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Old 08-05-2018, 07:57 AM   #1 (permalink)
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The laws of thermodynamics and ice chest coolers

I keep seeing DIY evaporative coolers using ice and\or frozen water bottles. This one claims to be "Very smart !" https://imgur.com/gallery/hwQbkDb

It definitely is not smart and I am not sure that it is very anything. Who puts spaces before punctuation?

Savages.

This is the only version that I have seen that blows cool air straight up, instead of in the general direction of carbon-based life forms.

There is a law irrevocably decreed in the intersewers that if someone makes something, people will insult them for not spending more money on something commercially available and vaguely similar. It is also human nature to latch onto bad ideas and defend them vehemently, so there are very mixed camps with these.

Yes, they do blow cool air, for limited time and duration, and each day you need more ice, which requires refrigeration, but they are trying to show how clever they are by not using refrigeration.

Directly.

People claim that you can pick up a window unit (if you have windows that you can use) for $100. The cheapest one that I saw at Walmart.com was $125.40 with tax, 5,000 BTUs, and allegedly adequate for 150 square feet. Of course, another question is how much electricity it uses, and people make various claims there, but I think there are too many factors to satisfy easily. How large is the room, how well insulated, are there any heat sources? How many BTUs do you need to reduce the temperature, and what is the humidity. How much does electricity cost?

"Just purchase a bag of ice every day." If you can run to a gas station and be back in five minutes that is one thing, but one five-minute thing every day for ninety days? If that bag costs $1.11 and you purchase ninety, you will spend $99.90, plus travel, etc., which is 80% the cost of that window AC unit, although that may cost another $20 a month in electricity.

How do they compare?

I read that it takes 144 BTUs to melt one pound of ice. A seven-pound bag would effectively have 1,008 BTUs, compared to that 5,000 BTU unit.

One guy said showed his big cooler with six three-quart water bottles that he froze every night.

How much does that cost?

People have made claims about the cost to make ice. Allegedly it is expensive. To make seven pounds you would need ten or twelve ice cube trays. That kid was putting eighteen quarts of water in his freezer each night. This says that it costs a little over half a cent to make a pound of ice, so perhaps 4 per night, but that is a commercial ice machine, and the refrigeration unit creates more heat.

People claim they make these for five, ten, or twenty dollars, but often include the cooler, fan, and perhaps other things that they had lying around, but others wouldn't necessarily have for free. This guy says he spent $100 and he includes everything:

He used a 20" box fan, ran plastic tubing through the ice, and connected that to copper tubing attached to the fan. The thing is, he is trying to cool down a two-car garage, which may not be insulated. The garage door definitely isn't insulated, but it has direct sunlight, it was 100 in Southern California (queue violin music) and the sun hitting his garage door hit 150, making his garage, where he worked, 110 - 120.

I think there are some more important questions.

Can he avoid working during the hottest part of the day? Can he spend $60 on foam board insulation? Are the walls insulated? It cost me just over $100 to insulate a wall and a half in my mother's garage. Is he committed to the cute windows? California Civic linked $13 window film, but covering them would be even better.

Can he put something in front of the garage door, with an air gap?

I see many ways that he could spend a hundred dollars and cool his garage more effectively. To properly cool his garage he would need wall and door insulation, window film (or cover), a 4" hole in a wall, and a $600 air conditioner. The Walmart one is rated 2.8/5. The Frigidaire one at Home Depot is rated 3.8/5.

The King of Random('s sidekick Nate) improved upon Household Hacker's design and tested variations:

This guy made a $350 one that uses rechargeable batteries for tailgating in Phoenix:

People politely insisted that he did not understand physics, but claimed that it would be a waste of energy to attempt to explain them to him.

Are the thermodynamic problems limited to freezers creating heat?

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Old 08-05-2018, 08:37 AM   #2 (permalink)
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No, you have heat and mass transfer limitations.
In a nut shell a cooler full of ice can only very slowly soak up a little bit of heat even with a fan blowing on it, from a relatively large room with bad heat infiltration.

Since I understand thermo a lot more than the average person but a lot less than a mechanical engineer I made my ice chest cooler rig to pump ice water through about 80 ft of plastic line that goes in contact with your clothing. I know enough to understand why a few pounds of ice will never be able cool a room or the inside of a car, but it can cool a person for a few hours.

You can effectively cool a room with ice and fans but it takes at least hundreds of pounds of ice and fans.
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Old 08-05-2018, 02:20 PM   #3 (permalink)
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Thank you for the explanation. This page described ice-based air conditioners that require far more room, but since they pay much less for electricity at night, they save money overall: https://home.howstuffworks.com/green...e-block-ac.htm
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Old 08-05-2018, 07:35 PM   #4 (permalink)
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Unless you are living in some screwed up like California, chanes are you pay the same for power night or day for residential service.

I already provided over on ecorenovator that adding evaporative cooling to home A/C with a mechanical txv can reduce power consumption by up to 25%.
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Old 08-05-2018, 07:46 PM   #5 (permalink)
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Thermostatic Expansion Valves? You can potentially save money in the Phoenix area by being on a time-of-use plan. I had a friend whose family turned off air conditioning from 9 - 3, which he hated for good reason. I believe that we were on a similar plan. Our landlord just asked us to not use appliances during the day, but as always, my roommates did as they pleased.
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Old 08-05-2018, 07:48 PM   #6 (permalink)
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I leave mine off most days now that I'm working nights.
Working nights saves at least $100 per month in power.
When I do run it, it runs at night.
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Old 08-06-2018, 11:00 PM   #7 (permalink)
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Quote:
There is a law irrevocably decreed in the intersewers that if someone makes something, people will insult them for not spending more money on something commercially available and vaguely similar. It is also human nature to latch onto bad ideas and defend them vehemently, so there are very mixed camps with these.
Why do you ask? Intellectual curiosity or you want to be cool?

My experience is that I use a whole house fan and only 'need' the air conditioner on the hottest days. The air pulled in by the fan can come across my back from a window, or straight down if I open a ceiling vent. It exhausts straight up into an aluminum mushroom cap. I've upgraded once already and I have a third vent to be installed as part of a solar collector.

The air conditioner is old and beat-up but it still works. I'm eyeing replacements against the day, I see them secondhand for $25-35.
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Old 08-06-2018, 11:27 PM   #8 (permalink)
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Old 08-07-2018, 02:16 PM   #9 (permalink)
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I picked up a fairly high efficiency GE A/C unit from Craigslist for $25. There are tons of fairly recent model air conditioners on Craigslist around here for pennies.

I make an effort to use shades and ventilation to control temperature in my apartment. I only run the air conditioner when I'm uncomfortable. Sometimes I dunk my head in cold water and point a fan at me, and let evaporation do the work.
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Old 08-07-2018, 02:28 PM   #10 (permalink)
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There is a high efficiency window unit made by LG that uses variable speed conpressors and expansion valve.

What I would do when I was using window units is I would take my old ones, sell them before they die and buy a new one every 2 or 3 years. Sell the old one so I wouldn't have to eat the entire cost of a new unit.
Now I use high efficiency split units.

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1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 1LE drive shaft, 4th Gen disc brake fbody rear end.
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