From the MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Evaporation is happening all around us all the time, from the sweat cooling our bodies to the dew burning off in the morning sun. But science’s understanding of this ubiquitous process may have been missing a piece all this time.
In recent years, some researchers have been puzzled upon finding that water in their experiments, which was held in a sponge-like material known as a hydrogel, was evaporating at a higher rate than could be explained by the amount of heat, or thermal energy, that the water was receiving. And the excess has been significant — a doubling, or even a tripling or more, of the theoretical maximum rate.
After carrying out a series of new experiments and simulations, and reexamining some of the results from various groups that claimed to have exceeded the thermal limit, a team of researchers at MIT has reached a startling conclusion: Under certain conditions, at the interface where water meets air, light can directly bring about evaporation without the need for heat, and it actually does so even more efficiently than heat. In these experiments, the water was held in a hydrogel material, but the researchers suggest that the phenomenon may occur under other conditions as well.
The findings are published this week in a paper in PNAS, by MIT postdoc Yaodong Tu, professor of mechanical engineering Gang Chen, and four others.
The phenomenon might play a role in the formation and evolution of fog and clouds, and thus would be important to incorporate into climate models to improve their accuracy, the researchers say. And it might play an important part in many industrial processes such as solar-powered desalination of water, perhaps enabling alternatives to the step of converting sunlight to heat first.
The new findings come as a surprise because water itself does not absorb light to any significant degree. That’s why you can see clearly through many feet of clean water to the surface below. So, when the team initially began exploring the process of solar evaporation for desalination, they first put particles of a black, light-absorbing material in a container of water to help convert the sunlight to heat.
Then, the team came across the work of another group that had achieved an evaporation rate double the thermal limit — which is the highest possible amount of evaporation that can take place for a given input of heat, based on basic physical principles such as the conservation of energy. It was in these experiments that the water was bound up in a hydrogel. Although they were initially skeptical, Chen and Tu starting their own experiments with hydrogels, including a piece of the material from the other group. “We tested it under our solar simulator, and it worked,” confirming the unusually high evaporation rate, Chen says. “So, we believed them now.” Chen and Tu then began making and testing their own hydrogels.
They began to suspect that the excess evaporation was being caused by the light itself —that photons of light were actually knocking bundles of water molecules loose from the water’s surface. This effect would only take place right at the boundary layer between water and air, at the surface of the hydrogel material — and perhaps also on the sea surface or the surfaces of droplets in clouds or fog.
In the lab, they monitored the surface of a hydrogel, a JELL-O-like matrix consisting mostly of water bound by a sponge-like lattice of thin membranes. They measured its responses to simulated sunlight with precisely controlled wavelengths.
The researchers subjected the water surface to different colors of light in sequence and measured the evaporation rate. They did this by placing a container of water-laden hydrogel on a scale and directly measuring the amount of mass lost to evaporation, as well as monitoring the temperature above the hydrogel surface. The lights were shielded to prevent them from introducing extra heat. The researchers found that the effect varied with color and peaked at a particular wavelength of green light. Such a color dependence has no relation to heat, and so supports the idea that it is the light itself that is causing at least some of the evaporation.
The puffs of white condensation on glass is water being evaporated from a hydrogel using green light, without heat. Image: Courtesy of the researchers
The researchers tried to duplicate the observed evaporation rate with the same setup but using electricity to heat the material, and no light. Even though the thermal input was the same as in the other test, the amount of water that evaporated never exceeded the thermal limit. However, it did so when the simulated sunlight was on, confirming that light was the cause of the extra evaporation.
Though water itself does not absorb much light, and neither does the hydrogel material itself, when the two combine they become strong absorbers, Chen says. That allows the material to harness the energy of the solar photons efficiently and exceed the thermal limit, without the need for any dark dyes for absorption.
Having discovered this effect, which they have dubbed the photomolecular effect, the researchers are now working on how to apply it to real-world needs. They have a grant from the Abdul Latif Jameel Water and Food Systems Lab to study the use of this phenomenon to improve the efficiency of solar-powered desalination systems, and a Bose Grant to explore the phenomenon’s effects on climate change modeling.
Tu explains that in standard desalination processes, “it normally has two steps: First we evaporate the water into vapor, and then we need to condense the vapor to liquify it into fresh water.” With this discovery, he says, potentially “we can achieve high efficiency on the evaporation side.” The process also could turn out to have applications in processes that require drying a material.
Chen says that in principle, he thinks it may be possible to increase the limit of water produced by solar desalination, which is currently 1.5 kilograms per square meter, by as much as three- or fourfold using this light-based approach. “This could potentially really lead to cheap desalination,” he says.
Tu adds that this phenomenon could potentially also be leveraged in evaporative cooling processes, using the phase change to provide a highly efficient solar cooling system.
Meanwhile, the researchers are also working closely with other groups who are attempting to replicate the findings, hoping to overcome skepticism that has faced the unexpected findings and the hypothesis being advanced to explain them.
The research team also included Jiawei Zhou, Shaoting Lin, Mohammed Alshrah, and Xuanhe Zhao, all in MIT’s Department of Mechanical Engineering.
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The paper: https://www.pnas.org/doi/10.1073/pnas.2312751120
Using sunlight to evaporate water would be a net cooling, no? And is using sunlight to create electricity also a net cooling?I found it strange that greenies would have people living in deserts. Then, I realized that desert peoples are under the control of the water supplier.
It doesn’t matter much anyways, the lunatics will do whatever the voices say.
Lunatics will soon outlaw lighted swimming pools as the increased evaporation must warm the atmosphere per global warming theory.
If the water vapor didn’t come from a positive feedback from CO2 it will be ignored as all other causes of warming are…
According to the article, there is no increase in the energy of the water molecules, so there is no cooling.
That is surely nonsense. What is happening is that the green photons are delivering energy that overcomes the van der Waals forces that hold the water in the gel and accelerates them via some mechanism to the typical speed of vapour molecules away from the surface.
ave v^2=3RT/M or 3×8.3×300/0.018 = 415,000 m^2/s^2
so K.E.~ 0.5x18x1.66E-27×4.15E5 = 6.2E-21 J/molecule, so it is only a fraction of the 3.5-4E-19 J per photon – or alternatively each photon can energise the ejection of a number of water molecules perhaps as a nano droplet, which would mean that individual molecules would not have to be separated at the cost of extra energy to overcome more van der Waals forces. The net energy balance of the process may succeed in transferring more or less than the energy of the incoming photon to kinetic energy of evaporated molecules, and on that will depend the temperature of the hydrogel. Some energy may be taken from the gel, or added to it.
Having dipped into the paper – thanks to Dan Hughes for finding the arxiv version
https://wattsupwiththat.com/2023/11/05/in-a-surprising-finding-light-can-make-water-evaporate-without-heat/#comment-3812415
I see they are indeed proposing that evaporation is of nano droplets.
shame that light is heat tho… you’d think someone would have caught that.
You just need a few sentient kangaroos and a girl driving a tank and you’ve got a decent graphic novel on your hands…
“”…hoping to overcome skepticism that has faced the unexpected findings and the hypothesis being advanced to explain them. “”
They’re up against ‘settled dogma’. Clouds etc just got that little bit more difficult to model
Light is heat, and vice versa.
What they really need to do is to explain the process in quantum mechanical terms. How does a photon of energy around 4E-19 J manage to act like a trigger to get the gel to fire out water molecules as if from a cannon? Understanding the molecular geometry at the surface is the starting point.
…interface surface ‘tension(s)’, as associated, is included in understanding molecular geometry.
zzebowa wrote, “Light is heat, and vice versa.”
Bing chat tells us, “Light and heat are both types of energy, but they have different properties. Light is a type of electromagnetic radiation that is emitted by hot objects, such as lasers, bulbs, and the sun. Heat is a type of kinetic energy that is related to the random movement of the particles in a substance. Light does not create heat, but it can cause heat when it is absorbed. When an object absorbs light, it changes the energy of the photons into kinetic energy of its own atoms. This kinetic energy makes the atoms vibrate faster, which we call “heat””.
Well Bing chat is wrong. Heat is energy in transit and only exists at the boundary of two systems or a system and surroundings. Once “in a substance” it is internal energy.
Further, heat can only exist if there is a temperature difference.
Heat is internal energy. It doesn’t matter what the internal energy of some other object is.
So MarkW you have a different definition of heat than my thermo book? You are wrong.
I agree. Heat is a boundary phenomenon. Heat is the energy that is transferred across a system boundary due to a temperature difference. Work and heat are path variables. Internal energy is a state variable. A bucket of hot water does not contain heat. But it has a higher internal energy than a bucket of ice cubes. This is elementary thermodynamics. It’s essentially taught on the first day of class.
light is also a boundary phenomenon, that’s how it exists: it is a packet of energy suspended in its un-aging relativity traveling to a compatible destination
Huh?
Light is a wave and a particle, now there is something else to figure out.
The point they are making is there is more evaporation than the heat present can explain.
Light behaves both as a wave and a particle, not necessarily is one.
Light can either behave as a particle or as a wave, but not both simultaneously. Electrons, which clearly are particles, can also behave as either a particle or as a wave, but not both simultaneously. In an atom, electrons behave as waves. The image of an electron orbiting the nucleus like a planet orbits a star would violate the physics of charged particles.
actually light is neither a wave nor a particle… it doesn’t exist between emission and absorption, those occur instantaneously, contiguously and continuously for it… additionally light can be facing the wrong way to be detected, which is mind boggling but fairly straight-forward
for it ^ immaterial of the duration of time outside its relativity ^ …
The energy of the photon is conserved as evaporated water but the photon does not have enough energy. That’s the issue ?
The phenomenon might play a role in the formation and evolution of fog and clouds, and thus would be important to incorporate into climate models to improve their accuracy,
Surely they would need to have some accuracy in the first place…..
Something else to be ignored in the climate models I should think.
The number of white polar bears reflecting light into arctic oceans should be a concern for green environmentalists, don’t you think?
This is the way science should be done.
Nah you gotta get a ‘sense’ about these things to appease the Groupthink doomsters even though the evidence calls BS on them-
“We don’t yet see a very clear signal of sea level rise but my sense is that it’s going to appear in the next 50 years.”
At Bengello Beach, longest-running coastal study in Southern Hemisphere finds ‘nature is the best healer’ (msn.com)
Like I gotta sixth sense erecting solar panels and windmills to appease the weather Gods isn’t working so needs to be scrapped for throwing climastrologists into volcanoes.
“my sense” says “coastal science expert”
hmmmmm….
Are they saying that all the experiments that developed our concept of thermodynamics were done in complete darkness so nobody noticed this before? I think there is something missing from their explanation and it involves energy from the sunlight.
Climate justice is a mysterious force.
Not again.
I’m desperately trying to think of ways to help these guys out of the breaking-the-First-Law-of-Thermodynamics hole they are digging.
What wavelength does the hydrogel tend to absorb at? Probably doesn’t matter, otherwise it would appear coloured. But ask the question anyway.
How long do their experiments run for? The hydrogel certainly presents a different, rougher, surface to the air than pure water. This means the top few microns are not in the same thermodynamic state as pure water. I wouldn’t be surprised if it evaporated faster at the beginning, then slow down.
It is a great trick I’ve seen in biochemistry papers. Run the experiment for a very short time, or a very long time. Stop it at the point where you can claim an anomalous result.
And again, how saturated was their ‘air’? How accurate is their knowledge of the light source and its spectral-distribution and intensities? If they are not measuring the IR with sufficient precision then this will cause errors.
You don’t try to find out ways to break the First Law. You try to find out what you are doing wrong.
This does make me think of those “cold fusion” experiments a few years back.
Cold fusion does, at least, not break the Laws of Thermodynamics.
It is a rate thing.
I’m not going to pay to read the full article, but the supporting information with this article is long on photographs and short on relevant data.
Figure S3 may be the only important one, but without the full text it is difficult for me to (quickly) understand what the significance is without further explanation.
I do note that the lower band for their temperatures are 30 Celsius where lines start to converge. Why not lower?
Quite apart from whether they are properly removing IR, it seems a bit too convenient that the effect is reportedly maximal at the same 520nm as many green diode-lasers.
Going a step further, and I may be out of bounds with this one, Green diode laser pens are reportedly potentially damaging to the human eye. But this is not because of the green light. It is because they use frequency doublers to convert IR to the visible green. It is the residual IR that does the damage because it is still the majority of the output.
A 1% error in IR will have a far greater effect than 1% of the green in their experiments.
Not saying I’m right, but these are questions I would want answered as a reviewer.
And I should give a shout-out to “auto” for his comment on the previous article for the most penetrating question:
“Is the effect reversible?”
Does it emit green light under reversible conditions?
Penetrating indeed.
Not necessarily. The apparent color is effected by the thickness of a material. Geologists have long used unglazed porcelain plates to obtain the ‘streak’ of a mineral. In general, minerals that are transparent in a thin-section (low extinction coefficient), will have a colorless streak. On the other hand, opaque minerals (high extinction coefficient), will have a strongly colored streak, usually black or red. Therefore, something that has a magenta color (minus green) in bulk, may appear colorless in very thin material or highly comminuted (pulverized) form.
An experiment we used to do in beginning geology classes was to apply an oxy-hydrogen flame to a colorless piece of pumice (glass) or white granodiorite (granular crystalline). Where the rock melted, it would become a black glass (equivalent to obsidian).
Yes, but that also depends partly on the sensitivity of the human eye.
I am more familiar with organic compounds. They can often appear white in the fine-crystalline phase, but more commonly yellow in solution. Offhand I can’t think of any dramatic colour changes that exceed this general observation.
I assume this is mainly due to reflection/refraction at crystal surfaces.
The large bulk crystal (rarely seen) or the solution phase probably gives a truer picture.
Plants are hydrogels….all plants.
Gaia is laughing at us
Yes – the energy converted to vaporize water via transpiration is thought to be greater than that of evaporation (5-10% more). The hydrogel plant analog must then by understood to be more resistant to vaporization, not less. More calories per gram lost (not fewer).
In budgeting schemes, consider that on a sunny day the plant may transpire more water than accounted for by its total mass. This is how Gaia does it: Maximum rate of energy transformation. It starts in the wilderness of the soil, and ends in the sky.
To make this discussion a bit more understandable to those without a lot of thermodynamic knowledge, I offer this:
The bottom line is that the researchers have found that the amount of energy required to evaporate water can be significantly lower under some circumstances. The net result is that the “efficiency” is increased. My Model A engine vs my new Subaru engine is a good analogy. For a gallon of gasoline, I can travel further with my Subaru engine than my Model A engine (ignoring weight, air resistance, etc). This is the result of nearly a hundred years of experiments and understanding of how internal combustion engines work.
Water evaporates in the tiny thin zone between the liquid water and air. This is not a hard line. The term “Knudsen Layer” is used to label this zone where liquid water and gaseous water (vapor) (the phases) is undefined. We can see it as a transition zone. The simple explanation of water evaporation is that in this Knudsen Layer, a few water molecules might have more zip (kinetic energy..ie moving around more) than the surrounding molecules. The liquid phase of water acts kind of like a sticky glue….ie the molecules want to be close to each other. At the Knudsen Layer, a few molecules of water might just have enough zip (kinetic energy) to escape the “glue”…basically moving much further apart from other water molecules we then call this water a “vapor”…ie a gas. The term “evaporative cooling” is used to describe the little bit in drop in temperature of the remaining liquid water. This was due to having the really zippy water molecule of water leave (become vapor/gas). The average “zip” of the remaining liquid water molecules is now slightly lower…and we measure this as a reduced temperature. We can increase the rate of evaporation by increasing the temperature of the liquid water (ie adding energy. Which if it increases the temperature, we call it heat). Lots of other variable also come into play (humidity, pressure, air (gas/vapor) movement (wind), etc.
What the MIT researchers have found is that if you physically change the configuration of the liquid water, basically making into jell, and you shine green light on it, then you can free more liquid water molecules into vapor than just simply adding bulk movement (ie heat).
A large number of questions loom. The evaporation rate of water under given circumstances is highly dependent upon how many vapor wafer molecules are very near the Knudsen Layer. The bulk term is “humidity”. If the very local humidity is low, then more water molecules escape…the higher humidity results in less escaping. At a given outside temperature we (humans ) are generally more comfortable in dry air than humid air, because our own evaporative cooling system (sweat) is more efficient. How is this affected by the gel?
MIT reasons that the increase in evaporative efficiency is perhaps influenced by the wavelength of energy (light) interacting with the Knudsen Layer. They guess that this is due to more liquid water molecules in this layer getting enough extra zip directly from the green light and hence more escape. The question is how is this possible if the water molecule is not really any better at absorbing green light than some other color (spectrum). From their experiments it is clear that the gel plays an important role.
At any rate, this is an interesting experimental observation. Unfortunately, they immediately then speculate they can change the world by increasing the efficiency of solar water desalination plants.
I suggest that they take this a tiny step at a time. Figure out exactly how and why this increase in efficiency is taking place, then slowly and methodically change the configuration (ie do more experiments) to see what can be learned. Unfortunately , this probably does not result in lots of research grant dollars…hence the desire to wildly speculate….
Ethan Brand
No, that’s not what they found. They found that their experiment doesn’t adequately account for the energy in the system.
Color me skeptical. I do not see how a photon can kinetically move water molecules. I await the physics.
An absorbed photon in the frequency range under discussion (visible light) almost always results in increased vibrational and rotation energy of the bonds between hydrogen and oxygen atoms in the absorbing water molecule. If this increase in molecular energy is large enough, it can be sufficient to change the phase of the molecule from that of liquid water to that of water vapor.
The thermodynamic phases are established by net energy (aka enthalpy) of a given molecule under given external conditions, such as pressure and concentration gradient . . . generally not from an absorbed photon creating translational motion (i.e., kinetically “moving” a water molecule via momentum exchange).
So not kinetic. But I thought water did not absorb visible light, or at least not in proportionally large quantities.
Well, I could have been more clear in my above post. The energy transfer from an absorbed photon does manifest as kinetic energy of molecular-bond vibration and rotational modes (as determined by available degrees-of freedom), but some smaller fraction of the total energy exchange can also result in increased translational kinetic energy (i.e., molecular velocity) increase. Maxwell-Boltzman molecular speed distribution statistics (some having “+” signs and some having “-“signs in 3D orthogonal coordinates) tend to smear out any absorbed-photon increase in transitional kinetic energy of the absorbing molecule(s).
As to your other question, you are correct that water (liquid or vapor) does not absorb incident photons significantly that are in the visible spectrum. But water vapor is a good absorber of photons at longer wavelengths, from near-IR to the LWIR spectrum critical to explaining the “greenhouse effect”. See the attached graph.
My son sent me an article on that yesterday, a process engineer asking the opinion of his heat transfer engineer father….my response was this:
”…sounds like techno wordplay to get some research grants from stupid bureaucrats ….photons can bounce molecules out of a water surface without “heating” it as measured buy a bulk fluid temperature thermometer. Water at it’s boiling point isn’t technically “heated” but molecules are bounced out of the surface, mostly by molecular collisions, but those molecular forces are the result of photons being exchanged….The whole basis of the Planck curve is how much electromagnetic radiation (photons) is emitted by a substance at a given temperature.
MIT seems to be full of grant seeking announcements these days…”
First comes the observation, then confirmation of that observation, then the explanation.
This photo-molecular effect may help to explain the fictitious discontinuity of ~ 20 Watts per square meter between surface and air adjacent, a problem which has always plagued surface energy budget schemes.
Photons “cleaving”, or knocking loose water molecules on the way by, without being absorbed. The holy grail for closing surface energy budgets?
revision: knocking loose may be a poor analogy. Cleaving is probably best. to slice off or split. not a “knocking” effect. knocking suggests kinetic jiggling, which is NOT the idea.
Inferring from surface energy budget challenges, the effect may have a power equivalent magnitude of ~20 Wm-2 globally averaged at surface (savings) in climate budgeting. No more need for the bizarre double accounting of (net) sensible heat (20 Wm-2) + 20 Wm-2 net radiation transfer from surface to air. It never made any sense.
In NASA scheme:
Surface Net Radiation = 398.2 Surface UP – 40.1 window – 340.3 Atmospheric Down = 17.8Surface Net Radiation = 0 = 398.2 Surface UP – 57.9 window – 340.3 Atmospheric Down
The virtual numbers really don’t matter. What matters is that there cannot be any radiative discontinuity.
For clarity, this [surface net radiation exchange = zero] pertains only to the thermal LW infrared (orange arrows) of course.
In straightforward terms, the surface and surface air adjacent can be allowed to be the same temperature for the intents and purposes of globally averaged energy accounting. This should be intuitively obvious, but the radiation accountants would have us believe otherwise.
What matters in all of those Kiehl and Trenberth-type diagrams, such as the one you posted, that purport to calculate a 0.6 W/m^2 or so net imbalance in Earth’s energy “budget” under assumed steady state conditions, is that the incoming TOA solar irradiance is not accurately known. Wikipedia gives a recent value of 1360.9±0.5 W/m^2 referenced to solar minimum conditions.
(Ref: https://en.wikipedia.org/wiki/Solar_irradiance )
Dividing by 4 to account for a near-spherical Earth being illuminated on only one hemisphere 24/7/365.25 gives an equivalent Incoming TOA surfaced-averaged irradiance influx of 340.2±0.1 W/m^2. Note that this value is 0.2 W/m^2 less than that given in text on the upper left face of the graph you presented.
This same reference goes to great lengths in discussing how solar irradiance varies annually as a result of Earth’s orbit around the Sun, due to variation with the ~11 year sunspot cycle, and due to other factors. Likewise, there is much discussion on the inaccuracies/uncertainties inherent in measuring TOA solar irradiance using space-based instruments and their needs for periodic calibrations. So even the above-stated ±0.5 W/m^2 uncertainty must be taken with a grain of salt.
Given this degree of uncertainty of power flux input, it is ridiculous for anyone to assert Earth’s “energy balance” can be calculated to yield an net imbalance value of 0.6 W/m^2 (see bottom left text on face of your presented graph).
Your observational uncertainties pale in comparison to the 20 Wm-2 faulty and fictional accounting in surface budgets. We can discuss the implications of fractions of Watts per unit area once we can see Watts What.
Well your derivation of “20 W/m^2 faulty and fictional accounting” appears to be understated.
In the K-T style “energy budget” you posted:
1) there is the red up arrow of “emitted by surface 398.2” and the red down arrow of “back radiation 340.3” from greenhouse gases, leaving a surface-to-atmosphere net radiation exchange of 398.2-340.3 = 57.9 W/m^2
2) of the total 398.2 W/m^2 “emitted by surface”, 358.2 is stated to be “absorbed by atmosphere” with the rest being shown as “atmospheric window 40.1”, which balance to within 0.1 W/m^2 round-off error.
3) there are NO other surface-to-atmosphere radiative exchanges given; note that the red up arrow labeled “atmospheric window 40.1” is surface radiation-directly-to-space, NOT surface-to-atmosphere
4) the red up arrow labeled “”total outgoing infrared radiation 239.9” accounts for atmosphere-to-space radiative exchanges of “emitted by atmosphere 169.9” and “emitted by clouds 29.9”, but it also includes the atmospheric window component of surface radiation of 40.1 W/m^2.
5) so, per the diagram numbers, the bulk ATMOSPHERE receives 77.1 W/m^2 of absorbed sunlight, far left yellow down arrow, 57.9 W/m^2 net from surface radiation exchange (per Item 1 above), 18.4 W/m^2 from “thermals (conduction/convection)”, second from right white up arrow, and 86.4 W/m^2 from evapotranspiration latent heat (change of state), far right white up arrow, yielding 77.1+57.9+18.4+86.4 = 239.8 W/m^2 total input
6) at the same time, again per the diagram numbers, the bulk ATMOSPHERE is said to be radiatively emitting 169.9 W/m^2 from its gases plus 29.9 W/m^2 from its clouds, summing to only 199.8 W/m^2
So there is a input-out discrepancy in calculated atmospheric power flux balance of 239.8-199.8 = 40 W/m^2, which is covered up by adding the 40.1 W/m^2 of “atmospheric window” radiation from Earth’s surface.
The atmosphere-specific accounting error in the K-T style energy budget you posted is actually 40.1 W/m^2 not 20 W/m^2.
Thank you for the interest and detailed response
My interest is specifically the thermal IR residual at the surface in this steady state diagram.
To narrow things down:
Why should the system choose to maintain a 17.8 Wm-2 discontinuity at the surface atmosphere interface?
398.2 – window 40.1 = 358.1 available to be delivered to the atmosphere adjacent to the surface, yet only 340.3 is coming back.
The virtual vectors of surface atmosphere exchange are: 358.1 Wm-2 “up”. 340.3 “down”.
net discontinuity 17.8.
What is this 17.8 Wm-2 mystery item?
When considering the overwhelming wash of other fluxes at the surface-atmosphere interface, in the turbulent mixed boundary layer, it seems to me it’s unlikely the system would be able to maintain this discontinuity. I expect it should not exist in reality.
That is not a discontinuity. It is an natural power flux imbalance. The physics of the Sun-Earth surface-atmosphere-deep space system doesn’t “choose” to maintain any “discontinuity” anymore that your body “chooses” to maintain a temperature imbalance with wintertime ambient air temperatures.
can you describe the nature of this natural power flux imbalance between the surface and air adjacent? I believe it may be an analytical artifact. The closure is already accounted for in the turbulent flux. Does a piece of wood maintain a temperature imbalance with air in steady state climatological mean? Thanks.
I don’t understand why you’re complaining about the IR window at 40 W/m^2. KT1917 screwed up the calculation of that value and everyone seems to be using it anyway.
no that is not the point.
You should make your point more explicit.
the surface cannot gain or lose energy radiatively with atmosphere. net surface-atmosphere radiative exchange must be set = 0, regardless of window.
Adjusting window is but one way to constrain the diagram, among many other ways.
The surface radiative constraint is LW up – LW down – Window = 0. The tiny residual of ~20 or whatever in the diagram is already accounted for in the (net) sensible heat term (up). Sensible heating eliminates the fictitious radiative flux (up) of the same magnitude. The diagram counts this power twice.
Net flux of mass and energy at surface air interface, or anywhere internally, is by non-radiative turbulent pathways. It is the exchange of total energy to and from space which is by radiation.
I noticed that climate scientists don’t seem to make any distinction between surface temperature and surface air temperature. They usually treat them as identical, when they are not.
As for the IR window, using KT1997’s numbers, I calculate 87 W/m^2. It’s not 40 W/m^2 or 38 W/m^2.
Silly Wikipedia average.
Take this link, for example: https://lasp.colorado.edu/data/sorce/tsi_data/daily/sorce_tsi_L3_c24h_latest.txt
On July 5, 2003 we have an Earth TSI of roughly 1315.9211 W/m^2. And on January 13, 2004 we have an Earth TSI of roughly 1407.1228 W/m^2. That’s a range of roughly 91.2017 W/m^2 during the year. If I average those two values, I get roughly 1361.5220 W/m^2. The link also includes error terms, which I didn’t bother with.
I’m not sure you can just average the minimum and maximum and get a valid result.
Not so silly Wikipedia.
Most scientific references to “established TSI” value (at top of atmosphere) will state that it is referenced to a Sun-Earth distance of exactly 1 AU and to minimum solar cycle conditions.
Therefore it is not surprising to find that “real-time” TSI varies from Earth’s solar orbit perihelion distance (= 0.9833 AU) to its solar orbit aphelion distance (= 1.0167 AU). Your straight averaging of the colorado.edu-provided values (1361.5220 W/^m^2) is very close to the upper value afforded by Wikipedia’s stated value of 1360.9±0.5 W/m^2.
BTW, do you really believe ANYONE has measured TSI values to precisions of .0001 W/m^2 out of a magnitude of 1361 W/m^2 (equivalent to a precision of 7×10^-6 %)? . . . just laughable!
And where was in Sun with respect to its ~11 year sunspot cycle back in July 2003–January 2004?
“BTW, do you really believe ANYONE has measured TSI values to precisions of .0001 W/m^2 out of a magnitude of 1361 W/m^2 (equivalent to a precision of 7×10^-6 %)? . . . just laughable!”
No, but using an average is just as laughable.
This is embarrassing. General chemistry 1st semester tells you what energy you need for a phase change and it’s never described as having to be from “heat.”
Flash: It is possible to accelerate the rate of evaporation of water in the absence of light or heat. All one needs is a cold wind. You then end up with ‘lake-effect snow’ or ‘freeze-dried fish.’
Rate of evaporation is a mass transfer problem. The amount of energy required to evaporate a specified amount of water at a given temperature and pressure is fixed. Fast or slow, that energy has to be supplied to move those molecules apart and overcome the attractive forces that make water a liquid or solid.
The point being, that energy can be in the form of kinetic energy of the air, or the radiant energy of photons of different wavelengths, not just heat (kinetic) energy of the moving water molecules.
Sorry. You’re talking basic chemistry/physics. They’re talking advanced Climate Science. Whole different entity – somewhat like astrology.
True, true!
Temperature is an average that reflects an underlying distribution of the kinetic states of molecules. This “we can’t account for the evaporation” nonsense ignores that fact that when you’re just on the edge of the temperature bound for a phase change, part of the system is already above the boundary.
“They began to suspect that the excess evaporation was being caused by the light itself —that photons of light were actually knocking bundles of water molecules loose from the water’s surface.”
What are these “bundles”? And why “bundles” at all, instead of individual molecules?
From the above article:
“In recent years, some researchers have been puzzled upon finding that water in their experiments, which was held in a sponge-like material known as a hydrogel, was evaporating at a higher rate than could be explained by the amount of heat, or thermal energy, that the water was receiving.”
And why, exactly did they need to hold the water in a hydrogel to conduct this experiment?
And how, exactly did they account for the (additional?) heat that the interface with the hydrogel structure contributed to the liquid water? (Hint: I suspect the maximum light absorption coefficient of the hydrogel used in the experiments will turn out to be right around the “particular wavelength of green light” mentioned in the above article.)
And did they account for the fact that the hydrogel composition almost certainly reduced the surface tension of the water it contained, thereby enhancing evaporation rate all other factors being equal?
And, finally, did they run the simple control experiment to measure the rate of evaporation from the aerogel compared to an equivalent surface area of free-standing water in the complete absence of incident light so as to isolate and document the sole effect of incident light energy versus other possible explanations for the experimental observations?
If the use of a hydrogel turns out to be a critical factor in increasing the rate of water evaporation via the so-called “photomolecular effect”, then good luck employing hydrogels on such a massive scale as required for commercial desalination, especially considering how the Na+ and Cl- ions will almost certainly attack the hydrogel structure and its water absorption properties.
Oh, well.
First make sure that conservation of energy is satisfied. Then check that the various laws of thermodynamics also are satisfied. Then we can talk.
How could the experimenters disobey thermodynamic laws? Of course they can’t…
An experiment cannot disobey any natural law. That is plain. But the interpretation of the outcome may. Especially if the experiment contains processes that are not fully accounted for.
The interpretation is a consequence of natural laws, it is not in spite of them. If we choose to obey them, we must concede that something else must be going on here.
I submit that the idea of “disobeying a natural law” or of ‘not being able to disobey” is an intellectual trap. It assumes that what is generally believed, the consensus, best fits reality and that anything contrary to that consensus must be wrong, no matter how convoluted a view must be taken to make it fit. It is the “scientific realism” philosophy of consensus rather than the Karl Popper philosophy of empirical falsification.
History has show that long entrenched ideas are often overthrown. At the beginning of the 20th century a widely held belief was that the only thing left to science was to add a another decimal point or two to already know physical constants. The idea that an entirely new framework is required to account for new findings can be difficult to absorb, and certainly should not be casually assumed, but one cannot be assured that new evidence will always lead down the expected path.
Thanks. I’m a believer in thermodynamic laws, and it helps me to constrain the nature of reality. Assuming both the experiment and foundational thermodynamic constraints are valid, it calls for an explanation.
Yes, the devil is in the details, as the saying goes.
Some very smart person or group will have to work out the quantum mechanical probabilities governing such photon-matter interactions as well as the statistical thermodynamics for the continuum phases at that complex water-gas interface to determine if net “photomolecular”-induced evaporation is possible for given boundary conditions of liquid water surface temperature, water partial pressure above the water surface, other gas constituents above the water surface, incident (ambient) light spectrum and power flux, and radiation sink temperature, if different from 3 K.
I gather the MIT authors of the above-referenced PNAS paper have NOT done such by lack of mention of rigorous theoretical explanation, and the above article citing “a team of researchers at MIT has reached a startling conclusion“ and “the researchers are also working closely with other groups who are attempting to replicate the findings, hoping to overcome skepticism that has faced the unexpected findings and the hypothesis being advanced to explain them” (in both quotes my bold emphasis has been added).
As David Dibbell posted separately above, “. . . those ‘cold fusion’ experiments a few years back.”
Great if it turns out to be real, greatly reduce the energy cost of desalination.
Amazing what science can discover when it’s not declared settled.
And here we were thinking that there was nothing else to learn …
Many years ago I read about an experiment that used sound energy to aid evaporation.
They believed that the shock wave that was sound energy in water, hit the discontinuity between water and air, it would impart enough energy to some of the water molecules to knock them out of the water, into the air.
One of a bazillion papers on this topic …
https://www.nature.com/articles/srep45864
arxiv version is here:
https://arxiv.org/pdf/2201.10385.pdf
As most research is found to contain today … nothing much new here … well except maybe they achieved the highest rate of evaporation to date, albeit by a known phenomenon for decades … most of this coming out of material/plasma sciences years ago. Daniel’s may have even mentioned it is his classic work on Solar Energy in 1967.
Here is Tu’s paper back in 2022 … this seems to be a hype-publication to get his hit count up … as the saying goes … publish “whatever” or perish …
https://arxiv.org/ftp/arxiv/papers/2201/2201.10385.pdf
I find it so surprising that this work has “proven a totally new phenomenon” …
In a surprising finding, light can make water evaporate without heat
A newly identified process could explain a variety of natural phenomena and enable new approaches to desalination.
David L. Chandler | MIT News
Publication Date October 31, 2023
I doubt the authors have ever looked in the “ancient” archives of research in plasma sciences .. or one of its applications … Directed Energy. Intense laser beams, strong electric fields (e.g. photons from laser light) have been known to evaporate on a time-scale much faster than bulk heating … i.e. direct absorption and molecular bond breaking of the hydrogen bonds (if the photon has the required threshold energy … there is also a polarization dependence). Here is a DoD/USN report on the modelling of the phenomenon that was being experimentally tested at my lab a decade or more before this work was published in 2019. Water absorption in the micron region is a huge issue in both DE and Laser Radar (LIDAR) Sciences.
https://apps.dtic.mil/sti/trecms/pdf/AD1073959.pdf
One thing I know for sure… none of the physics, such as these, are in any GCM model/code.
Here are other papers on/around this topic predating Tu’s work … tons more if you do a google scholar search…
https://pubs.aip.org/aip/apl/article-abstract/116/25/253903/39148/Does-sunlight-always-accelerate-water-droplet?redirectedFrom=fulltext
https://onlinelibrary.wiley.com/doi/full/10.1002/eom2.12157
https://www.sciencedirect.com/science/article/abs/pii/S0017931022001909
Cheers!
-Doc Elihu
“. . . .none of the physics, such as these, are in any GCM model/code.”
Not even cumulus convection is in GCMs. Their resolution is too large to include it, so they use parameterization and statistical effects to represent it.
The lack of certain cloud physics is very troubling … but as a computational physicist (one of many hats I wear) I would be(am) insulted by this find … https://journals.ametsoc.org/configurable/content/journals$002fclim$002f34$002f8$002fJCLI-D-20-0281.1.xml?t:ac=journals%24002fclim%24002f34%24002f8%24002fJCLI-D-20-0281.1.xml
Drift in a chaotic system? I’m shocked! (not)
I have this strange feeling of deja vu, like I’ve seen this somewhere before…
“without heat” uhhh, got bad news for these people who failed high-school physics with this paper.
I have come late to this thread, but I looked at the equation that explains the “thermal limit” and it is, as suspected, a statement of the first law of thermodynamics. What explains this allegedly counterintuitive observation is that L, the latent heat of the transition from liquid to vapor in the special circumstance of water adsorbed weakly to the gel, or perhaps at a variety of interfaces, is not the L that pertains when heating bulk liquid water to vapor.
There is an analog here. When taken beyond its critical pressure and temperature water is a high density fluid, not a vapor, without a latent heat at all. Strange things happen at the critical point.
I think the number of possible uses of these findings is going to have to pass through a filter of which uses end up promoting various sorts of perpetual motion schemes. The idea that we can evaporate large masses of water with little energy input, then obtain the release of the typical latent heat upon recondensing this same mass of water, without further effect, looks like a perpetual motion machine to me. I’ll have to look at the experimental design.
Percy Spencer, EM radiation causes water to heat. Also candy bars. Raytheon/American Radio, Inventor of the Microwave. 1945,
James Clerk Maxwell, light has mass, 1861-1864.
Johannes Kepler, Comet tails stream away from the sun, 1610
Surprise? Was there a reason that visible light might be different today?
Yes … Dark Light …. the cousin of Dark Matter and Energy.
By definition, not visible light, then?
Part of the surprise of this observation is that water is quite transparent to visible light. IR and microwaves are another matter. Thus, it seemed unexpected that visible light would have the measured effect.
Temperature being a necessity to evaporate water doesn’t seem to me to compute.
I dry laundry outside on a cloths line. It does dry faster in summer when there is more sun and the temperature is higher, but an adequate breeze seems more important. Certainly, neither the air nor the sheets and towels, nor the water in them, ever reaches anywhere near 100C.
70%, roughly, of the globe is ocean. Evaporation occurs constantly over much of it but neither air temperature or water surface temperature ever come near the boiling point of water. Wind creates spray. Water droplets in the air get further dispersed through turbulence. Small enough quantities, with a limit of single molecules, can stay mixed with the air for an extended period of time. Dust particles, much larger than molecular size, are readily raised to considerable height by air movement, so small water droplets could certainly also be.
Does 20C water in my laundry or at the ocean surface actually somehow heat to 100C, then gain another 540 calories per gram, to become vapor? Isn’t this the same puzzle as colder air heating a warmer surface to even a higher temperature through IR emission at air temperature?
“Does 20C water in my laundry or at the ocean surface actually somehow heat to 100C, then gain another 540 calories per gram, to become vapor?”
Don’t confuse evaporation with boiling point. Even snow evaporates. Swamp coolers use evaporation to cool.
That’s my point. It seems to me that in various situations liquid water can change to the vapor state on much less energy gain than the 540 calories per gram heat of vaporization of water, which starts at 100̊C. This would also mean that the calculations of water condensing to liquid at high altitude might release much less energy than what seems to be generally believed.
The latent heat of vaporization of 540 cal/gm as well as the phase change (i.e., “boiling”) temperature of 100 °C for water are only applicable under ambient pressure of 14.7 psia. And yes, such values are less for “high altitudes” due to reduced ambient pressure.
Take liquid water at any given temperature in a sealed container and start evacuating the overhead gas in that container . . . as the pressure is reduced the water will start to “boil”, even at bulk water temperatures as low as 1 °C.
To find the latent heat of vaporization or corresponding heat released from condensation for specific temperature and pressure boundary conditions, one needs to consult P-h or T-s phase diagrams for water, or the “on-line calculator” equivalents of such.
(ref: https://en.wikipedia.org/wiki/Phase_diagram# )
You have to have some source of energy to change state of water. In the case of your laundry this source is the internal energy of the air passing through and by your clothing — the air emerges cooler from the evaporation.
Yes, some energy, but maybe not as much energy as the heat of vaporization of water is claimed to require.
I’m surprised that chemists haven’t weighted in on much of this nonsense. I can place a glass of water on my kitchen counter, and in days or weeks it will all evaporate. There’s no need to boil it, and everything remains at room temperature.
Well, not quite: there will be a small be detectable decrease in the temperature of the liquid water that remains in the glass. That is because the process of water evaporation from the liquid’s surface extracts energy from the liquid. That temperature drop will reach a steady state “equilibrium” value since it also establishes a temperature gradient between the water and the ambient air (across the glass wall) that will transfer heat into the water that offsets the heat being lost due to evaporation.
The water continues to evaporate from the glass over “days or weeks” because it never fully saturates the air in the hypothetical kitchen.
However, if you were to place a glass of water, say, containing 300 cc of liquid water, into a hermetically-sealed box of, say, 1000 cc total internal volume, you would find that the water will never completely evaporate from the glass, independent of the ambient temperature being anywhere in the range 1 to 99 °C, because such a volume ratio does allow the evolved water vapor to reach 100% relative humidity level of the gas inside the box. At that point, the temperature gradients will disappear.
Heh! It’s another nit-picker!
I would like to try that box experiment, but I don’t have anything that will measure cubic centimeters. Is it alright that I use milliliters? When the metric system was first set up, milliliters and cubic centimeters were supposed to refer to the same volume, but they aren’t exactly the same now. My measuring device is only plus or minus 5%. Is 300 milliliters plus or minus 15 milliliters okay?
I guess I could assemble a box that’s 10 centimeters by 10 centimeters by 10 centimeters. My carpentry skills aren’t really that great. I believe that would be approximately 1000 cc. I found a glass that would fit into that box, but when I poured 300 ml (plus or minus 15 ml) into it, it overflowed. Apparently, the glass can only hold 280 ml (plus or minus 14 ml). Probably 250 ml is more likely (plus or minus 12 to 13 ml). Is that okay?
Another problem is that the box is probably not hermetically sealed. Can the box be partially hermetically sealed or is that like being partially pregnant?
When I place the glass of water on my kitchen counter, how long do I need to wait before it equalizes with room temperature? Charging a capacitor through a resistor takes an infinite amount of time. The capacitor is never fully charged. However, after five time constants, the capacitor IS considered fully charged. What is the time constant of equalizing the temperature of the room temperature with the glass of water?
“. . . because it never fully saturates the air in the hypothetical kitchen.”
I didn’t think my kitchen was hypothetical. Thanks for letting me know. Is my kitchen counter hypothetical too?
One of the problems I have is trying to figure out the physics around my glass of water. There’s a water vapor presence over the water in the glass. If saturation is achieved, then when a water molecule leaves the liquid, a water molecule in the overlying vapor will return to the the liquid. That would be called equilibrium. However, as the vapor diffuses into the room air, equilibrium is never achieved.
“. . . there will be a small be detectable decrease in the temperature of the liquid water that remains in the glass.”
I love this silly statement. You previously stated that more than four significant figures is laughable. I would think it would take more than four significant figures to measure that “so-called” detectable decrease in temperature. And when a water molecule left the water in the glass, it would immediately un-equalize the energy in the glass, and by definition, it would no longer have a thermodynamic temperature.
I have a thermostat that sets the temperature back at night. And my dogs love to have the patio doors open. My kitchen counter is 191 inches from the patio doors—that’s about 485 centimeters.
So when does that glass of water ever reach equilibrium?
So many wasted words . . . so little rational thought . . . so many absurd, tangential statements.
But thank you providing such humor for me and others!
“So many wasted words . . . so little rational thought . . . so many absurd, tangential statements.”
I see you’re projecting again. This easily describes most of your comments. I’m still trying to get around the fact that I have a hypothetical kitchen.
Here, I’ll help you out:
First you post “I can place a glass of water on my kitchen counter, and in days or weeks it will all evaporate. There’s no need to boil it, and everything remains at room temperature.”
—> That’s one amazing kitchen you have there that over the period of “days or weeks” “everything remains at room temperature” (quoting your words, and noting specifically the use of the word “temperature”, not “temperatures” that would indicate a range of variability).
Then you post “I have a thermostat that sets the temperature back at night. And my dogs love to have the patio doors open.”
—> Again, what an amazing real (not hypothetical) kitchen you have that remains at room temperature despite a change in thermostat setting (assuming of course that you have working HVAC) and where inside/outside convection from open patio doors have no effect on your real kitchen temperature that remains unchanged.
You should patent the design of your “real” kitchen . . . or maybe not :-))
Thus spoke whats-his-name!
I’m sorry, bad pronoun. I should have said, “Thus spoke whats-its-name!”
Bad grammar. Should be “Thus spoke what’s-its-name.”
LOL!
Mr. Layman here.
I remember back in one my first classes on science (grade school? HS?) being surprised to learn that a pot of boiling water will still pick up some H2O molecules from the air. And also that a chunk of frozen ice will still release some H2O molecules into the air. Liquid H2O does the same, releasing and picking up at about equal rates.
If I remember correctly, it had to with molecules always moving, sometimes from one state to another.
Some form of energy causes that motion. Light is energy. Heat is energy. All light doesn’t produce heat.
Why couldn’t it increase the motion of the H2O molecules (with more molecules changing state) without measurably producing heat?
Again, “Mr. Layman here”.
All solids and liquids have a property known as their “vapor pressure” at a given temperature. “Vapor pressure” encompasses the atomic/molecular scale thermodynamics (maybe even quantum mechanics) that drives matter to move from a volume of very high density (a solid or liquid state) toward a volume of very low density (a vacuum or low pressure ambient gas) . . . commonly known as a “concentration gradient”. The gas phase, by nature, requires that the associated atoms or molecules have greater kinetic energy (contained in all available degrees of freedom) than they have in solid or liquid form.
There is no getting around the fact that such delta-energy is either taken from the bulk material that is not evaporated or is provided by external input of energy.
“Heat” in a substance (solid, liquid or gas) is just a convenient term to encompass the total kinetic energy that is tied up in the rotational, bending and stretching vibrational modes of atom-atom bonds of poly-atomic molecules as well as the translational (linear velocity) modes of both atoms (e.g., He) and poly-atomic molecules, collectively referred to as mechanical degrees-of-freedom.
“Heat” does necessarily reflect the total energy carried by an atom or molecule, as energy can also be “tied up” in their electron shells, such as being elevated to an “excited state” (as in a laser) or to different levels of ionization (as in a fluorescent light).
4 related Preprints https://scholar.google.com/scholar?oi=bibs&hl=en&cites=821270015002238579&as_sdt=5 e.g.
Photomolecular Effect Leading to Water Evaporation Exceeding Thermal Limit
https://arxiv.org/ftp/arxiv/papers/2201/2201.10385.pdf
On the Molecular Picture and Interfacial Temperature Discontinuity During Evaporation and Condensation
https://arxiv.org/ftp/arxiv/papers/2201/2201.07318.pdf
Maybe it is a discovery, but why is the hydro gel important? Water can unzip glass, so it would not be surprising if it has some undiscovered secrets.
Using light instead of heat to evaporate water? Aren’t we really talking about energy, which at the lowest level is photons absorbed or released, even in solids, because you cannot have fractional photons regardless of how the energy is transmitted/conducted.
As such, making a distinction between light and heat with regard to evaporation seems a false distinction.
An interesting problem perhaps unresolved. What happens to the momentum of light at the air-water interface. Could this explain the observations?
Researchers find that light energy alone can cause water to change state, blimey, they will be discovering sublimation next!
Sublimation can occur in the complete absence of incident light. All that is required, thermodynamically, is that the ambient pressure be lower than the vapor pressure-at-the-given-temperature of the substance being considered.
Simple home experiment: wrap a piece of “dry ice” (solid CO2) in totally opaque paper . . . you will find that is still sublimates quite rapidly at typical room temperature and room pressure conditions.
QED.
You can’t exceed the thermal limit, i.e. heat of vaporization of water. If the researchers do a careful energy balance, the amount of water evaporated will equal the amount of heat and light energy added to the system. There may be an effect where water is volatilized as a very small droplet, so not a true vapor, and this could require less energy. As with other things that sound too good to be true, they are.
I would recommend watching Dr. Gerald Pollack’s video – https://risingtidefoundation.net/2023/10/15/the-fourth-phase-of-water-beyond-the-three-you-already-know-rtf-lecture-with-dr-gerald-pollack/
Anthony,
I know better than to dispute Willis’s very shaky grasp of Heat Transfer. Heat is energy. The way to measure Heat is Temperature, which is the Average Kinetic Energy of the molecules of an object.
Light is Radiation. There are three ways to transfer energy between objects: conduction, convection, and radiation. If there is one thing that has been studied extensively, it is how to make water evaporate/boil.
i think that “evaporation rate” actually refers to Pan Evaporation, dependent on many variables. A veteran engineer, this is the first time I have ever seen the phrase “evaporation rate,” meaningless without specifying many’ many other variables.
A real clunker here.
MR Moon
“The way to measure Heat is Temperature, which is the Average Kinetic Energy of the molecules of an object.”
This is not what the kinetic theory of gases says. Heat is not temperature. Using SI units, Kelvin does not equal joule. And temperature does not equal average kinetic energy. Heat is energy and can be measured by BTUs, calories, joules, foot-pounds, watt-hours, newton-meters, ergs, dyne-centimeters and so on. But temperature is not energy. The MORE correct statement is that temperature is PROPORTIONAL to the average kinetic energy. Even that statement isn’t exactly correct.
I didn’t say Temperature is Enegy. Work on your reading skills
I’m sorry. I guess I have extremely poor reading skills. So when you say “heat is temperature,” you didn’t say that heat (which is energy) is temperature? And the clause “which is the Average Kinetic Energy . . .” doesn’t refer to the preceding word which is “temperature?” I should really work on my poor reading skills.