Repost from four days ago: https://news.ycombinator.com/item?id=44060712
Also, they do a really good job of making it sound like it violates thermodynamics. Since it doesn't, and dehumidifiers already do a good job of getting water out of air for the energy price you have to pay, there has to be some other selling point. Right? But I'm not sure I see it.
> dehumidifiers already do a good job of getting water out of air for the energy price you have to pay
They do a terrible job. Condensate dehumidifiers are as expensive to run as an AC, produce unwanted heat, and are noisy. Dessicant dehumidifiers are even less energy-efficient.
If there's a way to extract moisture from the air with less energy and less noise, that would be huge.
Less energy would definitely be a huge plus but unless this violates our understanding of thermodynamics there will still be unwanted heat put out into the air. The heat from a dehumidifier comes primarily from the latent heat in the water being released so that the water can become liquid. This heat must be released somehow in this process unless they actually did find something that breaks our understanding of physics.
Obligatory Technology Connections video on the topic: https://www.youtube.com/watch?v=j_QfX0SYCE8
That's true, but I was under the impression that most of the heat generated by current compressor dehumidifiers is just waste heat from the mechanical operation of the compressor itself. The phase change heat is there too, but it's significantly less. So there should still be a lot of room for improvement, theoretically.
Yeah, there's certainly some of that, but going off of Alec's numbers and trial above you end up with a larger portion coming from the latent heat than from the mechanism, for the same reason that a heat pump is more efficient than an electric heater: it's condensing more water than it is running mechanisms.
That said, his demo is not under typical operating conditions in that a dehumidifier is normally expected to actually be able to catch up and reduce humidity in the room, while his demo ensures that the humidity levels stay high throughout the hour. So it's likely that under normal operating conditions the mechanism's proportion of the waste heat is higher than it is in his demo.
> That's true, but I was under the impression that most of the heat generated by current compressor dehumidifiers is just waste heat from the mechanical operation of the compressor itself.
Nope. It's almost all (>80%) latent heat. I believe, the theoretical limit is around 90% for typical room temperatures.
>The heat from a dehumidifier comes primarily from the latent heat in the water being released so that the water can become liquid.
A dehumidifier movies heat from one side to another using electricity to do the work. One side gets cold so the water can condense on it, while the other side gets hot from extracting the heat from the cold side. Heat is still generated from this process even if there are 0 water molecules in the air and no water is collected. The water does not create the heat, the electricity does.
I don't think there has to be any heat involved with collecting water molecules in the air into a larger volume of water, depending on the process used.
> I don't think there has to be any heat involved with collecting water molecules in the air into a larger volume of water, depending on the process used.
For the water to condense, there must be heat given off, unless I’m fundamentally misremembering my high school physics class.
state change takes 63 calories per cubic centimeter of H2O unless I am misremembering my high school science class.
at least for water solid to liquid
Both processes create/release heat, and in Alec's tests in the linked video the bulk of the heat from the dehumidifier running in a humid space is coming from latent heat released from the water. That may not be true in regular operating conditions, but there will always be a substantial amount of heat released from the water when you trigger a phase shift.
> I don't think there has to be any heat involved with collecting water molecules in the air into a larger volume of water, depending on the process used.
The only other option is to increase the pressure in the room or in a space within the room, which this material pretty clearly isn't doing.
It’s not just the electricity creating the perceived heat https://en.wikipedia.org/wiki/Enthalpy_of_vaporization
If water changes phase from a gas to a liquid, it releases a large amount of energy (enough heat to heat five times as much water from 0 to 100°C). That's likely far more heat than the electricity generates - though, of course, the heat released by electricity is very real as well, and as you say will even happen if the air is completely dry.
Possibly this doesn't happen if the condensation happens in a capillary (there is some funkyness related to energy levels), but then it must stay trapped there.
> If there's a way to extract moisture from the air with less energy and less noise, that would be huge.
Less noise: I agree, but you still need some air flow so the corners of the room that are far away also get dehumidified. Perhaps a slow fan in enough, and when you run them slowly they are quieter.
Less energy: It's not clear that this uses less total energy. It's easier to imagine what is happening if you compare it to a high tech Dehumidifier Bag. https://www.amazon.com/Wisesorb-Moisture-Eliminator-Fragranc... But instead of sending the drops down, they get attached to the device. You can use it only once unplugged. Then you have to buy a new one or use energy to extract the water (like boiling the water of the dehumidifier bad until you get the crystals again). It's not clear if building a new copy of this is cheaper than building some new calcium chloride salts, and/or if regenerating the new device is cheaper than regenerating the calcium chloride salts (that is usually not done).
So, is this new method less noisy and/or more energy efficient? The article doesn't really say.
>without the need for any external energy.
That sure seems to imply that there's no need for a noisy and power hungry compressor.
It also seems to imply that it violates thermodynamics.
It takes energy to condense water, so where does it get it? If it isn't external, it runs out.
Condensation of water is exothermic. If you add a cold thing to the environment, ie, remove heat from the water, the water will condense all on its own. This reaches steady-state when the cold thing is warm and the water is condensed. No thermodynamics issue!
They’re claiming they have a material that will do it at higher temperatures. Assuming such a material gets hotter as it works, there’s no thermodynamics problem here.
> They’re claiming they have a material that will do it at higher temperatures. Assuming such a material gets hotter as it works, there’s no thermodynamics problem here.
That's ok, but the amount of water you get is fixed - the process can't continue. You install the device into your room, it condenses 1L of water as droplets on its surface (or, more likely 1ml of water), and it's now done, that's all the water it's going to remove/produce, if this is the right explanation. It would be perfectly equivalent to bringing in a cold slab of metal from your fridge into your room - it will condense some water as it gets hotter, and it will eventually get as warm as the room and stop condensing anymore water, forever.
Conversely, if the process were continuous (say, as long as you remove the condensed droplets, new droplets form), as they seem to claim, that would very likely violate thermodynamics again.
What about entropy? The reason you need to power AC for example is that if you didn't you could reduce entropy and generate power for free, and basically reverse time.
The amount of entropy in a closed system is not allowed to decrease, but it is allowed to be constant. The amount of energy released by the phase shift is equal to the amount of energy absorbed by the object as heat.
Sure, but then the temperature of the object has to increase. And as its temperature increases, it has to stop condensing water - at the very least, once it reaches ~100C, it will instantly boil off any extra water that it condenses.
You are super confident a thing that exists can't exist
https://en.wikipedia.org/wiki/Air_well_(condenser)
Check out the passive section of the above
Especially for somewhere like UAE that in summer has the upsetting combination of massive humidity, no rain, and huge water demand.
Someone here once posted a link to a story[0] about the Persian Cooling Towers, that you see all over the ME.
Thousands of years old, I think.
[0] https://www.bbc.com/future/article/20210810-the-ancient-pers...
Perhaps OP was describing "swamp coolers" — or evaporative cooling?
That's the exact opposite of extracting moisture from the air.
Running a swamp cooler + dehumidifier that doesn't heat up would mean being able to do self contained cooling which would be a massive thing. Currently the only way to do that is with desiccant which needs recharging too often to be practical.
Tech Ingredients demonstrates how this works: "Revolutionary Air Conditioner!"
> If there's a way to extract moisture from the air with less energy and less noise, that would be huge.
I vote we write to our legislators to update the laws of thermodynamics to enable this. Typically I would agree we should leave well enough alone, but in this case it seems like the benefits outweigh the costs.
That's not how the laws of thermodynamics work.
In reality, you would need to convene an international consortium to approve to the change, and the Chinese wouldn't sign on unless we agree to a temporary suspension of Newton's third law.
We would only need a committee's approval if changing it would break things.
I say we skip that process, test it in a lab somewhere in rural midwest where nobody lives, and see if gravity starts changing or whatever. As long as cows don't start to float in a 3 mile radius within 4-5 hours, that's probably good enough validation to move forward with changing thermodynamic legislation.
[edit] we should also probably make sure the boiling point of water stays the same
I’m not sure the constitution grants Congress that power.
They already weighted in on the definition of π so why not?
It almost certainly doesn't violate our understanding of thermodynamics, but it's not clear that it would have to in order to condense ambient water vapor from the atmosphere.
From the paper [1]:
Remarkably, when these amphiphilic nanoporous PINFs are exposed to high yet subsaturating conditions [i.e., relative humidity (RH) < 100%], macroscopic water droplets appear spontaneously on the film surfaces without the need for cooling, as illustrated in Fig. 1C and shown in Fig. 1D.
Well, when water changes from vapor to liquid, it releases heat. The heat has to go somewhere.
Sorry, I don't know the correct physics lingo. Heat of enthalpy or formation or whatever.
Tangent but this may solve a mystery of mine. When I make scrambled eggs I add a little bit of water to make them fluffy. When I turn off the heat there’s a puff of steam I can’t explain. Since it seems to me more heat is needed to produce that extra puff of steam. However, maybe the fast condensing of the water vapor that happens when I turn off the stove produces a. I start of extra heat and therefore the steam?
Is it gas stovetop?
If so, it could be that the water vapor coming from the eggs no longer mixing with the hot gases coming from the flame around the pan, allowing it to drop below the dew temp (?) and allowing it to condense right above the pan. IOW the water vapor is always there, you just can't see it until it is able to condense in lower ambient temps.
I don't think the fast condensing of steam would cause enough heat to cause more steam, though. Because the reason it condenses fast is because there's heat being removed from the system (cooling to the room, and no longer more heat getting added). So the heat is already "spoken for" - it's the reason that steam turned back into water.
The other commenter that wonders if it's a gas stove might be onto something.
Maybe the drop in temp stops a Leidenfrost effect that was reducing the surface area of the food that was contacting the pan.
If water vapor is condensing on the material, wouldn't there be a transfer of heat energy?
The idea here is that you don't need to cool down the air to get water.
First you get water, and as a result material heats up a little bit, then it can cool down passively back to ambient.
> there has to be some other selling point. Right? But I'm not sure I see it.
Windtraps [0].
or moisture vaporators
Those exist, but, as the GP points out, they're called "dehumidifiers". Or sometimes clothes driers. The question was, what makes this new dehumidifier any better than existing dehumidifiers.
Windtraps are passive, like this material. They work solely on temperature differential and wind.
I'm not aware of any passive, solid state dehumidifiers which are not chemical, which condense water to a chemically loaded solution, which what a Windtrap is not.
There is no way to make a passive, solid-state humidifier that doesn't involve some chemical reaction with water. Thermodynamics dictates that precipitating water from a non-saturated atmosphere must generate large amounts of heat (assuming pressure doesn't change) and so can't be a self-sustaining process.
So, even though it may not be 100% clear how from their description, their device is nothing but a novel way of making a dehumidifier that needs some kind of active component - perhaps the AC that is keeping the temperature steady in their experiment (since water condensation generates large amounts of heat, it's likely that, without the AC, temperature would rise and their water droplets would evaporate right back).
> There is no way.
No, there's a way without running afoul of thermodynamics. You need to bleed the heat to a cooler surface efficiently, and you can do it without any external power.
You can use heat pipes to effectively wick away heat to a heat sink, like the Earth itself. Similar systems exists for cooling and heating, which uses buried pipes to extract or dump heat to the Earth's crust. You can sink the heat similarly without any external power (sans wind to push air through the material).
In the Windtrap example, the other side of the opening is a deep well basically. Cooler than outside world. The rocks sink the heat probably, too. Sı it's possible to create self-sustaining process without external electricity. Yes, an heat-pipe is not solid-state per se, but it's insulated and works on the principle of heat difference only.
> their device is nothing but a novel way of making a dehumidifier that needs some kind of active component...
No, their paper say that they forced air through it and it worked on a temperature differential. Maybe a compressor or Peltier device can acclerate the process, sure, but sinking the heat to the earth and blowing air through it will work equally well.
Have a friend who designs heat-pipes for space applications. That things are way faster than we see on computer applications, but equally more expensive.
None of this matches the setup the paper describes, though of course you are right that bleeding heat into the Earth is a time-tested and known to function way to build such systems.
From the article:
> Before they understood what was happening, the researchers first thought that water was simply condensing onto the surface of the material due to an artifact of their experimental setup, such as a temperature gradient in the lab. To rule that out, they increased the thickness of the material to see if the amount of water collected on the surface would change.
There's a temperature gradient in their lab setup. i.e. one side of the material is cooler than the other side. This is where I extrapolated that you can increase the performance of the material by bleeding the heat into the Earth.
This would be passive and solid state.
Except we know that is impossible from simple thermodynamics, so there's no point in contemplating this "what if".
Have you done any work with this material?
No, but I don't need to work on a perpetual motion machine to know that it doesn't work.
My question would be do you have enough experience to know that it is a perpetual motion machine. I read the article and nothing sounds like perpetual motion to me. The atmosphere is always dynamic and my reading of the article was that it is using 2 different changes to harness these changes. Surface tension and atmospheric pressure seem to be the mechanisms.
I was trying to figure out if your comment was informed.
> Repost from four days ago
Thanks! there were a few comments there and we'll merge them hither.