aboveground or near-surface pressured-air pipelines, which can be cost-effective for about 2 to 4 hours of energy.

EPRI studies have found that approximately three-fourths of the United States has geology that is potentially suitable for locating reliable underground CAES systems (EPRI, 2008). The Alabama Electric Cooperative built (with EPRI support) the first U.S.-based CAES plant, with a capacity of 110 MW for 26 hours. Because the plant was the first of its kind, the cost was high ($800/kW). With new CAES plants projected to cost in the range of $500–600/kW, CAES will be a viable option for providing the backup power that compensates for the electrical-output variability, for example, of a large wind farm. A 300 MWe CAES storage facility can utilize wind power to compress air, and then, during low wind periods, the compressed air can provide the combustion air for a natural-gas-fired combustion turbine providing up to 10 hours of backup capability for the wind farm.

Another possible storage technology for use in the grid is batteries, which rely on electrochemical processes to store electricity. There is a wide variety of battery types with potential for large- to small-scale dispatchable storage. Examples include lithium ion, sodium sulfur, zinc bromide, nickel metal hydride, and vanadium. In general, present battery technologies are expensive ($400/kW for 2 hours), incur high losses as the batteries are charged and discharged, and have reliability issues. In addition, battery storage requires AC/DC converters, which at present add $100–150/kW to the cost and about 4 percent to the in-out losses. However, more R&D and the mass production of standard power-electronics building blocks should bring converters’ costs and losses down to less than half by 2020 and to one-quarter by 2035. Also, in converter-based FACTS applications, batteries can be added at little extra converter cost.

There are significant advantages to battery storage. Batteries are modular and non-site-specific, meaning they can be located close to intermittent-generation sites, near the load, or at T&D substations. Battery storage technology can provide needed reliability and flexibility to the T&D system if it can be economically developed in the 100 MW range. Some battery storage technologies, such as sodium sulfur batteries, have been demonstrated and should be available for deployment before 2020. For example, American Electric Power plans to increase reliability by deploying 25 MW of sodium sulfur batteries in its distribution system by 2010 (Bjelovuk, 2008). Meanwhile, extensive R&D is in progress on lithium ion, nickel metal hydride, and other types of batteries. These technologies

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