Tom Lombardo for Enegineering.com:  Grid-level energy storage takes many forms, including flow batteries, Li-ion batteries, pumped hydro, compressed air in underground caverns, and even flywheels. Toronto’s Hydrostor just added another tool to the arsenal: underwater compressed air energy storage (UCAES). Hydrostor recently activated a pilot UCAES plant - the first of its kind - that will provide grid-level storage for the city of Toronto. In addition to supplying the city with cost-effective energy storage, the system will allow engineers to study its behavior and optimize the design. Can UCAES become a viable energy storage technology?
The idea has been around for many years: when supply exceeds demand, use the excess energy to run an air compressor and store the air in an underwater balloon. When power is needed, open a valve and let the compressed air run a turbine to generate electricity. The principle is simple, but the economic feasibility has yet to be demonstrated. Hydrostor hopes that their facility becomes the proving ground.  Cont'd...