Comment by amluto | Hacker News Reader

Comment by amluto | Hacker News Reader

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I expect that the bulk of the improvement is in avoiding a bunch of the complexity that’s needed to do effectively the same thing at sea level.

Normally, in a desalination plant, you have feed water entering a membrane at a pressure P_feed. Brine comes out at P_brine and permeate (the desalinated output) at P_permeate. P_feed is very high, P_brine is nearly as high as P_feed, and P_permeate is much lower. The flow rate of the feed and brine is considerably higher than the permeate, and one tries to adjust the parameters to get the permeate flow rate as high as possible for a given feed rate because obtaining feed water is expensive and disposing of brine is expensive.

One can engage in trickery. There’s a very clever device called a pressure exchanger that uses the large P_brine to help pressurize some of the incoming feed water. One might imagine a simpler hack of making P_permeate negative so that the feed and brine could be at low pressure, but that’s not going to work (water will not remain liquid at excessively low pressure, and negative pressure has all kinds of problems).

Now move the whole device deep under water. The feed water is (a) all around you and (b) already at plenty of pressure to let P_feed be the ambient pressure. You need to pump feed water in or brine water out to get P_feed - P_brine to be correct, but no pesky pressure exchanger is needed. You need to pump the permeate out — one might think of pumping it “up”, but really the only hard work is producing the pressure difference P_feed - P_permeate or so — water is buoyant to an extent that almost exactly negates its weight. (You’re moving the permeate up, but the pressure difference between the plant and the air helps you out. This is just like how swimming from the bottom of a pool to the surface while carrying your entire body weight is easy, while you almost certainly could not swim well enough to lift your body weight entirely above the water.)

For bonus points, it seems likely that one could dispose of the brine water immediately outside the plant on whichever side is downstream relative to the ocean currents.

> one might think of pumping it “up”, but really the only hard work is producing the pressure difference P_feed - P_permeate or so — water is buoyant to an extent that almost exactly negates its weight

If you package the permeate in a balloon (hopefully a very strong one!) and let the balloon rise to the surface, buoyancy is very relevant.

If you instead pipe it -- to simplify the analysis, let's say it's going straight up a vertical chimney to the surface -- it doesn't seem like buoyancy is relevant.

Take a vertical water-filled pipe sealed at the bottom and open to the air on top. The water N meters from the top of the pipe will be the same pressure as N meters inside the ocean -- even if the pipe's nowhere near an external body of water! A water column self-pressurizes due to the potential gradient of Earth's gravity.

Now put the bottom of the pipe at the bottom of the ocean, you can unseal it and stick a pump on it.

You put 1 kg of water into the pipe N meters below the surface. You take 1 kg of water out at the surface. And repeat in a cycle. Some part of the system has to be doing enough work to lift 1 kg of water N meters per cycle. That work has to come from the pump -- where else could it come from?

I'm skeptical of any notion that water "floats" to the surface of the pipe "for free"!

If you have two pipes of the same height, one filled with fresh water and one with salt water, the pressure will be greater at the bottom of the salt-water pipe because salt-water is denser. Connect them at the bottom with a pipe and water will flow from salt to fresh until the pressures equalize. But connect them with a membrane, and this is countered by the osmotic pressure of fresh water trying to get to salt water, so you don't get any magic flow for free. You have however got a pressure gradient for free - just not enough to desalinate.

If you put this in the ocean, you can remove the salt pipe and get the same effect. But if you want continuous fresh water, you need to further increase the pressure difference across the membrane by continuously lowering the height of the fresh-water column by pumping water up and out of the top. That takes energy, but not as much as it would take if we had to raise the pressure on the salt-water side.

Other things equal, a low P_permeate would be best - because raising it means you're operating at greater depths, which generally costs more.

The brine water is denser than the surrounding feed water - with a bit of clever design, gravity-driven circulation could remove the need for pumps (aka expensive points of failure) on that side of things.

Pumping it out means you need head at the pressure p_feed and leaving it on the fresh side of the membrane lowers the pressure gradient. You're not saving energy, energy must be put into the system to be consumed by the work done in the reverse osmosis process.

At-least in small-scale boat water making setups, the limitation is energy use of the pump. If you put it deep underwater, do you not remove the need to even have a pump, thus it will require almost zero energy to operate?

it seems that you would remove the need to have a pump to build pressure, but you would still need a pump to move the product water ( perhaps creating the negative pressure on the outflow side of the membrane in the same step )

Way less energy, apart from the challenges of operating at depth. There would be power delivery constraints, and the basics of plumbing.

It would have to be shown with a large engineering margin that the exhaust salt would have known quantifiable effects.

It too might need to be physically subsea buried.

I dug around and found: https://www.flocean.green/subsea-desalination (Scroll down to 'Subsea SWRO') for their explanation which is interesting; so yes they take advantage of the pressure - but the other thing they say is because they're taking it from the deep low oxygen area they don't have to fight with sealife etc. (It seemed easier to look for that rather than fight the paywalling)

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