Tidally Pumped Moist Air Duct

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.