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Imagine a space station orbiting a star.
The station is not orbiting a planet.
The station has cylinder shape and is orienting its side to the sunlight.
The station is rotating just like any O'Neil Cylinder
It is filled with water. Assume salt water, just like Earth seas.
Near the central rod there are radiators. Likewise in the inner part of the structure there are radiators.

The wanted result is to have:

  • general cooler temperature near the bottom (around 4°C the maximumm density of water)
  • water in proximity of the outer structure is going to be warmed by sunlight
  • outer structure is going to be cooled by the water
  • warmer water (less dense) is going to go 'up ward'
  • as the warmer water reaches the central rod area it gets in contact with radiators that absorb the heat and cool the water down
  • the cooled water sinks again, closing the cycle

See crude drawing: water space station - section

Design aim: Would convection cycles actually happen?
In other words what conditions should be adjusted to have convection?
What we want to achieve is an environment which will not develop extreme conditions (too cold / too hot).
Plus: Is the cycle in the drawing in the correct direction?
Note that my drawing is just to give a clarification of the question. To my understanding there are going to be many convection cycles of different sizes as the outer structure (and especially its radiators) is bound to have different temperatures in different places with H being the likely hottest and C the likely coolest.

Consider that:

  • The star and orbiting distance can be planned to have the ideal amount for sunlight. But the station is not meant to change orbit. Just keep it stable.
  • rotation speed should ensure some artificial gravity. But it is not necessary to reach 1g. Just in the 0.7 - 1 range (by the way fishes don't do so well in space).
    Rotation speed and section radius can be adjusted to achieve the wanted result. Assume a minimum radius of 400 meters.
  • outside structure can be engineered in order to have a variable amount of insulation / conduction. It would be possible to increase insulation on the shadow side and increase conduction on the sunlit side for instance. Still the sunlit side is going to be warmer until it rotates to shadows.
  • heat at the central rod can be used for energy production though the station has other main power generators far more powerful

Costraints:

  • the whole habitat should be able to sustain itself with minimal additional energy input (for instance to control the radiators and their flow).
  • the structure was built with very advanced technology (imagine the feat of just bringing all the necessary materials in orbit, some of it from another star system far far away...) but is running on tech not so much more advanced than our own, e.g. nuclear fusion for main power. Consider current known physics for running operations. No magic alien tricks.
  • keep the maximum size of the station within reasonable limit for narrative reasons. Let's say max length: 100km, max radius 2km. Min length: 2km, min radius 400 meters. But I don't think it matters and am not looking for the exact numbers.

Additional clarification: what you see in the drawing is just a section of the station where the water is. There are other compartments along the cylinder with the necessary equipment to run the station. For instance Fusion reactors.

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  • $\begingroup$ Why makes you suppose the skin temperature would be lower ? The heat will come in from the sunny side, on the opposite side, the water would cool down.. Rotating the cylinder, the temperature gradient is toward the sun, the rotation causes moderation of the temperature by convection.. but I doubt if a center radiator is needed. You may want controlled heating, to prevent water on the opposite side of the sun to freeze. This is not an answer, I am not an expert in energy and water. But I doubt if above construct is sensible. $\endgroup$
    – Goodies
    Nov 11 at 14:51
  • $\begingroup$ I suppose by 'skin' you mean the outer structure. My thinking is water is going to rotate inside the cylinder but as it warms it will tend to rise to the center. Without radiators the warm water is not going to be in touch with the radiators on the cool side. The heat will build up at the core. $\endgroup$ Nov 11 at 18:35

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The water will mix itself

In a rotating system of liquid where it's all the same temperature, you will eventually reach of point of equilibrium where all water will stay more or less in the layer it is in... but not in a system being heated from 1 side like this.

As water "rises" towards the middle, its angular velocity will still be that of the outer water. As you decrease radius and maintain angular velocity, your rotational period decreases. This means that this whole system will not move together like a wheel, but it will have many currents moving at different speeds at different layers. These differences will cause vortexes that will keep the warm and cool water constantly mixing; so, you don't need to over think the temperature thing. It will self regulate.

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This design is fairly bad.

Getting heat out at the central rod is a problem. You will need a set of fins at the middle with similar area to the inside of the cylinder. You will need a heat pump of some kind. You will need radiator fins somewhere outside the cylinder, to radiate the unwanted heat to space. Those will need shade from the sun, and will need an area comparable to the area of the inside of the cylinder.

And that process will use a lot of energy. Recall that, in the summer, you need to pay a lot for electricity to run your A/C. And you can't exactly generate electricity off the heat rejected by your A/C. You absolutely cannot generate power using this system. It will be a net consumer of power, by a wide margin.

Massively easier to control the desired temperature of the system by controlling how much sunlight falls on it. Say by a sunshade. And if you need to circulate the water you can just put in a fairly small pump.

If the goal is living space for aquatic life, you probably want the water to be in a layer on the inside of the cylinder. That way, the rotation can provide something useful in terms of an acceleration. You can then access the water from the inside surface, which can be uncovered if you want. The water is held to the inside surface by the rotation. You can provide heat, light, and the correct atmosphere inside the cylinder.

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  • $\begingroup$ Water temp is about 4°C min and about 20°C. "~" meant 'about'. Maybe it is difficult to see on small devices? $\endgroup$ Nov 11 at 15:12
  • $\begingroup$ Oh! Sorry, I saw the ~ as a minus sign. My fault. I will take that out. $\endgroup$
    – Boba Fit
    Nov 11 at 15:14
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Yes it will work as you describe.

Picture a cylinder as a centrifuge. If you centrifuge a tube with fluid and bubbles the bubbles will move towards the center, being less dense than the fluid. Note in background of image how the syringe with fluid and bubbles is laid in the centrifuge.

centriguge bubbles https://www.youtube.com/watch?v=P_rs_Q5Z5G4

Water warmed by the sun at the outer edge of the cylinder here plays the role of bubbles. Being less dense than cold water this warm water moves towards the center, displaced by more dense water pushed to the periphery.

At the center you remove heat. Now cold and denser the water moves back out to the periphery.

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