This is an interesting idea that came out of the Advanced Research Projects Agency - Energy conference last week. Atmocean has developed a system called WEST (Wave Energy Seawater Transmission) which is an interesting take on wave energy devices in that it captures CO2 while generating energy. But the most interesting part of the system is the energy storage portion which was developed by Bright Energy Storage Technologies. From their website:
Bright Energy Storage Technologies is developing an ultra-low-cost underwater energy storage system. Energy from the electric grid, or from an offshore renewable energy source, compresses air, stores it in vessels at the bottom of a body of water, and then generates electricity when released back to the grid. Bright Energy storage systems yield dramatic reductions in the net cost and availability of grid-scale energy storage. They can be deployed offshore, in lakes, and in many reservoirs.
Storing air at hydrostatic pressure, in equilibrium with the surrounding water allows the use of thin-walled storage vessels, and simpler, more reliable compressors. A simple, proprietary reversible air compressor runs at near-constant temperature and uses surrounding water as a heat sink and source, eliminating the need for fossil fuel.
The stored energy could come from any renewable source, not just wave power. An obvious contender would be off-shore wind farms, but it could come from shore-side generation as well and might even work in deep enough lakes.
This information comes from a post on Matter Network where you can see some other interesting inventions from the ARPA-E convention like new ideas for geological compressed air storage and retrieval, printable LED light "paper", and micro chillers that use fuel cell technology.