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Exergy (Gillett)
A cogent 2002 footnote definition, by American geologist Stephen Gillett, of exergy as a term not to be used, particularly in a chemical thermodynamic sense, from his article “Entropy and its misuse: Energy, free and otherwise”. [8]
In thermodynamics, exergy, symbol Ξ or B, is the maximum work done by a system during a transformation which brings it into equilibrium with a reservoir. [1] Exergy, by convention, is positive when putting the system into equilibrium with the reservoir results in doing work on the surroundings.

It its often claimed that exergy is synonymous with: “available energy” (Gibbs, 1876), "free energy" (Helmholtz, 1882), or “available work”, among others. [2] The term exergy, however, is used predominately in ecology, economics, and branches of mechanical engineering; often merely presented as a simplified or verbalized and equationless version of the free energy as used in the chemical-physical sciences.

The term exergy was coined by Slovene mechanical engineer Zoran Rant, in his 1953 PhD thesis “Exergy: a Useful Concept” completed at Chalmers University of Technology, Goteborg, as follows: [6]

“These claims show that aie will be the most appropriate suffix. Since the concept of a work is examined, it is necessary as the root syllable (as genus proximum) the Greek word erg (appear on) [see also: ergal] for this. Well, the correct prefix is to be selected, which highlights the specific nature, the specific difference. It is necessary that the requirement that the new term to describe the work that can be extracted from a system. AAUs is called in Greek AEK 'before consonants and AEX "before vowels.

This is the new concept of exergy, has met practically all the demands made and the letter 'x' distinguishes it clearly from the related concept of energy, so that ruled out despite the analogy in word formation, any confusion remains. The expression can be imported into any Germanic, Romance or Slavic languages, it is, e.g. German to exergy, in English exergy, exergy in French, in Spanish exergia, in Italian and Slavic essergia [and] eksergija.”

In 1956, Rant was using the term exergy to denote “technical working capacity” or “technical available energy”; soon thereafter becoming very popular. [3] The concept of exergy or maximal work, however, as introduced by Rant, supposedly, is a redressing of earlier works, in particular: American engineer Willard Gibbs (“available energy of the body and medium”, 1873), German physicist Hermann Helmholtz (“free energy”, 1882), and French physicist Louis Gouy (“possibility of obtaining work”, 1889), among other variants. [4]

A significant promoter of the concept is Swedish physical engineer Goran Wall who completed his 1986 PhD on “Exergy as a Useful Concept”. [6] Wall argues that the concept of exergy applies to society and aspects such as morality. [7]

Gouy-Stodola theorem
A folk theorem attributed independently to Gouy, in 1889, and Slovak mechanical engineer Aurel Stodola, in 1910, called the Gouy-Stodola theorem or the “Law of Gouy and Stodola”, states that the entropy production is the exergy loss divided by the temperature of the surroundings; thus minimizing loss of exergy is equivalent to minimizing entropy production. In equation form, this theorem reads:

Blost = ToSgen

where Blost denotes the potential work or exergy lost by the system in a transformation process, To denotes the temperature of the system’s surroundings, and Sgen is the entropy generated in the transformation. [2]

There was a short-lived Exergy: an International Journal (2001-2002), but which became incorporated into the journal Energy.

1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. Baumgärtner, Stefan, faber, Malte M., and Schiller, Johannes. (2006). Joint Production and Responsibility in Ecological Economics, (section: Engineering Thermodynamics: The Exergy Concept, pgs. 54-55). Edward Elgar Publishing.
3. (a) Rant, Zoran. (1956). "Exergie, ein neues Wort fur "Technische Arbeitsfahigkeit" (Exergy, a new word for "technical available work")". Forschung auf dem Gebiete des Ingenieurwesens, 22: 36–37.
(b) On Exergetics, Economics and Desalination – by Goran Wall and Mei Gong.
4. (a) Gibbs, J. Willard. (1873). "A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces" (pgs. 49-50), Transactions of the Connecticut Academy, II. pp.382-404, Dec.
(b) Gouy, Louis G. (1889), "Sur L'Energie Utilizable" (On Utilizable Energy), Journal de Physique 8, 501.
5. Stodola, A. (1910), Steam and Gas Turbines, McGraw-Hill.
6. Wall, Goran. (1986). “Exergy: a Useful Concept”, PhD thesis, Chalmers University of Technology, Goteborg.
7. (a) Wall, Goran. (1995). “Exergy and Morals: Second Law Analysis of Energy Systems: Towards the 21st Century, eds. E. Sciubba and M.J. Moran, July 5-7, Rome: Esagrafica-Roma., pgs. 21-29.
(b) Wall, Goran. (1997). “Energy, Society and Morals”, Journal of Human Values, 3(2): 193-206.
8. (a) Gillett, Stephen L., (2005a). “Entropy and its misuse, I. Energy, free and otherwise” (abs). Ecological Economics (received Jan 2002), 56, 58–70.
(b) Gillett, Stephen L. (2005b). “Entropy and its misuse, II. Matter matters less. Ecological Economics.

Further reading
● Demirel, Y. (2004). “Exergy use in Bioenergetics”, (Abstract), International Journal of Exergy, Vol. 1, No. 1, (pgs. 128-46).
● Sato, Norio. (2004). Chemical Energy and Exergy: An Introduction to Chemical Thermodynamics for Engineers, (pg. 99). Elsevier.

External links
Exergy – Encyclopedia of Earth.

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