end of 2026. The next goal is spheres with a diameter of 30 meters. The German Federal Ministry of Economics is funding the project with almost 3.4 million euros, while the US Department of Energy is providing around four million US dollars.

The Dutch Ocean Grazer project is pursuing a similar approach. However, it relies on a closed pressure bubble that does not require any exchange with the surrounding water. This prevents marine life from entering. The StEnSea concept, on the other hand, relies on a close-meshed sieve to prevent this.

This article first appeared on t3n.de .

(vbr)