"All measurements were performed at 20° ± 0.2°C maintained by an air circulation system unless otherwise noted. The temperature of the films was controlled using a heating/cooling unit (THMS350V, Linkam Scientific Instruments, Salfords, UK) when necessary."
So the latent heat is conducted away by the cooling apparatus, it's just not explicitly stated, to sound more sensational.
This is really moist air that's only barely short of forming dew. A lot of people are focusing on sensational "violation of physics", when it's an incremental improvement on process that happens naturally.
> And no, this is not something permitted by the second law of thermodynamics.
If that's true we just need to balance energy, which the cooler does.
My other comment here (and and a reply to a similar question) has more detail [1], but in short: this is true for capillaries and pores, it is not true for "collectable" droplets on a flat surface.
My understanding of it now is that since it can work at a higher temperature in an environment where the ambient temperature is low enough the latent heat can be passively radiated away. Even if using an active heat pump the higher temperatures would allow for a more efficient process. A closed system would eventually reach an equilibrium but there is no need to maintain a closed system.
Devices like that would be essential during 'wet bulb' days where the temperature and water content of the air created dangerous conditions for people. A passive device that takes no energy and just sucks water out of the air? Could be a lifesaver.
The researchers also noted that the water that was expressed to the surface of the material did not evaporate (as one would expect). There some interesting speculation as to why that is. It wasn't clear whether or not the water would move across the nano-structure if it was affected by gravity (aka dripping) but I can imagine several ways to transport it off the surface so I'm sure the researchers can too.
[1] The description in the paper is that capillary action forces the vapor into the interior of the structure where it collapses into liquid.
This is basic thermodynamics, you can do however much hydrophobic/hydrophilic nanomaterials, but you won't get condensation unless you somehow conduct away the latent heat. This can be done by storing energy in the material itself (that's how desiccants work), or by providing a temperature gradient (a cooler).
> This can be done by storing energy in the material itself (that's how desiccants work)
This is exactly where the energy goes. From the paper (in it's Materials and Methods section) -- All measurements were performed at 20° ± 0.2°C maintained by an air circulation system unless otherwise noted. The temperature of the films was controlled using a heating/cooling unit (THMS350V, Linkam Scientific Instruments, Salfords, UK) when necessary.
So the hypothesis is that the heat in the water vapor goes into the nano-pore material, which in their experiment they were actively maintaining at 20 degrees C. So yes, they are actively removing the heat created by the phase change.
One difference with desiccants is that once they are saturated you have to restore them through heating them up, but this stuff doesn't have that property. And while it may sound like nonsense it was reproduced in another lab[1].
Apparently capillary condensation is a thing, its the popping out of the liquid water that was unexpected.
[1] With a material that could potentially defy the laws of physics on their hands, Lee and Patel sent their design off to a collaborator to see if their results were replicable.
There is no cube. The droplet's are attached strongly to the surface.
If the droplets drop to a cube, you can replace the cube with a cotton mat and let the water evaporate and get a low temperature mat. And then use the difference of temperature to generate electricity https://en.wikipedia.org/wiki/Thermoelectric_generator and turn on a lamp. And now you are breaking the second law of thermodynamics.
Consider a typical unplugged dehumidifier with Calcium Chloride. It generates water that drops to a cube, but it's salt water that evaporates less than fresh water, so you can't do the trick.
If you use silica gel, the water is trapped inside the material, so there is no cube.
With this new material the droplets are on the surface, but they refuse to fall down.
With an AC you get a cube full fresh water, but it obviously work only while plugged, so there is no magic.
> And while it may sound like nonsense it was reproduced in another lab [1].
They reproduced the visible droplets in the surface of the material. In neither lab they had a cube filing process. The sentence you quoted in [1] is very misleading.
You coat a piece of aluminum with nano-pore material and hang it vertically. Air flows over it and droplets appear on its surface (based on the paper). You also hang a frame of vertical wires (unenergized just small diameter wires, kind of like a screen but without the horizontal members) in front of the sheet by 1/2 the droplet's diameter. The wires don't touch the surface, they are suspended 1/2 droplet away.
Now when a droplet forms, it grows and intersects the wire (which is not hydrophobic) Surface tension puts the droplet around the wire and it slides down to the bottom of the wire frame, impacting any other droplets that had formed below it.
The resulting liquid water drops off the bottom of the wire frame into a catch pan below.
If one of these assemblies generates net water production from RH 70% air then an array of then would generate more water.
What am I missing?
1. Have the drops fall on some surface and let them evaporate. This can happen because the relative humidity is below 100%.
2. This surface will get cooled by the evaporation.
3. Now use that temperature gradient to get free energy!
1. Drops fall from the sky
2. They collect and flow down a river
3. We use that river to generate hydroelectric power to get free energy!
Water vapor, in air, has both thermal and potential energy that under the right conditions can be converted into a more useful form. We agree on that yes?
In case of hydroelectric power, there's a temperature gradient, driven by the Sun. Water evaporates in higher temperatures, radiates the heat into space, and falls out as rain.
"Water evaporates in higher temperatures, radiates the heat into space, and falls out as rain."
The paper says, "Water vapor in the nano-pores radiates its heat into the material and comes out to the surface as liquid water."
So you don't believe that the researchers experiment did what they say it did?
That's fine, typically in science you go and see if you can reproduce it.
So you don't believe that the researchers correctly described what was going on when it did what it did?
That's fine, typically in science you go and propose a way to falsify their hypothesis and test that.
My point was simply, if the researchers were presumptively accurate in their understanding (that's the principle of giving them the benefit of the doubt), then it would imply their material would pull liquid water out of the air below the temperature and conditions in which it would normally precipitate out.
They go to some length in their exposition to describe how they think it does that and where the energy comes from and where it goes. But if you don't believe them, then sure.
Then the _material_ is a store of energy. Once it's exhausted, the condensation will stop.
> So you don't believe that the researchers correctly described what was going on when it did what it did?
The article is very low-quality. They must understand that their work implies the conservation law violations, so there must be some unaccounted source of energy. But they have not attempted to find it.
And it can be as simple as energy from the moving air. Or maybe an electrostatic charge, or something similar.
Once the energy source is identified, they should have calculated the efficiency of their setup, compared to regular dehumidifiers.
> Then the _material_ is a store of energy. Once it's exhausted, the condensation will stop.
The paper points out that the sample was surface that maintains a particular temperature (20 degrees C in this case). The water condenses, the material heats up, the thing its sitting on removes that excess heat to maintain the temperature. No violation of CoE or TD.
Without that temperature controller, the material would presumably continue to store the heat, which would make it hotter than the ambient temperature. By how much is, as you point out, something to be characterized.
Thermodynamics says that the heat will equalize, so that excess heat will conduct to the air around it (it's not in a vacuum so it doesn't have to radiate it). That will lower the temperature of the material which will then condense more water and heat up again. My original thought was you could enhance that conduction by putting a heatsink on one side of the material.
The paper states that inside the pores they have managed to create a space that changes the parameters around the vapor carrying capacity of the air which results in the water condensing even though it would not have condensed outside those pores. Then they go on to describe how the effects of hydrophillic and hydrophobic materials, used in conjunction, create spaces near the molecular limit of water molecules and how the forces acting on that water might result in it condensing. When the vapor does condense, the heat goes somewhere, and they assume its going into the material (reasonable assumption in my opinion) and that their temperature controlled platform is then removing it. I found the description of how that water expresses to the surface a bit more "hand wavy" but that they observed liquid water on the surface, and that it is somehow coming from the material they created, seems reasonably well supported.
I think for the purposes of this discussion we're done. I really do appreciate that you are skeptical and feel that some of the more well tested laws of physics are being violated :-). Since we can only go on what they wrote up, I did make the presumption that they too know the laws of physics and have a good faith belief that they are not being violated either. It is one of the things I look for in papers that talk about things like this. Also the journal where they published their paper, Science Advances, is a refereed journal so I would presume that the reviewers were also satisfied they weren't violating any well known laws of physics. Doesn't mean that you should believe what they say, just that it's not obviously wrong.
> When water droplets reach a certain size, the system reaches a steady state. As the volume of voids decreases with increasing ϕPE, the growth and coalescence of water droplets are slowed down.
That does not break the current laws of physics.
Form the press release:
> these films could be integrated into passive water harvesting devices for arid regions
I asume "harvesting" mean we can collect the water and drink it or use to irrigation or something interesting. Not just absorbing it like silica, even if the unusable water is visible.
Passive as using the day-night temperature different to collect water: It has been done.
Passive as a continue stream of running water: It breaks the second law of thermodynamics.
The new material is very hydrophilic, so the water prefer to be attached to it than been vapor.
If the wire is even more hydrophilic then the droplets will jump and collect around the wires, but they will be so attached that they will not fall down from the lower extreme of the wires.
If the wires are not so hydrophilic, the water will prefer to keep attached to the surface, or even the droplets will be smaller to avoid the wires and the collection will stop earlier.
Tweaking smartly the hydrophilic values and separations between the wires and the separation with the surface you may get interesting capillarity effect, but the water will be trapped again.
Anyway, it's difficult to look at all the details, but at the end of the day "The second law of thermodynamics. It's now trivially easy to create a free energy:"
It sounds good on paper because everybody knows that water can travel up a wick. But of course, if the end of the wick in the upper reservoir is submerged in the water, then water will just as happily travel _down_ the wick. And if the end of the wick is in free air, then water will not drip from it because the same capillary forces prevent it.
But the paper suggest that it will condense at ambient anyways, because it gets warmer so radiation to ambient is enough for it to work.
Their experiments suggest that tiny water droplets appear inside the material at 70% RH (relative humidity). If this is true, then I expect there is a way to extract the droplets using very little energy. Ideas:
- make open collection points on the film
- use ultrasound to bounce the droplets around and consolidate them
- make the film on a material that can be saturated with water so the new droplets can easily join the flow
In theory, if that makes it hotter than ambient air in the process, that would be a good thing - usually we have to cool things down below ambient air to get moisture out.
Not a good thing if you want to measure maximum moisture extraction, but cooling something to ambient temperatures is a much easier task.
Is there a corollary to Betteridge's Law that says that popular science journalism will always overatate the result?
It could still be a useful material, but the science would be bad.