Available & Unavailable Energy (Availability)

Definition

In thermodynamics, Available Energy (also known as Exergy or Availability) is the maximum portion of energy that can be converted into useful work under reversible conditions between a system and its surroundings (the environment). Conversely, Unavailable Energy (Anergy) is the portion of energy that cannot be converted into useful work due to the limitations imposed by the Second Law of Thermodynamics, typically manifesting as heat rejected to a sink at a constant temperature.

Main Content

1. Concept of High-Grade vs. Low-Grade Energy

• High-Grade Energy: This is energy that can be converted completely into work, such as kinetic energy of a wind turbine or potential energy of a falling weight.
• Low-Grade Energy: This is thermal energy (heat). According to the Second Law, heat cannot be entirely converted into work without a heat sink; hence, part of it is always "unavailable."

2. The Role of the Surroundings (Dead State)

• The "dead state" or "environment" is the reference point (usually atmospheric pressure and temperature). A system is said to have zero availability when it is in thermal, mechanical, and chemical equilibrium with the environment.
• Any energy present in a system beyond the state of the environment is considered "Available Energy."

3. The Second Law Limitation

• The Second Law dictates that the entropy of an isolated system always increases. Because entropy is a measure of "disorder," as entropy increases, the quality of energy decreases, transforming "Available Energy" into "Unavailable Energy."

       Total Energy (E)
      __________________
     |                  |
     | Available Energy | (Work potential)
     |    (Exergy)      |
     |__________________|
     |                  |
     | Unavailable Energy| (Heat rejected)
     |    (Anergy)      |
     |__________________|

Visual representation of the division of energy.

Working / Process

1. Assessment of Initial State

• Determine the total energy content of a system based on its internal energy, pressure, and temperature.
• Define the "Dead State" ($T_0, P_0$), which acts as the universal sink for rejected heat.

2. Calculation of Reversible Work

• Assume a perfectly reversible process (Carnot Cycle) where the system moves from its current state to the dead state.
• In this ideal scenario, the maximum possible work ($W_{max}$) is extracted. This $W_{max}$ is equal to the Available Energy.

3. Calculation of Unavailable Energy

• The remaining energy that cannot be converted to work is calculated by the product of the temperature of the surroundings ($T_0$) and the change in entropy ($\Delta S$) of the system.
• Equation: $Unavailable Energy = T_0 \times \Delta S$

Advantages / Applications

• Energy Management: Helps industries identify which energy sources are being wasted and where efficiency improvements can be made.
• Thermodynamic Optimization: Used in power plant engineering to maximize the output of turbines by minimizing the destruction of available energy (Exergy destruction).
• Sustainability: Assists in evaluating the true "quality" of fuels, allowing for better resource management and less reliance on high-entropy processes.

Summary

• Available energy is the portion of total energy that can be converted into useful work, while unavailable energy is the portion lost to the surroundings as heat.
• The Second Law of Thermodynamics dictates that energy transformation always results in some level of unavailable energy.
• Availability is a measure of the "quality" of energy rather than just the quantity.
• Important terms to remember: Exergy (Available Energy), Anergy (Unavailable Energy), Dead State (Environmental equilibrium), and Entropy (Measure of energy degradation).