Advanced Topic 409 of 1102 Thermodynamics & Energetics
Exergy
The maximum useful work obtainable from a system as it comes to equilibrium with surroundings.
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In-depth explanation of Exergy
Exergy is the maximum useful work obtainable from a system as it is brought into equilibrium with a specified environment, usually at the surroundings' temperature and pressure. It measures the quality of energy rather than its quantity. High-temperature heat contains a large fraction of exergy because it can drive a heat engine, while the same amount of heat at ambient temperature has essentially none. Irreversible processes destroy exergy, and the amount destroyed equals the entropy generated multiplied by the environment temperature. Exergy analysis identifies where energy systems waste their useful potential.
Going deeper: Exergy
Key ideas: Exergy measures the useful part of energy relative to the environment, so high-temperature heat and electricity have high exergy and low-grade warm water has little. Destroyed exergy equals T0 times entropy generated. Efficiency by exergy often exposes losses invisible in an energy balance.
Common mistakes and connections: Do not confuse exergy with energy, which is conserved, whereas exergy is not. A device using heat at low temperature to heat a room can have high energy efficiency but low exergy efficiency, since high-quality energy has been wasted on a low-grade task.
Visualization of Exergy
Typical power plant converts roughly a third of fuel exergy to work; the rest is destroyed as irreversibility or lost as low-grade heat. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Electricity is almost pure exergy, while warm bathwater at about 40 degrees Celsius carries only a small fraction of useful work because it is close to ambient temperature.
Frequently asked questions about Exergy
What is Exergy?
The maximum useful work obtainable from a system as it comes to equilibrium with surroundings.
Is Exergy a beginner, intermediate or advanced topic?
Exergy is a advanced topic in Thermodynamics & Energetics. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Exergy?
Study these earlier topics in Thermodynamics & Energetics first: Entropy of Mixing, Entropy and the Arrow of Time, Heat Death Concept.
What comes after Exergy?
The next topics in Thermodynamics & Energetics are Joule Experiment and Standard Conditions.
Can I practise Exergy with quizzes?
Yes. The interactive lesson includes a self-check question on Exergy, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Exergy fits in Thermodynamics & Energetics
Exergy is topic 81 of 91 in Thermodynamics & Energetics.