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Deep-Sea Desalination Pulls Fresh Water from the Depths
- pmontra 1y agohttps://archive.is/20250811174135/https://www.scientificamerican.com/article/deep-sea-desalination-pulls-drinking-water-from-the-depths/ https://archive.is/20250811174135/https://www.scientificamer...
- aaron695 1y ago[dead]
- dan353hehe 1y agoI 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?
- deleted 1y ago[deleted]
- refulgentis 1y agoI am not sure why getting it up is >= the energy to create the pressure force it through the membrane.
- snappr021 1y agoTheoretically, 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.
- sikonomial 1y agoYou 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 agoThat would be a perpetuum mobile. You either have a pressure difference at the membrane or between outside and inside the tube.
- sikonomial 1y agoThe 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 agoThen 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 agoYou 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]
- patall 1y agoIt'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.
- themafia 1y agoIsn'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?
- impossiblefork 1y agoIt'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.
- amluto 1y ago> 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.
- jmyeet 1y agoOne 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...
- sikonomial 1y agoYou 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 agoThat 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 agoIt 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 agoIf 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.
- hulitu 1y ago[flagged]
- lucketone 1y agoBecause people want to pay 40$ for a T-shirt, not 400$. Edit: People are greedy
- amluto 1y agoI 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.
- trebligdivad 1y agoI dug around and found: https://www.flocean.green/subsea-desalination 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)
- ZeroGravitas 1y agoMaking it more complicated to deploy in order to save energy costs seems like the wrong direction. They should be making it cheaper instead and slapping a bunch of cheap solar down to power it with the money saved.
- jasonkester 1y agoThis seems like it would work nicely if you removed the concept of pipes and pumps, and replaced them with containers and gravity. I imagine a barrel of air at the surface with an osmosis filter at the opening and a big ass rock tied to it. Kick it off your barge, let it drop to the bottom and fill with filtered water. Then cut the string and let it float up for collection. Seems like you could do that pretty cheaply.
- mattmaroon 1y agoHow do you get the floating barrel of air down there? If it floats when full of fresh water it definitely floats when full of air right?
- jasonkester 1y agoI thought the “tied to a giant rock” part sufficiently explained how to get it down there.
- mattmaroon 1y agoWhat is the energy expenditure of getting this rock there? The size of the rock is directly proportional to the amount of freshwater this container can hold right? How much energy does the barge, or whatever pulls it, spend getting itself and the rock and the container into place and back out? What is this container made of that it can be large enough for this to be feasible, it is full of only air, and it won’t just collapse under pressure at depth? How much does it weigh? We might be talking a much bigger rock than you are envisioning. You’re glossing over all sorts of energy input and engineering issues, at some point it’s easier to just pump the remaining stuff up
- cable5 1y agoYou can use the hydrostatic pressure (about 60bar) with a simple hydraulic intensifier to lift RO permeate from ~600m to ~100m depth, then finish the last stretch with a small, diver-serviceable booster. Basically you only need power to pump the final 100m which isn't bad.
- brilee 1y agoDesalinated water is also less dense than normal seawater, so the water column inside the output pipe would create a pressure imbalance with the water column outside the pipe, assisting in the outflow? I'm having trouble figuring out how to resolve this seeming perpetual motion machine
- josefx 1y agoI think it would stop in an isolated setup once most of the water is desalinated.
- shiandow 1y agoThat still makes no sense, water can't desalinate itself in the same way it cannot spontaneously cool itself.
- XorNot 1y agoNot in a static system, but the ocean isn't static - there are currents. Until the membrane fouled, if you sank a system like this to the bottom, fresh water would naturally spill out at the surface while brine built up around it. If the brine doesn't flow away (brine is weird like this) then eventually the system hits equilibrium and stops. But if ocean currents (powered by the sun, tectonics etc.) keep removing brine at the bottom...then it can in fact run indefinitely because there is an energy input.
- shiandow 1y agoA steady supply of salty water doesn't help enough in the same way a steady supply of warm air cannot cool a house. The problem with your system is that it you can power an engine with the flow of salt ions and that really isn't the kind of thing you are supposed to be able to do to something that happens spontaneously. And really water spontaneously desalinating is about as clear a violation of the second law of thermodynamics as you can get. With the scale of latent energies involved it would be like water flowing up a 70 meter wall. Look maybe I am missing something somewhere that secretly compensates for the apparent decrease in entropy but I am not seeing it. Brine will flow away e entually, the water returns to the ocean and in the meantime you can power your power plant by salinating the water, indefinitely.
- wewewedxfgdf 1y agoCause it's not an ecosystem, right? It's just a resource for us to drain.
- semitones 1y agoI think there's enough water in the ocean for us to try
- catlikesshrimp 1y agoThe problem is not killing what lives in said water. And many times we the public become aware something damages the ecosystem when the damage is extensive, and some people never acept the fact, much less any responsibility. Edit: In this specific case, the best case scenario is saving half the energy expenditure. Larger and more intrusive infrastructure with unknown effects for saving less than half the energy. Let's hope they always use renewables, btw.
- khalic 1y agoI totally agree about the consequences needing to be understood, but in the end, if it comes to this, it’s water. We can’t survive more than 3 days without water. We will boil the oceans if we need to
- throwawayffffas 1y agoWhere do you imagine the water goes after you pee it out?
- phyzome 1y agoInto the sewage treatment plant, which produces almost-drinkable water. That's what we should be reclaiming with reverse-osmosis, but instead we dump it into the ocean. (The main problem with desalination is not so much that you're taking the water as that you're then dumping concentrated brine into the ecosystem.)
- victorbjorklund 1y ago
- cable5 1y agoWhy not use the deep ocean hydrostatic pressure with a simple hydraulic intensifier to lift RO permeate from ~600m to ~100m depth, then finish the last stretch with a small, diver-serviceable booster? This removes deep rotating equipment, cuts energy dramatically versus full-depth pumping, and keeps maintenance simple and close to the surface.
- sitkack 1y agoThey don't talk about pollution, some pollution will drop off while coagulating microplastics can be much higher. The whole ocean is basically a fractionating column. Of course they are going to want to dump the salt in the bottom to complete the mass transfer loop of the upwelling water. This is going to mess up the whole thing. Humans should be operating in closed water systems. We would have to do that anywhere else we go, we should be turning Earth into well run spaceship.
- hagbard_c 1y agoFilter it out already, problem solved. Look for solutions, not for problems. If microplastics do indeed concentrate in the depths this would offer a chance to take them out of the environment, the same goes for other pollutants.
- westurner 1y agoAren't there radioisotopes in sea water? If you're filtering microplastics out of deep sea water you might as well collect those too? "Fungus breaks down ocean plastic" (2024) https://news.ycombinator.com/item?id=40676239 https://news.ycombinator.com/item?id=40676239 > Of course they are going to want to dump the salt in the bottom to complete the mass transfer loop of the upwelling water. This method of desalination is designed to limit hyperaccumulation of salt in the ocean and the apparatus: "Extreme salt-resisting multistage solar distillation with thermohaline convection" (2023) https://www.cell.com/joule/abstract/S2542-4351(23)00360-4 https://www.cell.com/joule/abstract/S2542-4351(23)00360-4 .. "Desalination system could produce freshwater that is cheaper than tap water" (2023) https://news.ycombinator.com/item?id=39507702 https://news.ycombinator.com/item?id=39507702 : > Here, inspired by a natural phenomenon, thermohaline convection, we demonstrate a solar-powered multistage membrane distillation with extreme salt-resisting performance. Using a confined saline layer as an evaporator, we initiate strong thermohaline convection to mitigate salt accumulation and enhance heat transfer. The thermal difference between the deep sea water and surface water (or waste heat heated water, or solar heated water) can be used to generate electricity. "140-year-old ocean heat tech could supply islands with limitless energy" https://news.ycombinator.com/item?id=38222695 https://news.ycombinator.com/item?id=38222695 : OTEC: Ocean thermal energy conversion: https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversio... "Ask HN: Does OTEC work with datacenter heat, or thermoelectrics?" https://news.ycombinator.com/item?id=40821522 https://news.ycombinator.com/item?id=40821522 .. "Ask HN: How to reuse waste heat and water from AI datacenters?" https://news.ycombinator.com/item?id=40820952 https://news.ycombinator.com/item?id=40820952 At 40-44% efficient given at least 1,435°C, Solid state thermoelectrics are more efficient than steam turbines at converting a thermal gradient to electricity. "Renewables Game-Changer? 44% Efficient TPV Cell" (2024) https://eepower.com/tech-insights/renewables-game-changer-44-efficient-tpv-cell/ https://eepower.com/tech-insights/renewables-game-changer-44... Thermophotovoltaic energy conversion: https://en.wikipedia.org/wiki/Thermophotovoltaic_energy_conversion https://en.wikipedia.org/wiki/Thermophotovoltaic_energy_conv... "Using solar energy to generate heat at 1050°C high temperatures" (2024) https://news.ycombinator.com/item?id=40419617 https://news.ycombinator.com/item?id=40419617
- portly 1y agoI wonder if capillary effect can be used? Or some other mechanism that trees use?
- khalic 1y agoYou’re describing osmosis filtering, so yeah pretty solid idea
- khalic 1y agoPressure differentials strike again, my physics teacher would be having a blast reading these comments. It’s really hard to wrap your head around