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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.
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
https://www.aquatec...rs-low-water-price/ 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
http://photos1.blog...urvival-still.0.gif 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
http://news.mit.edu...r-desalination-0207 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]
[link]
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Would you care to hazard any numbers? |
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Certainly, but not today. |
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[Bun], you're channeling Aragorn again, stop it.... |
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<Notes with interest that spellchecker recognises "Aragorn" as a valid word/> |
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//Tidally Pumped Moist Air Duct// Is this not a euphemism? |
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No, a euphemism is a brass musical instrument, a bit like a small tuba. This is obviously similar but more like a glass flute. |
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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. |
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What, like "Arsenal 3, Leicester City 2 ... Sheffield Wednesday 1, West Bromwich Albion 2 ..." ? |
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If you write to the BBC they might let you have a go. |
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Those are certainly numbers. |
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Well considered, well written, and interesting. You may have
all the buns I'm authorized to give. |
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This isn't a thing? This should be a thing. |
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What's missing is numbers. It will move a pitifully small
amount of water relative to the cost of construction. |
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You make it sound like Prince Harry's prostate ... |
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Wasn't that a music hall song? |
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//What's missing is numbers// |
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So ... Sheffield Wednesday need more shots on target? I think
that's where we were going with this. |
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That, and we need to quantify pity. |
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Well, if it's anything like mercy, you need some sort of rain gauge ; it droppeth as the gentle rain from heaven ... |
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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. |
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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. |
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Wikipedia says: "Coastal tidal ranges vary globally and can differ anywhere from near zero to over 16 metres"
So 16 m it is. |
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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. |
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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. |
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More efficient strategies may exist. |
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<Collective tut-tutting and head shaking/> |
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And now, some disagreeable facts. |
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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*. |
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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. |
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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. |
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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. |
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You will be asked questions later. You will be expected to justify your answers. |
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*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). |
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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. |
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We'll have that inland sea filled in no time, provided we don't stop
for tea. |
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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. |
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//<Collective tut-tutting and head shaking/>// |
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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. |
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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? |
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//So, go away and recalculated ...//(sic) |
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Those who can, do; those who can't, talk about Prince Harry's
prostate. |
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It's a fascinating subject and worthy of discussion, for reasons that will become obvious. |
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Now now children. Don't make me turn this dirigible
around. |
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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. |
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I have a vague recollection that at least one was built,
but don't quote me on that, pending research. |
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Meanwhile, [link] is a proposed floating desal dome
that uses surrounding mirrors to concentrate solar heat. |
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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. |
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Course, you don't then get the lovely highly productive
linear aquaculture greenhouse. |
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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. |
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Given my known previous proclivities for
geoengineering megaprojects, may I propose Spencer
Gulf, South Australia, as the reference location for tide
and insolation numbers. |
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Or, of course, any of the Mediterranean coasts. |
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//any of the Mediterranean coasts.// |
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Except that the med is practically non-tidal. |
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I wonder if this idea could be blended with those oscillating
heat pumps I recently found out about? |
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// the med is practically non-tidal // |
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Is it? Hmm. Red Sea perhaps then? Lots of sun there,
and parched populations either side. |
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// oscillating heat pumps // |
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Perhaps we should construct a greenhouse roof over the
Suez Canal. |
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// So a square metre might yield a teaspoon [per day]
// |
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Or nearly six litres per hour [link] |
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