3 ms·

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.

patall 1y ago
That would be a perpetuum mobile. You either have a pressure difference at the membrane or between outside and inside the tube.
- sikonomial 1y ago
  The process would be like this: 1. Take in salt water 2. Spend some energy to separate salt from water. 3. Put fresh water into a container. 4. The container containing fresh water will raise to the surface, since it is less dense than salt water. There is no perpetual motion.
  - patall 1y ago
    Then you could also do it at the surface. But they do it a depth because they want a pressure difference on the two sides of the osmosis membrane. You somehow need to generate that pressure difference and the energy you need for that is minimum equal to the amount you need to move the freshwater. Oh, and you will have to do it continuously, not with a 'container'. Existing desalination plants produce hundreds of thousands of cubic meters of fresh water per day.
    - XorNot 1y ago
      You pump water off the top of the pipe, reducing mass and pressure at the bottom and thus allowing for desalination. It's a classic distance x force trade off: it's easier to use a static membrane, and a low pressure pump then build a high pressure pump at the surface. Nothing in this system is 100% efficient, so how you organize your components can make a huge difference.
  - deleted 1y ago
    [deleted]
  - mattmaroon 1y ago
    You have had to spend energy to get the floating container to the bottom. If you filled it with something heavier than water, or left it open to the elements to sink, you still would have to spend a bunch of energy to pump it clean at the bottom. Probably still easier to just pump the water up.
    - sikonomial 1y ago
      The container doesn't have to float. The container could have density of 1020.00001kg per m3 and it will sink. Saltwater is 1020kg per m3 Then when you fill container with fresh water 1000kg per m3 it will float.
      - mattmaroon 1y ago
        It has that density when full of air? What about when it’s full of highly pressurized salt water? Or, if it’s open to the environment on the way down, how does it evacuate the salt water and how much energy does that take? Even if all this wasn’t a perpetual motion machine, which it is (the sea water is just part of the machine), wouldn’t it be easier to just float some solar panels to power a pump?
        sikonomial 1y ago
        The container can be similar to a hydration bladder (Google what it looks like) that is slightly more dense than salt water. 1. At bottom you fill it with fresh water 2. It floats to the surface 3. At the surface you just empty it and remove the fresh water 4. It starts sinking 5. Jump to step 1
        mattmaroon 1y ago
        Right. You’re glossing over the energy required to fill a bladder at sea depth with enormous pressure on it. That requires a pump and a lot of power, just like pumping it to the top does.
      - patall 1y ago

patall 1y ago

That would be a perpetuum mobile. You either have a pressure difference at the membrane or between outside and inside the tube.

sikonomial 1y ago
The process would be like this: 1. Take in salt water 2. Spend some energy to separate salt from water. 3. Put fresh water into a container. 4. The container containing fresh water will raise to the surface, since it is less dense than salt water. There is no perpetual motion.
- patall 1y ago
  Then you could also do it at the surface. But they do it a depth because they want a pressure difference on the two sides of the osmosis membrane. You somehow need to generate that pressure difference and the energy you need for that is minimum equal to the amount you need to move the freshwater. Oh, and you will have to do it continuously, not with a 'container'. Existing desalination plants produce hundreds of thousands of cubic meters of fresh water per day.
  - XorNot 1y ago
    You pump water off the top of the pipe, reducing mass and pressure at the bottom and thus allowing for desalination. It's a classic distance x force trade off: it's easier to use a static membrane, and a low pressure pump then build a high pressure pump at the surface. Nothing in this system is 100% efficient, so how you organize your components can make a huge difference.
- deleted 1y ago
  [deleted]
- mattmaroon 1y ago
  You have had to spend energy to get the floating container to the bottom. If you filled it with something heavier than water, or left it open to the elements to sink, you still would have to spend a bunch of energy to pump it clean at the bottom. Probably still easier to just pump the water up.
  - sikonomial 1y ago
    The container doesn't have to float. The container could have density of 1020.00001kg per m3 and it will sink. Saltwater is 1020kg per m3 Then when you fill container with fresh water 1000kg per m3 it will float.
    - mattmaroon 1y ago
      It has that density when full of air? What about when it’s full of highly pressurized salt water? Or, if it’s open to the environment on the way down, how does it evacuate the salt water and how much energy does that take? Even if all this wasn’t a perpetual motion machine, which it is (the sea water is just part of the machine), wouldn’t it be easier to just float some solar panels to power a pump?
      - sikonomial 1y ago
        The container can be similar to a hydration bladder (Google what it looks like) that is slightly more dense than salt water. 1. At bottom you fill it with fresh water 2. It floats to the surface 3. At the surface you just empty it and remove the fresh water 4. It starts sinking 5. Jump to step 1
        mattmaroon 1y ago
        Right. You’re glossing over the energy required to fill a bladder at sea depth with enormous pressure on it. That requires a pump and a lot of power, just like pumping it to the top does.
    - patall 1y ago