Editing Exergy (section)

===Examples===
For two thermal reservoirs at temperatures ''T''<sub>H</sub> and ''T''<sub>C</sub>&nbsp;< T<sub>H</sub>, as considered by Carnot, the exergy is the work ''W'' that can be done by a reversible engine.  Specifically, with ''Q''<sub>H</sub> the heat provided by the hot reservoir, Carnot's analysis gives ''W''/''Q''<sub>H</sub>&nbsp;= (''T''<sub>H</sub>&nbsp;− ''T''<sub>C</sub>)/''T''<sub>H</sub>. Although, exergy or maximum work is determined by conceptually utilizing an ideal process, it is the property of a system in a given environment. Exergy analysis is not merely for reversible cycles, but for all cycles (including non-cyclic or non-ideal), and indeed for all thermodynamic processes.

As an example, consider the non-cyclic process of expansion of an ideal gas. For free expansion in an isolated system, the energy and temperature do not change, so by energy conservation no work is done. On the other hand, for expansion done against a moveable wall that always matched the (varying) pressure of the expanding gas (so the wall develops negligible kinetic energy), with no heat transfer (adiabatic wall), the maximum work would be done. This corresponds to the exergy. Thus, in terms of exergy, Carnot considered the exergy for a cyclic process with two thermal reservoirs (fixed temperatures). Just as the work done depends on the process, so the exergy depends on the process, reducing to Carnot's result for Carnot's case.

W. Thomson (from 1892, Lord Kelvin), as early as 1849 was exercised by what he called “lost energy”, which appears to be the same as “destroyed energy” and what has been called “anergy”. In 1874 he wrote that “lost energy” is the same as the energy dissipated by, e.g., friction, electrical conduction (electric field-driven charge diffusion), heat conduction (temperature-driven thermal diffusion), viscous processes (transverse momentum diffusion) and particle diffusion (ink in water). On the other hand, Kelvin did not indicate how to compute the “lost energy”. This awaited the 1931 and 1932 works of Onsager on irreversible processes.