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B Eco-Cooler: plausible? Or snake oil?

  1. Jul 12, 2017 #1

    DaveC426913

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    This invention has made the rounds on YouTube and elsenet.

    In a nutshell, pop bottles embedded in a hut wall draw air in, and supposedly cool the air inside. As far as I can tell, they are claiming it is not just a cooling effect (due to increasing skin evaporation) but a temperature drop, measurable on a thermometer, by as much 5C.

    Certainly, there good reason to be skeptical.

    Can anyone shed some light as to whether there are established principles involved (venturi effect? adiabatic cooling?) - that make this at least plausible?

    Actual demo starts at 1:05
    At 1:25 they get at least one fact wrong in their explanation: they say the bottles "compress and cool" the air. No. If you compress air, you raise its temp.
    At 1:45 they claim a measurable temp drop of 5 degrees.


    Discuss!

     
  2. jcsd
  3. Jul 12, 2017 #2

    russ_watters

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    The way you worded the claim, yes, but the video itself if just chock-full of no.

    If you embed the bottles in a wall, you open up a path for airflow where none previously existing. Warm air exits the top of the hut (or wherever there is another opening) and cooler air enters. But of course, opening a window would be better.

    The explanation they provide is that the bottles act as converging-diverging nozzles, which cools the air. That is, in fact, how air liquefaction plants work. But here's the catch: they start with highly compressed air. Air that has not been compressed does not expand by any meaningful amount. Compressibility is ignored in aerodynamics for pressures that generate airflows below 220 mph. This device operates way, way below what is needed to produce meaningful cooling.
     
  4. Jul 13, 2017 #3

    sophiecentaur

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    Doesn't that involve Joule Kelvin cooling? As I remember, the energy has to 'go somewhere' and in gas liquefaction, there is a heat exchanger before the expansion nozzle, to bring the air temperature down near ambient. Then the JK expansion leaves the air colder. Where is the equivalent heat sink in this system?
    If the conical path tends to speed up the air a bit, cooling by perspiration could be increased and cool people down better. The well know Swamp Coolers achieve a similar thing by passing air over a wet straw bale or equivalent. But the only work where the relative humidity is low so that the emerging (damper) air is still acceptably dry. I have tried one in the UK and it is useless but they are popular in parts of Australia, I believe.
    As russ says, a window would achieve much the same thing (but would let the Sun in if not placed optimally)
     
  5. Jul 13, 2017 #4
    Probably more the result of a clever marketing self promotion.
    Give the guy credit who came up with the advertising campaign.

    PS. If its free and doesn't work as "advertised" who can you sue or demand from to get your money back.
    And prove that it didn't cool your house down if installed, versus what the interior temperature could have been without one.
     
  6. Jul 13, 2017 #5

    russ_watters

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    Yes. The effect is actually very similar to a refrigerator/air conditioner. You could make an open-cycle air conditioner that uses the air itself as a working fluid and works as [partly] described in the link. But here, there's no compressor and no heat exchanger, both of which are required for it to work.
    Yes, but only insofar as velocity can be a proxy for volumetric flow rate. If moving the air faster gets it where it needs to go, that's great, but in terms of cooling a room, even the claimed velocity increase comes at the expense of volumetric flow rate, which decreases the amount of cooling done in the room.
     
  7. Jul 13, 2017 #6

    sophiecentaur

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    except that it could cause more evaporative cooling of sweat. In the 'west' we would use an electric fan perhaps.
     
  8. Jul 13, 2017 #7

    russ_watters

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    I'm really not trying to be nitpicky here; it is volumetric flow rate, not velocity, that determines cooling rate. They are often proportional and can be used interchangeably, but in this case they are not.
     
  9. Jul 13, 2017 #8

    sophiecentaur

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    Is that true for forced evaporation? It doesn't feel right to me. But perhaps there's some compromise here. Perhaps a faster, narrow flow would have more velocity and, hence greater local flow rate.
     
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