# Eco-Cooler - world’s first zero-electricity air cooler made from plastic bottles

#### Gundersen

##### Senior Member.
I stumbled upon the following video on Facebook:

To be honest, this makes very little sense in my head.

I tried to find something regarding heating/cooling of air by compression/decompression, but I came up short. I can easily imagine this working like a fan, creating ventilation. But cooling the air? Furthermore, I would also guess that the degree of actual compression would be quite insignificant. I would quickly call BS, but I might be too early. I would have researched this myself but had a hard time figuring out how to find the relevant physics.

#### Gundersen

##### Senior Member.
Some more googling, I found a principle called Adiabatic processes.

Wouldn't this mean that the initial compression would increase temperature equal to the drop in temperature you would gain from decompression? So no net loss in temperature?

#### Hofnarr

##### Member
Hm.. you can start fires with air compression.
https://en.wikipedia.org/wiki/Fire_piston

I too, can't get my head around how this should work..

#### JRBids

##### Senior Member.
Unless there is a breeze there would be no air going thru the bottles. The homes look congested and up against each other. I can't see how this would work at all.

#### jonnyH

##### Senior Member.
I would also guess that the degree of actual compression would be quite insignificant
I don't think it would come close to that required to achieve the claimed reduction in temperature:
My interpretation could well be wrong, but according to this:

the Joule-Thomson Coefficient for air (mostly nitrogen) at 300K (27°C/80°F) is about 0.36K/bar. This means that at 300K a 1bar change in pressure equates to a 0.22K. The change in pressure required to get the 5°C (5K) reduction would therefore be:

5 / 0.22 = 22.7bar

This is just an estimate as the Joule-Thomson Coefficient decreases when temperature rises, so the hotter the air is to start with, the greater the drop in pressure required to make a 5° fall in temperature. 22.7bar is a conservative estimate as this is reportedly being used where temperatures routinely exceed 300K and the system will not be 100% efficient.

So, in order to achieve a 5°C cooling effect the bottles would have to compress the incoming air to at least 24bar* so that when it expands to normal atmospheric pressure you get the pressure drop needed. That means (in terms of pressure) that the bottles are working at near the operating limits of this thing:

*and somehow avoid the inevitable heating effect of pressurising the air.

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#### Mick West

Staff member
To cool air you have to move the heat somewhere. You can certainly cool a house by compressing air so it's hot, passing it over some kind of heat sink to pipe away the heat, and then expanding it again. It would not be very efficient though, and this "invention" lacks any significant compression, and any heat transfer mechanism. The incoming air can not be any cooler than the outside air.

I suspect from the description: "the same cooling effect as a person blowing air with pursed lips" that all this does is focus the breeze into more intense spouts of air that then feel cooler.

Another possibility is that it provide airflow through a window that normally would have the sun shining though it. So if you compare these possibilities:
1. Window is covered with no airflow or sunlight coming in, the house is an oven
2. Window is closed, but sun is shining through, heating the house, worse case, greenhouse-like
3. Window is open, sun is heating the house, wind is cooling it
4. The "eco-cooler" is installed, sun is blocked, wind is cooling
Then #4 would be coolest, with the additional benefit of a focussed breeze.

#### txt29

##### Senior Member.
I agree with Mick, that creating shade while preserving the air flow is probably the major benefit. It is effect similar to Venetian blinds, used since centuries:

Additionally, as Mick also mentioned, the focused air flow helps too. Focused faster air flow creates so-called "wind-chill" effect - the room temperature with fast moving air does not sink, but it feels cooler, because it helps the sweat evaporation and removing body heat faster than standing air.

The Bernoulli's effect used in some types of passive ventilation (using the difference of air flow and pressure on the roof for sucking the air from the surrounding through the building) is likely not significantly involved in this case. See http://sustainabilityworkshop.autodesk.com/buildings/stack-ventilation-and-bernoullis-principle for some examples of passive ventilation based on Bernoulli's principle and stack effect.

So basically, I'd tell the invention can certainly help in some cases, but I would like to debunk the example with hot and cold air blown with open resp. pursed lips. The arguments used in the video are definitely wrong. The temperature felt on the hand has nothing to do with the compressing the air. This exact problem is very well explained for example at http://www.thenakedscientists.com/H...our-breath-sometimes-warm-and-sometimes-cold/, so I just copy and paste a part of the text here:

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#### Hofnarr

##### Member
1. Window is covered with no airflow or sunlight coming in, the house is an oven
2. Window is closed, but sun is shining through, heating the house, worse case, greenhouse-like
3. Window is open, sun is heating the house, wind is cooling it
4. The "eco-cooler" is installed, sun is blocked, wind is cooling
Then #4 would be coolest, with the additional benefit of a focussed breeze.

i totally agree with, that. nevertheless. option 4 would work without the bottles. Just the board with holes.
Even better without, since there is nonair heating up inside the sun drenched bottles, or am I missing something?

#### Mick West

Staff member
i totally agree with, that. nevertheless. option 4 would work without the bottles. Just the board with holes.
Even better without, since there is nonair heating up inside the sun drenched bottles, or am I missing something?
No, a board with holes would create somewhat less airflow, as there's less catchment area facing the wind.

This all seems like it should be easy to test though.

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