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Q & A: entropy and efficiency of heat engines

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Most recent answer: 04/13/2013
Q:
what happens when the entropy in the steam turbine increases? Why is it recommended to go for isentropic expansion in steam turbine?.
- Kirtan (age 20)
India
A:
There's a general expression for the efficiency of a heat engine in converting heat flow (Q) from a hot reservoir (TH) to a cold reservoir (TC) into work (W). If everything works ideally, you get the Carnot efficiency:

W/Q = 1- TC/TH.

However, things don't work ideally, there's always some net production of entropy, S. Then the expression becomes:
W/Q = 1- TC/TH - STC/Q. 
So to maximize efficiency you need  to minimize the entropy production, S.  

For gas expansion, there are two types of processes that keep net S unchanged. One is the adiabatic process, in which the entropy of the gas doesn't change. What it gains by taking up more volume it loses by cooling down. This is a good process for use in a turbine, because it can happen quickly, since it requires no heat flow with the surroundings.  The other process, isothermal, assumes heat flow with surroundings always at the same temperature as the gas. That's ordinarily too slow for use in a turbine.

I've read that some practical turbines use other expansion patterns, deliberately sacrificing some thermodynamic efficiency for other operating and construction conveniences.

Mike W.

(published on 04/13/2013)

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