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It seems like it should be possible to have almost entirely passive (minimal moving parts or machinery) solar desalinization that would be ideal for desert countries bordering the ocean. e.g. Northern Africa, Middle East, parts of Australia, etc. I finally settled on a rough sketch of how I think this could possibly work. I can think of no better community to help determine if this is feasible, and if so, how much fresh water would it produce?

To be up-front, I was a great physics & chemistry student in college, but in my job as a software engineer, my physics skills have atrophied for 15+ years. Bear with me please.

Imagine a large, very gently sloped concrete slab, oriented North-South, built in a desert climate. In order to give us hard numbers to work with, we'll say it's 1000m long, north to south, and 400m wide, east to west, with short sidewalls. The slab is covered in a black/dark material that would absorb sunlight and heat up water over it, such as simple black plastic, possibly laid over a cheap insulator like EPS (styrofoam) sheets. A simple canopy over it is glass or plastic and serves to allow light in and heat the space. I believe it's important that the structure be fairly well-sealed to prevent escaping heat & steam.

On the higher end of the slab, a drip or spray system slowly pumps in raw seawater. This is one of only three simple mechanized systems. The slope of the slab causes the water to run toward the other end, heating more as it flows downhill, eventually evaporating / turning to steam.

Deep below the slab is an underground cavern. Possibly the cheapest way to construct this would be with a tunnel boring machine of the most economical size (e.g. The Boring Company standardized on ~12 ft / 4m tunnels for economy). The tunnel should be deep enough that it is consistently a cool, stable temperature. Typical tunnels of this type are built water-tight, which is ideal here.

Along the slab at regular intervals, metal pipes would protrude well above the surface of the slab, but below the canopy, and extend down through the roof of the tunnel below. The metal surface of the pipe would encourage dispersing heat through the soil, cooling the air/water, and causing condensation. The concrete walls of the tunnel below would also help cool the air/water further. As the water heats during the day, air pressure would increase over the slab and push air down the pipes; the cooling process would "pull" the air down, creating a runaway effect.

A pump station would be necessary to pump the fresh water above ground from the tunnel/cavern. And at night, the cooling process above-ground may cause the cold, wet air to get sucked up the pipes. Likely a very simple flapper functioning as a one-way valve on the bottom of the pipes would be needed, and to avoid damaging the canopy with negative pressure, one-way valves letting air in at night.

Salt would accumulate on the black slab, increasing the albedo. To fix this, in the early morning hours just before the sun rises, a deluge of seawater would "wash" the salt off the surface from the north to the south, where drains would open to either the ocean or a nearby salt collection pond.

That's actually all there is to it. Seawater slowly sprays into a massive, otherwise sealed space. The seawater heats under the blaring desert sun, reaching sufficient vapor pressure to push down the metal tubes. As the humid air pushes down the metal tubes, the temperature drops causing condensation and also "pulling" the air down with it, creating consistent downward pull of that hot, steamy air. The clean water accumulates in a cavern below and is pumped out. It requires almost no moving parts or power to operate, and it is not particularly expensive to build. In fact, there's no reason to build a concrete slab - simply grading and compacting sand should form a sufficient base for the center of the chamber.

Would this work? I can't be the only person to have thought of this, so I wonder what I'm overlooking.

Obvious concerns/questions:

  • What is the correct combination of materials to cover the base and build the canopy out of to cause the chamber to heat the water as fast as possible? I assume other industries have had need of a similar design, so what is needed is likely well-established. If so, how much solar energy would be absorbed per sq meter?
  • Given the answer to that, what would the heat loss be through the canopy? I estimate this to be the single biggest loss of efficiency in the system, and, therefore, is perhaps crucial to estimating output.
  • Can the chamber be sealed well enough that positive air pressure is generated, forcing the hot air down the tubes, starting the chain reaction of "pulling" it down?
  • Will the heat overwhelm the chamber's natural cooling and stall the process?
  • How rapidly will the albedo of the chamber increase due to salt deposits? This could be a huge issue or a non-issue - it's a big unknown.
  • How many and how large do the pipes need to be?
  • Due to the breakdown of materials in the harsh desert sun, how often would the materials specified in the first bullet need to be replaced?

Finally, here is a simple diagram to help visualize the concept:

Diagram

To sum it up, how much fresh water would such a system produce?

Here are some useful facts I accumulated:

  • According to Wikipedia, the daily mean horizontal irradiance in some of the places most in need of more fresh water reaches over 6.4 kWh/m2 of insolation. In SI Units: times 3.6 MJ/kWh = 23 MJ/m2/day
  • Assuming the seawater is 25°C, it takes 2.57 MJ/l to fully boil water to vapor, but those numbers are using fresh water, not seawater, and I cannot find a conversion for that.
  • According to the Wikipedia page for solar water heaters, they vary from 81% to 64% efficiency typically. The higher end there involves insulating the heater with a vacuum behind glass, so that would likely be too costly for this project - much lower efficiencies of converting solar insolation to useful heat would be expected, I think.
  • In any case, at 64% efficiency, on an average day it might achieve 23 MJ/m2/day / 2.57 MJ/l * 0.64 efficiency = 5.7 l/m2/day, not accounting for important inefficiencies mentioned above.

EDIT: If by some small chance this idea is useful and feasible, I have no desire to be the one to take it to fruition. I give the idea freely and hope someone with more vested interest would run with it. I have no interest in making money with this.

EDIT2: Obviously this is a commercial scale solar still at its root. I cannot find any evidence that those have been attempted. Also, with the single exception of a diagram on Wikipedia from the Solar Still page, I cannot find any examples where the collection basin was deliberately underground to increase condensation. A typical still has the water collecting on the canopy, where it would remain hot in direct sunlight. In a desert climate where the ambient temp might be well over 40C, this is a rather inefficient design. Moving the hot air underground for condensing, and perhaps insulating the canopy to some level, might make this vastly more efficient than typical Reverse Osmosis desalalination. Perhaps.

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  • $\begingroup$ all you've done is upsized a solar still. Generally, these things are best described as sure it would work at some level, but if it paid out someone would already be doing it. But you aren't the first to think this way. Google large scale solar still. $\endgroup$
    – Tiger Guy
    Jul 4, 2022 at 3:30
  • $\begingroup$ @TigerGuy Thanks. In my experience the notion "if it could be done, someone would be doing it" is a fallacy. I'd like the discussion or answer to be on the scientific merits. And yes, it absolutely is a large scale solar still. I am not aware of any that have been built nor have I found answers on why it wouldn't be cost-effective. $\endgroup$
    – pbristow
    Jul 4, 2022 at 4:08
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    $\begingroup$ @pbarranis, the google search shows that it has been considered. Certainly no one here is going to price out structures for building this thing out, nor are they going to go design one to give you numbers. The tech is simple, it's building it that's hard. $\endgroup$
    – Tiger Guy
    Jul 4, 2022 at 18:37
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    $\begingroup$ There is an assumption here that a chamber 'at sufficient depth' will be cool and constant. That assumption is false. Depth is warmer, and you need a method of cooling, Some systems use cool night air: other systems use evaporative cooling, which is the opposite of what you have proposed. $\endgroup$
    – david
    Jul 9, 2022 at 3:23
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    $\begingroup$ One thing you haven't considered is how would salt be removed from the your device & what would be done with it. Salt water goes in, it gets heated, water evaporates & cools elsewhere & salt gets deposited in the chamber. Salt will accumulate & will eventually have to be removed or it will clog up the system. How do you plan on removing the salt? $\endgroup$
    – Fred
    Apr 1, 2023 at 0:37

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A better overall system can be achieved by lowering the pressure in the evaporation chamber. This way, captured heat isn't lost to the environment because the whole thing runs at ambient temps or thereabouts. This is how desalinators work on ships using very low quality heat recaptured from engine cooling water. Systems that work off the temperture gradient between surface seawater and deep seawater exist in India. Systems that work off reactor cooling water also exist.

So capture the heat with a conventional closed-loop solar collector system, or harvest waste heat from a powerplant. Use a heat exchanger in the low pressure evaporator tanks, which can be quite small. Cascade several evaporator tanks at decreasing pressures to use the solar/reclaim heat loop more effectively. Use a final air-to-water or water-to-water condenser to condense the potable vapor from the vacuum pumps. You want the tanks at close to ambient conditions during the evaporation process.

These systems are both very simple and cost effective from a land use and capital expense standpoint.

https://en.wikipedia.org/wiki/Low-temperature_thermal_desalination

And this is why it doesn't make sense to use a huge concrete pan -

https://www.fluencecorp.com/concrete-industry-water-use/

You really need to do a cost study. It wouldn't surprise me to find that it would be cheaper to fly fresh water in by aircraft than do what you are suggesting.

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  • $\begingroup$ Thanks - very helpful. I mentioned towards the end of the question "there's no reason to build a concrete slab - simply grading and compacting sand should form a sufficient base for the center of the chamber". It could also be built on the roofs of large warehouses. The example size I gave of 1km x .4 km is roughly the size of Tesla Giga Texas. $\endgroup$
    – pbristow
    Jul 5, 2022 at 0:03

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