Basically you put a reverse-osmosis membrane at least 231 meters deep in the ocean. If you put it deeper, you could potentially extract both freshwater and energy.
Also - 231 meters deep is where, in theory, you'll just start getting fresh water coming through your membrane. Which is 231m deep. Fresh water that you'll spend plenty of $energy to pump up to the ocean surface.
(For reference - Hoover Dam, when brim-full, has a slightly lesser drop from the water surface in the reservoir to the output of the hydroelectric turbines. You'll be pumping $energy in, to lift each ounce of your "free" fresh water up to where it is useful.)
[Edit - from some quick calculations, and the density of sea water relative to fresh water...it looks like the "don't need to pay to pump the fresh water back up" version of this scheme will need to run its osmotic membranes at the bottom of the Challenger Deep (~11,000m), or something pretty close to that.]
Sounds like a cool sci-fi premise.
Best case scenario, pumping 1 kg of water 230 meters costs 2.3kJ.
Elsewhere in this thread it was mentioned that typical desalination approaches need 10-15kJ/kg.
All of these sorts of repairs are also required for land-based water treatment facilities, of course, but now you have to perform them all at 230m underwater. That’s gotta be mega-expensive. Furthermore, all of your equipment needs to be able to survive at those depths, with saltwater attacking exposed surfaces and seals, requiring much more expensive materials and tighter tolerances.
It really doesn’t seem very economical to me!
I think a good analogy for this is oil extraction. These days we extract oil from places where it's more expensive and difficult to do so, because we can no longer meet demand with the cheaper options (many of which have simply dried up over time). But we'd never go after the expensive-to-extract oil 40 years ago (or whatever) when cheaper options were available that could meet demand.
You add your osmosis filter at the end of a long flexible hose. For maintenance, you reel it back in.
From what I can gather, oil rigs have profit margins below 20% (at best) and margins decline as the pressure in the well head drops, until they become totally unprofitable and shut down to wait for oil prices to go up. Water prices would have to increase by more than 2 orders of magnitude to make this strategy begin to approach profitability!
A cubic meter is ~264 gallons.
Here in my area of California (CA has lots of water agencies, so YMMV) 1000 gallons costs $38.66 (varies based on total consumption).
So that's $10.21 for a cubic meter.
...straws surrounded by air don't work beyond 10 meters, because the external pressure at the bottom of the pipe doesn't exceed 1 atmosphere.
Obviously this isn't true if the pipe does down into the ocean.
If you have a pump 230 meters below sea level trying to push water up a pipe, to get water to the surface that pump must be exerting the same amount of pressure as you'd be experiencing 230 meters below sea level (since otherwise it wouldn't be strong enough to push up the water column). So you might as well just take the pump up to sea level and have it pump water through the membrane, since it would be exerting the same amount of pressure.
Or alternatively, you could just find a hill that is 230 meters high, pump sea water to the top of it, and then have the sea water come down in a pipe and go through a membrane in the bottom. Should be totally equivalent without requiring anything underwater.
The reason you don't need to "pump" to force the water through the reverse-osmosis filter when you're down that low is because the overall pressure at that depth is sufficient to push the water through as-is. Merely raising water up 230m in the air and "dropping" it through a pipe into a filter sitting on the ground would not give you the same amount of pressure.
Put another way, the pressure of the water in a 230m-tall pipe on the filter on the ground is much lower than the pressure of all the water 230m under the surface of the ocean pushing on that submerged filter. While the pipe ensures that the water stays confined, it is not putting pressure on the water in the same way all the water in the ocean is putting pressure on the water that's being pushed into a 230m-submerged filter.
I imagine there is some way to do what you describe above-ground, my my intuition is that it would require essentially recreating a large ocean, suspended in the air (not as large as the Atlantic, say, but still fairly large). Much more efficient to just use the ocean we already have.
For the submerged filter approach, I think the biggest challenges are probably maintenance and keeping things stable and functioning at the pressures present at >230m below the surface. Those challenges might make it infeasible. I don't think the need to pump the desalinated water back to the surface is all that large a problem in comparison.
So you couldn't run a filter or anything, unless it's literally magic.
If someone put a pump at -230m and pumped out the water on one side of the membrane so that on one side the pressure was 'ocean' at -230m, and the other side was mean sea level, then yeah the membrane would work.
Notably, this requires roughly the same amount of work and energy as pumping water from the ocean at sea level into a filter so it has roughly the same pressure you would find if you did the first thing.
There is no free lunch.
The water at the ground level on the ocean will have the same pressure as the water at ground level in the pipe. The water at 230m down in the pipe will be the same as the water 230m down in the ocean.
Even if the pipe is capped on both ends.
The post I replied to said the opposite - that the pressure at the surface of the ocean would be the same as the pressure at the end of a pipe going under water, if there was a pipe.
Which clearly isn’t true or we’d have a trivial perpetual motion machine.
Edit: it looks like they changed their post?
which really puts into perspective how insane desalination, with it's order of magnitude higher energy costs is. why spend all that energy when we could simply build more infrastructure to capture more peak runoff from the sierras
If we use the water at depth. I think the way to go is underwater living pods.
If we go this route, I propose we also kickoff a solid Merpeople program (Mermaids, Merman, etc). The goal woukd be to gradually move the human race to be Merpeople. One day we will be viewed as the Neanderthals and Merpeople the people. As everyone knows, the hallmark of any reputable Merpeople program I'd breathing underwater, indefinitely, in Salt water. If we've been able to achieve that it's reasonable to assume we'd also be able to engineer the ability to "drink" seawater directly.
All this to say, I guess there isn't really an application for this.
We need way more daylight than most people realise, look at what goes into a submarine who are already living in a what is effectively a cramped mobile blacked out cave.
Living under water in pods has been done before but damp was the biggest issue from what I remember.
I'd imagine there are stepping stones along the way. We won't make it all the way to fish people right away, we'll likely have amphibian people first. Those people, amphibian people, should be pretty good in living in damp environments.
All that to say, I think we're good.
I like the idea of wave energy but the economics are really difficult in large part because the ocean is a difficult environment.
If the membrane requires 231 meters of pressure differential, it would stop moving water when the water was still about 230 meters from the surface.
I didn’t get to the whole paper, but I imagine the challenges of keeping that membrane clean are remarkable.
Ok, I’m being brief but you seem genuinely curious, so, lemme explain what I mean:
In thermo, you would draw a line around a closed system, and say basically conservation of energy applies in this system. Energy in = work out + losses.
In a system where one input is the ocean, you basically just consider that infinite because of the size differences. One dinky little pipe compared to the mass flow into the system from the ocean, that math would be silly to bother with. That said, to swat down the stupid “this is perpetual motion” comments from people with Reddit engineering degrees, you also have to add in the energy input into the system of the sun, which is keeping the water from freezing over the course of 1 million years as it runs someone’s sarcastic water wheel.
Adding salt is endothermic, it absorbs energy.
Most people have this mentally backwards since it's currently so hard to separate salt and water and they don't really notice the water getting cold when they stir in salt. But... there's no reason a process couldn't separate them and give off energy from a thermodynamics point of view.
The separated state has an easy way to receive energy to overcome that hump. It just needs agitation. There should be a way to reverse it and gain energy but we haven't found a good way yet.
That is separating salt from water releases energy. There's nothing perpetual motion about a process that separates fresh water and salt and also gives energy. I'm not sure if the above works or not but physically there's no issue with this.
Think about it - if it really worked you have invented a perpetual motion machine, since you could just feed the fresh water through a little water wheel back into the ocean.
You don't need to do any maths at all to know that his maths has gone drastically wrong somewhere.
Saying “maths” instead of just math sounds quite odd, why are you making it plural? Is solving one equation “a math” and solving multiple equations “maths”?
> So, get off your ath, let's do some math.
Still a minority in people right?
The sun isn't what causes pressure in the ocean...
The Anglo-American linguistic divide is always fun to examine. We don't always come out ahead, but now and then we get winner like "acclimate."
If you don't like something coming between "mathematics" and "are", how about "I can prove Fermat's Last Theorem, but the mathematics are pretty high level." One could argue that many people would say "the math is pretty high level", but I think I've heard each before. It doesn't feel odd to write at all.
Maybe a single mathematic would be a single concept/subject, like arithmetic or algebra?
The four mathematics are arithmetic, geometry, music, and astronomy.
> maths
That's what it's called in the UK (and maybe Australia & NZ). Shortening of mathematics. I could say the same about "legos". :)
It may be wrong but it really isn't "obviously delusional."