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I'm teaching a class on the subject of "energy quality". I'm supposed to tell them that energy quality is a measure of how easily (effective I guess) the type of energy can be converted to different types of energy, e.g. exergy. The book I'm using claims that energy quality depends on entropy but doesn't say how.

The only definition of energy quality I can find is $\frac{\textrm{Exergy}}{\textrm{Energy}}$. Can energy quality be defined in terms of entropy as well? Is there a connection between exergy and entropy?

I cant find anything about energy quality in my books on thermodynamics so maybe it doesn't have an exact meaning...

Maybe energy quality is connected to entropy due to the second law of thermodynamics in the sense that conversion of high entropy energy results in even higher entropy energy?

I'm confused.

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  • $\begingroup$ Which book are you using? $\endgroup$ – Shah M Hasan Sep 4 '18 at 11:09
  • $\begingroup$ I'm teaching 16-17 year olds in the danish "Gymnasium" so it's a simple book that do not teach entropy etc. $\endgroup$ – bgst Sep 4 '18 at 12:16
  • $\begingroup$ Get yourself a thermodynamics book (e.g., Fundamentals of Engineering Thermodynamics by Moran, et al) that defines and covers exergy. Exergy certainly depends on entropy. $\endgroup$ – Chet Miller Sep 4 '18 at 12:31
  • $\begingroup$ Exergy looks like an obsolete term in the same way that erg is an obsolete term for the measurement of energy. $\endgroup$ – Mozibur Ullah Sep 4 '18 at 13:06
  • $\begingroup$ @MoziburUllah I don't think that's really a fair comparison. Exergy isn't a unit of measurement. It's a way to describe the usable energy, and in context can save a good deal of confusion. It came up all the time in an energy management course I took. $\endgroup$ – JMac Sep 4 '18 at 14:17
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I think energy quality can be related to change in entropy using a carnot cycle in T-S diagram.For the same amount of heat added and the same sink temperature, if you consider two sources at T1 and T2.T1>T2 then,the entropy change during heat input for T1 will be lesser, so the heat rejected with source at T1 will be lesser than the other source.This means more work done with T1 source.Thus heat at higher temperature is of higher quality.

I think,to learn about entropy without much mathemathics, the book "The Laws of Thermodynamics: A Very Short Introduction" by Peter Atkins will be helpful.

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I found my answer on the wikipedia page on exergy. It states:

...exergy is always destroyed when a process is irreversible, for example loss of heat to the environment (see Second Law of Thermodynamics). This destruction is proportional to the entropy increase of the system together with its surroundings.

So if energy quality is defined as the ratio of exergy to energy, the loss of energy quality must be proportional to the increase of entropy.

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Exergy depends on entropy. So by defining energy quality as Exergy/Energy, one may argue that energy quality depends on entropy.

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  • $\begingroup$ This answer really needs more elaboration IMO; this doesn't really say anything the OP didn't mention in the question. $\endgroup$ – JMac Sep 4 '18 at 11:21

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