TourCompressed air energy storage
   
 Cylinder pressure p1 MPa
 Ambient pressure p2 MPa
 Cylinder volume v1 10-3 m3
 Cylinder temperature T1 K
 Specific heat capacity cp kJ/(kgK)
 Specific heat capacity cv kJ/(kgK)
 
 Gas constant R=cp-cv kJ/(kgK)
 Isentropic coefficient k=cp/cv -
 mass weight m=pV/(RT) kg
                   Isothermal expansion Adiabatic expansion
 Expanded volume v2 m3 m3
 Energy released W kJ kJ
 Temperature T2 K K

Compared to batteries, compressed air is favorable because of a high energy density, low toxicity, fast filling at low cost and long service life. These issues make it technically challenging to design air engines for all kind of compressed air driven vehicles (http://en.wikipedia.org/wiki/Compressed-air_energy_storage).
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