Researchers at MIT are working on a way to solve the intermittency problem of offshore wind by creating an energy storage system made from huge concrete spheres.
The spheres, which weigh thousands of tons each, would be placed on the seafloor under floating wind turbines. They would anchor the turbines as well as store energy for them, which would be released as needed.
When wind turbines produce excess power, it would be diverted to drive a pump that's attached to a sphere, removing seawater from a 30-meter-diameter hollow sphere - about the size of the dome on the US Capitol. Later, when power is needed, water would be allowed to flow back into the sphere through a turbine attached to a generator, and the resulting electricity would be sent to shore.
One sphere in 400-meter-deep water could store up to 6 megawatt-hours of power. If spheres were attached to 1,000 wind turbines, that could replace an average coal or nuclear plant. And the energy source would be available (or taken off-line) in minutes, unlike coal or nuclear, which take hours to ramp up.
http://www.sustainablebusiness.com/...
Upside, those spheres don't have to serve as anchors to a wind farm. You could sink some into the Ocean (or great lakes) then store power from onshore wind. Downside, this proposal uses a LOT of concrete. About the same as found in Hoover dam. Concrete is a major emitter of CO2. This can reduced by using concrete made, in part, from fly ash from coal power plants. At about $12 million a sphere, designers estimate a price of $0.06 per KWH for stored energy. That might be a bit steep on top of the price of the wind turbine.
The suggested sphere has three meter thick walls. I can't help but wondering if there might not be a solution using a different material that would work work better.