BeginnerTopic 28 of 105Thermodynamics & Fluids
Exergy
The maximum useful work available from a system relative to the environment.
In depth
Exergy (availability) is the maximum useful work obtainable from a system as it comes to equilibrium with the environment (dead state T₀, P₀). Flow exergy ψ=(h-h₀)-T₀(s-s₀)+V²/2+gz. Second-law efficiency=exergy output/exergy input reveals real improvement potential.
Key points
- Heat at low T has little exergy
- Exergy destruction identifies losses
- Eco-design tool
ExampleA 90 °C hot water stream carries much less exergy than the same energy in steam at 500 °C.
What is Exergy?
The maximum useful work available from a system relative to the environment.
Why is it important to understand exergy?
Exergy measures the useful work potential of energy, so it shows which losses really matter when we assess efficiency.
What is the difference between energy and exergy?
Energy is conserved, while exergy is the part that can be turned into work relative to the environment and is destroyed by irreversibility.
Why is exergy analysis useful?
It shows where and how much useful work is lost, which points to the components where improvement would help most.
How is Exergy used in practice?
Typical applications include comparing a heat source at high and low temperature by its exergy, assessing the quality of waste heat for reuse in a process and finding which component in a plant destroys the most useful work.
What should I learn before Exergy?
Study "Isentropic efficiency" first (topic 27 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Exergy?
Heat at low T has little exergy. Exergy destruction identifies losses. Eco-design tool.
Can you give an example of Exergy?
A 90 °C hot water stream carries much less exergy than the same energy in steam at 500 °C.
What comes after Exergy?
The next topic is "Irreversibility and lost work", which builds directly on this one.
How difficult is Exergy?
It is classed as Beginner in Thermodynamics & Fluids (topic 28 of 105). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.