Talk:Exergy

message: Just a reader leaving a quick note here: Thanks for writing! However, the article seems to jump around excessively. The core concepts should be brought into the front, in a concise form, before going into the ramifications... (just my 2 cents)  —Preceding unsigned comment added by 71.139.177.7 (talk) 04:47, 29 May 2010 (UTC)
thread_title: Needs more focus

message: What happens when there is no exergy left? Is there a way to prevent this??  —Preceding unsigned comment added by 67.53.37.220 (talk) 08:00, 16 April 2009 (UTC)
thread_title: Finite exergy?

message: I found one or two articles about this and emergy and energy quality that actually seem to make sense to me and seem to contain something approaching rigor. So I'm doing a rewrite starting with exergy because it's the oldest and most rigorous concept. Flying Jazz 12:00, 28 December 2005 (UTC)
thread_title: Rewrite in progress

message: I suggest that we merge the energy quality article with this article, and develop a clearer statement of the two. Sholto Maud 05:53, 4 December 2005 (UTC)
replies: text: That's a really bad idea. From what I can tell, exergy has been around since the 1800s, emergy is a pretty recent concept and it might turn into an important concept, and "energy quality" relates the two using a really awful term instead of the better term of transformity. This seems like important stuff and it needs to be presented separately and presented well so that both scientists and laymen can distinguish between what's been used for power plant designs for generations (available energy=exergy) and a promising protoscience that started out being a little goofy. Flying Jazz 11:56, 28 December 2005 (UTC)
replies: text: Now that I've gotten wrapped up in writing this thing, I see that  energy quality might not be such a bad term.  Oops. But I do think things should be presented separately.  Flying Jazz 01:48, 2 January 2006 (UTC)
thread_title: Merge

message: The sentence:
:This work potential is due to either a potential due to a force, temperature, or the degree of physical disorder.
Needs rewording, but I can't figure out how to fix it.RJFJR 22:27, Feb 12, 2005 (UTC)
RJFJR, I attempted to help - Wes, Aug 15, 2005
Is there a 2nd Law of Efficiency?RJFJR 22:27, Feb 12, 2005 (UTC)
Do you mean: Is there a 2nd law efficiency?  Yes, as defined in "Thermodynamics - An Engineering Approach" 4th edition, by Cengel and Boles (2002), p.400:
"...we define a second-law efficiency ηII as the ratio of the actual thermal efficiency to the maximum possible (reversible) thermal efficiency under the same conditions:
ηII = ηth / ηth,rev"
This is a concept I used quite extensively in the exergy analyses in my PhD thesis. I found it to be a more "true" representation of the energy conversion efficiency - it compares the system with the best possible (reversible) performance. Davo100 03:18, 27 March 2007 (UTC)
thread_title: Comment

message: There are two fomulas listed under Exergy of heat available at a temperature, one is for heat into an object and one for heat out of an object.  They both have terms that amount to  Tc/TH.  Can they be combined into one formula and ignore whtehr the object is warmer or colder than the environment? RJFJR 05:27, Feb 19, 2005 (UTC)
I think the same. B = Q(1-Tr/Tsource) shall we edit ? --Napishtim (talk) 14:07, 16 December 2007 (UTC)
thread_title: formulas

message: Question: Is the exergy the same thing as the thermodynamic Availability function,
:A = E - T_R S + P_R V - \mu_R N\,
where changes in A correspond to changes in the amount of useful work that can be extracted from a system with respect to an reservoir of infinite capacity at temperature TR, pressure PR and chemical potential μR ?
(Reference: Waldram, Theory of Thermodynamics, pp. 75, 205). -- Jheald 11:24, 5 December 2005 (UTC)
replies: text: Looks the same to me. The rewrite is making this clear Flying Jazz 11:56, 28 December 2005 (UTC)

text: When I see A, I think of Helmholtz free energy, but I think some people use F for helmholtz, and those might be the same people that use A for availability (which is another name for exergy). Just to confuse things more, the helmholtz page relates helmholtz to availability too when temperature doesn't change. Also, some people the past used B to indicate any thermodynamic potential (where I've used X). But the short answer to your question is yes. Flying Jazz Flying Jazz 22:02, 30 December 2005 (UTC)
thread_title: Availability

message: The 1911 Britannica article "Energetics" is of quite interesting historical value for this page.  As well as a good survey of how the concept of Available Energy developed, clarified and then took hold during the 19th Century, particularly noting the work of Rankine and Kelvin in the 1850s, it's also of interest for showing that circa 1911 the concept of Available Energy seems to have been seen (at least by the author) as very much the best way to think about the efects of the Second Law.  The original article is notably clearly written and reads very well.  Unfortunately, however, none of the characters used for variables has been OCR'd at all well, which makes most of the scanned version sadly hard to follow.  The original printed text is well worth looking out, however.
::Also relevant for the energetics page! :) Sholto Maud 06:35, 11 January 2006 (UTC)
thread_title: 1911 Britannica

message: The article says that (A) "if there is an energy transformation, the second principle of energy flow transformations says that this process 'must involve the dissipation of some energy as heat'. Measuring the amount of heat released is one way of quantifying the energy, or ability to do work and apply a force over a distance". 
The entropy (second principle of energy flow transformations) article says (B) ""Ice melting" - a classic example of entropy increasing". 
If A & B, then ice melting (as an example of entropy increasing) must involve the dissipation of some energy as heat. How does one measure the heat released when ice melts? (or would it be neg-exergy? Sholto Maud 05:34, 11 May 2006 (UTC)
thread_title: Re: Energy quality types

message: The statement on "cold exergy" is incorrect.
X/Q = 1-To/T is valid even if T < To.  The numerator and denominator do not magically reverse in this case.  However, the signs of energy and exergy are now opposite, indicating they flow in opposite directions.
The thermal exergy of a substance below the reference temperature can have an absolute exergy greater than the energy it contains if the temperature is less than half of the reference temperature (absolute scale).  Don't worry, the amount of energy flowing in at the reference temperature is still greater than the numerical value for the exergy.
If you don't get to it, i'll change it when I have time.
Wes Hermann 04:18, 3 December 2006 (UTC)
thread_title:  Cold exergy 

message: I think it's OK for a complex concept to have a long intro. Most people seem to think that exergy is a more complex concept than entropy (because it's introduced later in textbooks and coursework after students are familiar with entropy), and look at the size of the intro on the entropy article. Much of this article is still sloppy because the most interesting bits are poorly referenced or unreferenced. That's probably mostly my fault for not finishing what I started. I really do hope to be able to come back to it soon. Flying Jazz 04:04, 16 November 2007 (UTC)
thread_title: Intro length and article sloppiness

message: As for the rest of what you are wrote...Sholto...some of the rest of what you posted up there amazes me. Someone who is a fan of negentropy or who discusses negentropy often could use the word "negentropy" in their definition of temperature, of entropy, of energy, of evolution, of cosmology, of multiplication, of Botswana, of fuchsia, or of Venus, and all of that could be verifiable. Do you think that this is a reason for negentropy to be mentioned in the introduction of all of those articles? Maybe you do. Maybe you're just picking on little out-of-way articles like exergy because the introductions to other articles are closely guarded by many editors and you thought you could get away with sticking negentropy into this introduction. And there it sat for about a year. So you were right until I poked my head back in here. Why not read the rest of the article, determine where negentropy would best fit into the bulk of the article, and place it there? If it works then editors like me will probably think it will serve the reader there.
But not in the introduction. Here's why. My "view" is that exergy is a well defined mathematical thermodynamic concept, and that view is supported by references given to Gibbs and to links to MIT open courseware. If your view is that exergy is something else or that there is confusion, then the burden is on you to find similar sources and similar links to the ones that I have provided that define the something-else or expand upon the confusion. But above you wrote "there is no verifiable peer reviewed, and apparently scientific literature, which explains the difference between the different contexts, definitions and points of view." So you can't, and you would be making a huge mistake if you pursue this line of thinking. Wikipedia is for readers who wish to learn things. It is not for you to run around planting negentropy references into introductions for some reasons that other editors can't fathom because you say it's all so unclear. Please don't stroll down that path.
Similar discussions are taking place and have taken place all over Wikipedia for years, and over and over, the person like me who provides links and references to rigorous definitions is seen as supporting the encyclopedia and providing the reader with content of substance that can help them understand difficult material. And the person like you who claims, "There is confusion! There are different contexts! And we must cover the controversy! It's all so unclear! We must tell the reader that it is unclear! It's so unclear that nobody had published anything about why it is so unclear!" is seen as being disruptive. Especially when the article is about a mathematically rigorous science topic that's been described in universities by equations for over a century.
If you are like the other people at Wikipedia who have done this kind of thing before (and many have), then your next step will be to track down the exact verifiable references and statements of a published negentropy fan. Maybe you'll even invite a negentropy fan onto Wikipedia and have them place something on this talk page. I'm not going to follow you down that path. I've spent too many hours arguing in science talk pages with people like that to ever do it again. Learn about the current article first before you contribute to making it more arcane and ambiguous. Read the MIT open courseware. Change the article for the better. Think of a real reader who doesn't know what exergy is, and write for them so they'll understand what it is. Don't imagine a mythical reader who learns about things by discovering that there is a lack of clarity. That reader exists only in your mind and in the minds of people who have typed similar things into talk pages and disrupted Wikipedia in the past. Write for the real reader, not that fantasy-land reader. If you don't write for the real reader, if you do anything else, anything else, even here on the talk page, I will consider it to be disruptive to Wikipedia and I'll begin to take steps to stop it immediately with no additional warning. If the diagram you're thinking of posting here fits the article and the reality of the rigorous thermo definition of exergy, then bring it here, add it to the article, write about it. You don't even need to talk about it in the talk page. If the diagram you're thinking of posting here does not meet that description, don't bother posting it because I won't bother discussing it. Flying Jazz 04:04, 16 November 2007 (UTC)
thread_title: Negentropy in the intro

message: I table the exergy diagram (previously mentioned) for discussion. ... I'm not sure I've got the energy systems language analog right though Sholto Maud 10:00, 16 November 2007 (UTC)
replies: text: Something similar to the top diagram must be in the article eventually because there are diagrams like that in the exergy literature over and over using numerical values. The confusing things (for me right now because I need a refresher course on this topic) is that the use of the subscript "0" in that top diagram seems to sometimes refer to "reference conditions" in the sense of "standard conditions" (like for H-H0 to the left) and sometimes to "reference conditions" in the sense of "environment conditions" (like when T0 appears). I also suspect that all of the terms must be differentials in order for this diagram to reflect the physics in a general sense. Simple processes would result in logarithms being involved being involved instead of (T-T0)/T in the final term on the top right of your diagram. A concrete example is given in http://www.mdpi.org/entropy/papers/e3030116.pdf . Someone (with a formal science or engineering background) needs to look over that paper or another paper with a concrete example and try to write something general and relatively simple for the Wikipedia reader. As for "energy systems language" and the bottom diagram, I don't know anything about it (yet) other than the fact that arrows point to and from the same boxes but an 8 year old could tell you that. Please don't incorporate the diagram into the article yet. It leads to more questions than answers with the current version of the article. Flying Jazz 14:37, 16 November 2007 (UTC)
replies: text: Ok, thanks for your comments Flying Jazz. Once formal scientists or engineers have made comment on the top diagram, I can re-do it according to their specifications and leave out the energy systems language analog at the bottom. But I couldn't work out where entropy fitted in though, looks like I also need a refresher course on this topic. -- Sholto Maud (talk) 21:39, 16 November 2007 (UTC)
replies: text:  This diagram is more valuable than the whole of the main article - surely it can be refined by someone who understands the subject and included?  — Preceding unsigned comment added by 193.34.187.245 (talk) 14:03, 4 April 2018 (UTC)
thread_title: Exergy diagram

message: There might be a problem with different people having different definitions for exergy.
From what I can deduce from my 2008 text book, the exergy of a system, depends not only on the system but also on its environment.  Hence for example the polar ice caps currently have exergy, since they are melting.
If the earth cooled down a little bit and they stopped melting, they would again be in equilibrium and suddenly loose their exergy.
It is kind of like the concept that Joe System is richer than Bill Environment, at least right now.
If Bill Environment suddenly gets more money, then Joe System is no longer richer than Bill Environment, so Joe being in a state of richer is a combination property.  This seems to me different than a state property which would be how much money Joe System actually has.
Seems like a poorly defined concept to me, unless I can nail it down in terms of a difference between thermodynamic potentials but even then it is hard to know which potential to pick, with the Gibbs free energy being the best candidate???
My thermodynamics text book shockingly does not have either Gibbs free energy or Hemlmoltz free energy in its index, but seems to have devoted an entire chapter to this new exergy buzz word.
I can supply more quotes later, but what I wanted to ask is, I have now taken the liberty of changing the definition of exergy, because before it was relative to some reference state which since it was a reference state was assumed to be constant, unlike the idea of an environment which can be changing with time.  My text definitely says relative to an environment, not a reference state.  —Preceding unsigned comment added by Hobojaks (talk • contribs) 20:43, 23 December 2008 (UTC)
thread_title:  Combination property 

message: This discussion refers to the section Comparison of energy and exergy as it was in the article revision of 18:39, 20 July 2009; I removed this section in the very next revision, and will now discuss why.
The section consisted of a comparison table followed by a short paragraph. The comparison table had two headings, “Energy of a system is...” and “Exergy of a system is...”, and in particular it included the following two dubious entries: “Energy of a system is... its ability to produce motion” whereas “Exergy of a system is... its ability to produce work”; and “Energy of a system is... different from zero (E=mc2)” whereas “Exergy of a system is... equal to zero when at equilibrium with the environment”.
The paragraph following the table read: 
::Exergy is a measurable value that is decreased during the conversion of useful energy to useless energy. Therefore, exergy measures the actual potential of a system to do work. The exergy consumed to create something, a product or service, is more than the work done to create it. Exergy is the work that can no longer be done elsewhere because the economic good was made. Exergy has been described as a measure of energy quality because of these traits.
While a section like this is a good idea in principle, there are a number of reasons why it should not stay in the article in its present form. My main concern with it is that it seems to me to be potentially very misleading or confusing, as I will explain shortly. Unfortunately, I myself don't know how to fix it, and so, to prevent the spreading of confusion, I decided to remove it, for now. But I also have a more formal reason for removing it: it is almost completely unsourced. True, it does have one reference, but the article in question ('“On an absolute thermometric scale”' by Kelvin himself) has very little to do with the subject-matter of this section. For example, Kelvin's article quite obviously cannot be a source for anything having to do with E=mc2, as it was written more than half a century before Einstein came up with that equation. Also, it is clearly not saying anything about products, services, or economic goods. More critically, the word 'energy' does not even appear in the article!
Apart from issues of sourcing, a number of statements in this section are potentially very confusing. Let's start with the statement that “Energy of a system is… its ability to produce motion” as opposed to exergy of a system, which, the section says, is “its ability to produce work.” Now exergy clearly does have something to do with work. However, on the face of it, the statement about energy being the ability to produce motion rather than work is highly problematic. Every elementary textbook defines energy as “the ability to do work,” and for what seem to be good reasons: kinetic energy is equal to the work that would have to be done by a net force to accelerate the object from rest to its current speed, whereas the difference in potential energy (which is necessarily due to a conservative force field) at two locations is equal to the work that has to be done against the force field to move the object from one location to the other (such that the kinetic energy is the same at the beginning and at the end); these definitions do not change in more advanced treatments of classical mechanics. Any statement that would deny these reasons (for saying that energy is the ability to do work) is surely a bit suspect, and is probably simply a case of an author not expressing him- or herself well. True, energy must be expended to set something in motion, but then again, sometimes energy must be expended simply to change the configuration of something (which has to do with the object's potential energy). For example, it takes energy to compress a spring or to form chemical bonds in a nitroglycerin molecule. In short, whatever the nature of the distinction between energy and exergy that the author was attempting to elucidate here, the text as written fails to explain it. 
(In quantum mechanics things are trickier, and we usually don't talk about work; but things are also tricky as far as the concept of “motion”. For example, in what sense does the electron, when in an eigenstate of hydrogen atom, “move”? However, the quantum mechanical conundrums are probably irrelevant to the present discussion, which is concerned with macroscopic phenomena only, and whatever quantum mechanics says must, at macroscopic scales, be indistinct from what classical mechanics says; see the article on the correspondence principle.) 
Next, since this article is hardly the place to treat relativistic thermodynamics, let's not mention E=mc2. Invoking it in the present context is useless, since the relevant microscopic theories for the purposes of this article are clearly classical Newtonian mechanics and (perhaps) nonrelativistic quantum mechanics. So if one is doing a comparison between energy and exergy, one should first do it in the framework of those theories. If there is something interesting to be said about exergy in relativistic thermodynamics, that surely merits at least a separate section. 
Further, regardless of whether one wants to include E=mc2 in the total energy, it is simply not true that energy must be different from zero. After all, some of the energy in the system might be potential energy due to an external force field. (The zeroth component of the energy-momentum four-vector, which is what most special-relativity texts mean by “energy,” contains the kinetic and internal energy of the object, but not the energy due to an external force field; see Eq. 7.140 in Goldstein, Poole, and Safko.) And in spatial relativity as well as in Newtonian mechanics, only differences in potential energy due to an external force field have physical meaning. To put it another way, which configuration or location corresponds to zero potential energy is an arbitrary choice. So say you have made a particular choice of which configuration or location of the system under consideration has zero potential energy, and with this choice, it works out that the energy of the system in that state is nonzero. Then I can always chose the zero of potential energy to correspond to a different configuration or location than you did, such that with that choice, the very same system in the very same state works out to have precisely zero energy (I can make it have any other energy, for that matter). And yet, as long as each of us sticks with our choice of where the zero potential energy is, all our subsequent calculations of observable quantities will agree. 
In general relativity things are even more complicated. The notion of total energy only makes sense in certain spacetimes (see the articles ADM energy and Mass in general relativity]), and whether energy (and mass) can be negative is an open question (see the article Exotic matter).
Finally, in addition to the comparison table, there was the final short paragraph that talked about services and economic goods. While I am unable to evaluate the quality of this paragraph, it nevertheless seems to me that it has no place in this section. To begin with, the information it is trying to convey is already covered in the section Applications in natural resource utilization. More importantly, however, the present section is meant to be some sort of intuitive, shorthand comparison, a summary, one that can fit into a table format; more extended discussions should be done elsewhere.
For all these reasons, the section in its present form seems quite deeply flawed. I wish I knew enough about exergy to improve it, but I don't. (I do know enough about energy to see that some of the things the section says about energy are dubious at best, as I have discussed above.) Thus, to prevent the spread of confusion and misinformation (about energy, at least) to which the section in the present form would contribute, I am removing the section from the article, for now.  Reuqr (talk) 01:18, 13 September 2009 (UTC)
thread_title:  Removing the section ''Comparison of energy and exergy,'' for now 

message: Some pictures would be nice. Narssarssuaq (talk) 06:10, 6 November 2012 (UTC)
thread_title: Pictures?

message: I feel very uncomfortable with the end of the second paragraph: "Exergy is always destroyed when a process involves a temperature change... For an isothermal process, exergy and energy are interchangeable terms, and there is no anergy."
The first sentence is misleading - since exergy can be decreased or increased when changing an intensive variable - and the second is just wrong, which is most easily seen when considering the process of irreversible expansion of a gas through an orifice: There is no associated temperature change - but there is certainly a loss of exergy = anergy.
Has anybody an idea how to correct this flaw? I have difficulties to find suitable references, since the subtle details of this topic are not adequately discussed in most thermodynamics textbooks.
AKoerber (talk) 17:01, 30 May 2013 (UTC)
thread_title:  Serious flaw 

message: The most important thing I want to know about the term exergy is who uses it?
This article badly needs a factual summary of what journals, textbooks, university departments, scientific blogs, etc., use the term.
Someone with access to JStor could search for the term and produce a histogram of uses per year.
Such a section might be branded "original research," but too bad, the information is really needed in this article.  Also, such a section could be submitted for publication to a general science journal.  American Journal of Physics comes to mind.
207.255.42.202 (talk) 04:12, 18 November 2013 (UTC)
replies: text: We already summarize usages of the term at Exergy#Applications. I do not see how graphs of the occurence of this specific word by journal or time would help communicate an encyclopedic understanding of the subject. VQuakr (talk) 04:30, 18 November 2013 (UTC)
thread_title:  Who uses it?  Inquiring minds (our readers) need to know. 

message: The intro is far too long - most of it should be redistributed under headings. However I'm not qualified to fix it myself. 90.198.224.197 (talk) 14:29, 26 August 2014 (UTC)
thread_title: Intro too long

message: In an isothermal process heat would be exchanged if there is a change of fase. There would be a change in exergy. Exergy would remain the same only on an adiabatic process: when there is no heat transfer, not on an isothermal process, when the system does not change it's temperature (but may change it's configuration). 
The phrase "Exergy is always destroyed when a process involves a temperature change is wrong. Exergy is always destroyed when there is heat exchange. It does not matter if there is temperature change or not. The author is confusing adiabatic processes with isothermal processes.  — Preceding unsigned comment added by 186.26.113.114 (talk) 22:29, 9 March 2015 (UTC)
thread_title: Isothermal? Adiabatic!!!

message: The first sentence is plain wrong.  Exergy is the maximum useful work during a process that brings the system into equilibrium with dead-state at (T_0, p_0, \mu_{i,0}).  The dead-state definition is more than simply a heat reservoir.  I suggest this page needs a substantial reworking. RobbieIanMorrison (talk) 22:10, 16 March 2016 (UTC)
thread_title:  Incorrect first sentence 

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thread_title:  External links modified (January 2018) 

message: The section A historical and cultural tangent is too own-research-y, philosophical, and hand-wavy to be included where it is. It reads like a very high-flying, professorial Apologetic for exergy that only makes sense if you already know the concept very well. Needs rewriting or removal.
2001:171C:2E60:D7E1:1027:D836:13FD:46EF (talk) 00:30, 17 January 2020 (UTC)
thread_title:  A historical and cultural tangent 

message: Why are any of energy, exergy, emergy, etc. of any interest if they are diluted by time?
The Carnot cycle is made up of isothermal and isentropic phases.  Perfect isentropy requires infinitely fast compression and expansion.  Perfectly isothermal phases however require infinitely slow compression and expansion.  It can be shown that the isentropic phases of the Carnot cycle exactly cancel, whence the mechanical energy obtainable from the Carnot cycle derives purely from the isothermal phases.  But if these happen infinitely slowly then the available power is zero.
Unless exergy takes time into account, I don't see how it is any better than the energy obtainable from the Carnot cycle, which is useless from a power perspective.  Vaughan Pratt (talk) 06:55, 8 December 2022 (UTC)
thread_title:  Shouldn't time be relevant?