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A Cold Day for a Kiddie Pool
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Picture a largeish, but very shallow body of water in a climate subject to winter freezing and summer thawing. The body is in fact shallow enough that the entire body freezes. Perhaps a very large kiddie pool. Let's make it cylindrical-shaped for simplicity.

Floating on the surface of this large body of water is Maxwell's demon, resting comfortably on a beach chair on top of a floating 'lid,' which covers the entire surface of the kiddie pool and basically seals it. Connected to the lid is a piston arm which apply the upward motion induced by the freezing of the pool water into some potential energy which is then converted in your method of choice. Perhaps hydraulic displacement or whatever. Effectively we have a large, squat cylinder which expands and contracts with the weather and supplies some power in doing so.

Despite resting easy on his beach chair, the demon is perplexed as to how to balance his energy books and explain matters to his head gargoyle who doesn't like to be trifled with when joules come up missing.

—RayfordSteele, Jan 10 2018

Liquid density&pressure https://www.enginee...pressure-d_309.html
[bs0u0155, Jan 10 2018]

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       hmmm, water gets about an 0.5% volume change between 4-15C which is better than the ice-water system because the phase change is a real energy suck. Also, 4-15 is totally feasible on a day/night cycle in many parts of the world.   

       So say you had some 10m tall tubes with pistons in, between min/max temp you would get a 5cm throw, which isn't great, but workable. The energy output would be the throw * the force. How large can the force be? well, liquids are non-compressible, so effectively infinite??? It's here that I faced a fork in the road, free energy Youtube video one way, self education the other.   

       Self-education yielded another high-school science lie, liquids compress, there's curves and everything <link>. So the energy output would, rather boringly, be limited to the low grade thermal energy input extracted very slowly and probably inefficiently. Sort of a Sterling engine, without all the fuss.
—bs0u0155, Jan 10 2018
   

       Make the vessels rigid and conical, with a float on the top.   

       Ambient energy harvesting from a negative temperature differential - nice.
—8th of 7, Jan 10 2018
   

       If you extrapolate those data points, the bottom of your garden variety oceanic trench is worth a 5% density increase, which should make it a little easier to float to the surface should you find yourself there unexpectedly.
—bs0u0155, Jan 10 2018
   

       // the demon is perplexed //   

       And I'm confused. Is the demon perplexed as to where his potential energy has come from? Surely it comes from the water, as it freezes?
—MaxwellBuchanan, Jan 10 2018
   

       I suppose the water, when freezing and thus lifting the demon, must do some work under pressure, and thus freezes a tad slower than when not under any more pressure than 1 atm.   

       bs0u, I’m most interested in the potential from the phase change. Forget the min max temp differential, the 9% ice expansion here is what’s interesting, where a reduction in thermal energy results in a potential energy rise.
—RayfordSteele, Jan 10 2018
   

       //I suppose the water, when freezing and thus lifting the demon, must do some work under pressure, and thus freezes a tad slower than when not under any more pressure than 1 atm. //   

       Quite so. And, reciprocally, you can melt ice by compressing it.   

       I'm pretty sure entropy is involved somewhere, but me and entropy have never seen eye to eye.
—MaxwellBuchanan, Jan 10 2018
   

       Don't worry, it'll approach your level of disorder eventually...
—RayfordSteele, Jan 10 2018
   


 

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