Compressed air energy storage

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Compressed Air Energy Storage (CAES) refers to the compression of air during periods of low energy demand, for use in meeting periods of higher demand. Typically, compression is done with an electrically powered turbo-compressor; and expansion is done with a natural-gas powered 'expander' (heater) which drives a combustion turbine.^[1]

Installations exist in Huntorf, Germany, and McIntosh, Alabama (USA).^[1] Additional facilities are under development in Norton, Ohio and Iowa. This 2700 MW Norton project has been started in 2001, but in early 2007 construction had not actually begun.^[2] Air is stored underground in a cavern created by solution mining (salt is dissolved away) or an abandoned mine. Plants are designed to operate on a daily cycle, charging at night and discharging during the day. Compressed air energy storage can also be used to describe technology on a smaller scale such as exploited by air cars.

1 Physics of compressed air storage
2 References
3 See also
4 External links

[edit] Physics of compressed air storage

Ideally reversible air compression and expansion is described by the isothermal process where the heat of compression and expansion is removed or added to the system at the same rate as it is produced. Compressing air heats it up and must therefore be able to flow to the environment during compression for the temperature to remain constant. In practice this is never the case, but approached for very small and slow, i.e. low power systems. Nevertheless it is useful to describe the limiting case of ideal isothermal compression of an ideal gas:

$W_{A\to B}=nRT\ln{\frac{P_A}{P_B}}$

W is the energy stored, n the number of moles, R the molar gas constant, T the temperature in Kelvin.

This works out to about 101 $ln{\frac{P_A}{P_B}}$ kJ/Nm3 at 0° Celsius or 110 $ln{\frac{P_A}{P_B}}$ kJ/Nm3 at 24° Celsius.

A Nm3 is a cubic meter of gas volume at normal, i.e. atmospheric pressure conditions.

As the isothermal process is reversible, the efficiency of compressed air storage approaches 100% and the equation above represents the maximum energy storable. In practice the process will not be isothermal and the compressors and motors have losses.