Here is a magnificent piece on energy storage: a perfect blend of the basic science and economics. It’s a very readable explanation of how and why storage will be required to integrate significant amounts of renewables on the grid.
Today, the vast majority of energy storage is accomplished with pumped hydro. The problems with this, however, are a) only certain (hilly) terrains are appropriate to implement this inexpensively, and b) the stored energy is not portable, i.e., it cannot be used in transportation, which accounts for 40% of our total energy needs.
This gives rise to chemical solutions, also discussed in the piece. Personally, I’m betting on Windfuels, a 2GreenEnergy client, which sports a remarkable breakthrough in synthetic fuels.