Science: Terraforming: Water
Tidally Pumped Moist Air Duct   (+4)  [vote for, against]
Build stepped ponds and pour watery air over them.

A passive system for purifying water and transporting it long distances, should you (for instance) have a large inland dry lake to fill, or wish to produce food on an otherwise rain- deprived coastline.

Inspired by and spun off from [FlyingToaster]'s Solar Desalination Aquaduct [link]

Cover an area of seawater by the shore with a greenhouse, to trap solar heat, and encourage evaporation. Construct it on footings fixed to the sea bed. The bottom is open to the sea, but the openings are permanently submerged below the low tide mark. The roof is above high tide, so trapping a variable volume of air between it and the sea's surface.

As the tide comes in, the sea level inside your aquadome rises and compresses the air above it, forcing that air - laden with moisture evaporated from the sea - into a duct leading onto the land. At the high turn of the tide, hatches open [*] to allow ambient air to flow back into the aquadome, rather than drawing it back out of the duct.

Moisture-enriched heated air from the sea is forced by air pressure from the sea-level greenhouse up into the duct. Over a complete tide cycle, the air is pushed further up the line half of the time, and is still the rest of the time.

The duct is a long skinny half-tube-shaped greenhouse, which leads to a receiving pond on land, at a slightly higher elevation, also enclosed. The greenhouse roofing covers this pond and continues on past it, to another pond at a slightly higher elevation than the previous, and so on and so on, to the geographic distance and elevation required.

Where and when the moisture condenses, it flows downhill to the previous receiving pond on the path. This arrangement acts as a one-way ratchet - condensed water can only flow back to the previous pond in the line, and no further.

The base of each pond is black - perhaps covered in crushed bluestone gravel or a concrete made of same. When the sun comes up, it re-heats the water, and warm, steam-enriched air flows on and up to the next pond.

Where your path crosses a ridgeline, water is allowed to flow downhill from there to a receiving pond at the bottom of the next valley, and then the stepped pond / greenhouse duct repeats from there to get up and over the next elevation.

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

For a financial return, the entire length of the greenhouse / pond system can be filled with high-value crops and freshwater fish. The water taken up by crops will mostly be transpired again, and continue on its way, with a slight delay. Salt-tolerant species should be selected for growing at the beach end, as salt spray will no doubt penetrate at least a short distance into the system.

Even the area of sea covered at the start of the system could be utilised for intensive aquaculture - growing kelp, for instance, or saltwater fish farming.

Scale and spacing of components is largely dependent on site and purpose. As the air pressure applied by the tide should be fairly low - a stiff breeze - I imagine that the greenhouse roofing should not need to be much more robust than your standard issue polytube frame with plastic greenhouse sheeting, sealed and anchored at the edges with dirt.

[*] Bar the opening and closing of these hatches, the system has no moving parts. Even they could be simple flaps which open and close in response to air pressure, thus requiring no external power source, and minimal maintenance.

If the seawater inlets are partly above the low tide mark, such that air can flow into the system once the water has dropped, we may not even need those flaps. Some efficiency in pumping will be lost, as air will be drawn back out of the tunnel for part of the cycle, but the gain in simplicity and durability may be worth it.
-- BunsenHoneydew, Feb 02 2020

Solar Desalination Aquaduct Solar_20Desalination_20Aquaduct
From the mind of [FlyingToaster] [BunsenHoneydew, Feb 02 2020]

The Australian Inland Sea The_20Australian_20sea
On dry lakes and the filling thereof. [BunsenHoneydew, Feb 02 2020]

Australian Inland Sea II Australian_20Inland_20Sea_20II
Ditto, with lots of numbers. [BunsenHoneydew, Feb 02 2020]

Solar Desalination Dome
Proposed, some maths, some disputes. If the link takes you to the front page rather than the article, click on [News] -> [Desalination] [BunsenHoneydew, Feb 13 2020]

Solar Still
Diagram of a simple small scale floating solar still for survival situations. Illustrates the proposed dome and gutter structure. [BunsenHoneydew, Feb 13 2020]

Seawater greenhouse https://en.wikipedi...Seawater_greenhouse
Open at both ends, utilises natural sea breezes. Seawater is first used to humidify and cool the air, then solar distilled to produce fresh water. Finally, humidified air is expelled to improve growing conditions for outdoor plants. [BunsenHoneydew, Feb 13 2020]

MIT passive solar desal
Cheap, simple, 5.7 litres / hr / sq m [BunsenHoneydew, Feb 13 2020]

Oscillating heat pipe https://www.electro...lsating-heat-pipes/
[bs0u0155, Feb 13 2020]

Would you care to hazard any numbers?
-- pertinax, Feb 02 2020

Certainly, but not today.
-- BunsenHoneydew, Feb 02 2020

[Bun], you're channeling Aragorn again, stop it....

<Notes with interest that spellchecker recognises "Aragorn" as a valid word/>
-- 8th of 7, Feb 02 2020

//Tidally Pumped Moist Air Duct// Is this not a euphemism?
-- MaxwellBuchanan, Feb 02 2020

No, a euphemism is a brass musical instrument, a bit like a small tuba. This is obviously similar but more like a glass flute.
-- 8th of 7, Feb 02 2020

With those flaps opening and closing under air pressure, I'm hearing the music of the spheres, but with cylinders. Still, I'd like to read the score.
-- pertinax, Feb 03 2020

What, like "Arsenal 3, Leicester City 2 ... Sheffield Wednesday 1, West Bromwich Albion 2 ..." ?

If you write to the BBC they might let you have a go.
-- 8th of 7, Feb 03 2020

Those are certainly numbers.
-- BunsenHoneydew, Feb 04 2020

Well considered, well written, and interesting. You may have all the buns I'm authorized to give.
-- Voice, Feb 04 2020

This isn't a thing?
This should be a thing.
-- 2 fries shy of a happy meal, Feb 05 2020

What's missing is numbers. It will move a pitifully small amount of water relative to the cost of construction.
-- MaxwellBuchanan, Feb 05 2020

You make it sound like Prince Harry's prostate ...
-- 8th of 7, Feb 05 2020

Wasn't that a music hall song?
-- MaxwellBuchanan, Feb 05 2020

//What's missing is numbers//

So ... Sheffield Wednesday need more shots on target? I think that's where we were going with this.

That, and we need to quantify pity.
-- pertinax, Feb 06 2020

Well, if it's anything like mercy, you need some sort of rain gauge ; it droppeth as the gentle rain from heaven ...
-- 8th of 7, Feb 06 2020

Interesting as the football results are, I'm not sure they are critical to understanding the TPMAC concept.
I will attempt a calculation of the maximum yield, disregarding all losses and attempting to pick a generous value from each relevant range.

It is obvious that the amount of air pushed through the system is primarily dependent on the size of the coastal acquisition chamber and the intertidal distance. Thermal expansion may contribute to air movement, but for the sake of this calculation I will disregard it.

Wikipedia says: "Coastal tidal ranges vary globally and can differ anywhere from near zero to over 16 metres"
So 16 m it is.

Also let us assume that the air in the system becomes maximally laden with water - this is called the dew point.
Again, wikipedia helps us with a handy graph from which I estimate that at 30 degrees C, about 3% of the air can be water, by mass. Furthermore, a random page on the internet claims that the density of air at 30 degrees is 1.165 kg/m^3.
So the amount of water is about 3.5 grams/m^3.

Therefore, given two tides a day you could shunt at most about 112 grams of water per square metre of enclosed coastal real-estate, using this air-carrier method.
Usually this maximum will not be achieved, since (quite apart from needing optimal conditions), some processes are assumed to occur here - in particular, the water must have been evaporated to create moisture-laden air at the start of the stroke cycle.

More efficient strategies may exist.
-- Loris, Feb 06 2020

// So 16 m it is. //

<Collective tut-tutting and head shaking/>

And now, some disagreeable facts.

The extreme tidal range is of the coast of Newfoundland - the Bay of Fundy - in a region which does not appear high on the list of "hot, sunny places" for your planet - if, that is, it appears at all. Also, that range refers to the extreme range at the spring tides - the average tide is only half that*.

For the system to work, a large amount of solar input is needed, combined with a reasonable tidal range. You need to look for coastline in the tropics, with very limited cloud cover. It's going to be a tradeoff.

Initial assessment suggests that the periphery of the Indian or Pacific ocean will meet your needs but a tidal range of 2 to 3 metres is the most you can expect.

So, go away and recalculated on a 12-hour day length, 25% average solar obscuration, a 2m tidal range averaged over a lunar month, and an appropriate solar gain for low latitudes. Then come back and present your results to the class.

You will be asked questions later. You will be expected to justify your answers.

*For further information and the relevant data tables, refer to "Operation Neptune planning", "Ovelord", and "Mulberry harbour" as well as the minutes of the "Extemporised Harbour Committee" 1943-44 (See also COSSAC).
-- 8th of 7, Feb 06 2020

So a square metre might yield a teaspoon, but a dispersed teaspoon, mostly stuck to the sides. And if you leave it going for a year, you might be looking at a teapot.

We'll have that inland sea filled in no time, provided we don't stop for tea.
-- pertinax, Feb 06 2020

There will be a way to use shapes to keep any moisture within the system from leaving during tidal outflows and inner surface area condensation has yet to be taken into account.
Not that this will make it viable... m'just sayin is all.
-- 2 fries shy of a happy meal, Feb 07 2020

//<Collective tut-tutting and head shaking/>//

Yeah. As I clearly pointed out, it was a theoretical maximum, not an expected yield - in fact I covered every one of the points you raise as an issue.

If you're claiming that the very low yield I calculated is too high, you've missed the point.
Maybe you should drop those advanced condescension classes in favour of remedial comprehension?

//So, go away and recalculated ...//(sic)

Those who can, do; those who can't, talk about Prince Harry's prostate.
-- Loris, Feb 07 2020

It's a fascinating subject and worthy of discussion, for reasons that will become obvious.
-- 8th of 7, Feb 07 2020

Now now children. Don't make me turn this dirigible around.
-- BunsenHoneydew, Feb 13 2020

While I can't find the link just yet [TBA], I recall an Israeli tidal desalination project which used the downstroke of the tide to lower the air pressure inside a sealed-top concrete dome, thus forcing condensation. A gutter around the inside base of the dome collected the distillate for conventional storage and transport elsewhere.

I have a vague recollection that at least one was built, but don't quote me on that, pending research.

Meanwhile, [link] is a proposed floating desal dome that uses surrounding mirrors to concentrate solar heat.
-- BunsenHoneydew, Feb 13 2020

It has occurred to me that the air pressure generated by a tidal dome could also be used as the driving force to pump liquid water through a pipeline - but I'm struggling to work out a mechanism for that which does not rely on moving parts. Some form of ram pump presumably, perhaps with inverted U airlocks to prevent backflow.

Course, you don't then get the lovely highly productive linear aquaculture greenhouse.

Perhaps the simplest solution is to cut out the middleperson entirely and build your aquaculture greenhouses at sea ... but then your inland dry lake filling up is no longer part of the end result. Besides which, that's something I'm sure I've seen proposed elsewhere.
-- BunsenHoneydew, Feb 13 2020

Given my known previous proclivities for geoengineering megaprojects, may I propose Spencer Gulf, South Australia, as the reference location for tide and insolation numbers.

Or, of course, any of the Mediterranean coasts.
-- BunsenHoneydew, Feb 13 2020

//any of the Mediterranean coasts.//

Except that the med is practically non-tidal.

I wonder if this idea could be blended with those oscillating heat pumps I recently found out about?
-- bs0u0155, Feb 13 2020

// the med is practically non-tidal //

Is it? Hmm. Red Sea perhaps then? Lots of sun there, and parched populations either side.

// oscillating heat pumps //

Got link?
-- BunsenHoneydew, Feb 13 2020

Perhaps we should construct a greenhouse roof over the Suez Canal.
-- BunsenHoneydew, Feb 13 2020

// So a square metre might yield a teaspoon [per day] //

Or nearly six litres per hour [link]
-- BunsenHoneydew, Feb 13 2020

random, halfbakery