The second-biggest form of bulk-energy storage, though it is dwarfed by PSH, is compressed-air energy storage (CAES).

ECONOMIST: Energy storage

There are currently only two CAES sites in the world -- in Huntorf, Germany and in McIntosh, Alabama.

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There are only two commercial CAES plants in operation: one in Huntorf, Germany, and the other in McIntosh, Alabama.

ECONOMIST: Energy storage

In existing CAES systems energy is lost as heat during compression, and the air must then be reheated before expansion.

ECONOMIST: Energy storage

General Compression, a Massachusetts-based company also backed by the DOE, has developed an isothermal CAES system focused on providing support to wind farms.

ECONOMIST: Energy storage

As with hydro storage, efforts are under way to adapt the basic concept of CAES to make it more efficient and easier to install.

ECONOMIST: Energy storage

Traditionally, CAES stows energy in a vast underground reservoir.

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The big drawback of CAES is its inefficiency.

ECONOMIST: Energy storage