Explain the concept of available energy in thermodynamics.

Short Answer:

In thermodynamics, available energy refers to the maximum useful work that can be obtained from a system as it moves towards equilibrium with its surroundings. It is the part of total energy that can be converted into mechanical work, while the rest is lost due to irreversibility or increase in entropy.

Available energy is important because it shows how efficiently energy can be used in real systems. Not all energy in a system can be turned into work, and available energy helps engineers understand how much energy is actually usable, especially in machines like engines, turbines, and compressors.

Detailed Explanation:

Available energy in thermodynamics

In practical systems, we often deal with energy in many forms — heat, kinetic energy, potential energy, and internal energy. However, not all energy can be converted into useful work. Some part of it is always lost due to friction, heat loss, or entropy generation. The concept of available energy helps us calculate how much of the total energy in a system can actually be used to perform work.

Available energy is also known as exergy or useful energy. It measures the maximum possible work output a system can give when it interacts with a reference environment (also called a dead state) — where the system and surroundings are in complete thermal and mechanical equilibrium.

What is Available Energy?

Available energy is the portion of the total energy in a system that can be converted into useful mechanical work under given environmental conditions. It is based on the second law of thermodynamics, which considers both the conservation of energy (first law) and the degradation of energy quality (second law).

The remaining energy, which cannot be converted to work due to entropy increase, is called unavailable energy or anergy.

So,
Total Energy = Available Energy + Unavailable Energy

Factors Affecting Available Energy

  1. System Condition:
    Temperature, pressure, and type of fluid affect the ability to do work.
  2. Surroundings (Dead State):
    Reference temperature and pressure (usually atmospheric) determine how much energy can be extracted.
  3. Irreversibilities:
    Friction, unrestrained expansion, mixing, and heat transfer with finite temperature differences reduce available energy.
  4. Entropy Change:
    Greater entropy generation means lower availability of energy for useful work.

Types of Available Energy

  1. Physical Exergy:
    Work obtainable due to temperature and pressure difference between system and surroundings.
  2. Chemical Exergy:
    Work obtainable from chemical reactions like combustion.
  3. Kinetic and Potential Exergy:
    Energy due to motion and height that can be fully converted into work.

Applications of Available Energy

  1. Power Plants:
    Helps determine the maximum work from steam or gas in turbines.
  2. Refrigeration and Air Conditioning:
    Measures how efficiently the systems use electrical energy to move heat.
  3. Heat Engines:
    Defines the upper limit of work output from fuel energy.
  4. Energy Auditing:
    Used to identify losses and improve energy efficiency in industries.
  5. Environmental Engineering:
    Evaluates how much energy from waste heat or renewable sources can be recovered.

Mathematical Representation

For a control mass system, the available energy (Exergy) can be written as:

Exergy = (U – U₀) + P₀(V – V₀) – T₀(S – S₀)

Where:

  • U, V, S = internal energy, volume, and entropy of the system
  • U₀, V₀, S₀ = properties of the environment (dead state)
  • P₀, T₀ = pressure and temperature of the surroundings

This formula shows how much energy is available above the environmental conditions, which can be used for work.

Importance in Engineering

Understanding available energy is very important because:

  • It sets a realistic upper limit on work output.
  • It helps compare actual performance with ideal.
  • It guides design improvements by showing where energy is lost.
  • It brings together both first and second laws into one useful tool.
Conclusion

The concept of available energy in thermodynamics refers to the maximum useful work a system can produce as it reaches equilibrium with its surroundings. It accounts for energy losses due to entropy and helps measure how much energy is actually usable. This concept is very important in engineering design, power generation, and energy efficiency analysis, as it combines the practical aspects of both the first and second laws of thermodynamics.