Plastic water bottle "air conditioner"- thermodynamics

Factual Questions
1

I’ve seen recent FB posts and internet sites for an “eco-friendly” DIY “air-conditioner” made from plastic water bottles.

Basically, one drills an array of holes into a board. Into each hole is placed the neck of an empty water bottle, with the bottom cut off. The board is mounted into a window. Air flows into the bases of the water bottles, is compressed in the neck of the bottles, and flows into the room (tin hut).

Supposedly, the device cools the interior of a tin hut by about 5 degrees Celsius. Unfortunately, a lot of conditions are not specified, most especially the baseline conditions.

It purports to utilize Bernoulli’s principle, but folks are skeptical. Here is some thinking from another board, which is largely dismissive, but I’m having trouble following some of the finer points. Bernoulli and Venturi are not so well understood by me. Some wonder if it is basically “opening a window”.

Question: I was wondering if it was at all possible that the compression in the neck of the bottle might slightly heat the air, passing a small amount of heat to the bottle neck and surrounding air. Then the air would cool slightly as it re-expanded. This was not suggested on the physics board. Maybe there is not sufficient time for this kind of heat exchange.

If this is based on some “for real” Bernoulli or Venturi air flow effect (or a “sun-shaded open window” effect) would you not get the same effect with a board of holes without the plastic bottles?

2

I’m going with bogus … as we increase pressure, we’re decreasing volume for the most part. There may be a slight increase in temperature, and a slight amount of this will be absorbed by the plastic … then as the pressure decreases, and volume increases, the final temperature of the air will be slightly lower.

… but 5ºC lower is complete bullshit.

My sense of this is that the air outside the tin shack is already 5ºC cooler, the apparatus does nothing … except perhaps to restrict the air flow into the building … which is counter-indicated, better to have the freer flow.

In a refrigeration circuit, when we compress our fluid, we’re actually compressing it so much that we’re changing it from a gas to a liquid. It’s this change-of-state that warms up the fluid by quite a bit. We then run the liquid through our radiator cooling it off. THEN we cycle the fluid through the compartment and release the pressure, causing the liquid to change back into a gas, and absorbing any energy it can out of the refrigerated compartment.

The critical step is the change-in-state, something completely missing with the apparatus described in your link.

I don’t think Bernoulli’s principle has anything to do with this … but I could be mistaken on that particular point.

3

Maybe it offers a little bit of cooling. It would have to be a really small tin hut to get a 5C drop in temperature. Any cooling of the air would be from the transfer of heat to the throat of the nozzle, the neck of the bottle in this case, so at some point that heat will just transfer back to the incoming air. The pictures are blurry, the video doesn’t play for me, but it looks like the whole bottom of the bottle is cut off which makes no sense to me.

4

From the flaky “One Green Planet” site you link to:

The Eco Cooler works by collecting hot air in the large opening of the bottle. As the air passes through the small neck of the bottle, it compresses and cools naturally. This simple invention drop indoor air temperatures by five degrees, bringing the average temperature down from a stifling 86 degrees to a much more comfortable 77 degrees.

First of all 86 °F (I assume, but they don’t state units) is hardly “stiffling” and is not dangerous, even for the elderly. Outside of direct sunlight, you can’t even suffer heat stroke unless the ambient temperature is significantly above body temperature.

Second, when you compress air (or another compressibly fluid) it heats up because of the mechanical work being done on it. It will cool when it is allowed to expand again, hence why the air coming from a compressed tank is cold, but in order go get net cooling in such a system it has to be allowed to reject heat into a low temperature reservoir. An refrigerating air conditioner works by taking a refrigerant fluid, such as R134a or ammonia, compressing it to a highly pressurized state which forces it to heat up well above ambient temperature, running it through a heat exchanger on the outside to reject this excess thermal energy to the ambient outside air, and then allowing it to expand over a heat exchanger on the inside which chills the inside air.

Third, this thing does nothing to “condition” the air; the air volume entering the device is the same as the air volume leaving the device; there is no thermal exchange mechanism as described above. What it does do, presuming there is a vent of some kind in the roof or on the opposite side of the building, is draw air in and accelerate it by dint of a thermal gradient inside the building. This sort-of forced aspiration may create an accelerated air flow which could have a convective cooling effect similar to fan, and this principle is used in many buildings specifically designed for natural convective cooling. The efficacy of this will depend on the specific geometry and insulation of the building so it isn’t as if you can just put this in any opening and get the same effects, something the linked article touches on not at all.

By the way, there is a design for an air conditioning/refrigeration system which operates without any moving parts (other than the working fluid) and requires only a heat source, which could consist of a salt or thermal reseroir heated by intensified solar energy. It was patented in 1930…by Leo Szilard and Albert Einstein. (Although it is commonly referred to as the “Einstein refrigerator” it was Szilard who did most of the engineering and actually constructed a working prototype, and was itself based upon a slightly earlier design by Swedish RIT engineering students Munters and von Platen.) This is the concept obliquely described in Paul Thereaux’ The Mesquito Coast and was developed for industrial applications by Electrolux for use where there was sufficient waste heat to power it, but there is no reason, other than cost and developing a suitable thermal source, that it couldn’t be used in other off-grid applications. And unlike this plastic bottle “cools by compressing” hand waving McGuyver-esque contraption, is actually based upon sound thermodynamic prinicples rather than handwaving.

Stranger

5

This is what I was going to say. I recall reading that gopher holes’ ventilation works the same way, one opening being higher than the other.

6

Stranger’s Einstein-Szilard refrigerator.

http://www.concretepiano.com/einstein%20sample.pdf

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiKsZXDl-7NAhWGGj4KHZLeBxgQFggeMAA&url=http%3A%2F%2Fphys.org%2Fpdf141581384.pdf&usg=AFQjCNHhPZIbLoYUoeZQlC3yHkxVFroRXg

from the last link (PHYS ORG 25 Sep 2008, Lisa Zyga):

7

That’s really the crux of the matter here; you’re not removing any heat from the equation- you’re compressing it slightly, and then expanding it slightly, without really dropping the heat from the compression before you expand it, unlike the way refrigerators/air conditioners do with their refrigerant.

I think Stranger’s probably right in that this contraption probably increases airflow and convection somehow, and that’s what’s causing the results.

8

If the pop bottle gadget works at all, which I’m inclined to doubt, I can only guess that the board shades the interior from the sunlight while still letting air in (through the bottles). The shade in combination with the air outside being a little cooler might help slightly.
?

9

This was proposed on the physics board.

In this case, would you need pop bottles at all? Wouldn’t a holey board do the job? Would a screen or an awning work better?

10

Maybe not dangerous, but hot as shit for me, with the humidity you get around here. I leak like a sieve at that temp. The difference between 77F and 86F is vast, and I would have no objection with calling the latter “stifling.”

11

Here’s other material on “for real” cooling systems.

http://sustainabilityworkshop.autodesk.com/buildings/stack-ventilation-and-bernoullis-principle

I found this part interesting- the solar chimney is often used in outhouses, and I imagine it could help a tin hut. Trombe wall would be better, but might be difficult to implement in an existing structure. I’m thinking the chimney could be used for a fire when needed. (apparently “hut lung” from burning fires in huts is a huge cause of lung disease in the developing world)

12

The bottles do accelerate the flow and enforce directionality, which may increase the effectiveness of the convction process.

For someone not performing activities requiring extreme exertion, temperatures below 37 °C are not dangerous, and in general, thermoregulation problems at a resting condition for the elderly tend to be at the cold side,where they can’t generate enough body heat to maintain core temperature. Hydration, of course, is an issue for older people, but evaporative cooling will only exacerbate this.

Stranger

13

<hijack>

Smoke doesn’t just mess with your lungs; the toxic components in the gases/particles can be absorbed into your bloodstream and compromise health in other areas of your body. This is especially a problem for women and girls, who do most of the cooking in the developing world. A lot of effort is being put into manufacturing and distributing clean cookstoves, which can do a lot to improve quality (and quantity) of life for these people.

</hijack>

14

Would you notice the acceleration if you were not very close to the bottle wall? I mean is this only effective if you are next to the bottles?

I guess if it turned a faint breeze into a gentle breeze, even if only at close range, that could be better than nothing.

15

Just to clarify (or is that muddy?) the waters, folks tend to get confused with Bernoulli and such. There is a drop in static pressure through the throat of a Venturi tube. Couple that with the Gas Law, PV=NRT and you get the conclusion that T must drop if P has dropped. What is missed is that although static pressure dropped (which is why Venturi tubes work like they do) total pressure remained the same. Total pressure is the sum of static pressure and dynamic pressure. What a Venturi does is trade some of the static pressure of the airstream for an increase in the dynamic pressure of its speed. Venturi tubes are isothermic at low (<M=0.3) speeds, for all intents and purposes.

Folks upstream (ha) have pointed out that acting as a fan-like device it probably feels nice to sit in front of. But there is no actual drop in temperature. As humans, being covered in moist skin, it can feel pretty terrific to be sitting in front of fan-like devices, of course.

16

:confused: The bottlenecks shouldn’t accelerate the flow. If the wind is blowing 10MPH toward the wall, then the maximum stagnation pressure upstream of the bottleneck should correspond to 10 MPH - and so we shouldn’t expect to see more than 10MPH inside the bottleneck.

A garden hose has head loss along its length; by attaching a restrictive nozzle at the end, you lower the total flow rate, which reduces head loss along the hose and puts most of the pressure drop at the nozzle, increasing the velocity of the exiting stream of water. But an ambient breeze is not like a garden nose; putting a restriction at the terminus doesn’t increase velocity at the expense of total mass flow rate, it just decreases total mass flow rate.

17

Well, yeah, that’s what I thought. So are we back to getting a screen window and an awning for our hut?

18

Well… :smiley: :wink:

19

I’m not seeing how the air passing through the bottles is going to accelerate either, considering the very low pressures of a breeze and the fact that the wind isn’t confined in a tunnel that must pass through that area blocked by the shack.

It’s the equivalent of holding a kitchen funnel in a river (wide side facing upstream) and expecting the water to shoot out the narrow end of the funnel at a higher velocity than the surrounding river flow. All that will happen is that the water will for a very short time initially get funneled down to the narrow end, build up pressure, and then that pressure equalize and most of the flow that would be going into the wide end of the tunnel will simply flow around the outside where the pressure is slightly lower. If anything, I’d expect a slightly lower velocity coming out the narrow end of the funnel due to all the friction/drag and turbulence caused.

Maybe the 5ºC drop mentioned comes from cutting a hole in the solid metal sides of these huts and allowing air flow… kind of like opening your oven door (which seems to me like what these tin shacks simulate). They’d probably be best off cutting a hole on opposing sides of the hut and leaving them wide open. But I’m thinking this is more of a feel-good project that gets a bunch of local people doing something in their community and gets them inspired to invent and be creative (despite the fact that the design doesn’t actually work as billed).

20

The 5 degree drop is problematic. My initial video didn’t work- I’ll look for another. But, at any rate, it didn’t show initial conditions. Maybe it is just like opening a window, except keeping benefits of shade and privacy, but losing some airflow.

The physics guys were having conniptions, speculating on the existence of chimneys, etc. but you can’t tell from the video. Just not much support for the “officially” proposed mechanism.