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Solar Desalination Aquaduct

From seaside to destination, distilled enroute.
  (+16, -1)(+16, -1)
(+16, -1)
  [vote for,

Filling up dry lake beds from the ocean seems a laudable idea in arid areas, as long as the water table and wind currents don't simply send the water back to sea. But salt water isn't all that useful because if there *is* a water table you've just poisoned it....

Proposed is a physical method of simultaneously transporting and desalinating seawater:

A clear fully-enclosing-pipe contains a black trough. Heated by external solar concentration, water evaporates from the trough and condenses on the walls of the outer pipe creating a stream of fresh water on the bottom *. The fresh water is dumped into the lake and the concentrated brine left in the trough is returned via a separate pipe to the ocean to either be dumped in (messing up the shoreline ecology somewhat), or put into evaporation ponds for mineral harvesting.

While at first it seems counterintuitive to pump the concentrated brine back to the ocean shore instead of simply letting the water boil off completely in the solar pipeline, there's several reasons why this is actually a good idea.

Evaporating seawater precipitates in the order
(ex Wikipedia:<link>)
1. Calcite (CaCO3) and dolomite (CaMg(CO3)2)
2. Gypsum (CaSO4-2H2O) and anhydrite (CaSO4).
3. Halite (i.e. common salt, NaCl)
4. Potassium and magnesium salts

Notice the stages 1 & 2 precipitants: they're just rock. Precipitated out enroute and swept along by the current in the trough, the fine sand can be filtered out of the black trough and deposited (or harvested) at the end of each section of aquaduct.

Halite and the other salts, stages 3 & 4, which are the bulk of the precipitants and what we don't want in (or even near) our freshwater environs, won't start precipitating out of seawater until 80% of the water has evaporated.

So 80% of the water goes into the freshwater lake and the remaining 20% transports its still-dissolved salt back to the ocean. While the control system for the ocean-to-lake flowrate would be calculated (time of year, temperatures, weather patterns) and modified by feedback sensors, it doesn't have to be too fiddly to operate near the optimum level, ie:

- if there's too much sun, and some haline salts start precipitating out too soon, we just start returning the black-trough water at an earlier point in the aquaduct

- if there's too little sun and there's more of the (albeit weaker) brine going back than is optimum... so what ?;

For periodic or emergency maintenance, there's already a "backflushing" system in place, ie: the brine-return pipe.

As a further advantage salt harvesting, if bothered with at all (though highly recommended), is done at the seashore, away from a freshwater ecology area, using a pond system a fifth the size of what would be required if the water came right from the ocean, since the returning brine stream is already concentrated.


* the actual design of the pipe/trough is still on the drawing board, but basically anything that heats up water in a black trough which then evaporates out of the trough and condenses on an ambient-air-cooled condensing surface and continues on its way in a separate portion of the aquaduct. I find the idea of a "u" within an "O" to be an easy way to visualize it... or a sideways "@"

FlyingToaster, Sep 28 2010

Seawater: order of precipitation http://en.wikipedia.org/wiki/Evaporite
The order in which dissolved solids will precipitate as seawater denatures [FlyingToaster, Sep 28 2010, last modified Oct 01 2010]

Insolation http://en.wikipedia...File:Insolation.png
Map calculated on weather patterns not actual ground level measurement but hey, if it's good enough for Wikipedia... [FlyingToaster, Sep 30 2010, last modified Oct 01 2010]

Inspiration Salt_20of_20the_20Earth
I had an aquaduct'ish idea, but all Loris's annos on the composition of saltwater as it denatures caused the light bulb to flicker about how to handle the actual salt part of saltwater, which precipitated this post... [FlyingToaster, Sep 30 2010, last modified Apr 14 2013]

Lake_20Death_20Valley ... only to find of course that the "brilliant flash" was proposed offhandedly by [8th of 7] in 2002 (third anno down), totally unfair: Borg with a time machine [FlyingToaster, Sep 30 2010, last modified Oct 01 2010]

Molten Salt Energy Conversion http://www.treehugg.../molten_salt_as.php
May be of use? [infidel, Oct 03 2010]

Dead Sea canal http://en.wikipedia...wiki/Dead_Sea_canal
[pashute, Dec 27 2010]

Bunsen's spinoff idea Tidally_20Pumped_20Moist_20Air_20Duct
Like this, only more differenter. [BunsenHoneydew, Feb 02 2020]

Solar desal tube https://www.researc...-solar-still-42.png
Illustration of apparently related idea [BunsenHoneydew, Feb 13 2020]

3U desalination https://www.deviant...larfilter-848799340
my take on this with open system air flow [wjt, Feb 15 2020, last modified Jul 15 2020]

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       I like it. Some numbers would be nice (eg, how much fresh water can a sensible-sized pipeline produce, given reasonably sunny weather?).
MaxwellBuchanan, Sep 28 2010

       hmm... I'll have to work on that a bit. I had had the general idea on the back half-burner for awhile as a mono-directional flow, ie: by the time it gets to the terminus it's all evaporated. But that was a bit of a Dune-esque vision: sand-hermits stationed every few miles, emptying the troughs nightly, and the purpose was to refill the underground lakes under the Sahara and the like.   

       Meanwhile, "sensible-sized" is almost entirely a function of location and bankroll. Can you afford a greenhouse 200 yards wide and 500 miles long ?
FlyingToaster, Sep 28 2010

       The math isn't complex for a rough figure.   

       insolation x area / LH vapourisation of water.   

       hmm... pretty impressive actually:   

       Based on 8kWh/day insolation at ground level (not unreasonable for an equatorial'ish target site), we get   

Volume: 4,567.5 cubic metres per year.
Flow Rate: average of 0.1477 litres/second or 2.56 US gallons/minute.

       so.... say 600km of 5m high reflective surface on a smaller pipe would yield...   

       13.7 million cubic metres of fresh water per year at an average flow of 7,680 gal/minute (so in reality, during the day it might peak at say 20,000 gallons per minute ?)
FlyingToaster, Sep 28 2010

       That is quite a lot of water indeed.   

       What sort of size lake are you wanting to fill?
MaxwellBuchanan, Sep 28 2010

       I'm worried. This is making sense to me. [+]
Grogster, Sep 28 2010

       [MB] Don't be a smart-ass: my face is not going to drop when I realize that it would take over 70 years for the example-spec'd system to move a cubic kilometre of water.   

       The original unposted idea was to have the UAE et al. spend all their oil money to turn the Sahara green over a period of centuries: a large working waterfall in every major city, slowly refilling and overflowing all the aquifers, etc.
FlyingToaster, Sep 28 2010

       But seriously, if you are wanting to fill a lake, how big?   

       I'm imagining a modest lake, 10km x 10km by 20m deep. That's 2x10^9 cubic metres of water, requiring something like 200 years to fill from your pipeline, except that this will be completely overwhelmed by evaporation from the lake itself.   

       I suspect that even a 1km x 1km x 10m lake (10^7 cubic metres) will not be sustainable, when the inflow has to battle evaporative losses in a sunny climate.   

       As a supply of fresh water for humans, such a system might work. A westerner uses something like 100 cubic metres per year, so your 600km pipeline will be enough to keep 100,000 people (roughly the population of Basingstoke, or a tenth the population of Perth) in comfort, as long as they don't want to irrigate crops and stuff.
MaxwellBuchanan, Sep 28 2010

       fine, let's redefine "sensible" to the tune of say the equivalent price of a six-lane superhighway in terms of cost per kilometre. That's quite sensible if a government takes the idea seriously.   

       Now we can easily have 200m worth of concentrators focussing on the pipe maybe with an order of magnitude fiduciarily to spare.   

       And now it's only going to take 5 years to fill your 100 sq.km lake.   

       Or supply fresh drinking water for 4,000,000 of your western standards water usage people, though on behalf of "western people" including you, I resent the inaccuracy and implied eco-racism, since the operative parameter is "people in countries with a wet weather system" or arguably "people in countries with a climate too wet to be able to take a dump on the sand and not worry about it" which certainly encompasses more than "western" culture.   

       //irrigate crops//   

       what, on a measly 12.7 litres/day per m2 of sunlight ? that's only, umm... 4.65 meters of irrigated land (based on a depth of 1m/year) for every m of sunlight... so even the 200m worth of sunlight on the "superpipe" would only produce 900 metres worth of metre deep "rainfall" per year...   


       okay [MB]: a 1-3km deep farm or greenbelt along the entire length would be nifty.   

       but you can sail on a lake.
FlyingToaster, Sep 29 2010

       //but you can sail on a lake.//   

       No, I can't. Powerboating is no problem, but the whole sailing thing - I never got the hang of it.
MaxwellBuchanan, Sep 29 2010

       To me this feels very similar to my "Salt of the earth" (née "Land from salt") idea. There are obvious differences in that I'm interested in the solids and you're interested in the liquid, but I wonder if you were spurred on by my annotation of a couple of days ago on the order of precipitates from seawater.   

       My concern with your strategy is that the initial stony precipitates would silt up the inside of the channel, rather than being swept along. That may not be a problem in practice.
If the rocky precipitates _do_ form fine, flowing particles, it may be desirable to filter them out at intervals along the pipeline. I believe this would be techologically trivial to do - then you can purify out the different products, which may be useful to nearby populations. Even if not, they could be discarded in appropriate places. You also absolutely don't want to have to pump water with solid precipitates in at any stage.

       I don't think you need to worry about the depth of a terminal lake - have it as deep as you like. In fact, consider putting it in a greenhouse too (this may negate the sailing option, though). On the other hand, deep in the arid interior of a continent, extra water-vapor in the atmosphere may be beneficial to the environment and yield extra rain.   

       If you're supplying freshwater to humans, it isn't as bad as Max suggests. The water would still be non-salty even after westerners have used it, so it could still be used for irrigation.   

       The other thing is the efficiency issue you mention. If the water has to be pumped, you absolutely want to maximise that. This ought to be possible with some form of flow regulation.
Loris, Sep 29 2010

       Wow, amazingly great idea. You sure about those pipe size x distance x temperature = gallons per minute numbers?
doctorremulac3, Sep 29 2010

       Imagine this pipeline stretching up and over an inconveniently large landform between the ocean and the target dry lakebed. Now imagine the pipeline is kept above 100C for all of the uphill run. The steam would rise to the top of the pipe, condense, and flow down the other side to the waiting dry land, without requiring active pumping.   

       Perhaps it can be allowed to drop below 100C daily to flush precipitates out of the uphill leg.
BunsenHoneydew, Sep 29 2010

       //daily to flush precipitates out of the uphill leg// try sitting down instead, and making sure the paper is conveniently located.
pocmloc, Sep 29 2010

       I am puzzling over how to get flow in opposite directions. You want your freshwater to flow away from sea and to lake. Your salt water will have to do this too, until you reverse the flow to clean out aggregate salts.   

       You could have it all one way if you use Death Valley or some comparable sub sea-level waste spot. I proposed this in Lake Death Valley but used reverse osmosis, which entails fussy filters.   

       Pipe as proposed leads from ocean to Death Valley. It is downhill so there is flow.   

       Accumulated fresh water is tapped along the way and used for irrigation.   

       Waste salts and brine accumulate in Death Valley - already full of similar stuff.   

       The beauty of this is that at night there will be no evaporation but there could still be flow, which can be used to purge any accumulated salts in the dry end of the tube.
bungston, Sep 29 2010

       pretty well everybody who annotated on this... yeah you're right :D   

       [Loris] oh definitely, I had "topping up the Saharan aquifers" on the backburner for awhile; your precipitate order, crowned by brine concentration figures, sorta nudged it over, err caused it to precipitate out as it were.   

       The entire post is really just trappings for the flow pattern and the solar-pipe design (which I've firmed up somewhat but I'd need a Wacom tablet to post it).   

       If you like you can have the brine after I'm finished with it to build your seaside salt castles :)   

yup again, the pumping points are definitely the place to dispose of the first two stages of precipitates.

       [doc] err... you mean the numbers I crunched ? They're honest figures for a middle-school science quiz, but an engineering company would just shoot me on general principles. In reality a half or third might be a bit closer but that's me guessing.   

       But if you mean the factors going into predicting how much fresh water you can get out of a system that runs from point A to point B over terrain C, that's pretty straightforward: you know in advance how efficient the actual pipe is going to be from design and testing, the seawater's specs are going to be constant (or at least predictable), so the only thing that's going to change is the amount of sunlight which is based on what time of day, what time of year, and the weather.   

       [bungston] what you propose is a use for the solar-pipe design sans brine-return. Nifty: I'll send you my bill :)   

       As far as the system operating uphill or downhill, there's a major difference between the two pipes. The solar pipe pair (seawater,freshwater) is actually an aquaduct, albeit a fancy covered desalinating one: it always flows downhill; occasionally there might be a waterfall or a point where the water is pumped up to the beginning of the next section, depending on the terrain and required velocity. The brine return pipe is a standard industrial pipe, it's pumped whenever needed, but it's a fully enclosed pipe. (post retitling mulled over... done).   

       I was going to mention that you wouldn't even need the brine-pipe to be coupled with the solar-aquaduct in any way (except for convenience since it's probably going back to point A), but you've made me think that it might be advantageous to have several points along the length as it approaches its terminus where the brine is connected to the pipe, to allow for really really sunny days... hmm. § x1
FlyingToaster, Sep 29 2010

       One could use the Salton Sea for the discharge point for brine from the Pacific. The aqueduct proposed here could not be used as a siphon, but one could siphon seawater to El Centro, California (below sea level) and from there via desalinating aqueduct to the Salton sea.
bungston, Sep 30 2010

       Loris, while it's definitely undesirable to mechanically pump water with particulates in it, there's an simple non-mechanical way to pump sandy water.   

       Specifically, mechanically pressurize some of the fresh water (of which we have a handy supply :) ), and use the power of that pressurized water to move the sandy water using an eductor pump.   

       Eventually, even an edcutor will wear down from the sediment, but not as fast as a conventional pump would.
goldbb, Sep 30 2010

       [bungston] If it has to go over a mountain then some energy could be recovered on the downhill slope.   

       But it occurs to me that siphons wouldn't be possible on any other system either: over 32' vertical and the going-up side will cavitate, thus desiphonizing the pipe.   

       Does the Death Valley water table run into the ocean ? or is it a self-contained basin.   

       and I just looked at "Lake Death Valley" and found my brilliant brine-return idea in an off-handed anno by 8/7 back in 2002... <sigh>   

       [goldbb] well, pumping sediment a few hundred miles is a bit of a waste of energy, and anyways it's just the rock sediments (the salt makes a roundtrip, dissolved), so, since it's an aquaduct not a pipe, the end of each segment seems a good place for the sediment to settle out... You'd still be pumping silt that didn't settle out of course, but hopefully
FlyingToaster, Sep 30 2010

       I am pondering price. A closed siphon pipe to bring seawater from Pacific to El Centro: OK, a big pipe. But how much would a clear pipe cost, and what would you build it from? It is 22 miles from El Centro to Salton Sea: good length for this project, traversing very hot and sunny desert which is also optimal. The Salton sea is currrently saltier than the ocean and getting saltier still, so if brine inputs were offset by addition of plain seawater at night it should not screw up the salton sea any more than it is.   

       Polycarbonate is tough and clear, but pricey and in the desert I bet it would not be UV stable for more than a year. I think glass is the way to go. It can be made clear, is cheap and is desert durable. Long glass pipes will break in earthquakes and so this would be short segments with flexible joints. Vandalism should not be a huge problem in this area.   

       I could not find good cost estimates for glass pipes. My google only finds bongs, which are much marked up for this use.   

       Finally I like the idea of one or more cleaner bots that would move along the 22 mile pipe to sweep off bird droppings and dust. It would be solar powered.
bungston, Oct 01 2010

       Unable to stop thinking about this, I realize that it could be done more cheaply by dispensing with the pipe. The main structure would be a metal halfpipe, (really a trough) ideally in continuity with the earth and with support structures designed to conduct heat into the dirt. Within the metal pipe is a black plastic (or ceramic) halfpipe which contains the seawater: this is intended to retain heat. By making it a halfpipe the top can be covered with cheap flat glass panes. Dispensing with curved glass and structural requirements will make this cheap.   

       The best of all would be realization of this using salvaged materials. Or most awesomely of all, using an existing concrete lined irrigation ditch instead of metal base pipe. One could make a small scale mockup, perhaps traversing a peninsula or small island and using a windmill and archimedes screw (better for saltwater, I would think) to get initial saltwater to height.
bungston, Oct 01 2010

       As far as your elevating unit, if needed, there are plenty of pumps with a minimum of wetted parts, pumping even gritty brine is a more or less solved problem. A peristaltic pump is the least failure prone (just replace the cheap tubing periodically), but other types do exist.   

       As far as the eventual use of the water, the most efficient as far as avoiding point of use evaporation would be to pipe it back into aquifers that have been overused.
MechE, Oct 01 2010

       /aquifers that have been overused./ this is solving the Tragedy of the Commons by fertilizing the commons. The problem is the bastards that overgraze the commons / overuse the aquifers and such an approach would facilitate their behavior.
bungston, Oct 01 2010

       its not a good idea to combine the simple purpose of a pipe with the complicated application of a solar still. Clear pipe that will not cloud or weaken in the sun must be made expensive materials (glass, polycarbonate) and moving saltwater any distance inland is a waste of energy. There is no way that the end product at the "lake" could be anything but strong brine which, for reference, cannot be rinsed away with more strong brine.   

       This is a giant mineralization still. The use of sea water might be very efficient but the use of energy and materials would be a simple waste.
WcW, Oct 01 2010

       [bungston] the "black pipe within a clear pipe" is only a visualization aid. The latest one I'm toying with is a black pipe and a finned shiny pipe; every x metres a shiny tube protrudes from the top of the black pipe and runs down to the top of the shiny pipe/trough; or maybe a continuous sheet of aluminum along the bottom of which the condensate flows to the freshwater pipe/trough.   

       // size// well, again from constant stirring, I've sortof got the idea of making consistent sized segments that are sized to be transportable by helicopter/tractor trailer/shipping container, so that would be the maximum length of the cover.   

       Continuing with the fixed-length segment idea, the precipitates would be passively sequestered in a sump at the end of each: what might pass through a pump would be only the particles still too small to sink to the bottom through the flow. So an issue but not a "gritty" one, more of a "silty" one.   

       //pipe it back into aquifers that have been overused// that was the original idea... still is, the "lake" was just a quick slap of paint to coincide with the "Great Australian Sea" referenced in Loris' Salt of the Earth.   

       [WcW] I think you're reading a different post/annos. Quick recap...

       - seawater goes in one end, 80% of the original volume comes out the other as freshwater. The minerals are waste product to the idea, though the mechanism makes them easily available for harvesting.   

       - as the seawater gets dewatered along the way, the rocky precipitates (the first to come out) are sequestered and disposed of in each section: they're environmentally neutral so if harvesting them isn't part of the project they can just literally be dumped onto the ground.   

       - *before* the concentration of seawater reaches the point where salts start to precipitate (which is at the 20%-of-original-volume point) the brine is sent back to the ocean (or wherever you want to). The salts never get a chance to precipitate out.   

       Maybe the rocky precipitates formed along the way could be made to form the permanent supporting structure: a temporary scaffold would be in place originally while the stalactites/mites slowly forms the permanent columns :). The temporary scaffolding could then be used on another project.
FlyingToaster, Oct 02 2010

       It seems to me that the pipeline aspect of this is something which can be optimised to suit the terrain.
Having the pipeline itself as the evaporation platform seems like an efficient use of space. However, this doesn't mean that we couldn't change the width of the pipeline. It would make sense to have particularly wide sections where conditions are optimal - the land is cheap, there is no shade from nearby mountains, there is a decent prevailing wind (to condense the water vapour), there's a suitable disposal mechanism for precipitates and so on.
Removing any particulates can be as simple as a long, deep settling trough (or several in parallel to allow for a cleaning rotation).

       Indeed, it seems to me that there isn't really any reason for the entire body of water to make a full round-trip. There must be pumping of saline at some point. If getting to the target area means pumping water uphill at any point, it would be better to return the concentrated saline before that point. We only need to send the pure water onwards - a significant energy saving. Similarly, if the concentrated salt-water needs to be pumped back to the sink (some other part of the ocean), then extracting the pure water before committing to that pumping would be desirable.   

       The upshot of this is that you'll probably end up with a series of covered desalination ponds somewhere near the coast, and a freshwater pipe heading inland. If space-efficiency bugs you, cover the pipeline with solar cells to help power the pumps.   

       It's not quite true to say that the rocky precipitates are environmentally neutral - you wouldn't want to just dump them into an acidic marsh, for example. I also think that you're likely to get a lot of large accretions rather than loose, flowing particles forming. Which I don't think is much of a problem - build the course to allow for buildup, and scrape it off periodically.
Loris, Oct 02 2010

       The idea is sill just a giant, inefficient, solar desalination plant. In cost of materials and energy it has no benefits over a conventional plant and I see no obvious benefit over simply pumping the now fresh water, from a conventional and efficient coastal de-sal plant to the desired location of end use. The expensive water from de-sal should be used on the most fertile soil available and in the most efficient manner, not simply used "along the way" the agricultural viability of land "along the way" is not likely to be a good use of the water. If you have ever contemplated the roof of an old glass greenhouse it becomes abundantly apparent that every surface of a solar evaporator suffers the effects of mineralization and would need constant cleaning.
WcW, Oct 02 2010

       //coastal desal plant// fine if the coast's weather is clear and sunny every day... and if the price of coastal real estate is cheaper than the price of inland desert and the neighbours don't mind... and a whole bunch of other non-sequiturs.
FlyingToaster, Oct 02 2010

       Refilling aquifers does not necessarily subsidize the commons. It can be treated as a water storage and or transportation system same as any other water distribution system, with people paying usage on their wells exactly as people pay usage on municipal systems.   

       Also UV stabilized acrylic (or even polycarbonate) isn't that expensive. It's not super cheap, but even compared to simple PVC pipe in similar diameters it's a fairly minor cost addition. Construction costs will be more of an issue than materials.
MechE, Oct 02 2010

       Actually I only see two problems with this:   

       One is that the black saltwater pipeline will be at 100C for almost its entire length: not a problem per se, but it will be losing heat through the walls. The obvious solution is for a transparent shield, but then you have to make sure it's clean all the time.   

       The other problem isn't a problem so much as a lack of regenerative efficiency, ie: the water vapour condenses into 100C freshwater... but unlike a shore-based plant there's no option for heat exchange with incoming raw seawater.
FlyingToaster, Oct 02 2010

       can anyone point out any benefit of this system over a single-point plant and a conventional pipeline?
WcW, Oct 02 2010

       [FT] Generally solar desal uses evaporation rather than boiling. As such, it uses the dew point and is considerably below 100 deg C.
MechE, Oct 02 2010

       //benefit of this system over a single point plant//   

       I'm wondering why you'd want a single point plant in the first place
FlyingToaster, Oct 03 2010

       Well, obviously, because you could fit an infinite number of them in a very small area.
mouseposture, Oct 03 2010

       Well... you could do that with a line if you curled it up tight enough.
FlyingToaster, Oct 03 2010

       //can anyone point out any benefit of this system over a single-point plant and a conventional pipeline?//   

       You mean apart from the ones FT mentions in a post above yours?   

       * Coastal weather may be cloudier
* Coastal real estate is often very expensive

       Well, off the top of my head:   

       * Coastal land may not be suitable for other technological, political or environmental reasons.
* Land at the coast is necessarily above sealevel. The hinterlands may not be, in which case less pumping could be required. If you can move water passively in-land, you then only need to return the 20% rejected water and precipitates. Yes, you may need to tunnel to arrange this, but that's a one-time cost.
* It's cool
Loris, Oct 03 2010

       you can locate a solar desalination plant offshore loris. it's actually easier. Weather at the coast is likely to be be similar inland until you reach a mountain range so not seeing the benefit there.
WcW, Oct 03 2010

       Use the salt you create from this project as a massive heat store, for generating electricity at night, effectively using the solar heat the system traps during the day. Salt is a good way to store heat and use solar energy. [link]   

       This would involve running some of your concentrated brine mixture through reinforced black PVC piping in the last stages of the pipeline during the day. The benefit of that would be very fast precipitation of the halite component, as the heated solution would evaporate quickly.   

       Then storing the heat you harvest from that in the salt stockpiles is a matter of heat transfer from the heavily concentrated brine as it feeds to its final precipitation ponds.
infidel, Oct 03 2010

       The pipe need not be complicated. Just use a large black pipe, that is half full of liquid and instead of pumping water, pump air... fast. This will aid evaporation and cause an inland current, assuming it's flat-ish.   

       At night, let the system stop, then start again the next day.   

       You would need a second pipe running back to the coast. This could be the same, but with a gentler breeze blowing inland.
marklar, Oct 03 2010

       Toaster, could you please summarize the discussion?   

       Is it feasable?   

       BTW, as far as I recall, Shimon Peres' idea of the "Red Sea Dead Sea" canal was very similar, except instead of an aquaduct and using solar power, to use the potential energy in a hydro-electric generator (similar to the Naharaim station demolished in 1948 by the Iraqi's) in order to desalinate the water using one of the two conventional desalination technologies. See link.
pashute, Dec 27 2010

       //summarize the discussion// umm...   

       The idea came through relatively unscathed:   

       Bunsen and bigsleep both proposed that any uphill runs (ie: pumped) be done as pipes instead of aquaducts which, while mitigating any transfer between the seawater pipe and the freshwater pipe for the duration would ease pumping complications, and any evaporation within the seawater pipe that occurred enupwardsroute would simply shoot into the construction when it resumed being a gravity-operated troughs thingy.   

       Everybody hated the idea of using clear plexiglass at any point in the evaporation>condensation process, so at this point I'm going with connected covered troughs rather than nested troughs.   

       //Is it feasable?// eh ?   

       I'm not sure I see any parallels with the DeadSeaRedSea pipe apart from both using the pipe itself for power (in different ways) for various purposes.
FlyingToaster, Dec 27 2010

       [WcW] While the entire coastline is great for supplying seawater, the weather will vary from place to place. But weather at the far end we can assume to be maximally sunny, simply for the reason that if it was cloudy we'd be getting rain and therefore wouldn't need to (re)stock freshwater.   

       ... and before anybody says "so build the desal plant there", that would be more construction. This idea takes advantage, not only of improved insolation, but of a pipe trace that has to be there anyways.   

       [marklar] musta missed your anno first time around... I know evaporation is pretty important, but I have no clue on the calculations... but why would we need a second line going back to the coast ? we already have one in the form of a brine-return pipe and there's no reason to evaporate that
FlyingToaster, Dec 27 2010

       Alright, I was going to post this as a separate idea, but this one seems so closely related to it that I'll anno instead. (Pending feedback from others - is it different enough to warrant its own post?)   

       Cover an area of coastal water with a floating greenhouse, to encourage evaporation. The floating greenhouse is linked by a greenhouse tunnel (ie, a long skinny half-tube-shaped greenhouse) to a pond on land, at a slightly higher elevation, also covered. The greenhouse tunnel continues on past this pond, to another pond at a slightly higher elevation than the previous, and so on and so on, to the geographic distance and elevation required.   

       Steam-enriched heated air from the floating greenhouse flows up into the tunnel. Where and when it condenses, it flows downhill by channels to the previous receiving pond on the path.   

       The base of each pond is black - perhaps covered in crushed bluestone gravel or a concrete made of same. Thus, as each pond fills with condensate, mostly at night, when the sun comes up, it re-heats the water, and warm, steam-enriched air flows on and up to the next pond.   

       When your path crosses a local height maximum, you can use downhill flow to the local height minimum along your chosen path, and then begin again to get over the next one.   

       Desalination becomes a non-problem, because the floating greenhouse is distilling pure water for you. Flushing by tides and currents should take care of the slight elevation in salinity beneath it.   


       If your sea level first stage greenhouse is not floating, but is fixed to the sea bed, you can use the tides as an air pressure pump. The bottom is open to the sea - as the tide comes in, it compresses the air trapped above it, thus forcing it into the tunnel. At high tide, you open hatches to allow air to flow back into the greenhouse as the tide recedes, so you're not sucking air back out of the tunnel.   

       This would probably also assist with making this system work better across dead level land, where you can't depend on heat-driven convection to lift your wet air through the system. [*]   

       Scale and spacing of components is left as an exercise for the reader.
BunsenHoneydew, Jan 28 2020

       [*] And right there is the major flaw in the first version of my idea.   

       Hot air will only rise when it is being displaced from below by denser, cooler air. I'm not sure the passive version can work at all, if it's closed to the outside world.
BunsenHoneydew, Jan 28 2020

       ^ If that's the case, three U's, the top one upside down with cooling fins below the middle one.
wjt, Jan 29 2020

       You've lost me there, [wjt]. Can you describe how your three "U"s work?
BunsenHoneydew, Feb 02 2020

       Just like the Three Seashells, [Bun] ...   

       Please, do try to keep up.
8th of 7, Feb 02 2020

       I'll spin mine off into another post, with credit. [link]
BunsenHoneydew, Feb 02 2020

       [BunsenHoneydew] Same as idea but clear pipe is a C on its side, opening face down, and black collecting tray spans dew fallout from C. tips of C could have vertical cooling cuts as they are below the lip of the internal saltwater tray. Frames would be needed to hold orientations. Cooler air can freely flow between all U channels and move hot pure water laden air against the clear covering channel.   

       This is going to lose some water but at least it is adding to inland weather.
wjt, Feb 04 2020

       Note: Your evaporation rate will be substantially higher if you:   

       1. Increase airflow across the surface of the water, to increase the interaction of warmer, drier air withthe water surface, to increase evaporation rates. If you don't then you will develop a sort of laminar atmospheric thermocline in a closed system, reducing evaporation rates marginally. (See [marklar]'s suggestion, above)   

       2. Actively reduce the relative humidity of the air in the system, by deliberately condensing the water out of the air as quickly as possible.   

       In broad terms, a cubic metre of air weighs about 1.3KG and will hold 1.175g or 1.175ml of water as humidity for every degree Celsius you raise the temperature of the air. (i.e. a cu.m. of air at 10degC will hold 11.75g of moisture at 100% RH, while the same cu.m. of air at 30degC will hold 35.25g @ 100% RH)   

       Ideally, you would fan-force warmed air, at temperatures up to about 60degC (feasible with black plastic components and full sunlight exposure) across the surface of the moving water, to maximise your evaporation > condensation > desalination cycle rate.   

       Beyond a certain max temp, I think you might run into pressure containment problems.
UnaBubba, Feb 12 2020

       // collecting tray //   

       The walls of your side-C are going to be the coolest spot in the system, because they are connected to and cooled by the outside air. Most of your condensation will happen there and run down the sides. An internal gutter each side is probably sufficient. That leaves the middle clear for your brine return gutter.   

       Related to this, consider an inverted V profile thus - /\ - for your greenhouse, rather than side-C. All condensates should then run down the sides rather than some dripping in the centre.
BunsenHoneydew, Feb 13 2020

       [BunsenHoneydew] I don't think I explained myself as usual , see link. There are air gaps between cooling fins and white collecting tray. Haven't put in a concentrated brine removal gutter. That would be worried about at destination.
wjt, Feb 16 2020

       // I don't think I explained myself as usual //   

8th of 7, Feb 16 2020

       Nicely true [8th] but you can't disagree that the brightest are the quickest to fully understand.
wjt, Feb 17 2020


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