Image: Pumping wind power underground

This diagram illustrates how a current project, the Iowa Stored Energy Park, plans to use compressed air to store power generated by wind turbines. Electricity from the wind turbines will power motors that compress air many times atmospheric pressure. The air is injected under ground into an aquifer, a dome-shaped structure made of porous sandstone. When demand--and price--for electricity is highest during the middle of the day, the compressed air is released and used to power a generator, and electricity is sold to the grid.

General Compression, a start-up that recently gained seed funding, intends to use compressed-air energy storage but is taking a somewhat different approach. It intends to integrate the compressor directly onto a wind turbine. Storage can be in geologic formations like aquifers, or pipelines.