Pumped hydroelectric storage makes up most of the total electricity storage capacity in the United States. However, non-hydro storage has doubled in electric power sector capacity from 160 megawatts (MW) to nearly 350 MW over the past several years, according to an article released today from the U.S. Energy Information Association.
Today, almost 100% of all U.S. electricity storage is pumped hydroelectric storage - which involves pumping water to a higher elevation reservoir during off-peak demand and then running it back down through a turbine during peak demand.
Nonhydro storage systems, unlike hydroelectric systems, include compressed air, batteries, and flywheels, and can provide several ancillary services to the grid, including regulating the grid's frequency on an intermittent basis (either second-to-second or minute-to-minute) and fast-ramping capacity to cover sudden and unexpected gaps between electric demand and supply.
Recent Federal Energy Regulatory Commission (FERC) orders support using electricity storage through creating market incentives for those services, and are in addition to state initiatives to develop energy-storage related policies. For example, New York proposed a new platform in 2014 for energy delivery, Reforming the Energy Vision, which explains the need for a competitive marketplace for distributed technologies, including energy storage.