Isentropic and Exergetic Efficiencies

[Newbie34]

Hi to all people here, i am new around here.

I've searched the forum about this kind of thread and wasn't successful on it. What i wanted to ask is, about isentropic and exergetic eff., where isentropic is compared to the ideal condition and exergetic is comparation of Product and Fuel.

Now for the question :

Can exergetic efficiency be greater than isentropic efficiency? If it can, in what conditions do they occur (like example in a turbine).

The question is just out of my curiosity, and although i think that Exergetic Eff, will not exceed that of Isentropic, i am not convinced thoroughly, are there special occasions?

Thanks for the reply.

 

[dcasto]

look up rankin cycle. You can get more heat out of the system than you put in. Well, not exactly, the extra energy comes from pulling energy in from the vaporization step.

 

[Newbie34]

What about in a component, not in a system, can it occur?

 

[dcasto]

clarify what you mean in a componet? Componet of a system? an air cooler will cool a stream down rejecting millions of BTU/hr, yet the motor requires a few thousand BTU's/hr of input.

 

[Newbie34]

Yes, a component in a system. Like turbine for example, how do it's exergetic efficiency can exceed that of Isentropic eff ?

The exergy of a turbine is work produced divided by the exergy at input substracted by exergy at output. While Isentropic is actual work produced divided by ideal work.

And also, how do these two efficiency relates?

 

[DCanelhas]

Exergy takes into account the availability or "quality" of energy, as far as i understand it. In other words Electrical energy and Thermal Energy are both measured in the same units but the process of converting between the two is more favorable from the most qualitative to the least, the difference being a measure of exergy.
However, with regards to how unsure i am on the subject, perhaps that whole statement should be phrased as a question, actually.

I do know a good source for learning more about the subject though, I have to remind myself to read though it:

http://www.exergy.se/ftp/introduction.pdf

 

[Newbie34]

Thanks for the replies, but i managed to figure it out myself, the answer is it can. This conclusion is obtained from the environment temperature. Once again thank you for the replies.