Reconsider Prob. 8-120 . The air stored in the tank is now...

Reconsider Prob. $8-120 .$ The air stored in the tank is now released through the isentropic turbine until the tank contents are at $100 \mathrm{kPa}$ and $20^{\circ} \mathrm{C}$. The pressure is always $100 \mathrm{kPa}$ at the turbine outlet, and all heat exchanges are with the surrounding air, which is at $20^{\circ} \mathrm{C}$. How does the total work produced by the turbine compare to the change in the work potential of the air in the storage tank?

Key Concepts

Ambient (Dead) State

The ambient or dead state is the reference state at which the system is in equilibrium with its surroundings. In exergy analysis, the difference between the system’s state and the ambient state defines the work potential. For instance, bringing the air in the storage tank down to the ambient conditions (here, 100 kPa and 20°C) helps in quantifying the total available work, since no work can be extracted once the system reaches the ambient state.

Turbine Work Extraction

Turbine work extraction involves converting the available work potential (exergy) of a pressurized or high-energy fluid into mechanical work. When air stored in a tank is expanded through an isentropic turbine, the process ideally converts the decrease in exergy into work. The comparison between the turbine’s work output and the change in work potential of the air provides a measure of how effectively the stored energy is utilized, highlighting the relationship between ideal exergy change and actual work produced.

Exergy (Work Potential)

Exergy is a measure of the maximum useful work obtainable from a system as it comes into equilibrium with its surroundings. It represents the work potential of a stored state relative to a specified environment (often called the dead or ambient state). In thermodynamic analyses, exergy quantifies how far the current state is from equilibrium and determines the maximum work that can be extracted under ideal, reversible conditions.

Isentropic Process

An isentropic process is a reversible adiabatic process, meaning there is no heat transfer and no change in entropy. In the context of turbines, it represents an ideal expansion of the fluid where all the available work potential (exergy) is converted into mechanical work, with no losses due to irreversibilities. Thus, when the process in the turbine is isentropic, the work output ideally reaches its maximum possible value.

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