Some of the evaporated vapor will recondense in the vat. It's not a one-way ratchet.
this post was submitted on 25 Jul 2025
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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.
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)
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.
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.
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.
Hmm seems like it's similar to Maxwell's Demon / Szilard's Engine.
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.