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.
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 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.
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.
Way less energy, apart from the challenges of operating at depth. There would be power delivery constraints, and the basics of plumbing.
It too might need to be physically subsea buried.
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?
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.
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.
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.
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.
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.
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.
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.
Nothing in this system is 100% efficient, so how you organize your components can make a huge difference.
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.
Then when you fill container with fresh water 1000kg per m3 it will float.
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?
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
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...
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.
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.
I guess many people have not been scuba diving since these concepts seem so foreign to them.
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.
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.
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.
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.
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
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.
(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.)
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.
"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