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For some reason I thought the below described method was common knowledge. But it is not easy to google, so maybe some good discussion will come of a posting.
Stretch a semipermeable membrane across the end of a pipe. This membrane would be the type used for reverse osmosis, allowing noncharged
water thru but not ions. Reverse osmosis is what happens next. Walk with your pipe to the end of an ocean pier and lower the membrane end into the water. At a certain depth, the pressure difference between the air filled pipe and the outside water will be enough to force fresh water thru the membrane, where it will accumulate in the bottom of the pipe. The membrane might need to be buttressed against the pressure with an underlying mesh or cap.
Attach pipe to pier. You now have a well. Pump out the water. If you need more water, put another pipe down. There is no accumulation of brine, although a large enough operation of this sort could increase the salinity of an area without much water movement. In which case you would need to lower the pipe further down.
Such an operation could be set up on an offshore oil drilling platform. The amount of water provided is limited only by the amount of pipe and membrane available.
addendum 8/11/06: [Freefall] posted an anno to this idea positing that given a deep enough pipe, a reverse osmosis desalinator could be built such that the freshwater rises to the top of the pipe and fountains up from the top. This is even slicker than my prosaic desalinator as above. Much of the discussion following that anno concerns [Freefall]'s freshwater fountain, not my idea as posted above.
Geothermal distillation
http://www.halfbake...ater_20for_20Africa
Inspired by interesting discussion here. [bungston, Oct 04 2004, last modified Oct 05 2004]
Howard's link
http://www.lhup.edu.../museum/osmosis.htm
link service provided free of charge [Freefall, Oct 04 2004, last modified Oct 06 2004]
Desalination company
http://www.lenntech...tion-RO-modules.htm
They assert 50-60 bar pressure needed for reverse osmosis pressure of seawater. [bungston, Oct 04 2004, last modified Oct 06 2004]
Pressure conversions
http://xtronics.com...ference/convert.htm
To convert bar into inches of water. [bungston, Oct 04 2004, last modified Oct 06 2004]
Ocean data
http://mbgnet.mobot...alt/oceans/data.htm
How deep is the marianas trench? [Freefall, Oct 04 2004, last modified Oct 06 2004]
Degassing Lake Nyos
http://www.pbs.org/...no/01/indexmid.html
Seltzer from a lake [Freefall, Oct 04 2004, last modified Oct 06 2004]
The Lake Nyos Fountain
http://perso.wanado...alb/nyos/webcam.htm
Webcam view of the Nyos fountain so the scientists can monitor it. [Freefall, Oct 04 2004, last modified Oct 06 2004]
Osmotic Energy
http://exergy.se/go...j/98/osmotic/#intro
A related subject [ldischler, Oct 04 2004, last modified Oct 06 2004]
Ocean Salinity Profile
http://www.windows....salinity_depth.html
Not constant, but close enough... [ldischler, Oct 04 2004, last modified Oct 06 2004]
Tremendous variability in salinity in Gotland Deep
http://www.smhi.se/...tkonferens/Ralf.pdf
See pages 12-14 (PDF file) [ldischler, Oct 04 2004, last modified Oct 06 2004]
Offshore Drilling
http://www.gsfdrill.com
Deep water drilling... alive and "well" (sorry about that) [zigness, Oct 04 2004, last modified Oct 05 2004]
APEC domestic water filters
http://www.freedrinkingwater.com/
77 cents per gal. Why are we sinking an 8km pipe? [FloridaManatee, Oct 04 2004, last modified Oct 06 2004]
A primer on Diffusion.
http://en.wikipedia.org/wiki/Diffusion
A law of Physics [jhomrighaus, Aug 09 2006]
Cesium Chloride Seperation
http://www.bio.com/...tocol.jhtml?id=p571
See Part C. [jhomrighaus, Aug 12 2006]
Solvation
http://en.wikipedia.org/wiki/Solvation
Where does the energy come from? [ldischler, Aug 14 2006]
Short name, e.g., Bob's Coffee
Destination URL.
E.g., http://www.coffee.com/
Description (displayed with the short name and URL.)
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Clever. You'd need a powerful downhole pump, though, to pump the water up that huge head. And if you had the pump, you could do it directly, and dispense with the pipe. (And this will be a very long pipe, since you have to go down a couple thousand feet. So, if you're not near a trench, there will be a long horizontal run.) |
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The pump would not need to be any more powerful than those used on dryland wells. I expect this device will actually be considerably shallower than dryland wells - you don't have to be that deep to have a really big pressurehead. |
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Eh? It's about 1/2 psi per foot, wherever you are. And don't forget—the pressure needed is much greater for seawater, as compared to purifying well water. Ironically, the pressure you need to pump it from the bottom of the pipe to the top is exactly the pressure you’d need to run the same set-up at sea level, so you gain nothing using the pipe. |
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Why would you need to pump from the bottom of the pipe? Am I missing something, or would the pipe not fill to sea level? |
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So the idea is to take advantage of the difference in density between sea water and plain water over a great depth to drive the flow? Could work, but as howard said, you'd need a L-O-N-G pipe. Plus, you occasionally need to replace the filters. |
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Edit: checked howard's calculation, I also get approx. 8000 meters (8268 meters, to be exact.) That's a lot of pipe, but it could work. |
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Assuming that we use a large desalinization plant at the bottom of the pipe, buoyancy of the pipe itself shouldn't be a concern. Assuming a non-reusable filter, one filter assembly could simply be attached to the outside of the pipe and allowed to sink down the line and hook up, allowing the old assembly to drop off the end of the line. If recyclability or disposal issues are a concern, the old filter assembly could simply inflate a buoyancy bag to return to the surface. |
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What you're missing is that the pressure is not hydrostatic pressure--it's a pressure differential. If you have an equal head on both sides of the membrane, nothing happens (except regular osmosis). You have to exceed the osmotic head to get reverse osmosis, and for seawater that's a lot of pressure. |
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ldischler, there is a pressure difference across the membrane due to the difference in density of seawater and fresh water. I'll show my math. |
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Assuming it takes 20 bar to begin reverse osmosis: |
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d=depth of pipe
P1=pressure at the inside of the pipe at the bottom
P2=pressure at th outside of the pipe at the bottom
p1=density of fresh water = 62.428 lb/ft^3
p2=density of sea water = 1.025 * p1
1 atm = 14.7 psi = 2116.8 lb/ft^2 |
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P1 = p1 * d
P2 = p2 * d = P1 + 20 atm
p1 * d + 20 atm = p2 * d
20 atm = p2 * d - p1 * d = d * (p2 - p1)
d = 20 atm / (p2 - p1) = 20 atm / (0.025 * p1) |
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d = 20 (2116.8 lb/ft^2) / .025 * 62.428 lb/ft^3 |
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d = 27126.3 ft
d = 8268.1 m |
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Sehr interresant... So what do you make of the 8000m number? |
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Yah, no fountains are going to shoot out of the ocean. I did some math. These folks (linked) assert 50-60 bar of pressure is needed for reverse osmosis of seawater. That is, to squeeze the freshwater out of the salty stuff. Converted, 60 bar is 24060 inches of water, or 2005 feet. So that is a pretty long pipe - I guess I am not going to be carrying it down to the pier. But 2000 feet should be achievable within a few miles of the california coast. |
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Assume a pipe 2050 feet long. When the freshwater on the inside rises up to a depth of 50 feet, the pressure difference between inside and outside will no longer be adequate to drive reverse osmosis. You will need to pump that water up from a depth of 2000 feet, which is exactly as [ldischler] said: damn deep. |
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The plus: no brine accumulation! Which I understand is a big problem on land. |
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So bungston, are you refuting your own idea? 2000 feet isn't going to get you much, but if you build your plant in the mariana trench (36,200 ft) or puerto rico trench (28,374 ft), you should be able to get an unlimited supply of free-flowing fresh water. |
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Sadly, if it does take much more than 20 bar to start the process, there's nowhere on the planet that this would work. |
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Freefall, it looks like you have the pipe full to the top with freshwater, and you are running this off the density differences between fresh and salt. Which is very cool, because you would not need a pump and it might overflow as a minifountain. It makes me glad I posted this. |
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But that is not the idea as posted. This is brute force reverse osmosis, run off the pressure difference between a column of seawater and a column (pipe) of air. You need less pipe to do it this way, making it more widely applicable. |
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Yes, the pressure difference between a column of air and water would let you do it, but then you run into the same problems already stated: you still need to pump the water up out of the hole, meaning it would be cheaper and easier to just have everything on land. |
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Even if my implementation isn't exactly what you had in mind, it's probably the most thought-provoking idea I've seen here in a while. I don't think I've ever seen an idea get posts fast enough to act as a near-real-time discussion. I only wish I had more than one bun to give. |
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Upon more thought, it may not take 27000 feet of pipe to do this. Think about dissolved gases. If the dissolved gases that permeate the membrane begin to come out of solution as they rise up the pipe, they may begin to act as a natural bubble pump that keeps it going. |
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A similar example was covered a couple years back where archaeologists uncovered that the area surrounding lake Nyos (in Camaroon) appeared to be hit by a massive explosion every several hundred years. The scientists discovered that the lake was unusually high in carbon-dioxide concentration, and the pressure at the bottom of the lake would cause the water to become supersaturated. At this point, an occurance of seismic activity would cause the CO2 in the lake to begin coming out of solution (like shaking a soda bottle). As it would rise up, it would become depressurized (much like taking the cap off said shaken soda bottle) and even more would come out of solution. Once this started, the entire lake would explode with expanding CO2, decimating the surrounding forest and any local villages. |
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The solution was remarkably simple: the scientists took a long pipe with a buoy at one end and jagged edges at the other (to make turbulence). When lowered into the lake, they pumped some water up through the pipe to start the process. As the water de-gassed inside the pipe, the difference in density was enough to keep the process going, resulting in a fountain shooting 20-30 feet in the air. |
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By continually bringing the supersaturated water up in a controlled manner, they're hoping to prevent another spontaneous degassing explosion. |
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I think that something similar may happen within such a long pipe, but it would take more knowledge than I have about dissolved deep ocean gases to answer that question with any confidence. |
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[Howard] and [freefall] Did you take into account the change in salinity with depth (which the web site mentioned but didn't quantify). |
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I don't actually know how much the salinity changes with depth, but I suspect that it would exactly cancel out the boyancy of pure water in salt water. |
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Consider this: If you put two tubes side by side, one very deap, and the other shallow, and it didn't cancel, then the water level in one would be different than the water level in the other. If that happened, you could connect the two and let the water flow, and harness a little energy. In the process however, you would be moving pure water from deep to shallow, increasing the salinity of the deep ocean until the extra pressure needed for reverse osmosis balanced out the height difference. In reality, nature has already reached that balance, so there's no energy there to be harvested. |
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Conclusion: no matter how deep you put the pipe, the fresh water level will always be at the same distance down, even when you take into acount the boyancy of fresh water. |
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It looks to me that howard and Freefall are right (except possibly for the assumption of 20 bar)—you could desalinate at no energy cost if the ocean were only deep enough. It's rather counterintuitive. And if there were sufficient dissolved gasses, that would make it a lot easier.
See also the link on Osmotic energy. The water flow in this pipe could likewise be used to generate energy. Where this energy comes from is a good question. (Negative heat of solution of the salts?) But there's a feeling of perpetual motion about it, because you could just dump the fresh water back in the ocean and keep going forever. God doesn’t like that. |
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If physics didn't exist it would be nessecary to invent them. |
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Alternative idea: lower a sealed pipe by rope. When it is full of fresh water, pull it back up. |
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The free energy implication is disconcerting, but I can't immediately see why it wouldn't work. I was wondering if you could model this with less extreme depths by using liquid much saltier than seawater, this making the difference more extreme. But I suspect as the amount of solute rises, the pressure needed to do reverse osmosis also rises because of the osmotic pull of all that solute. This is, I think, how the bubbles help - instead of increasing the density of the salty column, with bubbles you effectively decrease the density of the freshwater column. |
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Yeah, [Freefall] I know I just reiterated what you said. But I was so proud that it finally made sense to me. |
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But back to the model issue. It seems like there must be some way to model this osmotic energy thing on a more reasonable scale. Maybe this could be done using really heavy molecules as your solute. As regards osmotic pull, a molecule is a molecule - as I understand it a 1 molar solution of NaCl and a 1 molar of PbCl would have the same pull. But the lead salt solution would be a lot heavier. |
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Sleeping on it, I realized there would only be a perpetual motion problem if the ocean were in equilibrium, which is isn’t, because it’s constantly stirred up by currents, and those currents are driven by the sun, and by volcanic, tidal, and seismic energy. So there’s nothing mysterious about it.
If the ocean were in equilibrium, the method wouldn’t work. If you totally isolated a column of seawater miles high, so that it became quiescent, subject only to ordinary diffusion, eventually salt would precipitate out at the bottom, and the water at the surface would become fresh. In that case it would be impossible to get a flow in bungston's pipe, since the osmotic head would always be precisely enough to keep it from working. So it’s only the stirring up of the ocean that makes it possible. Mechanical energy that eventually derives from natural sources. No extra magic needed. |
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Would it precipitate though? And what percentage? There's a coefficient of chemical balance in operation here of some sort. |
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You could even use this notion to back calculate what the equilibrium profile of solute would be.
But getting back to the main idea, another way of making the pipe idea practical would be to build it near a volcanic vent, allowing the first leg of the pipe to pass through superheated water, while insulating the rest of it. At 80C you’ve already doubled the density difference, and potentially you could go much further—to several hundred degrees. |
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I'm with [Ling] on this one. Who needs a pump or a perpetual diffusion machine, when we could go deep-sea fresh water fishing? The cylinder could be small enough to be carried on sailboats, or on inflatable emergency rafts. |
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A small cylinder...and a thousand feet of rope. |
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In defense of the idea as it has evolved:
First, the link is wrong.
Second, the pipe is not buoyant.
Third, the whole point of this is to do it without a pump.
And fourth, the economics are insane, sure, but so what? Eventually, the cost of oil will reach $100/barrel, and then all sorts of formerly insane technology will get funded. |
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[Toadinnov] Isn't that exactly what they do when they drill for oil offshore? Save the contamination worries, of course. |
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[laughs last] I didn't think about the emergency use for fishing for fresh water, but you are right: what a wonderful application. |
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Just to be clear, there are 2 ideas cohabitating here - my original supersimple version, which uses a pump and 2000 feet of pipe, and the more exotic one, which uses no pump, 27,000 feet of pipe and holds the promise of free energy. |
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As regards the simple version and [Toads] complaints. I agree with expensive and large, but not sophisticated. It is a pump and a pipe! The best way to deal with the troubles of oceanic pipes is to adapt oil well platforms for this use. At the very least, such an outfit could supply the oilmen with water, as opposed to having it shipped in. I agree that transoceanic pipes can be dodgy. But still, such pipes could be built with 1960s technology. |
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Back to the free energy. Salts do not precipitate out of solution. Absent evaporation, a flask of salt solution will stay salty. A solution is different from a suspension. Answering [ld]'s overnight thoughts on the free energy aspect lets consider two tall pipes: one full of saltwater and within it, another full of freshwater. The reverse osmosis membrane is at the bottom. Freshwater spills out the top of the interior pipe, mixing again with the saltwater. God frowns. |
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[ld] proposes that absent weather, the fresh and saltwater will stratify out in the salt tube, leaving the top fresh because of replenishment from the fresh tube, and the bottom of the salt tube presumably extra salty. This is an osmosis topic! The fresh water will be osmostically pulled down into the saltier stuff. Conversely, mass action will make the dissolved salt ions move up into the fresher stuff. No wind, rain, agitated jellyfish or other agents are needed. It still seems like there would be perpetual motion. |
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True, bung, you’ll never see a glass of salt water stratify, because random thermal motions won’t let it. To see that happen, you’ll have to have something as deep as an ocean. Look at my link showing the Salinity Profile. The deep ocean is saltier, becoming less salty as you approach the surface, and this trend continues until about 500 meters from the surface, where it suddenly gets salty again (because of evaporative concentration). Without evaporation, the curve would continue off to the left as you follow it from the bottom. And this effect would be much greater if there were no currents mixing things up.
As for salts not precipitating out of solution, sure they do, if the concentration is high enough—if the water becomes supersaturated. |
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"...lets consider two tall pipes: one full of saltwater and within it, another full of freshwater. The reverse osmosis membrane is at the bottom. Freshwater spills out the top of the interior pipe, mixing again with the saltwater. God frowns."
But you're still presuming the presence of gravity, expressed as pressure. It's the difference in pressure that causes the flow. |
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It's the whole Lake Nyos thing, but with H2O instead of CO2. You can accomplish the same effect for the same reason, you just need more relative pressure. |
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With the 8000m pipe idea, it is interesting to think about where the energy is coming from.
In the two pipe theory where "god frowns", the salt concentration at the bottom of the outer pipe will increase as fresh water is pumped into the top. The Osmotic pressure will increase and stop the flow, until the fresh water and salty water mix again.
So the driving energy is in the mixing of fresh and salty water, against the natural tendency for more concentration at lower levels.
Or something like that. |
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There is no correlation to be drawn from the Lake Nyos example and the pumpless version of this idea, unless someone identifies an area of ocean, possibly around an area of constant intratectonic activity, where dissolved gas concentrations are extraordinarily high. |
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It seems like the "perpetual motion" osmotic energy machine is actually driven by the forces which mix up the ocean - currents, winds, tides etc. So it actually could run forever, or at least until those things stop. |
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I am thinking about the Black Sea, which I understand does have a supersalty, anaerobic dead zone in the depths. This happened as a result of a lack of mixing. If the osmotic engine is positioned in a deep ocean trench (is it would have to be), it could happen that the deep pocket containing the tube end might get saltier. |
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//Oil platforms work in shallow water// |
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[Toadinnov], Ever hear of deep water drilling? see link. |
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<quoted from link> Our fleet includes... Three dynamically positioned, ultra-deepwater drillships, two of which are designed for the ultimate drilling capability of up to 12,000 feet of water <qfl> |
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Also, a pipe is not necessarily bouyant just because it's not full of water... I'm not sure what you mean by that one. The pipes used in oil exploration are a lot of things, but they sure aren't bouyant. |
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I've been out on 10 different oil rigs by the way -- I will agree that the economics are rather severe. |
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I've met him. He's no dummy. He debunks dumnbass inventions for a living. Most of what he throws out are proposals for free energy devices. |
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Whatever. He just didn’t seem very thoughtful. More of a knee-jerk debunker. And I’m wondering if that’s his job. Does somebody actually pay him to debunk?
BTW, I started off thinking this idea was bunk too, but then some people here--people a lot smarter than me--convinced me I was wrong. |
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At first glance, it seems like it would be possible. However, taking into consideration the increased salinity at depth (which would likely increase osmotic pressure to well above 20 atm) and the possibility of stagnation at the bottom of the trenches (the only places deep enough for this to work) I'm thinking this idea may not be feasible. |
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While there may be locations that would make this more feasible (possibly shallower water near volcanic vents) it seems to be stuck in the realm of technologically possible but logistically impossible. |
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Did that [Toadinnov] delete his account? Hopefully not because of stuff in this idea! A shame either way. |
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As he said, he gets someone proposing this idea, or utilising the "free energy" it provides every now and then and has to explain why it doesn't work. He's a scientific reviewer for a government innovation and joint venture department. It's a shame to lose someone who actually has a clue. |
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Congratulations to all who contributed to his demise. |
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What's a shame is that people like him--only interested in casting aspersions and ridicule--are in such positions in government. And it's amazing that he'd seen this idea before--and shot it down--and yet had so little understanding of it. |
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I guess the other side of the argument is that without people like him we would have governments investing in all sorts of harebrained schemes, like $9,600 wrenches and $25,000 ashtrays for fighter aircraft. |
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Drill holes in the side of the pipe, cover entire pipe with membrane, more fresh water. + |
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Yeah, that's what [Toadinnov] apparently thought. |
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It's just a fundamental difference in attitude, I think. I see the proposal and try to think of solutions to the problems of depth and buoyancy. He sees them as reasons why it wouldn't work. Not that this is a bad thing - the world needs people like him or people like me would be selling perpetual motion machines on the internet. Just different. |
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I worked on a large reverse osmosis project in 1999 and the engineering necessary is staggering for what seems a simple process in terms of physics. In the end, the engineering consultant scrapped the HP stages, relying on filtration and cationic systems. |
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So I'm afraid I'm with the naysayers. |
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First, if the pipe is full, there will be no pressure differential across the membrane. Therefore no reverse osmosis. If the pipe is to be emptied and the pressure maintained, you're pumping water up an 8km pipe. |
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Secondly, have you any idea what an 8km depth submarine pipe costs to build and maintain? You'd be making the most expensive water on the planet. |
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Finally, there's only a few places in the world you could even test it. |
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I'm having trouble finding financing for even straightforward biomass and wind projects. They don't make financial sense in many parts of the world. |
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As for an 8km depth pipe? Dream on! |
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Just don't delete your account when the cloudcuckooland types start slinging insults, [FM], please. |
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//As for an 8km depth pipe? Dream on!
// |
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So would that be a pipe dream? + |
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//So would that be a pipe dream? // |
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Only if the Brits force their trade on China again. |
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Hello [Manatee]. I have 2 things to correct in your complaint.
1. "If the pipe were full, there would be no pressure differential." The pressure differential is from the difference in weight between a column of fresh and a column of salt water. The difference is not huge, which is why such a long pipe is necessary to parlay that small difference into enough pressure to run r.o. |
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2. "If the pipe were empty you would have to pump the water up 8 km.". Actually only 2000 feet - see my math above. The difference in weight between a column of saltwater and a column of air is a lot greater, so the pipe does not need to be as deep. There are lots of places where open ocean is 2000 feet deep. |
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I would be interested to see a summary of the engineering challenges your team encountered in what I presume was a land-based r.o. desalination project. |
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[DeGroof] - that is a good link. It lists 27 bar as the osmotic pressure for seawater. I used 60 bar to get my 2000 foot pipe. But the excess pressure is only necessary because the osmotic pressure of concentrated seawater is greater, and so the system needs to exceed the osmotic pressure of the most concentrated fluid to keep that fluid from pulling fresh water back across the membrane. |
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In the open water pipe, no concentrated saltwater accumulates. Therefore the pipe only needs to generate 27 bar - 910 feet deep! |
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Convert it to steam—a truly halfbaked suggestion! (If you did that, you would already have fresh water.) |
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There is another aspect of reality that has been overlooked in this discussion. It's the biota of the ocean. Almost anything that lies in the ocean gets encrusted with innumerable living and formerly-living things (think barnacles). How do you keep the fancy semipermeable membrane clean so that it can work? My only answer is to sink the pipe into a "dead" zone of the sea, such as the anoxic area in the Gulf of Mexico. Then at least you'd have fewer critters gluing themselves onto the membrane. |
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Why not set up a "vortex" pump using the temperature differential of the water. A lot cheaper and less clumbsy than 8000 metres of pipe |
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More on vortex pipes, please, good [tasman]. |
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I’m bumping this idea up because it’s so brilliant and counter-intuitive. And because it occurred to me just now that the discussion about salinity equilibrium could explain the formation of salt domes. The usual thinking is that a sea got cut off and dried up. But if the oceans were frozen over (the Snowball Earth theory) there would have been no solar input to the oceans for millions of years, and no mixing currents to keep salt from precipitating out. The upper water would have become fresh, while a layer of salt would have covered the bottom everywhere. This would have been reversed when the Earth warmed up again, except where sediments covered it up quickly, such as near the outflow of rivers. |
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What if you created a cone shaped outer body inside of which you construct a helical, spiral membrane. Attach this assembly to a pipe with a free spinning coupler. This whole rig can then be suspended in an area with a current flow. As the current passes through the membrane the assembly would rotate like a turbine, this rotation would power a positive displacement type pump that would accomplish 2 goals, 1 it would move the fresh water up the pipe and 2) it would reduce pressure inside the membrane to increase production at shallower depths and allowing for maximum production at all times. This would also increase production by ensuring fresh seawater at the interface at all times and at a slightly higher pressure as the water is forced through the cone. |
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The motion of the assembly should also drasticly reduce the buildup of encrustations on the membrane. A manual backflush cycle could be setup as well that would reverse the flow of the pump to over pressure the membrane and help keep it clean between cleaning cycles. This might allow for high production and significantly lower depths with no outside input of energy. Placed in a location with a constant flow, these modules could produce constantly. |
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Serviced by a floating barge that is very low to the waters surface would allow for thier placement in locations near land with no deleterious visual impact. Exisiting pipeline technology as is used for oil wells would allow the water to be pumped back to land. |
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Fine, if you must. But it ruins the simplicity of the idea. |
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As presented the idea is grossly inefficient and doesnt really address the cited objections. I was offering a variation that is really quite simple and addresses many of the issues cited. |
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The concept is simple, but execution is anything but. |
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[ldischler]Why did you bump the idea if you are going to poo-poo any discussion of the topic. |
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//Why did you bump the idea if you are going to poo-poo any discussion of the topic.//
If it seemed I was poo-pooing, it was because your idea is only peripherally related to the original idea. There's so much that could be discussed that it would make more sense to post it separately. (Anyway, this is the halfbakery, not a corporate brainstorming session, so poo-pooing is allowed.) |
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//promise of free energy// |
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Its not free energy it would just be free to us. You still need to expend energy dissolving the salt into the water to create the salt water in the first place. There is also a small cost in the passage of the membrane as well. But if, Theoretically you could find a deep enough place in the ocean you could get a steady flow of water to form at a pressure slightly lower than the overall differential less the Osmotic presure of the seawater. But as stated you would need 10s of thousands of feet to accomplish this. |
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As to the points on Stratification of Salt water, this is not a process that occurs spontaneosly. If you begin with a fixed concentration volume of Saltwater and place it in an enclosed tube, the concentration throught the tube will be the same for all time. This is basic diffusion. The solution is at equalibrium becase it is a solution, not a suspension(which is a Physical Process) Stratification in the ocean occurs due to temperature differentals and Large masses of water. It can be simulated in a lab but as time goes on the solution will equilabrate and a uniform density will occur. |
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//it was because your idea is only peripherally related to the original idea// |
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As is 90% of the discussion on this page if that is your assesment. |
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//As to the points on Stratification of Salt water, this is not a process that occurs spontaneosly//Actually, it does, at least on large scales. It's ocean currents that keep things stirred up. Even so, there is quite a bit of stratification, esp. in the more quiescent waters of the Arctic ocean. Too much stratification, and this idea won't work. (And that this idea works implies that there must be stratification, absent mixing currents.) |
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[ldischer] Please link some data that shows that a homogenious solution of saltwater placed in a column will spontaneously stratisfy. This certianly occurs in the ocean but it has a lot to do with moving water masses, temperature differentials and currents. In a homogeneous, static solution this is not the case. I could be wrong, but 10 years in chemistry and a degree later, i,m not sure how it would happen in the situation I described. |
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Look at page twelve of the Gotland Deep link, which shows the bottom salinity to be twice the surface salinity. This is more than the open ocean, presumably because there is less mixing in this area. That kind of variation is almost universal across the oceans, although the degree is generally less severe. See the "Standard Salinity Profile" in the link above it. Except for the surface water (made salty by evaporation--more evaporation than precipitation), the ocean typically gets saltier with depth, even with currents that tend to mix it up. Take out the evaporation effect (as in the cold Baltic waters), and the red curve would continue bending to the left, all the way to the surface. |
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[jhomrighous], I like your spinner. It is an idea in and of itself but I appreciate your putting it under this heading so discussion of membrane desalination can occur in one place. |
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No-one doubts that one can use pressure to carry out reverse osmosis desalination. The question is whether you can rig it to operate without obvious energy inputs (the energy input here is probably the mixing of the ocean by currents etc, as discussed above). Your hybrid solution harnesses energy inputs from currents to sidestep problems with depth and mixing. As a different issue, it may be a good idea to have membranes in motion to avoid fouling with ocean life. |
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[bungston] Thanks that means a lot coming from you. |
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[ldischler] I have already stated that Ocean stratification is a proven process. The question is not does it happen it is a question of in a closed system(addressing some earlier discussions) will it stratify and i still say no to that. |
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Okay, have you ever seen that demonstrated? Can you provide a link where someone has isolated a solution for a time and measured the concentration top and bottom with precision instruments, and shown that there was no gradient? Because if there isn't, then we've got a perpetual motion machine here. |
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Has anyone accounted for the Osmotic potential of the water back the other way? This would directly counteract the incoming flow of water once equilabration of the water column pressures has matched up(Fresh water taller than salt) Water will flow one way or the other until everything balances out. The more I think about it the more I think that this would equalize and no water would flow unless you removed water from one side or the other(requiring energy input) |
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If you started with a to matched columns one salty one not.(the fresh one taller by the prescribed amount to generate equal pressure at the membrane) with a membrane in between and a valve, what would happen when you opened the valve? |
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The answer is nothing, the colomns are balanced and there is no flow. |
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If you shorten the fresh column a little you would get some flow towards the fresh side but eventually it would equilibrate at the new level as the salinity of the salt side was reduced. |
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If you removed some fresh water then you would get flow towards the fresh side which would stop when the salinity stablized. |
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Remove salt water and flow would reverse to the salt side of the system until diluted sufficiently to equilabrate again. |
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I would expect some harmonic cycling in a non idealized system but in all cases it would eventualy cease to flow. |
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In a real world trial in the ocean I suspect it would look an awful lot like a perpetual motion machine, but in reality, the energy budget would be negative. |
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//The answer is nothing, the colomns are balanced and there is no flow.// You're neglecting the difference in density. Look at Freefall's calculations above. |
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No Im not.
In my example I cited the Fresh tube being higher than the slat tube. This difference was for the exact differential in height required to equalize the pressure at the interface of the tubes. From there follow the scenarios and you will see what will happen if tried. |
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If the columns are of equal height at outset you will see flow for a period of time sufficient to dilute the salt water to a concentration at which the flow of water at the membrane can no longer be sustained, at which time the flow will cease and the system will equilabrate.(it may oscilate in a non ideal scenario but it will equilabrate in the end) Keep in mind that such a scenario could lead to water flowing from the fresh to the salt side which could permit salt into the freshwater side of the system. Either way in the end it will equilabrate. |
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Freefalls calculation explains why it will look like a perpetual motion machine in the real world, but left to run for a sufficiently long period of time(eons and eons i suspect) the Ocean would eventually equilabrate with the water in the tube and the flow would stop.(RO pressure would increase as salinity increased, or vice versa the osmotic presure from the Fresh water would increase until eventual the system equalizes) |
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See link for discussion of diffusion which governs the Osmotic process. This is an energy cost free activity. |
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Perhaps another way to look at this is to say that, left to its own in a fully sealed system(neglecting heat losses) you would have a perpetual fountain, but the moment you try to extract ANY energy from the fountain you go negative an d the process stops(ie a turbine would increase backpresure and would force the flow back in the other direction and so no energy can be extracted.) |
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Also of note is when I say equilabrate that could mean either pressure equilabration, concetration equilibration or a combination of both depending on the scenario. |
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This is tough to visualize. |
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But it isn't two columns. Just one for the fresh side. The other side is the ocean. The top level of each is constant. Water spills from the fresh side continuously, with no energy input. That's what's so neat about it: fresh water by reverse osmosis, for free. (This is not the original idea--it's the better idea Freefall came up with. Bung ought to note that at the bottom of the idea.) |
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Not free but at a cost that is not visible to your eye because it is spreadout over the entire ocean. |
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I agree that it is a very cool concept and could actualy work(assuming the engineering obstacles could be overcome) But it is certinaly not a perpetual motion machine. Which was what I was hoping to show with the closed loop example(perhaps I have failed miserably to do so however) |
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Bungstons point is still valid, this is brute force RO and as such i suspect that even a relatively small system of several hundreds of feet could be used to generate fresh water. |
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It's perpetual, but it's not perpetual motion, because the energy source is the non-equilibrium state of the ocean, which gets that way from mixing currents. And the energy for that ultimately comes from the sun. |
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It is brilliant, and counterintuative. I'm still not sure I understand it though. |
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[ldischler] and [jhomrighaus], could you tell me if I'm getting close: |
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If I imagine a simplified model -an isolated system with a column of sea water and a column of fresh water linked by an osmotic membrane with a fixed pressure difference across it - at the top of the column, you get a head of fresh water, which could drive a turbine. |
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You could allow the fresh water (after extracting energy from it) to pour back into the sea water column. It looks like a perpetual motion machine now - it appears that we can extract work from the system, with no energy input. |
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But, after running for a while, the water at the bottom of the salt water column has become more saline - eventually, too salty for RO to function at that pressure. For the system to continue running, the fresh water at the top of the sea-water column has to be mixed with the salt water, either by stirring (requiring kinetic energy) or by diffusion (requiring thermal energy) |
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From the isolated system model, it looks like the energy in the ocean-bound system would come from the mixing of the ocean by currents. |
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So the energy output is actually coming from the gravitational potential energy of Na+ and Cl- ions (and others) that make the upper ocean waters more dense than they would be if allowed to settle without solar input. |
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Is that about right? I really have trouble getting my head around this. |
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[+] for the idea, and if I could, i'd [+] for the mental exercise too. |
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That's it, except the column of sea water is just the sea itself (which I'm sure you meant). It'd be interesting to calculate the potential energy for the oceans--huge, I'm sure.
A similar effect has been suggested for the internal heat source of Jupiter--stratification of the originally well-mixed He and H2. |
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To my knowledge there is no Gravitational Potential Energy associated with Na or Cl ions in solution. Thier mototion and diffusion within the water solution is an energy nuetral process. As such the static Model(and im am guessing here) may flow if left in an undisturbed state. The instant that any amount of energy is extracted will shift the balance and the entire process would stop and equilabrate. |
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I think the Salinity stratification in the ocean is significant at an operational level for this system, but when looking at an ideal closed loop system it does not enter into the picture.(if you took a colomn of ocean and isolated it, it would eventualy equilabrate and become uniform over its entire volume, as is dictated by the laws of thermodynamics.) |
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I am going to try and do some calculations on my lunch break and see if i cant quantify this question somewhat. |
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[ldischler] I think the thing that is confusing is that stratification comes about through the interaction of a complex system of energy flow causing variable potentials to move about the system. I think the other key is that these are short term(relatively) phenomnons and given time would give way to equilabration.(like food coloring in water, eventualy it is evenly distributed) This is the second law of thermodynamics at work and is the eventual state of the universe(a still quiecent pool of matter evenly distributed and equal in makup at all points devoid of all motion) |
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You have it backwards. It's the relative lack of stratification that's due to mixing currents. In a classroom you can say that diffusion is the only effect, and you won't be far off. But on large scales, you can't ignore gravity. Look at the earth itself, which was once molten. Did it stay all nice and mixed up? |
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Sorry [ldischler], I should have been clearer: I meant to describe an imaginary system of two columns of water isolated (from the ocean/surroundings), to understand where energy was crossing the boundary of the system. |
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From that simplification, it was clear that energy must enter the salt-water column from the surroundings (from outside the system), causing the salinity at the top of the column to be higher than it would be otherwise (by stirring and/or diffusion). |
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So I saw that a column of sea-water of uniform salinity has a higher energy than a column with a salinity gradient from top to bottom, and that effectively, this is gravitational potential energy (of the ions contributing that density). |
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So the energy available from a “real” system – and there would be a real, useable energy output – comes from the elevated density of sea water in the upper layers of the ocean, elevated relative to the density gradient that would develop if there were no solar input to the ocean. |
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[jhomrighaus] – it’s true that there are a number of factors influencing salinity distribution - solar evaporation will tend to produce a higher salinity near the surface; Mixing by ocean currents will tend to produce homogeneity, as will diffusion. All of these factors oppose the tendency towards the gravitational distribution of low density to high with increasing depth – it’s gravitational “rest state”. |
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Each mechanism depends on energy input to the ocean from the sun. The energy is expressed as the displacement from it’s rest state. |
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Hmm. Just done some rough calculations on the depth required. Taking a mean density of sea water as 1025 kg/m^3 and fresh water as 1000 kg/m^3, to develop a 60 bar pressure difference across the membrane would need a depth of around 24,000m. Challenger Deep is around 10,911m. |
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True, density will change with temperature and pressure, but I’ve assumed that the relative effect between fresh and sea water of these is negligible. |
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Even if osmotic membranes were greatly improved, it doesn’t look promising. Shame. |
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Thanks, Frankx. That’s a lucid review. And yeah, the gods seem to have conspired against this. The absolute minimum pressure differential is >24 bar, but probably closer to 40. That makes the situation better, but still, the ocean is not that well mixed, and there are salinity and temperature gradients working against it. |
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Croissant for making me think hard. |
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Is the best refutation that this perpetual freshwater well would not flow in a perfectly still ocean, because in the absence of stirring, the concentration of salt would go up with depth, so the osmotic pressure would go up at exactly the same rate as the water pressure? ? |
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(I'm Ignoring the practical fact the fact that the ocean is not deep enough.) |
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If Salinity varies by depth due to gravity(as has been positid and I have refuted) Could someone please supply the calculation that indicates the salinity level at a given depth? |
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If this is the way salt water behaves in an isolated column of know length then this vavlue should be able to be calculated. |
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I have thrown the gaunlet and await someone to pick it up! |
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Just saying it ain’t so is not a refutation. But I’ll argue it like this. Lets say you’re right, without mixing currents, in a quiescent ocean, a salt gradient won’t exist because of diffusion. Okay, that’s great, because we can set up this long pipe and start making fresh water (given a deep enough ocean, of course) and let's say we dump it back in the ocean where it becomes salty again because of diffusion. We can do this forever! But that can’t happen, can it, because it’s perpetual motion. So one of two things are wrong: either this pipe cannot make fresh water after all, or there must be a salt gradient in your quiescent sea that prevents us from making fresh water this way. And this salt gradient should be just sufficient to keep this idea from working. |
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The concept of a salt gradient goes against the concept of osmosis, which it would seem most posters here believe in. Just as we are using reverse osmosis to remove fresh from salt water, in a column of salt water, the osmotic pull of lower salty water should pull freshwater down from the top, with a consequent even distrubtion of salt. My google for ocean salinity shows the main differences in the ocean to be at the top, probably because of evaporative losses as was mentioned above. |
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Imagine a two column problem: one full of seawater and one full of salt. They are long and connected at the bottom with a membrane. A fountain results and the fresh overflows into the salt column at the top. The top of the salt column would become less salty and the bottom more so, and the fountain would decrease. However, the second law (entropy) states that even without external currents, ions in solution tend to move towards an area of less concentration, and so _even absent external currents and stirring_ the salt column will tend to equilibrate. As it does, and the bottom of the salt column gets less salty, the fresh fountain will periodically dribble over into the salt. Forever? |
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It looks like perpetual motion. It seems to me it is actually running off of entropy. Entropic engines are really heat engines. It seems to me that over time, these two columns will freeze and that will be what stops the flow. |
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It freezes! What ever happened to the heat? Did we destroy it? What if we fed in a little heat so that the water never freezes? Could we continue destroying heat forever? No, so the problem is still there--either this doesn't work, in spite of the math that says it does, or there's a gradient. |
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[bungston] That is exactly what i think would happen, Diffusion and by extension Osmosis are "energy free" processes and so do not consume energy on thier own, however molecular motion is a function of thermal energy input and so with no input of thermal energy I believe that a freeze is what would ultimatly result. IF you could preserve the heat in a perfectly insulated environment then the fountain could theoreticaly "flow" forever, this is decieving though because it appears like perpetual motion but in reality is balanced motion. As nothing is ideal loses due to friction(water flowing in columns, molucules bumping etc.)you would slowly lose energy through heat loss and the system would stop and eventualy freeze up along witheverything else in the universe. |
&nb |
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