What is availability (exergy) and how does it differ from energy?

Availability or exergy is the maximum useful work obtainable from a system as it reaches equilibrium with its surroundings. Unlike energy which is conserved (first law), exergy is destroyed in irreversible processes (second law). Exergy analysis identifies where thermodynamic inefficiencies occur and quantifies improvement potential. A system can have high energy but low exergy if at environmental conditions. Exergy destruction represents lost opportunity for work, helping engineers optimize thermal systems by targeting major irreversibility sources.