I have a buddy with a fish tank, and he maintains that if he keeps a bubble bar on the surface of the water, he keeps the water temperature several degrees below room temperature (he said that if the room temperature is 73, the water temperature is maintained at 66-68 using this method). I think he has an AC vent pointed at the tank, but granted, it’s been a loooooong time since thermodynamics for me.
So what say ye?
Evaporative coolers work very well. I know that the water in the swamp cooler where I used to live was not 110º on a 110º day. It seems to me that the evaporation caused by the bubbler would cause the surface of the water to cool down. The heat from the water below would move to make up the difference, where it is cooled. But is the evaporative cooling enough to cool down the water in the aquarium faster than the room is warming it up? I don’t know. Air is a poor conductor of heat, and the surface water is in direct contact with the water below it.
In the dry southwest we manage to get nearly a 20’F drop with swamp coolers when the humidity is low. In Albuquerque this means they work well enough for all but a handful of days per year.
Watching this thread:
Today I was watching the water episode of Good Eats, and Alton mentioned these porous crocks that people keep water in, and he said that the water in the crock could get colder than the ambient temperature, due to evaporative cooling of the water that was soaking through to the outside of the container. I don’t hold stock in all of his scientific claims, though, and was curious about this.
Isn’t this what causes “lake effect” weather?
I’ve had a hard time finding the article in the past, And I’m not going to look for it now, but some scientist harnessed just that to make a small fridge for use in 3rd world countries. He did this by nesting two pots inside each other with about a one inch gap between them. He then filled the void between them with wet mud and put a cover over the top on the inner pot and he may have insulated the whole thing leaving only the ring of mud uncovered. As the water evaporated it cooled down the inside and apparently not only worked quite well, but it’s basically free. Just have to come up with water to keep the mud wet.
That is how evaportive cooling works. When water changes state from liquid to vapor it requires 970 BTUs per pound. That heat energy comes by cooling the air or the water.
Definitely possible - evaporation consumes a huge amount of heat. Thermodynamically, here’s what you’re looking at:
In a 73 degree F house with 50% relative humidity (assumed) and little direct radiation, your evaporation rate is around 2.7 mm/day (Penman-Monteith equation with an assumed net radiation value of 100 Watts/square meter). Penman-Monteith isn’t the best approximation to use, since it was developed for agircultural water estimates, but it’s a decent first order equation, and I already have a spreadsheet set up for it. A 30 gallon aquarium has a surface area around 30" x 12" (~2300 square cm), so he’s losing about 630 cc of water/day - 0.63 kg.
Water has a latent heat of evaporation of 2,260 kilojoules/kg - this is the amount of heat consumed during the phase change from liquid water to gaseous water vapor. Evaporating that much water requires around 1,400 kilojoules (kJ) of energy - that has to come from somewhere. In this case, it comes from the energy stored (in the form of heat) in the water within the tank.
Water has a heat capacity of 4.18 Joules per cc per degree C - this means it requires 4.18 joules of energy to change the temperature of cubic centimeter of water by one degree celsius. A 30 gallon tank holds ~115,000 cc (30 gallons x 3.84 liters/gallon x 1,000 cc/liter). If you assume the tank is well mixed (reasonable, since there’s almost certainly a bubbler or other circulating pump in the tank), removing 1,400 kJ of heat from 115,000 ccs of water reduces the temperature by 2.9 degrees C, or a little more than 5 degrees F.
73 ambient and 68 water temperature sounds pretty reasonable. If you want to double check (and you’re feeling ambitious), find out how much water he adds to the tank and how frequently he adds it - you can recalculate everything with the actual evaporation numbers.
More than most want to know about Evaporative Cooling:
Clay wine coolers have been around for a long time. The bottle goes in the water that’s in the terracotta. People have been cooling with evaporation for a long time.
evaperational cooling can!
My cite? Human homeostasis working in 99+ degree heat.
The best way I’ve heard of to think of evaporative cooling, BTW, is as follows: inside a container of water, you’re going to have the molecules moving around at all different velocities. Some of these molecules are going to be moving fast enough to escape from the inter-molecular forces keeping them inside the container; these are the ones that evaporate into the air. However, these same molecules take “more than their share” of the total energy with them when they leave, since by definition they were the most energetic ones in the liquid. Thus, the average energy of the molecules remaining in the liquid goes down. (As an analogy: if all the rich folks in the U.S.A. decided to move to the Cayman Islands, the average wealth of residents of the U.S. would go down.) This decrease in average energy per molecule translates to a lowering of the temperature as the water evaporates.
Back when I was a child in Australia, my father bought a canvas water bag for long trips, which you simply hung on the front of the car. Some of the water leaked through the canvas and evaporated, and the rest of the water was cooler than the ambient temperature (which might have reached 40 Celsius).
(And, as The Tao’s Revenge points out, no human could survive for an extended period in temperatures of 40+ without evaporative cooling built into the human body).
That is a good description, best I have ever heard.