Underwater energy storage using compressed fluid

A compressed fluid energy storage system includes a submersible fluid containment subsystem charged with a compressed working fluid and submerged and ballasted in a body of water, with the fluid containment subsystem having a substantially flat portion closing a domed portion. The system also includes a compressor and an expander disposed to compress and expand the working fluid. The fluid containment subsystem is at least in part flexible, and includes an upper portion for storing compressed energy fluid and a lower portion for ballast material. The lower portion may be tapered proximate the flat portion to prevent it from being collapsed by ballast materials. The region between the fluid and the ballast has exchange ports to communicate water between the inside and outside of the containment subsystem. In other embodiments, an open-bottomed fluid containment system is held in position underneath a ballast system by a tensegrity structure.