Short Answer:
Exergy is the part of energy that can be converted into useful work, while anergy is the part of energy that cannot be used to do work. Exergy shows the usable or high-quality portion of energy, and anergy shows the wasted or low-quality portion.
In any real process, total energy remains the same, but only a part of it (exergy) is useful. The rest (anergy) becomes unavailable due to entropy generation and system losses. Understanding this difference helps in improving the efficiency of machines and energy systems.
Detailed Explanation:
Difference between exergy and anergy
In thermodynamics, energy is always conserved. But not all energy can be used to do work. That’s where the concepts of exergy and anergy become important. These two terms help us separate usable energy from unusable energy in a system.
Exergy and anergy together make up the total energy in a system:
Total Energy = Exergy + Anergy
This classification helps engineers and scientists analyze and improve the performance of engines, turbines, and power plants, where energy efficiency is important.
What is Exergy?
Exergy is the maximum useful work that a system can deliver when it comes to equilibrium with its surroundings. It is also called available energy.
- Exergy is a measure of energy quality.
- It depends on the condition of the system and its surroundings.
- Exergy is destroyed when there are losses due to friction, mixing, or heat transfer through a temperature difference.
- In ideal, reversible processes, exergy is fully converted into work.
Example:
In a steam turbine, the high-pressure steam has high exergy. As it expands and does work, some exergy is converted into mechanical output.
What is Anergy?
Anergy is the part of energy that cannot be used to perform useful work. It is the unavailable energy that remains after the system reaches equilibrium with its surroundings.
- Anergy is linked with disorder or randomness in energy, caused by entropy.
- It increases when energy becomes more spread out or degraded.
- Anergy remains in the system but is not useful for doing work.
Example:
After steam has expanded in a turbine and becomes low-pressure saturated steam, it still contains energy, but most of it is anergy — it cannot be used for further work.
Key Differences Between Exergy and Anergy
- Usability
- Exergy: Usable, can be converted into work.
- Anergy: Unusable, cannot be converted into work.
- Quality
- Exergy: High-quality energy.
- Anergy: Low-quality, degraded energy.
- Dependence on Surroundings
- Exergy: Depends on system and environment condition.
- Anergy: Remains as unuseful part regardless of process.
- Destroyed or Conserved
- Exergy: Can be destroyed due to irreversibility.
- Anergy: Increases as exergy is destroyed.
- Relation to Entropy
- Exergy decreases when entropy increases.
- Anergy increases with entropy and energy losses.
Practical Use in Engineering
- Efficiency Analysis:
Exergy helps in checking how much of the input energy is actually used. The more the anergy, the more the energy waste. - Design Improvement:
Engineers aim to reduce anergy and increase exergy output by using better materials, insulation, and system design. - Environmental Impact:
Lower anergy means better energy utilization and less fuel consumption, which reduces emissions and pollution.
Real-Life Example
Suppose you have 100 units of heat energy in a power plant:
- Only 40 units may be converted into electricity (exergy).
- The remaining 60 units are lost as heat to the environment (anergy).
By improving design or using combined heat and power systems, some of that anergy can be recovered or reduced.
Conclusion
The difference between exergy and anergy lies in their ability to perform work. Exergy is the useful part of energy that can be converted into work, while anergy is the useless part that remains due to entropy and inefficiencies. Understanding this difference helps engineers make machines more efficient, reduce energy waste, and build sustainable energy systems.