The Fraunhofer Institute is planning to test a new storage concept in a German lake before the end of this year. The storage idea, which involves placing hollow concrete globes on sea or lake beds, resembles an underwater balloon technology already developed by Ontario, Canada-based Hydrostor. In reality, the two concepts are quite different, however.

While Hydrostor’s balloons use water pressure to deliver a novel form of compressed air energy storage (CAES), the Fraunhofer project, called StEnSEA (Stored Energy in the Sea), is essentially a variant of pumped hydro.

“Instead of a lower and upper reservoir, the system uses a pressure tank as a lower reservoir placed on the seabed at 600 to 800 meters,” said Matthias Puchta, head of the Department of Energy Storage Systems within the Fraunhofer's energy process engineering division. “The water column above the entrance of the concrete sphere acts as an upper reservoir, without severe ecological and visible impact.”

The idea is the brainchild of physicists Dr. Horst Schmidt-Böcking and Dr. Gerhard Luther, and is expected to have a similar efficiency to conventional pumped hydro plants.

It is designed to use materials and system components that are relatively inexpensive, so costs should be in the same range as traditional pumped hydro.

“Because the pressure difference between the inside and the outside of the sphere almost stays constant, alternating loads to the sphere are small,” Puchta said. “Hence concrete can be used as pressure tank material, which is also heavy enough to safely install the sphere on the seabed without anchors.”

Depending on the number of hours of operation a year, Fraunhofer estimates a 400-megawatt subsea storage farm could deliver a levelized cost of storage of between €40 to €200 ($50 to $230 U.S.) per megawatt-hour.

This compares to an unsubsidized levelized cost of storage of between $188 and $274 per megawatt-hour for traditional pumped hydro, as calculated by Lazard.

A worldwide survey had identified potential StEnSEA sites with a cumulative storage capacity of around 817 terawatt-hours, Puchta said.

The StEnSEA concept is being developed by Fraunhofer in association with Hochtief, a construction company that has assembled a 1:10-scale sphere for testing at 100 meters of depth in a pilot funded by the German Federal Ministry for Economic Affairs and Energy.

If the test is successful, Fraunhofer intends to move to a pre-commercial pilot within three to five years, followed by full commercialization.

In the meantime, Hydrostor’s underwater CAES-based technology is also moving quickly toward commercial viability.

Curtis VanWalleghem, Hydrostor’s CEO, told GTM the company would be announcing a new commercial contract and various partnerships in early September. “We will begin marketing the technology as commercially ready this fall,” he said.

Hydrostor finished assembling a demonstration system eight months ago, delivering energy to the grid via local distribution company Toronto Hydro. It has already signed up one commercial customer, a water and electricity provider on the island of Aruba in the Caribbean.

Hydrostor’s technology, which like StEnSEA uses mostly off-the-shelf components, is a good fit for long-duration, grid-scale applications requiring upward of 10 megawatt-hours of storage close to deep water or caverns, said VanWalleghem.

It has no emissions, net positive environmental impacts and can last more than 30 years with unlimited cycling, he claimed.

StEnSEA requires deep water and all its mechanical parts are located at depth within the dome. Hydrostor’s compressor, air turbine and other major components were all designed to be onshore and connected to the balloons by a drilled airline.

“The two technologies are fundamentally different,” VanWalleghem said.

Although VanWalleghem would not disclose figures for the levelized cost of storage, he said: “Hydrostor is less than half the capital cost of lithium-ion batteries on a turnkey project basis, and lasts more than three times as long.”

However, CAES companies have long claimed superior cost and performance -- and have not lived up to their hype. Will an underwater approach bring a different outcome?