Exergy

In thermodynamics, the exergy (in older usage, available work and/or availability) of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir. When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its environment. Exergy is the energy that is available to be used. After the system and surroundings reach equilibrium, the exergy is zero. Determining exergy was also the first goal of thermodynamics. The term "exergy" was coined in 1956 by Zoran Rant (1904–1972) by using the Greek and meaning "from work", but the concept was developed by J. Willard Gibbs in 1873.

Energy is never destroyed during a process; it changes from one form to another (see First Law of Thermodynamics). In contrast, exergy is always destroyed when a process involves an irreversible process, for example loss of heat to the environment (see Second Law of Thermodynamics). This destruction is proportional to the entropy increase of the system together with its surroundings. The destroyed exergy has been called anergy. For an isothermal process, exergy and energy are interchangeable terms, and there is no anergy.

Exergy analysis is performed in the field of industrial ecology to use energy more efficiently. Engineers use exergy analysis to optimize applications with physical restrictions, such as choosing the best use of roof space for solar energy technologies. Ecologists and design engineers often choose a reference state for the reservoir that may be different from the actual surroundings of the system.

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