Theoretically, as water is pumped from the surface of the desalinated pipe, the resulting pressure imbalance drives water through the lower desalination filter at high pressure, continuously restoring the water level at the top.

It's not the pressure difference that other comments write, that does not make sense.

I would assume it's the result to waste water ratio. Afaik, reverse osmosis produces 3 to 4 litres of waste water per liter of fresh water. Since you do not have to pressure the waste water, only depressure the fresh water, you save energy.

It's that you have the pressure difference for almost free-- you get it without investing anything more than the work required to filter the water, whereas you otherwise have to invest enough to put it under pressure.

Suppose that you've got a pipe to the deep sea and a filtration system at the bottom, then a pump on the surface, so that the pipe is mostly filled with air.

Then you have a sufficient pressure difference for the membrane at the bottom and what goes through the membrane only has to go through the filter system.

Meanwhile if you want to achieve this on the surface, then it has to go through the filter, then through a high-pressure pump. The pressurized water will contain salt and some will go through the membrane, so it will be enriched in salt. So now you have a choice: keep letting it try to get through the membrane, or feed it back through the pressure recovery system and use that to repressurize new water.

Since the pressure exchanger is something like 90% efficient, you don't just feed everything back through the pressure exchanger immediately.

Meanwhile, when the membrane is at the bottom of the sea, you can feed in as much new water as you like.

I had this idea many years ago, but didn't think it was worth pursuing, so it's nice to that it's being tried.

> Suppose that you've got a pipe to the deep sea and a filtration system at the bottom, then a pump on the surface, so that the pipe is mostly filled with air.

That buys you nothing: you would expend exactly the same amount of energy to remove a given volume of permeate from the pipe this way (to keep the pipe from filling with permeate and to get the water to the surface) as you would to pump that volume of permeate through a normal water-filled pipe. In fact, it would be the same pump at the same speed. The only difference would be the pipe arrangement and the pumping system.

Where the pump is located is indeed not critical, it's where the filter is located that is critical.

The filter cannot be on the surface. If we didn't have it at the bottom we would not be able to have flow on the high-pressure side of the pipe that is not through the membrane.

This flow is why this thing has an advantage, and it's because of this flow that the saltwater on the high-pressure side is not much saltier than seawater.

I should perhaps clarify. Filling the pipe with air is unhelpful. The pump (or at least the wet part of the pump) on the surface is actively counterproductive — pumps are much, much, much better at producing high output pressure than at producing suction, and you can’t suck very hard on water anyway until it boils.

Almost all modern “deep well” pumps are at the bottom of the well, and a 50 foot well is “deep” for this purpose.

Ah, yes. I understand now.

So you propose basically pumping into the return pipe from some kind of membrane chamber and making it as on the surface-- just lift the pressure away.

Ah. Yes, then the air pipe I imagined serves no function, and presumably these real machines that are discussed in the article are of the sort you describe.

Isn't one of the issues here the pressure gradient across a very long segment of pipe? How easy would this be to build and how hard would it be to maintain?

I am not sure why getting it up is >= the energy to create the pressure force it through the membrane.

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.

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

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.

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.

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.

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.

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.

I think I see what you do not understand. Your freshwater is at surface pressure, not at depth pressure. You cannot just displace the salt water from your container, you need pressure to displace the saltwater and put the freshwater out of the filter chamber and in the container. That does not just happen because you cannot do it in the filter chamber as else, that filter chamber would lose its pressure differential and not work anymore. Sorry, but your idea is not made for reality :)

The idea is a perpetual motion machine (the water of the ocean is just part of the machine) and I’m trying to show OP that. They’re just skipping an energy intensive step in their heads with every idea.

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?

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

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.