# Thermodynamic cycle and entropy

## Main Question or Discussion Point

Alright, I'm confused about why thermodynamic cycles can never have 100% efficiency. There reversible processes right? And a reversible process has zero change in entropy, so then why can't they achieve 100% efficiency. I thought entropy was the measure of energy that cannot be used. How would you measure efficiency if the system increases in entropy?

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rock.freak667
Homework Helper
In a thermodynamic cycle, not all of the energy inputted is converted into work. Some heat is always rejected. Not all processes are 100% reversible.

russ_watters
Mentor
.....so why no try buiding a cycle with all reversible processes, so no heat is rejected? You'll find you can't. Real processes that can be combined into real thermodynamic cycles will always result in a cycle with some entropy.

Try drawing a cycle with no heat rejection on a t-s diagram and see if you can find real processes (devices) that do what the t-s diagram suggests.

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