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Air (an ideal gas) is stored in a rigid closed tank of volume V at absolute temperature of T0 and pressure P. Ignoring the effect of motion and gravity, what will be the specific exergy (availability per unit mass) of air? [Given: Environmental absolute temperature is T0; Environmental pressure is P0; Gas constant is R.]

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Concept:

Exergy is the maximum useful work possible when a system is brought to equilibrium with its surroundings.

For an ideal gas, the specific exergy (availability per unit mass) is given by:

For an ideal gas with constant specific heats, the entropy change is:

For a rigid closed tank, volume remains constant, and thus:

(since no change in enthalpy in a rigid tank)

Therefore, specific exergy simplifies to:

Using the relation for an ideal gas at constant volume:

Substituting this into the equation:

 

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