Evaporative Cooling and Propylene Glycol

name         Nathan
status       student
grade        9-12
location     IL

Question -   I am a high school senior.  I have been studying
environmentally-friendly heating and cooling alternatives.  One of
the best ideas I have seen is evaporative cooling, where a fan
blows through a wet screen and the evaporation of water cools the air.

However, as you know, the problem with evaporative cooling is that
it does not work well when the humidity in the air is already
high.  Also, the increased dampness can lead to increased mold and
mildew growth.

I would like to do an experiment, trying to perform evaporative
cooling with propylene glycol instead of water.  Propylene glycol is
non-toxic, non-flammable, and should not be affected by humidity,
since it actually absorbs water vapor (instead of adding to dampness).

But I know that propylene glycol has a very low vapor pressure, so I
wonder if a fan blowing air through a propylene glycol-wetted screen
would evaporate enough propylene glycol to cause a cooling
effect?  Can you shed any light on this question?  If not, can you
suggest any other non-toxic, non-flammable (i.e. not ethyl alcohol),
non-water liquid that might have the right properties?
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What you are contemplating is to take some liquid and dump it into 
the air as vapor.  You are entirely correct that a liquid with a low 
vapor pressure will not be very effective around room 
temperature.  Another issue you ought to be concerned about has two 
sides, both bad: (1) the liquid you evaporate will be gone from you 
after you evaporate it, so you'll have to keep buying more, and (2) 
the liquid will afterwards be in the air, so it better not have any 
polluting characteristics.  Unless you use something like liquid 
nitrogen (asphyxiation hazard), you have problems.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming
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I do not think there is a good way yet, Nathan.
It is a problem of low rate, like you said.
If there was some technology that increased the surface area _and_ 
the airflow over it,
then there would be more evaporation, more BTU's/hr of cooling,
but the amount evaporated in a cubic foot of air would be so low
that the temperature depression in degrees C would be unnoticeably low.

And there is probably no other liquid that meets your specifications.
You can just about list all the molecules with high vapor pressure, 
or at least their chemical families.
None of the inert gas elements are heavy enough to be liquid at room 
temperature.
Almost all inorganic small molecules except water _are_toxic, and 
organic ones are flammable.
Organic molecules with hydrogen (H) replaced by fluorine (F) or chlorine (Cl)
are less flammable but often toxic,
and if stable enough not to be flammable, they are ozone-depleters, 
almost because they are stable.
Only the exceptional sledgehammer provocation of 
"vacuum-ultraviolet" starts to break them up.

Fluorine substitution is the least toxic.
Some modified Freon such as HFC113  (C2F4H2 or C2F2H4)
would be OK in small uses, because it is not immediately toxic or 
flammable or ozone-depleting.
You can buy it in freeze-spray cans.
Probably cannot afford enough of it to do evaporative cooling of a 
room, though,
and that would require enough to start being a little toxic.

Maybe CO2 is an idea for you.  Compressed, almost liquefied, 
generates lots of cold when released.
LN2 or liquefied air, too.  Or dry ice or water ice.
But then all these things must be manufactured with refrigeration equipment.
If using refrigeration equipment, you might as well have it do air 
conditioning.

You might have here an indirect "functional discrepancy" (my term).
Water is non-toxic and plentiful because it is the local environmental liquid.
It is useable for cooling because it has a modestly high vapor pressure.
Put "plentiful" and "vapor pressure" together,
and you have humidity : partial pre-saturation of the atmosphere by 
the coolant.
In short, any liquid that is OK to use in an open evaporative cooler
is also likely to be subject to variability of local weather.

You might start thinking of "swamp cooling" not as throwing around a 
plentiful resource,
but as an opportunistic exploitation of a non-equilibrium condition 
common in that environment.
i.e.,  imagine a world with a hot dry crust over cold water,
then everybody would have their cold source right nearby in the 
downwards direction.
The point is to look around for another common non-equilibrated 
situation, not necessarily based on evaporation.
The temperature difference between air and the ground six feet under 
comes to my mind.
It is not always one way, such as hotter or colder, but we can still use it.
Ice could be shipped in from the north pole, if transportation that 
distance was cheap enough.
Oceanic shipping might have enough economy of scale to make 
north-pole ice useable, if everybody lived at the dock.
It might be the last 100 miles by gas-engine truck that makes ice 
counterproductive.
Perhaps it costs just as much gasoline energy to ship it as to make 
it locally.
Since the transportation costs are too high, it is not a "locally 
common" condition.
Snow is locally common in a lot of places...I wonder what would 
happen if you made a huge
"snow cellar" and filled it every winter?

Jim Swenson
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The short answer is NO because of the low vapor pressure of 
propylene glycol. The cooling arises from the heat required to 
evaporate a given amount of liquid coolant. If no liquid evaporates, 
no cooling occurs. Propylene glycol's vapor pressure at ambient 
temperatures is way too low to allow a significant amount of 
vaporization. As you suggest, if the relative humidity approaches 
100% even water will not work, because none can evaporate because 
the air is already saturated with water vapor. Neglecting that 
problem, water has almost all the properties required for an 
evaporative coolant: high heat of vaporization, significant vapor 
pressure, not toxic, not flammable, inexpensive. So water can be 
used in an open system, and design parameters are relatively simple.

Other coolants can be used -- and are (in air conditioners, 
refrigerators etc.) -- but for one or more of the advantages of 
water above, these alternatives require a closed recirculating 
refrigerator. I cannot think of an alternative that beats water.

Vince Calder
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