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.
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)
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.
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.
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?
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.
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?
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.
One 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.
You're not really displacing all that much mass though surely? The column is surrounded by water on all sides, i.e. you're removing relatively little mass from the top of the tube, and the entire ocean is pressing in on the rest of it.
Not quite. You need a pressure difference at the bottom. The fresh water will come from reverse osmosis at lower pressure than the ocean at that depth. And it will be not just little lower pressure, but pressure created by ~500km of water, because if not this then why to go to so deep?
Right, which would be provided by the pipe walls / container membrane resisting ocean pressure, and then reducing the weight of fresh water pressing down on the top (by removing it via pumping off the top).
That 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.
It 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.
If 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.
So then you need to fill the containers with something more dense, in which case you need to expend energy bringing that to the middle of the ocean where this whole contraption is going to be. You also need a way of expelling it from the container so that it can be replaced by freshwater, and then you have pollution concerns since it will sink to the bottom of the ocean, where it will pile up over time.
The container collapses onto itself. You can for example Google a hydration bladder to get the idea. At the surface you just squeeze the container flat to remove all the freshwater and then it will sink on its own.
Making 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.
Why 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.
Desalinated 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
The minimum pressure differential needed to perform reverse osmosis is bigger than the pressure differential between a collumn of fresh and a collumn of salt water of equal height.
Not 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.
A 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.
You're functionally drawing solar energy off the system very inefficiently (if you wanted kinetic motion).
A different way to look at the problem is that you can't have water spontaneously move up hill, but if you dam a river you can absolutely extract useful energy from it.
A turbine underneath the ocean could extract energy from ocean currents and this is the same problem.
So according to you you could simply lower a membrane and a pipe into a still body of salt water and it would spontaneously separate into sweet water and brine until the build up of brine prevented this from continuing?
Yeah I am still not seeing it. If that were the thermal equilibrium I don't see how it wouldn't separate spontaneously, or why you can mix salt and water with no input of energy whatsoever.
It goes against anything I know about entropy and osmotic pressure.
At this point you are simply arguing reverse osmosis is impossible. There is no functional difference between mechanically creating a pressure differential across the membrane with a pump, and lowering a membrane deep enough that the pressure differential can drive the process.
This 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.
You 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.
What 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
The 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.
I 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
Into 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.)
They 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.
Filter 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.
> 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.
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.
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)
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?
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.
One 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-khali...
You're not really displacing all that much mass though surely? The column is surrounded by water on all sides, i.e. you're removing relatively little mass from the top of the tube, and the entire ocean is pressing in on the rest of it.
Not quite. You need a pressure difference at the bottom. The fresh water will come from reverse osmosis at lower pressure than the ocean at that depth. And it will be not just little lower pressure, but pressure created by ~500km of water, because if not this then why to go to so deep?
Right, which would be provided by the pipe walls / container membrane resisting ocean pressure, and then reducing the weight of fresh water pressing down on the top (by removing it via pumping off the top).
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 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.
It 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.
If 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.
You can just have the container be slightly more dense than water and it will just sink.
I guess many people have not been scuba diving since these concepts seem so foreign to them.
So then you need to fill the containers with something more dense, in which case you need to expend energy bringing that to the middle of the ocean where this whole contraption is going to be. You also need a way of expelling it from the container so that it can be replaced by freshwater, and then you have pollution concerns since it will sink to the bottom of the ocean, where it will pile up over time.
The container collapses onto itself. You can for example Google a hydration bladder to get the idea. At the surface you just squeeze the container flat to remove all the freshwater and then it will sink on its own.
Making 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.
Why 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.
https://archive.is/20250811174135/https://www.scientificamer...
Desalinated 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
The minimum pressure differential needed to perform reverse osmosis is bigger than the pressure differential between a collumn of fresh and a collumn of salt water of equal height.
I think it would stop in an isolated setup once most of the water is desalinated.
That still makes no sense, water can't desalinate itself in the same way it cannot spontaneously cool itself.
Not 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.
A 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.
The system isn't closed.
You're functionally drawing solar energy off the system very inefficiently (if you wanted kinetic motion).
A different way to look at the problem is that you can't have water spontaneously move up hill, but if you dam a river you can absolutely extract useful energy from it.
A turbine underneath the ocean could extract energy from ocean currents and this is the same problem.
So according to you you could simply lower a membrane and a pipe into a still body of salt water and it would spontaneously separate into sweet water and brine until the build up of brine prevented this from continuing?
Yeah I am still not seeing it. If that were the thermal equilibrium I don't see how it wouldn't separate spontaneously, or why you can mix salt and water with no input of energy whatsoever.
It goes against anything I know about entropy and osmotic pressure.
At this point you are simply arguing reverse osmosis is impossible. There is no functional difference between mechanically creating a pressure differential across the membrane with a pump, and lowering a membrane deep enough that the pressure differential can drive the process.
Reverse osmosis without work is indeed impossible.
This 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.
You 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.
How 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?
I thought the “tied to a giant rock” part sufficiently explained how to get it down there.
What 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
Cause it's not an ecosystem, right? It's just a resource for us to drain.
I think there's enough water in the ocean for us to try
The 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.
I 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
Humans are part of the ecosystem. Unless you would claim we are not living things.
Dont worry. We are not going drink up the ocean. You will pee it out again.
Where do you imagine the water goes after you pee it out?
Into 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.)
Pressure differentials strike again, my physics teacher would be having a blast reading these comments. It’s really hard to wrap your head around
They 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.
Filter 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.
Aren'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
> 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 .. "Desalination system could produce freshwater that is cheaper than tap water" (2023) 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 :
OTEC: 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 .. "Ask HN: How to reuse waste heat and water from AI datacenters?" 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...
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
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I wonder if capillary effect can be used? Or some other mechanism that trees use?
You’re describing osmosis filtering, so yeah pretty solid idea
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Because people want to pay 40$ for a T-shirt, not 400$.
Edit: People are greedy