The most important part of a CAES plant is somewhere to store the compressed air. Small-scale CAES plants - with storage capacities of up to
20 MWh - can use overground storage tanks but large, utility-scale plants need underground caverns in which to store the air. The natural gas industry has used underground storage caverns for years to store gas; these same caverns can provide ideal storage facilities for a CAES plant. However the demand for such a cavern limits the development of CAES to places where such storage caverns are available.
A number of different types of underground cavern can be exploited. The simplest is a man-made rock cavern. This must be sited in an impervious-rock formation if it is to retain the compressed air without loss.
Salt caverns have been commonly used for gas storage. These are created by dissolving or dry mining salt to create a suitable enclosure. Salt deposits suitable for such caverns occur in many parts of the world.
A third type of underground storage is found in porous rock bounded by an impervious barrier. Examples can be found in water-bearing aquifers, or as a result of oil and gas extraction. Aquifers can be particularly attractive as storage media because the compressed air will displace water, setting up a constant pressure storage system. With rock and salt caverns, in contrast, the pressure of the air will vary as more is added or released.
All three types of storage structure require sound-rock formations to prevent the air from escaping. They also need to be sufficiently deep and strong to withstand the pressures imposed on them. It is important, particularly in porous-rock storage systems, that there are no minerals present that can deplete the oxygen in the air by reacting with it. Otherwise the ability of the air to react with the fuel during combustion will be affected, reducing the power available during the generation phase of the storage generation cycle.
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