Question

What is the significance of availability (or exergy) in thermodynamics?

• A. It measures the total energy content of a system.
• B. It quantifies the maximum useful work obtainable from a system.
• C. It is the heat lost during a process.
• D. It represents the entropy generated in a process.

Hint:

Energy is always conserved (First Law), but exergy is not conserved; it is destroyed by irreversibilities (Second Law).
Therefore, exergy analysis is highly useful for identifying where actual thermodynamic losses occur in a plant.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the thermodynamic definition and physical significance of "availability" (also known as "exergy").

Step 2: Key Formula or Approach:
According to the Second Law of Thermodynamics, not all heat energy can be converted into useful work.
Exergy or availability ($A$) is defined as the maximum theoretical useful work that can be obtained as a system undergoes a reversible process from its initial state to a dead state (thermal and mechanical equilibrium with the environment).
\[ A = (U - U_0) + P_0(V - V_0) - T_0(S - S_0) \]

Step 3: Detailed Explanation:

• Option A (Total energy content) is a description of the internal energy or enthalpy, which is governed by the First Law of Thermodynamics and does not account for the quality or usability of that energy.

• Option C (Heat lost) refers to thermal energy transferred across a boundary due to a temperature difference.

• Option D (Entropy generated) is a measure of irreversibility within a system, which actually destroys availability.

• Option B correctly identifies availability as the maximum work potential of a system at a specified state, relative to a specified environment (dead state). It quantifies the "quality" or "usefulness" of energy.

Step 4: Final Answer:
Availability quantifies the maximum useful work obtainable from a system.