Exergia

Exergia, or exergy in English, is a thermodynamic measure of the maximum useful work that can be extracted from a system as it comes into equilibrium with a specified reference environment. It complements energy, which is conserved, by accounting for the quality or usefulness of energy. In real processes, exergy decreases due to irreversibilities such as friction, mixing, finite temperature differences, and heat transfer to or from the environment.

Exergy exists in two broad components. Physical exergy arises from differences in temperature, pressure, and mechanical state between the system and its environment. Chemical exergy accounts for the composition of the system relative to the environment and represents the potential to do work through chemical reactions. In open (flow) systems, exergy is carried by moving streams, and the exergy of a process includes both physical and chemical contributions.

A common way to describe exergy is through an exergy balance. For a control volume, the rate form is Ein Xdot minus Eout Xdot minus Xdest, where Xdot is the exergy flow, and Xdest represents exergy destruction due to irreversibilities. An exergy efficiency or availability efficiency is often defined as the useful exergy output divided by the exergy input, providing a measure of how effectively a process converts available environmental resources into work or usable energy.

Applications of exergy analysis include evaluating and comparing energy systems, diagnosing where losses occur, guiding design improvements, and informing sustainability assessments. By focusing on the quality of energy and the sources of irreversibility, exergy analysis helps optimize processes beyond what conventional energy balances can reveal.