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Negative electricity prices registered in nearly all European energy markets
(www.pv-magazine.com)
this post was submitted on 14 Apr 2024
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The point is that even with 100% efficiency (as in extracting 100% of the water the air holds) you couldn't get enough water out of the air because the dry air simply doesn't hold that much water.
At, say 20°C, 100% humid air only holds about 17l of water per m³ or in other words, if you are in a dry climate where the air has maybe 30% relative humidity even a device extracting 100% of the humidity from the air would yield only maybe 5l per m³ of air it processes. You would literally need to pull in 200 m³ of fresh air (as in air that has not been through your device before) to get a single m³ of water and any practical device operating at less than 100% efficiency would yield lower results than that. Also, forgot using this at much colder temperatures than that since that drastically lowers the water carrying capacity of air.
That’s 17g or 17cc per cubic metre, not 17 litres!
In dry countries the relative humidity can be low - sub 40% is common. That said, the absolute water carrying capacity increases rapidly with temperature, so at 40C, the saturation mass is more like 50g/m3 and at 40%RH it still carries more water than 100%RH air at 20C.
You are correct, I got the wrong numbers. But with your numbers of course it makes even less sense.
You are right that you need to dehumidify a large volume of air (like, more than 150 cubic metres) to get enough water for a person, in a rationing scenario. That isn’t necessarily a huge problem; fans can move large volumes of air quite efficiently and dehumidifiers work more efficiently at higher temperatures, but your main point that it isn’t scalable on a population level is definitely true - it’s something you can do for off grid or survival situations, but not easily for domestic water supply.