[bdevine]

Ok, I have a dumb question about this sort of technology. Aren't these devices essentially just borrowing from tomorrow by capturing moisture that would have eventually collected as clouds? Are there any long-term ramifications to using these?

[cornholio]

Clouds are quite cheap. Most of them drop most of their water over areas that would not significantly suffer from a marginal reduction of rainfall (the oceans, the arctics, mountain ranges).

And a dryer average atmosphere should be partially compensated by increased evaporation from the oceans. Overall, this technology should be applicable on a continental scale before it shows any measurable effect on other areas.

[ethagknight]

A far more accurate response is “we have no idea and no way of knowing there wouldn’t be major negative ramifications but it would seem like...”

My biggest gripe with climate science in popular media is the presentation of sparse data and unproven models as fact that should be take at face value. Milking clouds in one region will have an impact elsewhere, that’s the only guarantee.

[thaumasiotes]

> areas that would not significantly suffer from a marginal reduction of rainfall (the oceans, the arctics, mountain ranges)

Agreed as to the ocean and the arctic, but precipitation over mountain ranges is generally recovered by lower-altitude communities when the water eventually flows down to them.

Snowmelt has been a major source of water historically.

[eloff]

Also the artics are essentially cold deserts, they don't get much precipitation and there is likely very little that could trace its origin to the hot deserts of the world.

[Cogito]

That's not really true.

Parts of the arctics are definitely deserts, but that is not true for all of the arctic regions.

Looking at the relevant parts of wikipedia [0] and [1], "annual precipitation averaged over the whole planet is about 1000 mm" while "parts of southeast Greenland [recieve] over 1200 mm". Antarctica has a lot less precipitation in general, but there are still large parts of it that are not classified as desert.

[0] https://en.wikipedia.org/wiki/Climate_of_the_Arctic#Precipit...

[1] https://en.wikipedia.org/wiki/Climate_of_Antarctica#Precipit...

[ianai]

Here’s hoping it somehow aids in carbon sequestration tech development...

[jerf]

In some sort of absolute sense, sure, water in the device is not water somewhere else. But with something like 326,000,000,000,000,000,000 gallons of water on Earth, it's well below the noise threshold. Even "dry desert air", if you work it out, has massive (heh, literally) amounts of water in it.

Plus the impact is even less than you think, because air that has been made dry is more able to pick up water, so you have to work even harder than you'd think to have any impact.

[ttul]

I used a moisture calculator to determine that 10% humidity desert air at 30 degrees celsius and 1013.25hPa air pressure contains 3.03g/m³ of water. At that rate, the air above a 100x100m football field (to altitude of 100m) contains 303kg of water, or 303L at 4 degrees celsius.

So you'd be "drying out" a very tiny cube of air to produce the water needed by an entire family per day even given desert conditions with one of these appliances.

In the first world, we use 100L/day for all our needs. That means if everyone in America lived in a desert, we might need 107,491,749,174,917.5 cubic meters of air to supply us with all our water. If that sounds like a lot of air, consider that Death Valley is 7.8e+9 square meters in area; thus, to supply ALL Americans with fresh water, you'd just need to suck Death Valley's air dry to an altitude of 10,000m or so.

[thaumasiotes]

> you'd just need to suck Death Valley's air dry to an altitude of 10,000m or so

It seems likely that, as the atmosphere becomes less dense with increasing altitude, so does atmospheric water. One cubic meter of air at sea level contains a lot more air than one cubic meter of air at 6,000m. Does your figure include that effect?

[ianai]

Anecdote but I routinely evaporate more water into air a mile higher than where I lived previously. It’s also much colder up here.

It all comes down to PV=nRT

If anything this would be much more like making sure the clouds drop their moisture where it’s sorely needed. I say this as someone who has many times seen the needed moisture fly by.

[tylerhou]

If you assume atmospheric water has the same density up to around 3000 feet, then you'd have to dry out 3 Death Valleys. Still pretty reasonable.

[portofcall]

At 3000 feet it’s about .9 atmospheres of pressure, but honestly humidity is incredibly variable. The lower pressure at 3k feet just places an upper bound on possible humidity, it doesn’t tell you what it is. My instinct is that you’re right, and we’re still talking about vast volumes of moist air. We could also “enrich” the air with solar powered evaporation of seawater and wastewater.

[jerf]

For the purposes of this discussion I'd say order-of-magnitude is close enough, and I doubt the number for Death Valley is off by more than two orders at most (taking into account the possibility that Death Valley is just too special to use the normal numbers on). It will take a lot of work to disrupt even a desert ecosystem by extracting water from the air, and we are precluded from even wanting to do that work by the immense energy expense it will involve.

[cwkoss]

Is that all personal water usage, or does that also include industry and agriculture?

[incompatible]

I guess a lot of it ends up back in the atmosphere once humans have used it. Perhaps some is piped in sewerage all the way to the nearest ocean?

[DougN7]

I've had the same thought about solar. The area under the panel used to get sunlight, and if nothing else, warmed up.

Not that it's necessarily bad, but widespread usage would have to have _some_ environmental impact.

[cirgue]

The same thing can be said of agriculture, and there are definitely environmental impacts, but we generally don't worry about the ground cover aspect.

[DougN7]

That is a really great point I had never thought of. Thank you.

[photojosh]

Now the panel warms up instead of the area underneath. What's the difference?

[dreamcompiler]

A small fraction gets turned into electricity. Which ultimately gets turned into...heat. So the net amount of heat reaching earth doesn't change.

[DougN7]

The final heat would most likely be produced in a different location. It would take a very large scale installation to have an impact. I guess I'm thinking of weather patterns when you see those images that say "if we covered 1/4 of Arizona in solar panels we could power the world!".

More of a thought experiment than anything I guess.

[adrianN]

Unless you build solar panels on top of a very low albedo ground, the amount of heat actually increases because less is reflected back to space.

[oh_sigh]

Maybe, but the scale here is gigantic, and it will take a lot of work for humans to make an impact. Consider that a single cumulus cloud can weigh 2 million pounds(that is, 900 cubic meters of liquid water).

If you were going to be worried about things like this, wind farms will probably pose a bigger problem than water collection, unless we start building large cities in the desert relying only on air moisture collection.

[pjc50]

.. in a different place. Damp air is pretty mobile. It also tends to equilibrium over bodies of water.

[natecavanaugh]

Long term ramifications I'm sure of. Long term harmful ramifications? I doubt it. That water has to go somewhere, whether it's captured in the ground or evaporates in the skies, the machinery of nature will keep ticking :)

[rukittenme]

There's no long term ramifications because these devices simply do not work as advertised.

[Zaak]

The MIT team explicitly addresses the limitations of refrigerated dew collection systems and claim that their system does not share those limitations.

In what way would this particular device not work as advertised?

[rukittenme]

It does share those limitations because physical reality requires it to! There is no technological way out. Thermodynamics asks for some amount of energy in exchange for some amount of condensed water. The amount of energy is astronomical compared to the meager amount of water.

You see the giant ass heatsink at the bottom of the image? Why do you think that exists? The water is condensing on a peltier heat pump. In a desert, to get a single bottle of water you're going to have to blow hundreds of thousands of liters of air (at perfect efficiency) over the pump. And that's for a single water bottle! Oh and this is assuming that your peltier device can get cold enough to produce a 100% humidity atmosphere. Because if it doesn't you get no water.

[nkurz]

The water is condensing on a peltier heat pump.

What makes you so confident that they are using a Peltier? Here's their description of the operation of the device:

During adsorption, air is circulated around the MOF layer and water from air is adsorbed. Passive radiative cooling lowers the MOF layer temperature below the ambient by dissipating thermal radiation to the clear cold sky to increase the effective RH for adsorption. During water production, the OTTI aerogel is stacked on top of the MOF layer to suppress convective heat loss from the solar absorber. The desorbed vapour is condensed on a condenser and the heat of condensation is rejected to the ambient by a heat pipe heat sink.

And here's the more complete description of the condenser:

The condenser of the device was fabricated with a copper plate (4 cm by 4 cm and 0.6 cm thick) attached to a commercial air-cooled heat sink (NH-L9x65, Noctua) to efficiently dissipate the heat from condensation to the ambient.

Are you still sure it's condensing on a Peltier cooler? The paper never mentions "Peltier". If the "condenser" is actually an electrically driven Peltier, this would seem like a fraudulently bad description, justifying retraction of the paper. Is it possible that you are wrong?

Still, I agree that you might be right about the physical limitations of scaling. You seem knowledgeable about the field, and I'd be interested to hear your impression after you read the actual paper: https://www.nature.com/articles/s41467-018-03162-7.

(I am genuinely interested in hearing your opinion about what they are doing, but would strongly suggest less overconfidence and more humility when offering bombastic pronouncements on papers you haven't read.)

[rukittenme]

> What makes you so confident that they are using a Peltier?

Because you can buy the exact device on Amazon (Noctua heat sink sold separately)[1]. Compare that image to the images shown in this MIT news article[2].

> The paper never mentions "Peltier". If the "condenser" is actually an electrically driven Peltier, this would seem like a fraudulently bad description, justifying retraction of the paper.

I would agree.

> The condenser of the device was fabricated with a copper plate (4 cm by 4 cm and 0.6 cm thick) attached to a commercial air-cooled heat sink (NH-L9x65, Noctua) to efficiently dissipate the heat from condensation to the ambient.

This describes a Peltier device exactly. Pass a current through it and one side will get hot and one side will get cold. Hence the heat sink. You need someway to dissipate that electrical energy.

Ask yourself, if the system works passively (i.e. ambient temperature), why do they need a heat sink? Would attaching a heat sink to my shed cool it off? I think the answer is clear.

Ignoring everything I said, to condense water from air you need two things. Air with water in it and a temperature differential. How is the differential generated? According to the paper, a condenser. How does every condenser generate a temperature gradient? Electrical current.

1. https://www.amazon.com/TEC1-12706-Thermoelectric-Peltier-Coo... 2. http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro...

[nkurz]

As long as you are clear that you are accusing the authors of outright fraud, I appreciate and applaud your logic!

I think you are wrong, though.

Ask yourself, if the system works passively (i.e. ambient temperature), why do they need a heat sink?

Because as you say, they need a cooler surface than the ambient for the water to condense on. Ambient temperature in this case refers to the temperature inside the solar chamber containing the saturated sorbent and insulated by the translucent aerogel. In the "legit" view, the heatsink is cooled relative to this ambient by being thermally coupled to the cooler outside air. In the paper, Figure 3 shows the temperature differential as about 40C: https://www.nature.com/articles/s41467-018-03162-7/figures/3

Would attaching a heat sink to my shed cool it off?

Well, if the inside of the shed has been heated by the sun so that it is warmer than the outside air, then yes. The heatpipes are effectively windows for heat to escape from the relative hot interior to the relatively cool exterior. Isn't this exactly how a passive heatpipe cooler like the Noctua works when installed as designed to cool a CPU? Some airflow over the radiating fins doesn't hurt, but convection takes care of this if the surface is large enough.

[flukus]

> What makes you so confident that they are using a Peltier?

The article contains a link to an older one and says the device is based on that: http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro... . Notice the heat sink? Notice quotes like:

In fact, the system doesn’t even require sunlight — all it needs is some source of heat, which could even be a wood fire. “There are a lot of places where there is biomass available to burn and where water is scarce,” Rao says.

That sure sounds and looks like a peltier device to me. MIT is gaining a reputation for crackpot stuff like this.

[nkurz]

Yes, there are previous devices that use a Peltier cooler to condense water directly out of the air. The article you link mentions these as a contrast to what this device is doing: "Another method of obtaining water in dry regions is called dew harvesting, in which a surface is chilled so that water will condense on it, as it does on the outside of a cold glass on a hot summer day, but it “is extremely energy intensive” to keep the surface cool, she says, and even then the method may not work at a relative humidity lower than about 50 percent. The new system does not have these limitations."

This approach uses a "sorbent" that adsorbs water at night, and then then uses sunlight to generate heat to drive the water out of the sorbent during the day. It does not use a thermoelectric cooler: 'The new system, by contrast, is “completely passive — all you need is sunlight,” with no need for an outside energy supply and no moving parts.' Rather than requiring active cooling, this approach requires heat to release free the water from the sorbent. The passive heat sink is used to help capture the water after the heat from the sunlight forces it out of the sorbent: "The desorbed vapour is condensed on a condenser and the heat of condensation is rejected to the ambient by a heat pipe heat sink."

So as best as I can tell, your theory that the paper (which you haven't read) is simply lying about what they are doing? Not impossible, but I think it would require some greater level of evidence the "Notice the heat sink". And while we're at it, how does "biomass available to burn" imply that they are using a thermoelectric aka Peltier cooler? https://en.wikipedia.org/wiki/Thermoelectric_effect

(Yes, I agree that it would be nice if the press release would actually link to the paper that correctly describes the apparatus, but its regrettable failure to do so does not give license to make up your own details as to how it works.)

[adrianmonk]

"They have developed a completely passive system that is based on a foam-like material that draws moisture into its pores and is powered entirely by solar heat."

http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro...

[tytytytytytytyt]

> The current version can only operate over a single night-and-day cycle with sunlight, Kim says, but “continous operation is also possible by utilizing abundant low-grade heat sources such as biomass and waste heat.”

Why can it only un for a single day on solar?

[hexane360]

It can operate for multiple days, but it only generates water once a day. At night the foam absorbs moisture, and then sunlight provides energy to release it as water.