Reversible Discharge from an Adiabatic Container
In the previous section we filled a container with compressed air. Expansion of the compressed air yields work. Thus, compressing air into a container can be used as a means to store energy. The reversible work potential of the air in the filled container is the closed system exergy of the compressed air (11.31),
For the evaluation we must consider the appearing quantities with some care: V is the system volume, which for the present notation is the container volume V0, while VE = mRTE is the volume of the system when expanded to environmental pressure and temperature pE = p0, TE = T0. Note that, due to the reversible adiabatic filling process considered, s = s0 = sE . With this, and the filling state m = nm0, T , p, as given above, the exergy of the compressed air becomes
So, not surprisingly, the work potential of the compressed air equals the work that is required for its reversible filling (with opposite sign). Thus, if all processes are fully reversible, compressed air storage gives a 100% efficient means of energy storage. All work required to fill the container could be taken out to produce work again, e.g. as electricity.