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Light can make water evaporate without heat

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Re: Light can make water evaporate without heat

#101

Post by geoffcampbell64 »

elil17 wrote: post #44
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Thermodynamics and evaporation are my day job and I think most other explanations here are missing the point. Evaporation normally occurs when individual water molecules have enough thermal energy to break their intermolecular bonds, leaving the bulk liquid and entering the air. In this case, they found strong evidence that water molecules were being removed in groups of several water molecules. Because intermolecula…

Is there anyway in which this effect could be used to create a new type of heat pump?

Re: Light can make water evaporate without heat

#102

Post by chimpanzee »

malfist wrote: post #95
montjoy wrote: post #71

> The researchers found that the effect varied with color and peaked at a particular wavelength of green light. I wonder if this affects photosynthesis in any way? IIRC plants are green because chlorophyll does not absorb that color. Maybe that’s partially because green light induces too much evaporation?

Maybe, but evaporation is critical for plants. They don't have a heart to pump blood and supply nutrients, they have to rely on evaporation to move nutrients from the roots up

I imagine the leaves being green would encourage this evaporation. The upper layers of cells are probably effectively translucent and might be awash in green light, perhaps increasing the rate of evaporation and improving overall transpiration.

Re: Light can make water evaporate without heat

#103

Post by stubish »

frud wrote: post #47
fspeech wrote: post #25

Earlier quoted context omitted.

Entropy favors the vapor form. It doesn't necessarily take energy to evaporate.

It takes [40.66 kJ/mol]( https://en.wikipedia.org/wiki/Enthalpy_of_vaporization ) to vaporize water. There are no shortcuts.

The shortcut is apparently breaking the bonds of groups of molecules, rather than supplying enough energy to break all the bonds of each individual molecule. But still technically correct, with the airborne groups breaking apart into individual modules cooling the air. But you don't have to supply all that energy and get to break some theoretical limits.

Re: Light can make water evaporate without heat

#104

Post by geoffcampbell64 »

I wonder how long it’ll be until someone figures out how to utilize this effect to create a heat pump! I would guess that this “photomolecular heat pump” might be easier to miniaturize since it has no need for a compressor. I wonder if other solvents or refrigerant might also be more efficient working fluids than water. Very exciting!!

Re: Light can make water evaporate without heat

#105

Post by elil17 »

geoffcampbell64 wrote: post #104

I wonder how long it’ll be until someone figures out how to utilize this effect to create a heat pump! I would guess that this “photomolecular heat pump” might be easier to miniaturize since it has no need for a compressor. I wonder if other solvents or refrigerant might also be more efficient working fluids than water. Very exciting!!

You'd definitely still need a compressor - otherwise there'd be no change in pressure driving a change in temperature. I guess you're suggesting that the photomolecular effect could be used to boil the refrigerant at the heat pump's evaporator. Boiling is a bulk process caused when the temperature of the fluid rises so much that all it's intermolecular bonds start to get broken. Evaporation is a surface process where a few molecules randomly break free of their intermolecular bonds and mix with surrounding air. Boiling, not evaporation, occurs in a heat pump's evaporator (despite the name). The photomolecular effect doesn't cause boiling, only evaporation. You wouldn't really want evaporation to happen in an evaporator because you'd need to have air around and then you'd be compressing a bunch of air along with your refrigerant, which would waste a lot of energy.

My first thought was some sort of cooling tower application. Cooling towers use evaporating water to cool various process fluids. But, when building a cooling tower, you want to pull down the temperature of the water by having evaporation absorb the heat in the water. This evaporation process actually reduces the amount of heat that gets absorbed from the water because it uses energy from light and heat from the air instead.

If this has engineering applications, it will likely be in places where the end goal is the evaporation of the water itself, such as a drying process or passive desalination.

Re: Light can make water evaporate without heat

#106

Post by elil17 »

elil17 wrote: post #44
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Thermodynamics and evaporation are my day job and I think most other explanations here are missing the point. Evaporation normally occurs when individual water molecules have enough thermal energy to break their intermolecular bonds, leaving the bulk liquid and entering the air. In this case, they found strong evidence that water molecules were being removed in groups of several water molecules. Because intermolecula…

One more thought for how to explain this, for those who don't want to worry about intermolecular bonds and what energy goes where:

Essentially they found that light makes a "splash" when it hits water (at least in certain circumstances).

Re: Light can make water evaporate without heat

#107

Post by mercutio2 »

tsimionescu wrote: post #40
mercutio2 wrote: post #28

Earlier quoted context omitted.

Enzymes catalyze the breaking of molecular bonds. The embodied energy of a phase transition does not pay attention to what path you took to cross it. There’s no “threshold” between phases of water. For H20 to move from liquid water to vapor, energy must be added. There’s no catalyst. So either we’ve discovered some new physics since I last studied thermodynamics, or this isn’t an accurate analogy.

> For H20 to move from liquid water to vapor, energy must be added. I think that is a wrong assumption. Liquids will naturally evaporate even with 0 external energy, assuming there is not too much pressure in the surrounding atmosphere.

The equilibrium partial pressure of water vapor with a source of water in another phase is not zero for most temperature and pressure regimes on earth, that is true.

But thermodynamics still hold; the water vapor is still in a more energetic state than liquid water or ice.

Re: Light can make water evaporate without heat

#108

Post by [deleted] »

montjoy wrote: post #71

> The researchers found that the effect varied with color and peaked at a particular wavelength of green light. I wonder if this affects photosynthesis in any way? IIRC plants are green because chlorophyll does not absorb that color. Maybe that’s partially because green light induces too much evaporation?

[deleted]

Re: Light can make water evaporate without heat

#109

Post by pard68 »

dheera wrote: post #68
pard68 wrote: post #27

Earlier quoted context omitted.

My 8th grade understanding is evaporation doesn't always mean steam.

Huh what? I thought steam === water vapor

No steam is specifically from heating water and often enough is a gaseous water. Water vapor (or "wet steam") is an aerosol of liquid water.

Re: Light can make water evaporate without heat

#110

Post by pard68 »

frud wrote: post #50
pard68 wrote: post #27

Earlier quoted context omitted.

My 8th grade understanding is evaporation doesn't always mean steam.

What exactly is the distinction between water vapor and steam?

steam is specifically from heating water and often enough is a gaseous water. Water vapor (or "wet steam") is an aerosol of liquid water. But both are means of evaporation.
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