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samskiter OP t1_j18nnn7 wrote

Reply to comment by incredibolox in Why do we use phase change refrigerants? by samskiter

So this is maybe a little bit of the crux of my question. A Carnot cycle is isentropic and I believe that phase changing would introduce randomness and therefore reduce theoretical efficiency limits.

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GenericUsername2056 t1_j18q7dc wrote

Instead of 'randomness' it's probably clearer to think of entropy as the amount energy unavailable to perform useful work. So pressure losses and frictional losses for instance consist of energy we cannot use to generate electricity. This is also why entropy must always remain equal or increase.

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seven_tech t1_j18u41q wrote

An ideal Carnot cycle is Isentropic. The real world isn't ideal. Entropy is increased in the real world because of loss of energy through friction, intra-molecular force in gas and other similar processes.

And again, liquid is a much better transfer medium for heat than gas inherently.

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samskiter OP t1_j193400 wrote

Yea that seems to be the conclusion, but was wondering about the why. Like why we don't try and get as close to the carnot cycle as possible. The replies with numbers helped a lot with getting a sense / intuition for the scales and sizes that make the liquid so much better

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seven_tech t1_j1anx4r wrote

That's kind of like asking why don't we try and get to the theoretical efficiency of an internal combustion engine - what does it solve? Liquid to gas refrigeration systems are inherently more efficient, so it doesn't serve any real purpose to try. Same as we know we can't keep using petrol, so there's no longer a point to try in ICEs.

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samskiter OP t1_j1cnzgp wrote

It's more like saying why we use 4 Vs 2 stroke.

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seven_tech t1_j1coweo wrote

Mmm, kind of? 2 stroke has its place though. It's reliable, cheap to build and easy to maintain vs 4 stroke. Which makes it perfect for simple engines like lawn mowers and yard trimmers.

Gas-gas refrigeration doesn't really have any advantages over liquid-gas. So we don't really use it since perfecting the gas-liquid version. They used to use air refrigeration cycles. But not really anymore because it's so bulky due to the sheer amount of space/gas needed.

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zebediah49 t1_j18wosy wrote

No, you can have an isentropic phase change cycle, such as the Rankine cycle.

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bl1eveucanfly t1_j19redl wrote

Your question has a fundemental misunderstanding of entropy, specifically in a thermodynamic sense.

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