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Temperature based Reverse Entropy Machine? (lemmy.world)
submitted 3 days ago by DoctorFoxy@lemmy.world to c/askscience@lemmy.world
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So, obviously there's a mistake in my reasoning somewhere. Maybe I've pointed it out already I'm not sure.

You have a vat with some liquid. The ambient temperature X (which is low and not useful) keeps it at X temperature, a certain base level of energy. Through random collisions as temperature works some will get more energy than the average and get enough energy to evaporate. You separate those passively since it's literally a phase change it could be done passively? (Leaving you with slightly less energy in the environment but which is in our case infinite since eventually you'd give the energy back before changing it a lot when doing useful things) with the now higher energy particles you have in a separate place?

It automatically turns high entropy useless environment thermal energy into higher more useful energy? (Cascade the same system many times for really high useful energy?)

This only works if the separation step can be done passively (or uses less energy than you gain from it) I guess but that seems maybe plausible considering the phase change?

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[–] TauZero@mander.xyz 6 points 2 days ago (1 children)

The people saying Maxwell's demon/"cannot separate" are mistaken. They've pattern-matched to the wrong concept. This is possible and is the way swamp coolers operate. They exploit the difference between ambient temperature and dew temperature of the liquid, say water. As long as relative humidity is below 100%, some water will evaporate, leaving your vat colder than environment. The separation is simple - the wind carries away the 100% humid water vapors, replacing it with fresh low-humidity air. You can then use the temperature difference to drive a stirling engine or something.

The point where your free energy ends is when you run out of water. You need to take your 100% humid air and cool it down somewhere else to get the water back by condensing it. In nature this happens automatically at nighttime when the heat radiates into the cold of space and air temperatures drop. If you are running a closed-loop cycle though, like on a spaceship, you need to provide your own source of environmental heat (the Sun) and your own heat sink (the cold of space), and at that point you should just be using a regular high-efficiency heat engine instead of this swamp cooler.

[–] MysteriousSophon21@lemmy.world 1 points 21 hours ago (1 children)

Evaporative cooling doesn't actually create free energy though. The cooling effect comes at the cost of the latent heat of vaporization - you're just moving heat from one place to another, not creating usable energy. A Stirling engine would need both a hot and cold resevoir, and the cooling effect alone dosn't create a temperature gradient that could produce net work.

[–] TauZero@mander.xyz 1 points 11 hours ago

Yep, that right here is the kind of pattern-matching you have to be careful about! Read what you wrote carefully:

cooling effect ... temperature gradient

The vat is literally cooler. You put a thermometer in it, will show a lower temperature than thermometer in air. This is not a fake effect "only shows lower because it's wet", it's a real temperature. You put your stirling engine cool coil in the vat and hot coil in the air, you got yourself a temperature gradient. A small one, maybe 10 degrees C, but more than zero.

[–] CanadaPlus@lemmy.sdf.org 7 points 2 days ago* (last edited 2 days ago) (1 children)

Yeah, it's a Maxwell’s demon with an extra phase change involved. Nobody's figured out that "passive separation" step in detail, and the second law basically states it's not just our lack of imagination. You seem to know that, though.

Think about it, you have your vapour in the top of the vessel at equilibrium with the liquid, and you close a valve to cordon it off. Then what? You pull it out and leave a vacuum in it's place? That change in pressure is going to involve quite a bit of work. There's also conduction that's working against you, so I'm not even certain the vapour is hotter to start with.

It's worth mentioning that unlike the other laws of physics, the laws of thermodynamics are emergent, and nobody's rigorously proven they actually work as described without some kind of handwave. The thing is, nobody doubts thermodynamics - the empirical evidence is mountainous and the almost-proofs are convincing. The missing part probably has to do with P and NP and pseudorandomness, which is one of the millennium problems.

[–] Successful_Try543@feddit.org 2 points 2 days ago

The thing is, nobody doubts thermodynamics

The law that entropy always increases, holds, I think, the supreme position among the laws of Nature.
If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations.
If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes.
But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.

Sir Arthur Stanley Eddington, The Nature of the Physical World (1927)

[–] tal@lemmy.today 8 points 3 days ago

Some of the evaporated vapor will recondense in the vat. It's not a one-way ratchet.

[–] threelonmusketeers@sh.itjust.works 3 points 2 days ago* (last edited 2 days ago)

You separate those passively since it's literally a phase change it could be done passively?

The phase change happens passively, the separation step, not so much.

[–] DoctorFoxy@lemmy.world 4 points 3 days ago

Hmm seems like it's similar to Maxwell's Demon / Szilard's Engine.