Lots of methods for storage are being tested. They're working on one called virtual storage that's supposed to be promising. I have no idea how it works.
I read about a company in the Netherlands that has come up with an undersea storage method. Many of their windmills are offshore in fairly deep water (the article didn't say how deep). They put some tough elastic bladders on the sea floor below the windmills. Excess power is used to force water into the bladders. When more power is needed, the weight of the sea water above the bladders squeezes out the water in them through tubes to an underwater turbine for generation. These need to be in fairly deep water to be practical. The Great Lakes might work fine. They say they're working on a similar method that will work in shallower water.
