[dan353hehe]

I think the claim about higher efficiency is due to the fact that the sea temp is stable and they don’t have to deal with algae blooms at the bottom of the ocean.

I don’t see how taking advantage of the pressure at lower depths makes much sense. The water would still need to be pumped to the surface, which I think would take as much energy as just pressurizing it.

Did I miss something?

[snappr021]

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.

[patall]

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.

[impossiblefork]

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.

[themafia]

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?

[refulgentis]

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

[sikonomial]

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]

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