by Allison Kerley
Slocum et al. (2013) propose a new design for an energy storage and generation unit composed of underwater concrete spheres and offshore wind turbines. The proposed design utilizes pumped storage hydraulics (PSH). During times of low energy demand from the grid, the cylinder would contain water at equal pressure with the surrounding ocean. In the proposed design, the floating wind turbines generate energy and the excess energy is used to pump water out of the storage sphere, creating a vacuum. When energy is needed from the sphere, the turbine would open, allowing water to pass through into the sphere. The proposed sphere design would have an inside diameter of 25 m, and would retain a 1/20th-atm environment when fully discharged. The proposed design could be used without alteration in depths between 200 and 700 m, and would continue to be economically feasible to a depth of approximately 1500 m. The authors tested a small-scale dry version of the proposed design, with the test sphere having an inner chamber diameter of 75 cm, with a ten meter height difference from the top of the pump and wind turbine to the top of the sphere. The test unit was found to have a low round-trip efficiency of 11%, which the authors attribute to their inability to use the most efficient pump and turbine technology due to the small size of their test model. They calculated that in a full scale model, the lowest round-trip efficiency would be 70%.
The steel-fiber reinforced concrete and glass-fiber concrete are indicated as the ideal materials for the creation of the sphere, as both have been shown to minimize cracking under high pressures. The authors also mention that further research is needed regarding corrosion of the pumps, clogging from sediment ingestion during turbine operation, and the effects of and on nearby marine life. However, they also propose the spheres as potential artificial coral reefs and marine habitat. In addition, the authors calculated a conservative 20 year lifespan for the spheres and turbines. However, as various concrete offshore platforms in the North Sea have continued to operate for over forty years, the authors estimate that depending on the materials used, their spheres and wind turbines could last a great deal longer than 40 years. At the end of the lifespan of the pump system, the pump/turbine units and transmission lines would be salvaged, but as the spheres themselves could be left in place as the structural materials are believed to be harmless to marine life. Lastly, the authors found multiple potential sites world-wide with wind speeds necessary to power the turbine, underwater terrain structurally sound enough to hold the units in place, and close enough to population centers for the energy to be able to reach consumers.
Slocum, A.H., Fennell, G.E., Dundar, G., Hodder, B.G., Meredith, J.D., Sager, M.A., 2013. Ocean renewable energy storage (ORES) system: Analysis of an undersea energy storage concept. Proceedings of the IEEE 101, 906-924. http://dspace.mit.edu/openaccess-disseminate/1721.1/78934