If air conditioner efficiency improves when it rains outside, why don't all air conditioners include water sprayers to help cool them?

submitted by magnora7 from self.science

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[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (3 children)

Interesting analysis.

Maybe it could only be activated on super-hot days, when the efficiency gains would be particularly high? And the high heat would also guarantee the water would all evaporate quickly so it wouldn't hang around for too long, which seems the source of a lot of the problems.

[–]Tom_Bombadil 3 insightful - 1 fun3 insightful - 0 fun4 insightful - 1 fun -  (2 children)

And the high heat would also guarantee the water would all evaporate quickly so it wouldn't hang around for too long

Not necessarily. It depends on the weather conditions.

If it's a humid day then the evaporation rate drops quickly,. Adding water on a 90 deg day with 85 deg dew point won't get you much.

All machine efficiency depends on your ∆T. So it will always work better on hot days with cooler water. If it's extremely humid then the evaporation rate will be lower.

Evaporation absorbs the most heat energy. 1cal raises 1g of liquid water 1deg C.
The latent heat required to evaporate 1 g of 100 deg C water to 1g of 100 deg C steam is 540 Cal/g. Same situation with ice to water is 80 Cal/g.

Do you have insulation in your attic? Check the R value for efficiency.

If not, then that helps quite a bit. Also, attic ventilation. Get a attic fan that you can use to increase ventilation. Particularly at night when the outside is much cooler.

Moving air transfers heat away if. Static air insulates (tiny pockets is air in Styrofoam). Sometimes the attic fan works against you. It depends.

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (1 child)

So I looked it up, it takes 2.45 Wh to raise the temperature of one gallon by one degree F. So if the water goes from 80F to 120F, that's 98 Wh. That's remarkably close to the 100 Wh break-even.

So if you had access to cheap water, or had expensive electricity, this might actually save money and use less energy overall. Or if the water delta is more than 40 degrees, that'd made it cost effective too.

[–]Tom_Bombadil 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (0 children)

It's way better than that.

That method is for heating water. Evaporation draws many times more heat energy away. Many.

The transition from liquid to gas is 540x the energy to raise the same mass 1 deg C.

You get the best results from evaporation. Your body cools when sweat evaporates. If it doesn't evaporate then you're hot and miserable.

Fans don't cool, but increase the rate of thermal transfer (increased air mass in physical contact per unit time), which also increases the evaporation rate. Moving air can also hold more moisture (from memory, needs verification).