Editing Exergy (section)

==Definitions==

Exergy is a combination property<ref>
{{cite book
 |title=Thermodynamics an Engineering Approach
 |last1=Çengel |first1=Y. A.
 |last2=Boles |first2=M. A.
 |edition=6th
 |page=445
 |isbn=978-0-07-125771-8
 |year=2008
 |publisher=McGraw-Hill }}</ref> of a system and its environment because it depends on the state of both and is a consequence of dis-equilibrium between them. Exergy is neither a [[List of thermodynamic properties|thermodynamic property]] of matter nor a [[thermodynamic potential]] of a system. Exergy and energy always have the same units, and the joule (symbol: J) is the unit of energy in the International System of Units (SI). The [[internal energy]] of a system is always measured from a fixed reference state and is therefore always a [[state function]]. Some authors define the exergy of the system to be changed when the environment changes, in which case it is not a state function. Other writers prefer{{Citation needed|date=February 2009}} a slightly alternate definition of the available energy or exergy of a system where the environment is firmly defined, as an unchangeable absolute reference state, and in this alternate definition, exergy becomes a property of the state of the system alone.

However, from a theoretical point of view, exergy may be defined without reference to any environment. If the intensive properties of different finitely extended elements of a system differ, there is always the possibility to extract mechanical work from the system.<ref>{{cite book |last=Grubbström |first=Robert W. |chapter=Towards a Generalized Exergy Concept |editor-last1=van Gool |editor-first1=W. |editor-last2=Bruggink |editor-first2=J. J. C. |title=Energy and time in the economic and physical sciences |publisher=North-Holland |year=1985 |pages=41–56 |isbn=978-0444877482}}</ref> Yet, with such an approach one has to abandon the requirement that the environment is large enough relative to the "system" such that its intensive properties, such as temperature, are unchanged due to its interaction with the system. So that exergy is defined in an absolute sense, it will be assumed in this article that, unless otherwise stated, that the environment's intensive properties are unchanged by its interaction with the system.

For a [[heat engine]], the exergy can be simply defined in an absolute sense, as the energy input times the [[Carnot's theorem (thermodynamics)|Carnot efficiency]], assuming the low-temperature heat reservoir is at the temperature of the environment. Since many systems can be modeled as a heat engine, this definition can be useful for many applications.

===Terminology===
The term exergy is also used, by analogy with its physical definition, in [[information theory]] related to [[reversible computing]]. Exergy is also synonymous with ''available energy'', ''exergic energy'', ''essergy'' (considered archaic), ''utilizable energy'', ''available useful work'', ''maximum (or minimum) work'', ''maximum (or minimum) work content'', ''[[Reversible process (thermodynamics)|reversible]] work'', ''ideal work'', ''availability'' or ''available work''.

===Implications===
The exergy destruction of a cycle is the sum of the exergy destruction of the processes that compose that cycle. The exergy destruction of a cycle can also be determined without tracing the individual processes by considering the entire cycle as a single process and using one of the exergy destruction equations.

===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.