# Exergy & gibbs free energy

1. Aug 27, 2008

### oldsloguy

Are the concepts of exergy and Gibbs Free Energy related? What are the differences between them? Thanks in advance for any help.

2. Aug 27, 2008

### oldsloguy

I'm also posting this question in the physics forum to see if anyone there can help.

3. Aug 28, 2008

### Topher925

Eh, sort of. The Gibbs free energy is the amount of available work for an isothermal and isobaric process. Exergy is more of a measurement of total available work until a system reaches equilibrium with its surroundings. You can't replace one with the other, but they are similar in concept.

I believe, hopefully someone else can verify this, that GFR $$\geq$$ Exergy of a system for an isothermal and isobaric process or system.

4. Aug 28, 2008

### oldsloguy

Exergy and Gibbs Free Energy

Are the concepts of exergy and Gibbs Free Energy related? What are the differences between them? Thanks in advance for any help.

5. Aug 28, 2008

### Andy Resnick

Re: Exergy and Gibbs Free Energy

"Exergy" is another word for "Gibbs Free Energy", AFAIK.

http://en.wikipedia.org/wiki/Exergy

6. Aug 28, 2008

### Topher925

I should also have stated that the amount of exergy a system has is not dependent on whether or not its an isothermal or isobaric process. It could be any type of process and it will still have the same amount of exergy regardless. The same can not be said for the Gibbs free energy.

7. Aug 28, 2008

### stewartcs

That sounds about right to me. Exergy is generally the total amount of useful work that can be obtained from a thermodynamic system, whereas Gibbs and Helmholtz are for specific processes.

CS

8. Aug 28, 2008

### stewartcs

Don't double post. If in doubt PM a Mentor and they will help you decide where to put it (or move it).

CS

9. Aug 28, 2008

### stewartcs

Re: Exergy and Gibbs Free Energy

Not exactly, Exergy is more of a general term used to describe the useable amount of work that can be extracted from a thermodynamic system whereas Gibbs is for a specific process (i.e. constant temperature and pressure). They are related though - conceptually.

CS

10. Aug 28, 2008

### oldsloguy

OOP, sorry. My thinking was that some people might not look at all of the different forums. BTW what is a "PM" and how do I do it?

11. Aug 28, 2008

### stewartcs

Private Message. Click on the name of a Mentor (or any user) and you will have an option to send a PM.

CS

12. Aug 28, 2008

### oldsloguy

Thanks for the responses. So would it be fair to say that the Gibbs Free Energy would be a specific case of the more general Exergy. That is that for isothermal and isobaric processes the GFE and Exergy would be equal and computed in the same manner?

13. Aug 29, 2008

### Topher925

I would have to say no. Exergy is dependent of the systems surroundings while the GFE is independent of the systems surroundings. You can have a system that is isothermal and isobaric where GFE = Exergy but your assuming the conditions are such that all available work can be extracted when the system reaches equilibrium. This isn't always the case as for a systems equilibrium may not allow all available energy to be extracted, which can be determined by the GFE, still assuming an isobaric and isothermal process of course.

14. Aug 30, 2008

### oldsloguy

Still a little confused. When you say "may not allow all available energy to be extracted", I think of "all energy available according to the 2nd law. So, if the energy is available according to the 2nd law, why wouldn't you be able to extract it as you bring the system to equilibrium? Can you think of an example that might be helpful? Thanks again!