Deep beneath the surface of the Pacific Ocean, approximately 2,000 feet down, a remarkable concrete sphere, comparable in size to a small house, is poised for a groundbreaking experiment. Designed to withstand pressures that are 77 times greater than those experienced at sea level, this structure is set to serve a unique purpose: storing renewable energy.
This initiative, known as StEnSea (Stored Energy in the Sea), aims to tackle a significant challenge in the clean energy sector: the effective storage of renewable energy during periods when solar and wind resources are not available.
A Revolutionary Energy Storage Solution
The concept behind StEnSea is elegantly straightforward. Imagine a hollow concrete sphere resting on the ocean floor. When there is an abundance of energy, such as from nearby offshore wind farms, seawater is pumped out of this sphere, creating a vacuum inside. When energy is required, the valve opens, allowing seawater to rush back in due to the immense ocean pressure. This influx spins a turbine, generating electricity. The system is designed for repeated use, capable of cycling hundreds of times each year.
In 2017, the Fraunhofer Institute for Energy Economics and Energy Systems Technology (IEE) successfully tested a smaller version of this system in Germany's Lake Constance. Now, preparations are underway for a larger prototype to be tested in the deep waters off Long Beach, California.
This new prototype will measure about 29.5 feet in diameter and weigh around 400 tons, positioned between 500 and 600 meters underwater. It is expected to begin operations by the end of 2026, with the capacity to store up to 0.4 megawatt-hours of electricity -- sufficient to power an average home for two to three weeks.
"This test run represents a significant advancement in scaling the technology," stated Dr. Bernhard Ernst, Senior Project Manager at Fraunhofer IEE. "As the global energy transition progresses, the need for efficient storage solutions will increase dramatically in the coming years."
Transforming the Ocean Floor into a Renewable Energy Reservoir
StEnSea is a modern adaptation of a century-old concept known as pumped-storage hydroelectricity, which typically involves elevating water into a reservoir and then releasing it to generate electricity. However, StEnSea innovatively utilizes the ocean depths instead of requiring two separate water bodies at different elevations.
This approach not only minimizes ecological impacts but also enhances public acceptance. Offshore locations are often near renewable energy sources, such as wind farms, allowing for efficient deployment without land consumption or community opposition. In this system, the deep ocean serves as the upper reservoir, while the concrete sphere acts as the lower one.
The construction of the prototype showcases a blend of cutting-edge engineering and innovation, with Sperra, a U.S. startup specializing in 3D concrete printing, taking the lead on building the massive sphere. The underwater motor pumps, crucial for the operation, are provided by Pleuger Industries, a Miami-based company with German roots.
"Pumped storage systems are ideal for storing electricity for short to medium durations," Dr. Ernst added. "However, their potential for expansion is limited globally."
According to GIS analyses by Fraunhofer IEE, numerous ideal sites for ocean-based storage have been identified, ranging from Norway to Japan and the U.S. East Coast. These locations are characterized by depths of 600 to 800 meters, where pressure and concrete strength can be economically balanced.
While the system's efficiency is around 75 to 80 percent--slightly lower than traditional pumped storage--the concrete spheres are projected to last 50 to 60 years, with turbine and generator replacements needed only every two decades.
The Future of Energy Storage
Although the current design allows for modest energy storage, the technology has significant scalability potential. A network of six large spheres could provide a capacity of 120 megawatt-hours and output of 30 megawatts, cycling 520 times a year. Such installations could facilitate energy arbitrage, allowing for the purchase of electricity when it is inexpensive and selling it when prices rise.
The economic outlook for this technology is promising, with costs estimated at 4.6 euro cents per kilowatt-hour stored, making it competitive against many existing battery technologies. The potential scale of StEnSea is particularly noteworthy; even a fraction of its estimated global storage capacity of 817,000 gigawatt-hours could revolutionize renewable energy management.
This innovative approach to energy storage began with an idea in 2011, conceived by physicist Prof. Dr. Horst Schmidt-Böcking and Dr. Gerhard Luther. Today, it is taking tangible form, illustrating a visionary step towards sustainable energy solutions.
As we strive for a decarbonized future, the challenge is not only generating clean energy but also effectively storing it. Perhaps the answer lies not on land or in traditional technologies, but beneath the waves, waiting to be harnessed.