Talk:Exergy

Needs more focus
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)

I'm another reader (from a biological background) trying to distinguish the terms "free energy", "available energy" and "exergy". There is a lot of good stuff here, but I note that there are still a number of loose ends in main text from work started in 2007 that sill hasn't been cleaned up. I had enough physics as an undergraduate to understand some of the issues, but not enough that I can tidy up the unfinished business. Thanks. —Preceding unsigned comment added by 124.180.182.165 (talk) 23:23, 2 March 2011 (UTC)

The article is a mess because Exergy is a mess ! Using Exergy (or Entropy) for thermodynamics is fair enough, but attempting to use it in Economics - i.e. saying that high Exergy efficiency is good and low Exergy efficiency is bad - is somewhere between dangerous and wrong! — Preceding unsigned comment added by 193.34.187.245 (talk) 13:43, 4 April 2018 (UTC)

Finite exergy?
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)

Rewrite in progress
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)

Merge
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)


 * 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)


 * 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)

Comment
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)

formulas
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)

I see this equation change was made, but nowhere in did I find a definition for Q —Preceding unsigned comment added by 158.12.37.51 (talk) 17:17, 24 March 2011 (UTC)

Availability
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 &mu;R ?

(Reference: Waldram, Theory of Thermodynamics, pp. 75, 205). -- Jheald 11:24, 5 December 2005 (UTC)


 * Looks the same to me. The rewrite is making this clear Flying Jazz 11:56, 28 December 2005 (UTC).


 * 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)

Cumulative Exergy
I'd like to suggest that there is a separate sub-section dedicated to the concept of "Cumulative Exergy" as it is used in engineering. And then another sub-section dedication to a consideration of whether the engineering application of "Cumulative Exergy" is formally analogous to the application of "emergy" in systems ecology. Sholto Maud 03:03, 10 January 2006 (UTC)


 * I think the rigor of the emergy concept should be discussed in the emergy article. This article might discuss cumulative exergy eventually in a little more detail in the context of changing a reference state or unifying multiple reference states to an earlier one. In general though, cumulative exergy loss (or gain) is just like cumulative money loss/gain, cumulative weight loss/gain or cumulative heat loss/gain, so it's not worth a separate section in my opinion. Flying Jazz 16:49, 10 January 2006 (UTC)


 * Re: emergy... agreed.
 * Re: cumulative exergy. 1. So can we assume that the cumulative exergy concept is mathematically the same no matter whether it is used by engineers, biologists, chemists or ecologists? 2. If consequence of irreversible cycle is cumulative exergy loss, does a reversible cycle have the consequence of cumulative exergy gain? Sholto Maud 06:33, 11 January 2006 (UTC)


 * 1. I don't know about that yet, but I suspect there won't be a simple yes/no answer to that question. 2. No. Exergy=available energy. That would mean available energy appearing out of nowhere. For a reversible cycle, there is no change in exergy. Flying Jazz 13:19, 11 January 2006 (UTC)


 * Thank you for your response Flying Jazz. You might be interested in the anonymous quote I have put up at Talk:Emergy. If my anonymous source (apologies for this anonymity) is right then it seems that emergy algebra and cumulative exergy algebra are one in the same. Iff this is the case, then cumulative exergy algebra appears to address the phenomena of "coproduction". Sholto Maud 22:25, 11 January 2006 (UTC)


 * Cogeneration is nice. So are other coproduction methods in engineering and in nature. Your anonymous source has told you that integrals are really just like sums. There's no reason to be so strange and mysterious about any of this. If you want more people to understand exergy and emergy concepts, you are going about it the wrong way. This is not ancient Greece and we are not probing Delphic mysteries. When I wrote there probably won't be a simple yes/no answer, I meant that if someone agrees with all the assumptions made then the answer is obviously yes, and if someone doesn't agree with a single assumption made then the answer is obviously no. An encyclopedia article shouldn't make that judgement for the reader in my opinion. Please show your source what I have written under the "Applications" section. This is not yet referenced and it's a first draft, but I think I can come up with support for most of the statements I've made. I'm curious whether you or your source or others find this section to have a neutral point of view as it proceeds from engineering to the cosmological argument. Flying Jazz 00:06, 12 January 2006 (UTC)


 * My apologies if I am going about this the wrong way, and it seems mysterious - I have not been well schooled in diplomatic emergy synthesis. But also I don't want to make stupid errors, so I go slow. :)
 * Re: Yes/No - I understand.
 * Re: Show my source - I'll refer them to the section, to be rigorous, I understand it is a draft, but, as you suggest, I think that the section should be very well referenced, with actual practical examples.
 * Re: Neutrality & general comments - there are a few comments which I feel hesitant about, but let me say that I think you have done some fantastic work so far, and I appreciate your contribution very much.
 * - Not sure if saying, "The combination of untestable hypotheses, unfamilar jargon that contradicts accepted jargon, intense advocacy among its supporters, and some degree of isolation from other disciplines have contributed to this protoscience being regarded by many as a pseudoscience." is neutral.
 * - Not sure if saying, "Testing this idea in living organisms or ecosystems is impossible for all practical purposes because of the large time scales and small exergy inputs involved for changes to take place. However, if this idea is correct, it would not be a new fundamental law of nature. It would simply be living systems and ecosystems maximizing their exergy efficiency by utilizing laws of thermodynamics developed in the 19th century." is neutral. I'd like to refer you to Bastianoni's article. It seems to me that $$k_iCE$$ in Figure 1 is an example of exergy flow using the energy systems language in biochemistry and biophysics. And also perhaps, this book might be good read as an example of testing the maximum power hypothesis.
 * - Rather than say, "There has been some progress in standardizing and applying these methods." it might be better to give examples of the progress.
 * The intensity of my emergy/cumulative exergy advocacy will reduce as this year proceeds. Good luck with this important work. Best Regards, Sholto Maud 23:09, 12 January 2006 (UTC)

You may be right about many of those comments. As I learn more (and relearn more of what I knew in college), I'll try to keep your comments in mind. However, your own statement about the energy systems language being important in that article does seem to support my statement that a problem many have with these fields is unfamiliar jargon to describe topics that are familiar using more standard (or simply older) language. I will consult Odum and his critics a little later after the math and engineering sections have a few examples in them. Don't worry. The guy won't be ignored when the article is done. Flying Jazz 23:11, 13 January 2006 (UTC)

1911 Britannica
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)

Free energy
Would there be any merit in combining the exergy article with the free energy article? They are almost identical in their definitions? Sholto Maud 00:45, 18 February 2006 (UTC)


 * No. In that case they should be edited to make them more different.  Exergy = Availability.  Free Energy = {Gibbs, Helmholtz} Free Energy.  Of course, Free Energy can be derived as a special case of the Availability function.  But IMO, discussion of exergy/availability under Free Energy shouldn't go much beyond that.


 * The two concepts are different, and usefully treated as such. If the articles have become too close to each other then some cutting back again to separate them is required. IMHO. -- Jheald 04:54, 18 February 2006 (UTC).


 * I agree with Jheald. The concepts are different and are used for different purposes and exergy should be understood on its own merits instead of being merged with other concepts.  That little subscript to indicate reference conditions makes a huge difference to everything!  By the way, I still regard this exergy article as incomplete and messy, and I wasn't the one who removed the tag.  Especially with respect to conservation issues, I think it's important that detailed and technically accurate information about this topic is "out there" online freely accesible to people.  My enthusiasm for the topic hasn't decreased, but getting done the first few sections took up too much of my time and I couldn't continue at that pace.  I hope to come back to this article in a few weeks and add examples and implications in text boxes and diagrams.  As it is right now, this article makes a lot of statements that I'm not sure should be included as written even though I wrote it but I hope to make detailed references with a copy edit at that time too. Flying Jazz 13:05, 18 February 2006 (UTC)


 * Ok cool. For one, I appreciate the attempts to clarify these concepts. If I may be useful as resident fool my summary is;
 * Reference conditions is what makes exergy different from free energy: exergy is applied to systems with a large scale reference environment, free energy is applied to systems with a small/no scale reference environment.?
 * When it says, "Unlike energy which is always conserved for a cyclic process, an irreversible cycle reduces exergy." is it correct to say that a simple electrical circuit with resistor and battery is an example of this, such that the electrical energy is conserved in the circuit/cyclic process, but the chemical energy driving electron flow is not conserved and is a non-cyclic (irreversible) process where "exergy is reduced" or lost as heat accross the resistor? Such a circuit seems to involve free energy but are the reference environments for exergy and free energy different or the same in this example? Sholto Maud 13:31, 18 February 2006 (UTC)


 * Didn't Socrates always pretend to be a fool at first...and then he started asking questions too... The reference conditions are what makes exergy different from every thermodynamic potential, including free energy. Exergy is applied by comparing a system to a reference environment.  It says what the system can do, what work it can perform, before it can't do any more because it just blends in with a reference environment that doesn't change because the event took place.  Free energy is only a property of the system and the environment is not involved in any way.  Free energy doesn't compare the system to anything else. If you have a mix of chemicals in an isolated box, you can use free energy to determine whether a reaction will occur.  There is a "before" and an "after" state for the contents of the box.  But in order to use the exergy concept properly, there has to be something else either inside or outside the box that acts on or gets acted on by the reaction.  Still, I think in many situations, free energy and available energy can be used as if they were the same thing when they are set equal to each other (see equation 6 and 7 in the current version of the article).  There might be some confusion about the math as it is now because the free energies of formation for chemicals are often listed in tables with respect to forming that chemical at 25 °C from elements at the same temperature, and the subscript 0 is used to indicate a "standard state" at a certain T and P, but this is different from "reference environment" in the exergy concept.  I changed the math in the article to "R" subscripts to reflect this and match the second law of thermodynamics article.
 * I'm not sure what you're asking exactly about the battery, but maybe the answer is in the last paragraph of the the Mathematical Description section. Flying Jazz 14:40, 18 February 2006 (UTC)


 * That explains alot, thanks Flying Jazz. I was really confused about the difference between this and free energy as well. So basically, free energy is the energy released during a reaction and exergy is the energy released ifa system shifted to the energy level of the surroundings? BTW, the 0 suffix is 25 degree C, 1 atm. kr5t

Re: Energy quality types
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)

I think one should look at Entropy as a measure of energy diffusion. At first in your ice cube, you have a temperature difference with the environment, which is then an energy difference. As ice melts, energy tends to be homogeneous in the system : entropy increases. Then, the sentence you mentionned is not exact. --Napishtim (talk) 14:21, 16 December 2007 (UTC)

B or &psi; or &Xi; or X?
These are the four letters I've seen used for exergy. I know that B is a magnetic field strength (units of tesla), so using B for exergy (units of joules) might confuse people. I want to use it anyway because my experience has been that the texts that refer to exergy most often and the most authoritatively (Szargut's "Exergy Method: Technical and Ecological Applications" is handy now) have used B and that's what I learned as a chemical engineer, so I'm reverting the introduction. E is an electric field strength (units of newtons per coulomb) and also energy (units of joules). Physicists seem used to this. The thermodynamics folks and the field folks should be used to having conflicting nomenclature by now, and these two subfields of physics don't interact often enough for it to be too much of a problem I think. Flying Jazz 06:46, 29 June 2006 (UTC)


 * Looks like it's called φ sometimes also. Flying Jazz 07:05, 29 June 2006 (UTC)


 * If I read the 2nd law article correctly and it's a thermodynamic potential, in statistical mechanics it's usually denoted as &Xi;. --0SpinBoson 13:44, 29 June 2006 (UTC)


 * Exergy+"Statistical mechanics" receives 888 google hits. Exergy+ecosystem gets about 25,000. Exergy+physics receives about 50,000. Exergy+engineering gets about 120,000.  The concept is used most often by engineers, so I'm reluctant to use the statistical mechanics variable. I also don't think putting subscripts and superscripts on a &Xi; looks right but maybe that's just me. Maybe I shouldn't have asked for help on the physics project talk page after all, but I would like to have rigorous math here before writing more about less rigorous subtopics, and I'd like to put the same letter on the 2nd law page. Flying Jazz 15:12, 29 June 2006 (UTC)


 * As long as you're consistent, it doesn't matter much I suppose. Just one of those things, like i vs. j for electrical circuits. --0SpinBoson 15:26, 29 June 2006 (UTC)


 * I found a good powerpoint presentation about this issue comparing 15 textbooks (with a silly joke in the middle). Their recommendation for textbooks was to use an "E" with a different font type, like normal Roman E for exergy instead of italic E for energy. However, they didn't say whether any of the textbooks they examined did this. Flying Jazz 15:12, 29 June 2006 (UTC)


 * When I did thermo it was A, for Availability; but IUPAC says that now has to be Free Energy Helmholtz Energy. Ξ was the Grand Partition Function, ie the partition function for a system with a variable number of particles.  I can't say I like B, because in physics there's almost always the chance of a magnetic field knocking around somewhere.  So that's why I used X in the second law article -- that and the fact that it seemd a nice everyday unthreatening sort of letter, and didn't seem to be taken by anything much else. Jheald 15:23, 29 June 2006 (UTC).


 * All four should be mentioned in the Exergy article, and perhaps also the 2nd law page. I have no preference as to symbol. In the absence of IUPAC standard, the idea that the most used symbol (in this case engineering useage) can be a proxy-standard seems to have merit. Sholto Maud 08:52, 30 June 2006 (UTC)


 * What was Zoran Rant's symbolism again? Perhaps the article should track the historical development (and changes) of the symbols: who used which one for what purpose... were they trying to emphasise something different with a different symbol or were they not aware of precedent symbols


 * I study energy eng. in italy..we have always used the &Xi according to Kotas..

Cold exergy
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)

I agree whith this, I changed the article. Further explanations are also given.--Napishtim (talk) 14:34, 16 December 2007 (UTC)

How this introduction got screwed up from a simple definition to a contradictory mess
The introduction to this article used to be:
 * In thermodynamics, the exergy B of a system with respect to a reservoir is the maximum work done by the system during a transformation which brings it into equilibrium with the reservoir.[1] The term was coined by Zoran Rant in 1956[2], but the concept was developed by J. Willard Gibbs in 1873.[3]

This was a factual definition of exergy. Then an anonymous user inserted
 * To put it simply, it is a measure of the actual potential of a system to do work. Energy that has a high convertibility potential is high valued exergy (electricity, mechanical work, fossil fuel deliver high valued energy). Reversely, energy which has low convertibility potential is low value exergy (heat close to room temperature). [2] Exergy analysis are used in the field of industrial ecology as a tool to both decrease the amount of exergy required for a process, and use available exergy more efficiently.

No. That is not putting it more simply. That is taking a nice, simple thermodynamic definition of "exergy" that physicists and engineers like and that ecologists should like and running off on the tangent of mentioning "convertibility potential" and adding the word "valued" to turn a definition into a mess. And what the hell is an "actual potential" of something? The actual is one thing. The potential is most likely another. Then User:MGTom added a huge chunk of text to the intro here. It's an introduction!

User:Sholto Maud, to his credit, removed all of that but replaced it with
 * Exergy is defined differently depending on one's point of view (POV). From the thermodynamic POV, exergy is defined as a measure of the actual potential of a system to do work. In the systems energetics POV, exergy has been defined as entropy-free energy, or neg-entropy. It is unclear whether these POV's are compatable.

What the heck? You want an encyclopedia that gives the reader two definitions and then tells the reader that it's unclear whether the two are compatible? That is NOT serving the reader. Exergy is one thing. One concept. If someone in systems energetics says that it is something different from what it is, it is not "unclear." That person is just plain wrong. What is more likely is that Sholto Maud is wrong by overgeneralizing what people in systems energetics say.

The current intro is:


 * Exergy is defined differently in different fields of study. In thermodynamics, exergy is defined as a measure of the actual potential of a system to do work. In systems energetics, exergy has been defined as entropy-free energy. The latter definition is also related to the negentropy concept which aims to measure the work performed by biological systems. It is unclear whether these definitions are equivalent.

Of course they're equivalent. If a process involves doing work by a transfer of energy with no entropic losses, then the maximum possible work done is the same as the work actually done. Energy transferred with no entropic loss could be called "entropy-free energy" by someone who is playing a little loose with words and ideas and probably most rigorous people wouldn't mind as long as their math worked out. Adding negentropy concepts to the introduction was a terrible mistake. Why do this, Sholto? Why take a strictly defined thermodynamic concept and try to obfuscate is as much as possible? Do you think this makes a better encyclopedia to tell people that something is unclear when it's not? I am reverting this introduction to what it used to be when it was clear. Flying Jazz 19:32, 15 November 2007 (UTC)


 * I grant a number of Flying Jazz's points.
 * With respects to obsfucation - what happens if when reading verifiable peer reviewed, and apparently scientific literature, one finds that there is one word (in this case "exergy", but not limited to this case) that is used in different ways depending on the context? Moreover, there is no verifiable peer reviewed, and apparently scientific literature, which explains the difference between the different contexts, definitions and points of view. Result: it is not clear that there is only one definition of exergy, nor is there any natural reason for thinking that any one context or point of view should be considered the standard definition above the other. Whether or not Flying Jazz believes the different conceptions of exergy to be equivalent, and whether or not this is actually true & mathematically provable, this opinion must be published in a verifiable, reputable/peer review scientific publication to be valid content in the article. Flying Jazz has not provided any verifiable reference that supports Flying Jazz's views. Futhermore, with all due respect, Flying Jazz's apparent clarity in fact covers over the reality of the current discourse around the exergy concept. Let the facts speak for themselves - some people use the word "negentropy" in their definition of exergy, while others do not. This is verifiable. But if it is obsfucatory then an obsfucate wikipedia article is an accurate expression of the current state of verifiable affairs. (P.S. Don't forget to provide verifiable references) :) Sholto Maud 22:39, 15 November 2007 (UTC)


 * PPS The intro is now far too long! Should try and keep it to a smallish paragraph.
 * PPPPPS Would a diagram help to illustrate the concept? I found one a while ago, I'll try and mock one up and paste it here for discussion. Sholto Maud 23:55, 15 November 2007 (UTC)

Intro length and article sloppiness
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)

Negentropy in the intro
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)

Exergy diagram
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)




 * 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)


 * 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)


 * 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)

Combination property
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)

Removing the section Comparison of energy and exergy, for now
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)

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

Serious flaw
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)

see "Isothermal? Adiabatic!!!" below

Who uses it? Inquiring minds (our readers) need to know.
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)
 * We already summarize usages of the term at Exergy. 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)

Intro too long
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)

Isothermal? Adiabatic!!!
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)

-- Similarly, in the introduction: "For an isothermal process, exergy and energy are interchangeable terms, and there is no anergy." The comment above "Serious flaw" comments on roughly the same thing. I believe the relevant qualifier is an isentropic process (an adiabatic process can still be irreversible and reduce exergy, like a shock wave in fluids). In the introduction, I've made the change from istothermal to isentropic. I don't think it can be wrong, because entropy increase and exergy reduction are linked.

Incorrect first sentence
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)

Can you explain what the dead state is? If it some choice of reference, could an infinite constant temperature environment be that reference? — Preceding unsigned comment added by 139.47.69.55 (talk) 19:08, 26 February 2021 (UTC)

Why is this an article
The correct term is "available energy". Zoran Rant was pretty much a nobody. Is this term really an accepted term? That needs to be proven or else the article should revert back to the more formal usage. — Preceding unsigned comment added by 107.77.213.225 (talk) 02:24, 8 July 2017 (UTC)


 * It's found on 173 pages of Moran et al., "Fundamentals of Engineering Thermodynamics", so it must at least be a practical engineering concept.


 * However, the current article defines it in the Mathematical Description section as being specific thermodynamic potentials (whose selection depends on the experimental setup and its purpose, I think), but in the Applications section, it gets identified with overarching economic and human goals. The problem is that the precise mathematical definitions of the first section have little intellectual contact with the "big picture" policy speculations of the second section.


 * This is the "Santa Fe transition", where the concept takes mushrooms and gets laid.


 * It is as if the term is being used by two unrelated communities. So I see your point.


 * 2001:171C:2E60:D7E1:1027:D836:13FD:46EF (talk) 00:19, 17 January 2020 (UTC)

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A historical and cultural tangent
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)

- To me, it reads less as pretentious, and more as somebody grappling with the philosophical ramifications of thermodynamics. Although I can sympathize, this Wikipedia article is not the place for it. The sections is based on just bits of the historical and philosophical picture. I propose to remove it completely, and have already removed the final paragraph. It made unsubstantiated claims about ecologists vs physicists calling exergy vs. entropy the more "anthropocentric" principle. I don't buy these supposed perceptions. It also made a lackluster history tangent, swerve towards the meaning of exergy in modern ecology.

Shouldn't time be relevant?
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)