I read with interest the other day about plans in northern Ireland for a relatively unique way of energy storage, and the first of it's kind within the UK. The plan would be to use cheap electricity to pump air into underground caverns. Then, when the electricity price increases, release this compress air through generators to produce electricity. In particular this will be useful as it will provide a good way of matching up electricity with electricity demand, which of course is one of the biggest problems with most forms of renewable electricity generation. This is not completely unproven technology, as there are also schemes in Germany and America, and so presumably there is less risk.
I also see another potential economic benefit of such energy storage schemes - as the energy is generally stored at peak output this means there will be less need for expensive grid works. For example, if a wind farm would normally produce a peak output of 30MW, and do so for a third of the time and for the other two thirds of the time under a conventional scheme where all eletricity is pumped straight into the grid, the local grid will need to be able to cope with 30MW of extra generation. However, if an energy storage scheme is used, the output could be changed to 10MW all of the time, meaning less has to be spent on grid re-inforcement and there will be greater predictability in production, meaning less need for back-up generators.
The only issue will be whether such storage schemes will qualify for ROCs, and if so at what rate.