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[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (10 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 -  (9 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.

[–]dark_devil_dd 3 insightful - 2 fun3 insightful - 1 fun4 insightful - 2 fun -  (1 child)

This guy thermodynamics. 👍

Did u study chemistry or engineering?

[–]Tom_Bombadil 1 insightful - 2 fun1 insightful - 1 fun2 insightful - 2 fun -  (0 children)

I'm a wainwright, with a cobbler minor.

;-)

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (4 children)

Yeah I might get an attic fan, I was looking in to that today too. My temp gun read out 165F on the attic ceiling, and 145F on the floor. Pretty dang hot.

I tried a little experiment and put a gallon of water on the external unit. It cooled it from 140F to 95F, and it evaporated in about 90 seconds because of the heat. Then about 20 minutes later it was back up to 140F.

From a cost perspective, a gallon of water costs 1 cent. 1 cent of electricity is 100Wh. I think it'd be very hard to save 100Wh with one gallon going from 80F to 140F, not sure how many Joules (or watt-hours) that is.

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (2 children)

Try the same experiment with a mister than can be directed at the cooling fins.

Dumping a gallon of water is a waste.

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

I sprinkled the gallon around the entire thing, I didn't just dump it

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

I'm not sure that I've ever heard anyone say they've sprinkled a gallon.

I'm sure you did, cause it was hot AF, and you were on a mission to fix it! ;-)

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Definitely go with the attic fan.

The attic fan should blow the air in the attic to the outside.
There should be a separate vent in the attic that allows air to be drawn from the outside in.
The fan will create negative pressure (it's actually not negative) inside the attic, which will draw in cooler air.

The same applies in your house with window fans. Open windows in separate rooms/areas. Blow the hotter air out of the house (from the hot room, which is typically upstairs or a sun lit room) and let the colder exterior air draw in from the other room.

Blowing cool air into the house is much less effective.

Allow the negative pressure to work for you.

[–]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).