Availability Exergy

[Hartlw]

Thermodynamics textbooks define availability (exergy) as the max amount of work that a system can perform as it goes to equilibrium with environment and interacts with it.

They also define difference in availability (exergy) between two states. Is the difference in availability (exergy) the max amount of work achievable between the two states? Why?

If I define "associativity" of a city as the shortest distance from the city to Chicago, it does not follow that the difference in "associativity" between two cities is the shortest distance between them.

 

[mcgyvr]

Homework? Sure looks like it.
Please read the forum rules.

 

[IRstuff]

In your Chicago example, the answer is no. Consider two cities on opposite sides of Chicago.

TTFN

FAQ731-376

 

[Hartlw]

This is not a HW problem. I am retired and reviewing Thermo. If my question is inappropriate, my apologies. Feel free to remove it.

I am reviewing from a number of Thermo texts. One text (Lee-Sears) says that the difference in availability (exergy) between two equilibrium states is the max work attainable between two states. I don't see it. Other texts don't mention it.

I am a Mechanical Engineer and not familiar with Thermodynamics. I thought the question might also be of general interest to Thermodynamics.

 

[MintJulep]

When ALL of the systems are in equilibrium, no more work can be done.

 

[Hartlw]

Single system between two different equilibrium states.

 

[chicopee]

I keep on seeing the word "exergy",what is that?

 

[Hartlw]

chicopee

It's something I picked up from some Thermo books. By definition, it's the max amount of work that a system can perform in coming to equilibrium with the environment. I really don't know if it's just an academic concept or something used in real world thermodynamics engineering like analysis of power plants.

 

[IRstuff]

See:

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

Based on that, the answer is also no, since entropy is included. Since you cannot make useful use of all the stored energy, unless you do directly from one state to another, there will be additional energy consumed.

TTFN

FAQ731-376

 

[dvd]

First place I encountered exergy was in an advanced thermo text by Bejan; I thought that book sucked. I like to use thermo to calculate stuff and solve problems. If I wanted to get into accountancy Iwould have. I don't view many of the terms, such as exergy, introduced into thermodynamics as helpful or relevant.

 

[mikanical]

I disagree dvd. The concept of exergy can be very helpful as it is an indicator of the quality of energy source or flow; something not accounted for in a 1st law analysis.

 

[willard3]

Which is why we have the second law of thermo.

First Law of Thermo: The best you can do is break even.

Second Law of Thermo: You can't break even.

 

[mjpetrag]

The amount of Exergy destroyed (ie the energy that is lost to the surroundings) is:

Xdes = (Temperature of surroundings) * (change in entropy generated)

The value for Xdes is usually expressed in units of Energy per unit mass.

Hopefully that helps.

-Mike

 

[Tunalover]

Hartlw-
A mechanical engineer SHOULD be familiar with thermodynamics if he/she were educated in an ABET certified mechanical engineering program. The thermo text is at arm's reach at least on a dusty shelf. The school I went to required three thermo classes.



Tunalover