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One thing I'd be curious to know about is the efficiency of pumping from 500 meters deep. That's a real issue. Pumping up becomes really inefficient. Large bui

by jmyeet 1y ago

One thing I'd be curious to know about is the efficiency of pumping from 500 meters deep. That's a real issue. Pumping up becomes really inefficient. Large buildings, for example, get around this by pumping to intermediate tanks [1]. This isn't really an option underwater so I'm curious how they'd handle it. Depending on how much more expensive that is to build and how much energy it consumes, this may just not be economical. [1]: https://www.sloan.com/sites/default/files/2016-06/burj-khalifa-water-system%206-22_0.pdf https://www.sloan.com/sites/default/files/2016-06/burj-khali...

XorNot 1y ago
You're not really displacing all that much mass though surely? The column is surrounded by water on all sides, i.e. you're removing relatively little mass from the top of the tube, and the entire ocean is pressing in on the rest of it.
- ordu 1y ago
  Not quite. You need a pressure difference at the bottom. The fresh water will come from reverse osmosis at lower pressure than the ocean at that depth. And it will be not just little lower pressure, but pressure created by ~500km of water, because if not this then why to go to so deep?
  - XorNot 1y ago
    Right, which would be provided by the pipe walls / container membrane resisting ocean pressure, and then reducing the weight of fresh water pressing down on the top (by removing it via pumping off the top).
sikonomial 1y ago
You don't need to pump up the water. Fresh water is less dense than salt water so it will float up to the surface on its own.
- adrian_b 1y ago
  That would work only if the fresh water is enclosed in some kind of container, like a plastic bag, otherwise it will mix with the salt water before reaching the surface. Perhaps the difference in weight between 2 columns of water of equal height, but where one of the columns is of fresh water and the other of salt water, which causes a difference in pressure at their bases, can be exploited somehow for pumping the fresh water, i.e. for pushing it inside a pipe towards the surface, but with some kind of piston that separates it from the salt water.
  - sikonomial 1y ago
    It shouldn't be too difficult to fill up a balloon like container at the bottom of the sea with fresh water. Once the container is filled it will float up to the surface. The container doesn't need to be super engineered, since it is filled with water so there is no pressure difference between the inside and outside.
    - solatic 1y ago
      If you have balloon-like containers instead of a pipe, then you expend energy to submerge replacement containers down to the depth at which desalination happens.
      - sikonomial 1y ago
        You can just have the container be slightly more dense than water and it will just sink. I guess many people have not been scuba diving since these concepts seem so foreign to them.
        solatic 1y ago
        So then you need to fill the containers with something more dense, in which case you need to expend energy bringing that to the middle of the ocean where this whole contraption is going to be. You also need a way of expelling it from the container so that it can be replaced by freshwater, and then you have pollution concerns since it will sink to the bottom of the ocean, where it will pile up over time.
deleted 1y ago
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by jmyeet 1y ago

XorNot 1y ago

You're not really displacing all that much mass though surely? The column is surrounded by water on all sides, i.e. you're removing relatively little mass from the top of the tube, and the entire ocean is pressing in on the rest of it.

ordu 1y ago
Not quite. You need a pressure difference at the bottom. The fresh water will come from reverse osmosis at lower pressure than the ocean at that depth. And it will be not just little lower pressure, but pressure created by ~500km of water, because if not this then why to go to so deep?
- XorNot 1y ago
  Right, which would be provided by the pipe walls / container membrane resisting ocean pressure, and then reducing the weight of fresh water pressing down on the top (by removing it via pumping off the top).

sikonomial 1y ago

You don't need to pump up the water. Fresh water is less dense than salt water so it will float up to the surface on its own.

adrian_b 1y ago
That would work only if the fresh water is enclosed in some kind of container, like a plastic bag, otherwise it will mix with the salt water before reaching the surface. Perhaps the difference in weight between 2 columns of water of equal height, but where one of the columns is of fresh water and the other of salt water, which causes a difference in pressure at their bases, can be exploited somehow for pumping the fresh water, i.e. for pushing it inside a pipe towards the surface, but with some kind of piston that separates it from the salt water.
- sikonomial 1y ago
  It shouldn't be too difficult to fill up a balloon like container at the bottom of the sea with fresh water. Once the container is filled it will float up to the surface. The container doesn't need to be super engineered, since it is filled with water so there is no pressure difference between the inside and outside.
  - solatic 1y ago
    If you have balloon-like containers instead of a pipe, then you expend energy to submerge replacement containers down to the depth at which desalination happens.
    - sikonomial 1y ago
      You can just have the container be slightly more dense than water and it will just sink. I guess many people have not been scuba diving since these concepts seem so foreign to them.
      - solatic 1y ago
        So then you need to fill the containers with something more dense, in which case you need to expend energy bringing that to the middle of the ocean where this whole contraption is going to be. You also need a way of expelling it from the container so that it can be replaced by freshwater, and then you have pollution concerns since it will sink to the bottom of the ocean, where it will pile up over time.

deleted 1y ago

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