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Old   January 29, 1999, 06:08
Default Energy Vs Exergy
  #1
Apple L S Chan
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Hello

From some CFD literatures, I have seen the word "exergy". Is there anyone please tell me the difference between energy and exergy.

Thank you very much!

Apple
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Old   January 29, 1999, 10:31
Default Re: Energy Vs Exergy
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John C. Chien
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If it is not a new car name, then it must be a name invented in China, because they seem to use a lot of " x " in the name. Why don't you type it into a word processor and use the online dictionary to check it. If you can't find it, then pick either one of the above answers. Or any other suggestions ?
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Old   January 29, 1999, 11:36
Default Re: Energy Vs Exergy
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Ridwan Setiadi Arrizar
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energy: is physical quantity, for example you can perform a energy balance to water that is used to cool of a heat exchanger, locically the water will rise in temperature, which can be determined by an energy balance.

exergy: is certainly not the same as energy so balancing is not possible and is (most of the time) hard to quantify. For exapmple: consider a certain industrial process were high quality products are made and the investment are low but great damaged is caused to the environment and the same process demading higher investments, producing the same quality products and causing lesser damage to the environment. Then it is (often) said that the exergy of the latter proces is higher. I personnaly think exergy is a doubtful term and is also (a bit) subjective

Hope I have helped you,

Ridwan.
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Old   January 29, 1999, 13:46
Default Re: Energy Vs Exergy
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John C. Chien
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I think, damage to environment is an irreversible process. An irreversible process will increase the value of Entropy. So, is it all right to say that " A process with lower Entropy increase has a higher Exergy value ". Well, I still think that anything with EX of XE in it must be a new car model. The model of my car is the EX model, and sometimes, it is the only way to spot my car in a big parking lot.
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Old   January 29, 1999, 15:26
Default Re: Energy Vs Exergy
  #5
Afshin Azari
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There is a book by Adrian Bejan on thermodynamics that has answered many perplexing questions that I had about thermodynamic concepts (exergy being one of them). If I remember correctly, the title is 'Engineering Thermodynamics'. The book is well written and the subject is well researched. It is definitely worth looking at. Good luck.
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Old   February 2, 1999, 02:35
Default Re: Energy Vs Exergy
  #6
Stana Todorcevic
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Hello Apple,

information on how efficiency of energy utilization in any kind of process can be investigated by means of exergy analysis, you can find in following articles:

DeNevers, N. and J.D. Seader, "The Two Lost-Work Statements and the combined First-and Second-Law Statement", Chem.Engng.Educ. 18, 146-148 (1984)

Debenedetti, P.G., "The Thermodynamic Fundamentals of Exergy", Chem.Engng. Educ. 18, 116-121 (1984).

Here, as you will see, exergy appears synonymous with "availability function", as well as "availability".

Also, more information on exergy, you can find in this book:

Szargut, J. and R.R. Petela, "Exergy".

Best Regards, Stana

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Old   February 2, 1999, 10:30
Default Re: Energy Vs Exergy
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John C. Chien
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I think, if you turn the mass into the energy, you get E=MC2. M is the mass, and C is the speed of sound. On the other hand, if you turn the "availability" into the product, you are going to get something like a new car model. When you run out of workers, steel, coal, etc., you run out of the availability. The assembly line will be shut down, company merged, workers laid off, because something is no longer available. By the way, don't try to read this message, I am just trying to fit my new car theory into the lost concept. Just an exercise, that's all.
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Old   February 2, 1999, 10:34
Default Re: Energy Vs Exergy
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John C. Chien
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C is not the speed of sound, C is the speed of light. I hope that Einstein is not surfing the Internet right now.
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Old   February 2, 1999, 12:19
Default Re: Energy Vs Exergy
  #9
Matej Forman
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Hi John has suggested that new car model is the thing I can touch as an exergy. I have been always taught that exergy is energy that I can use without breaking 2nd thermodynamics law. And there may be a problem. I am a PhD student and I cannot effort any new car (= exergy) without breaking some law. Well it's all I want to say about the car - exergy concept.

Have a nice car

Matej
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Old   February 2, 1999, 13:03
Default Re: Energy Vs Exergy
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John C. Chien
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Great! I think a PhD is not just = a dissertation. A creative mind is always helpful, after all, laws are man-made. Anyway, don't listen to me if you are concerned about breaking the law.
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Old   February 9, 1999, 17:53
Default Re: Energy Vs Exergy
  #11
R.kurz
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Exergy is a widely used definition in Thermodynamics: It acknowledges the fact that according to the 2nd Law of thermodynamics (that's the one with the entropy) does not allow all energy conversions that are allowed by the 1st law (e.g. there is no way to convert heat entirely into electric power). Exergy then is the part of the available energy that can be completely converted into other types of energy, while Anergy describes forms of energy that cannot be converted in any other form of energy (for example haet at ambient temperature). And: Energy=Exergy+Anergy
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Old   February 10, 1999, 23:04
Default Re: Energy Vs Exergy
  #12
Pyung Suk Jung
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Hi, John,

Don't try to underestand what is exergy.

A new concept is not underestood but familiarized.

For example, they think they underestand what is energy.

Even Feynman, the Nobel Prize winner, was very difficult to explain energy, and could give only as something conserving in his book Lectures on Physics.

In fact, exergy is easier than energy, the internal energy.

Exergy is "the possible maximum available work(like shaft work) from an interaction between an object and a reference huge system (atmosphere)".

It is useful in engineering sense to indicate the limit of shaft work obtainable from an object like hot water in the atmosphere. Why atmosphere? It can be any huge system whose state is fixed constant if you are a mathematician.

The problem is that the exergy in CFD.

Exergy has many versions like a cameleon changing its color corresponding to circumstances. Essergy, Flow exergy, available energy, Availability, Exargy function... Even the free energies such as Helmholtz free energy and Gibbs free enthalpy are regarded as the mutations of exergy sometimes.

Exergy is a state quantity ( variable, property, coordinate )of a system like energy, entropy or pressure. Therefore it can be used as a measure to show the state of the object.

There are several good points if you calculate the exergy in your analysis. One thing is that exergy may show the distance from the final state of the system because it stand on the Second law of thermodynamics like entropy. Another is that your writing seems to be fascinating not because it is really fascinate but because it is decorated with something mysterious arround you. The only problem is that you should provide the usefulness of your calculation. But don't worry. Because only few people will ask you. They are afraid to ask something they don't know(familiar) at all. At least you can get the fame to have tried something new arround you. If you can find anything useful or convenient in calculation by using exergy, it must be great. But I believe that nothing can be found there except a change of variable because exergy is not use as a acting variable but an additional estimation after the calculation finished in CFD.

Any way, we can give physical meaning of exergy, but how is energy? Absolutely we can't. The only thing is something conservative and mathematical meaning that energy is a kind of property, the value of which is given by the present state only and not by the past history.

You are asking the difference between energy and exergy. It is similar to the difference between Celsius temperature and Fahrenheit temperature. They are nothing but different measures, variables to be used to show the state of a system. Exergy is smaller than energy in general just like Celsius temperature is less than Fahrenheit temperature.
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Old   April 5, 2011, 10:32
Default Negative Exergy
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xergy, can be negative for biogas at low pressure(metan)???I did some energy-exergy calculation for biogas compressor. I obtained negative values at inlet and outlet states.(at 3.2 kpa pressure)Is İt possible? İF yes,why???
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Old   April 5, 2011, 22:18
Default Definition of Exergy
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Ahmed
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Read This
http://en.wikipedia.org/wiki/Exergy
http://www.mdpi.org/entropy/papers/e3030116.pdf
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Old   April 5, 2011, 23:45
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Thanks. I've been wanting to know the difference myself.
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