Energy storage on the bottom of the sea
Why not store energy In hollow tanks on the bottom of the sea? The German Fraunhofer Institute started a test with spherical storage tanks.
In November 2016, the first tank has been placed at the bottom of the Bodensee, near the German border with Switzerland.
Ultimately, the aim is to place the tanks in the area of offshore wind farms. But for now, the institute choose the lake because it is more practical for the tests.
“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.”
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.
The test-storage tank
The spherical bottom tanks are hollow. If there is a surplus of electricity, that energy will be pumped to an electric pump that empties the tanks. The production of electricity is done by filling the tanks again with water pressure. The water will start a generator that produces the stored electricity.
The test tank has a diameter of 3 meter. Eventually, the drums will be ten times as large. A 30 meter drum has a storage capacity of 20 MWH and a discharge time of four to eight hours. It is primarily intended for long-term storage of large amounts of energy.
According to the Stensea concept (Stored Energy in the Sea), a windpark would request about eighty large storage tanks. But it is easy to scale the system.
The charge and discharge cyclus
A major limitation of the system is that it works mainly at water depths from 600 to 800 meters. The water column is then high enough to supply pressure for the generators:
- the existing pump generators are designed for a water column of 700 meters deep
- at that depth, constructors can use a simple concrete structure. There is no need for high-strength concrete
- windparks near the coast usually have water depths of around 30 meters. It’s uncertain wether the pump generators can be redesigned for this kind of depths.
- At the test site in the Lake Constance, the water is about 100 m deep.
Costs storage on the bottom of the sea
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. Ultimately the costs will decrease by the scale of the systems and the value of the energy with this kind of energy system.
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.
If the test is successful, Fraunhofer intends to move to a pre-commercial pilot within three to five years, followed by full commercialization.