Researchers test first sphere power plant off California

Underwater energy storage Researchers test first sphere power plant off California

2024-11-20 Source: Fraunhofer IEE | Translated by AI 3 min Reading Time

The Fraunhofer Institute for Energy Economics and Energy System Technology (IEE) has developed an underwater energy storage system that transfers the principle of pumped storage power plants to the seabed. After a successful test in Lake Constance, the researchers now plan a prototype test off the California coast.

In the "StEnSea" project, researchers will anchor a hollow, 400-ton concrete sphere with a diameter of nine meters at a depth of 500 to 600 meters. The storage is charged by pumping it empty. As water flows in, electricity is generated—it is discharged.(Image: Hochtief) — In the "StEnSea" project, researchers will anchor a hollow, 400-ton concrete sphere with a diameter of nine meters at a depth of 500 to 600 meters. The storage is charged by pumping it empty. As water flows in, electricity is generated—it is discharged.
(Image: Hochtief)

"Pumped storage power plants are particularly well-suited for storing electricity over several hours to a few days. However, their expansion potential is severely limited worldwide. Therefore, we are transferring their functional principle to the seabed—the natural and ecological restrictions are much lower there. Additionally, public acceptance is likely to be significantly higher," explains Dr. Bernhard Ernst, Senior Project Manager at Fraunhofer IEE.

In this project, Fraunhofer IEE is working with the US-based startup Sperra, which specializes in 3D concrete printing for applications in renewable energy. The second partner is Pleuger Industries. The German-rooted company, headquartered in Miami, is among the leading manufacturers of submersible motor pumps, a key component of the StEnSea sphere storage system.

Gallery

Charging: Using an electrically powered pump turbine, the water is pumped out of the sphere. Discharging: The water flows back into the empty sphere, causing the pump turbine to operate in reverse as a turbine and generate electricity via a generator.(Image: Fraunhofer IEE)

Production of the 30m spheres for a StEnSea park.(Image: Sterra)

Field test with a three-meter sphere in Lake Constance.(Image: Fraunhofer IEE)

This is how an installation in deep water together with floating wind turbines could look.(Image: Sperra)

The partners have selected a coastal area off Long Beach near Los Angeles as the storage location. They plan to have it operational by the end of 2026 at the latest.

Sperra will manufacture the concrete sphere in Long Beach using 3D printing, possibly in combination with traditional concrete construction.
It will have an opening at the top into which an underwater motor pump, also known as a pump turbine, will be installed in a pipe.
When a valve is opened, water flows through the pipe into the sphere.
The integrated pump runs in reverse and operates as a turbine.
The water drives the motor, thus generating electricity.
This discharges the storage.

The technology is based on a work by Prof. Dr. Horst Schmidt-Böcking and Dr. Gerhard Luther from 2011. An underwater cable establishes the connection to the power grid on land or to a floating transformer station of an offshore wind farm. To store energy, the motor pump expels water from the sphere against the pressure of the surrounding water. The cycle can then begin again.

In a field test with a three-meter sphere in Lake Constance, researchers from Fraunhofer IEE, together with partners, have already demonstrated that this concept works well.

Water depths of 600 to 800 meters are ideal

The capacity and performance of the sphere storage systems primarily depend on two factors: the volume of the spheres and the column of water pressing on them. Experts at Fraunhofer IEE have calculated that water depths of 600 to 800 meters are ideal locations from an economic perspective. This is because parameters such as pressure, necessary sphere weight, and required wall thickness are in optimal proportion to each other. Moreover, conventional submersible motor pumps can still be used at these depths. Additionally, there is no need to use high-strength specialty concrete here.

Huge global potential

According to calculations by Fraunhofer researchers, the global storage potential of this technology is a total of 817,000 gigawatt hours. At the ten best European locations, it is still 166,000 gigawatt hours. For comparison: the capacity of existing German pumped storage power plants on land is just under 40 gigawatt hours.

Cost and capacity

The researchers at Fraunhofer IEE estimate the storage costs at around $0.046 per kilowatt-hour, with investment costs of $1.43 per kilowatt of power and $166,86 per kilowatt-hour of capacity. The lifespan of the concrete sphere is between 50 to 60 years. The pump turbine and generator need to be replaced every 20 years. The efficiency, based on an entire storage cycle, is somewhat lower than that of a traditional pumped storage power plant, ranging from 75 to 80 percent. This calculation is based on a storage park with six spheres, a total output of 30 megawatts, and a capacity of 120 megawatt-hours, with 520 storage cycles per year.

The StEnSea sphere storage systems are particularly suitable for two business models: on the one hand, for arbitrage trading, i.e., buying electricity at low and selling at high market prices, and on the other hand, for providing balancing reserve, with which grid operators stabilize power grids.

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