Question

The maximum amount of low grade energy, which is convertible into work is known as

• A. Anergy
• B. Exergy
• C. Entropy
• D. Energy loss

Hint:

Remember the simple word formula: $\text{Energy} = \text{Exergy} + \text{Anergy}$. - $\text{Exergy} = \text{Available energy (can be converted into work)}$. - $\text{Anergy} = \text{Unavailable energy (cannot be converted into work)}$.

Answer 1

The Correct Option is B

Concept: Thermodynamics categorizes energy based on its capacity to perform useful mechanical work:

• High-Grade Energy: Energy forms that can theoretically be converted entirely into work (e.g., mechanical work, electrical energy, water power).

• Low-Grade Energy: Energy forms that cannot be fully converted into work due to Second Law limitations (e.g., thermal energy, heat from combustion).
When a system interacts with the environmental dead state ($T_0, P_0$), low-grade heat energy ($Q$) can be split into two fundamental structural portions: \[ \text{Total Energy} = \text{Available Energy (Exergy)} + \text{Unavailable Energy (Anergy)} \] Exergy represents the maximum portion of this low-grade thermal energy that can be transformed into useful work via an idealized reversible Carnot engine operating down to the dead state temperature $T_0$: \[ W_{\text{max}} = \text{Exergy} = Q \left(1 - \frac{T_0}{T}\right) \]

Step 1: Evaluate definitions for each provided option.
Let's analyze the precise engineering meaning of each terminology selection:

• Anergy: The unconvertible, unavailable portion of energy that must be discarded to the surrounding environment as waste heat.

• Exergy: The maximum useful work potential available from a given energy form relative to a specific environmental dead state.

• Entropy: A state function tracking the degree of molecular disorder and quantifying the irreversibility within a system.

• Energy loss: A general term representing energy escaping across boundaries, which does not define a maximum conversion property.

Step 2: Map the question to the correct definition.
The question asks for the maximum amount of low-grade energy that can be converted into work. Based on the fundamental definitions of availability analysis, this is the exact description of

Exergy, corresponding to Option (B).