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.
- 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:
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.