Pumped hydro energy storage has been around in the US & Europe since the 1920’s. There are over 40 pumped hydro facilities throughout the US. The concept is pretty basic, pump water uphill to a reservoir then when electricity demand is high the water is released back through the system to help balance the grid. The technology has new found interest with the advent of Wind & Solar power, which has created a new boom in the need for energy storage. A pumped hydro storage & renewable energy hybrid system could provide the large scale storage we would need to curb our fossil fuel dependency from coal & natural gas electricity power plants.
More info on Pumped Hydro Energy Storage can be found here.
Another similar concenpt, has been tested with underwater energy storage spheres with the same basic principles. Earlier this year the German research institute Fraunhofer Institute for Wind Energy & Energy Systems Engineering successfully tested a three meter diameter sphere that contained a pump and turbine. Like the traditional land based hydro pumps shown above, the sphere technology takes advantage of when power costs are low, water is pumped out of the spehere, and when electricity is scarce water can be let back into the sphere. The institute envisions dozens of these underwater spheres, which would create an “energy park”, which could be connected to an offshore wind farm and could store as much as 20Mwh of power each. More information on this project can be found here.