MOF behaves like a sponge, but wouldn't feel like a sponge. Squeezing it would be nice, and could definitely eliminate the hassle of having to heat it up.

As for the energy, the thermodynamics of dehumidification basically requires an external energy source. To cool the air, you have to have a heat engine which removes the active ambient thermal energy out of a system. Such a system would look like a traditional dehumidifier hooked up to solar panels. The issue with that is the associated capital expenditure costs to build up such a system, as that already costs significantly more than "some random metal sponge" (assuming we could make it at scale).

For now, the only ways to cool the air down would be to use traditional refrigeration techniques, or peltier coolers. Peltier coolers are super inefficient, and traditional heat pumps require alot of energy. When in a low humidity environment, the coefficient of performance for heat pumps goes way down because the outdoor temperature could be very high, and the humidity very low. To reduce the air temperature to below dew point would mean cooling the air to near 0c, which is pretty much putting a freezer in a desert.

Solar energy is free, but absorbing it and converting it into useful work takes a good bit of engineering effort to make happen. What MOFs and similar materials can take advantage is being able to be left out in the sun like a sun dried tomato and covered in a black painted cover. Couldn't be simpler!

Haha yes salt pools are fun, but wouldn't always be the right kinds of salts required to create batteries. Unfortunately real chemical processes require very high purity raw ingredients, and using the reject water from an desalination plant probably wouldn't cut it. Although if someone figured out how to make a battery out of that, that could have big potential! You'd get all the water and energy storage you would need.

You're absolutely correct that there has to be water in the air. However part of the trick to these panels is that they're not steady state. They have a day cycle and a night cycle. During the night is where they do most of the work of absorbing the water from the air. Over a number of cycles I have overseen, the humidity in the air rises dramatically during the night. This helps these panels in terms of air extraction, since they work on a humidity basis, rather than a total-air-water-content. Think dilution or osmosis when it comes to the actual absorbtion mechanism.

When you do the math, it also doesn't really seem like there's alot of water in the air. Only something like 10-40 grams of water, especially depending on the outdoor temperature. We ran indoor tests with a panel a few sqm in size, and even in a small indoor warehouse, it was not able to dehumidify the warehouse to any significant levels. Maybe at most 5% humidity delta. However air is not static, and wind is always blowing, even when it seems really weak. There's a huge amount of atmosphere above the ground, and unless the panels can absorb the water from the clouds too, the localized de-humidification that happens isn't going to be significant. It's like trying to suck up all water on a beach. The waves are going to replace it shortly enough.

So the one practical limit of these panels that is most frequently missed is the solar aspect. The MOF materials are like a sponge. You can absorb all the water in the air, but you still need to take the water of the MOF. The limit depends on the sensible and latent heat of the water, while in the sponge. MOF doesn't actually really change the boiling point of water at all, so you're really essentially creating a water distillation tower. In 1sqm of land, the most irradiance you're going to get is about 1kw/sqm. 1kwh can boil about 10 liters of water. Taking that into account, over a 8 hour solar day. That means at most a single square meter of solar panel could generate 80 liters of water per day. It's alot, but considering solar losses, glass loss, and thermal loss, more practical limits would probably be like 40 liters. The MOF material also required sensible heat as well, so already a huge portion of incoming solar energy is gone to heating the environment and raising temperature.

In all, you'd have to cover a huge amount of acres before this would dent the atmosphere in terms of humidity. The 1000 liters a day can really only happen when you have a large solar collection area, plus absorbtion surface area to back it up.

Technically yes! To put it in perspective, there's about 2.5kg of water in the atmosphere per m^2 of earth surface area. If you put enough panels across the earth, you could probably do a decent job at taking some of the water out of the air.

We have to look at another factor affecting the water in the air. As we take water out of the air, it's not really a finite resource. Most water in the air generally comes from the sun evaporating the oceans. If we take the water out of the air, the sun will put the water back. There's always a balance of humidity and quantity evaporated. When the humidity is lower, the sun would have an easier time evaporating more water due to the osmosis of the water from the source (ocean) going into the air. Osmosis is a kind of log graph, so even if the humidity is lower, the exponential tail means the solar evaporation and humidity pretty much balances out at the end of the day.

It's similar thing to taking water from a river. If we take all the water from a river, can we dry up downstream? Yes! But considering the height of the atmosphere, it's like standing at the edge of the river trying with a bucket and trying to scoop everything up. Unless these water-from-air harvesters can reach all the way to the clouds, we probably won't dry anything up.

https://www.youtube.com/watch?v=EGTRX6pZSns