Editing Exergy (section)

{{Short description|Maximum energy available for use}}
{{redirect|Available energy|the concept in particle physics|Available energy (particle collision)}}
'''Exergy''', often referred to as "available [[energy]]" or "useful work potential", is a fundamental concept in the field of thermodynamics and engineering. It plays a crucial role in understanding and quantifying the quality of energy within a system and its potential to perform useful work. Exergy analysis has widespread applications in various fields, including energy engineering, environmental science, and industrial processes.

From a scientific and engineering perspective, second-law-based exergy analysis is valuable because it provides a number of benefits over energy analysis alone. These benefits include the basis for determining energy quality (or exergy content<ref name="dx.doi.org">{{Cite journal |last1=Wright |first1=S. E. |last2=Rosen |first2=M. A. |last3=Scott |first3=D. S. |last4=Haddow |first4=J. B. |date=January 2002 |title=The exergy flux of radiative heat transfer for the special case of blackbody radiation |url=http://dx.doi.org/10.1016/s1164-0235(01)00040-1 |journal=Exergy |volume=2 |issue=1 |pages=24–33 |doi=10.1016/s1164-0235(01)00040-1 |issn=1164-0235|url-access=subscription }}</ref><ref name="ReferenceA">{{Cite journal |last1=Wright |first1=S. E. |last2=Rosen |first2=M. A. |last3=Scott |first3=D. S. |last4=Haddow |first4=J. B. |date=January 2002 |title=The exergy flux of radiative heat transfer with an arbitrary spectrum |url=http://dx.doi.org/10.1016/s1164-0235(01)00041-3 |journal=Exergy |volume=2 |issue=2 |pages=69–77 |doi=10.1016/s1164-0235(01)00041-3 |issn=1164-0235|url-access=subscription }}</ref><ref name="ReferenceB">{{Cite journal |last1=Wright |first1=Sean E. |last2=Rosen |first2=Marc A. |date=2004-02-01 |title=Exergetic Efficiencies and the Exergy Content of Terrestrial Solar Radiation |url=http://dx.doi.org/10.1115/1.1636796 |journal=Journal of Solar Energy Engineering |volume=126 |issue=1 |pages=673–676 |doi=10.1115/1.1636796 |issn=0199-6231|url-access=subscription }}</ref>), enhancing the understanding of fundamental physical phenomena, and improving design, performance evaluation and optimization efforts. In [[thermodynamics]], the exergy of a [[System (thermodynamics)|system]] is the maximum useful [[work (thermodynamics)|work]] that can be produced as the system is brought into [[Thermodynamic equilibrium|equilibrium]] with its environment by an ideal process.<ref>Çengel, Yunus A. Thermodynamics : an Engineering Approach. Boston: McGraw-Hill Higher Education, 2008.</ref> The specification of an "ideal process" allows the determination of "maximum work" production. From a conceptual perspective, exergy is the "ideal" potential of a system to do work or cause a change as it achieves equilibrium with its environment. Exergy is also known as "availability". Exergy is non-zero when there is dis-equilibrium between the system and its environment, and exergy is zero when equilibrium is established (the state of maximum [[entropy]] for the system plus its environment).

Determining exergy was one of the original goals of [[thermodynamics]]. The term "exergy" was coined in 1956 by [[Zoran Rant]] (1904–1972) by using the Greek ''[[wikt:ἐξ|ex]]'' and ''[[wikt:ἔργον|ergon]]'', meaning "from [[Work (thermodynamics)|work]]",<ref>{{Cite journal | last1 = Rant | first1 = Zoran | title = Exergie, Ein neues Wort für "technische Arbeitsfähigkeit" | journal = Forschung Auf dem Gebiete des Ingenieurwesens | volume = 22| pages = 36–37 | year = 1956}}</ref>{{ref label|rant56|3|a}} but the concept had been earlier developed by [[Josiah Willard Gibbs|J.&nbsp;Willard Gibbs]] (the namesake of [[Gibbs free energy]]) in 1873.{{ref label|gibbs1873|4}}

Energy is neither created nor destroyed, but is simply converted from one form to another (see [[First law of thermodynamics]]). In contrast to energy, exergy is always destroyed when a process is non-ideal or [[irreversible process|irreversible]] (see [[Second law of thermodynamics]]). To illustrate, when someone states that "I used a lot of energy running up that hill", the statement contradicts the first law. Although the energy is not consumed, intuitively we perceive that something is. The key point is that energy has quality or measures of usefulness, and this energy quality (or exergy content) is what is consumed or destroyed. This occurs because everything, all real processes, produce entropy and the destruction of exergy or the rate of "irreversibility" is proportional to this [[entropy]] production ([[Gouy–Stodola theorem]]). Where entropy production may be calculated as the net increase in entropy of the system together with its surroundings. Entropy production is due to things such as friction, heat transfer across a finite temperature difference and mixing.  In distinction from "exergy destruction", "exergy loss" is the transfer of exergy across the boundaries of a system, such as with mass or heat loss, where the exergy flow or transfer is potentially recoverable. The energy quality or exergy content of these mass and energy losses are low in many situations or applications, where exergy content is defined as the ratio of exergy to energy on a percentage basis. For example, while the exergy content of electrical work produced by a thermal power plant is 100%, the exergy content of low-grade heat rejected by the power plant, at say, 41 degrees Celsius, relative to an environment temperature of 25 degrees Celsius, is only 5%.