4-12 An underground air storage has a volume of one million cubic feet and initial conditions 300 psia and 150°F. The discharging takes place until the air pressure inside drops to 200 psia. Assuming negligible heat loss from the storage system, estimate (a) the availability of the air leaving the underground air storage, (b) the change of the availability in the air storage, and (c) the second law efficiency for the discharging process.