A few months ago, a video was forwarded to me about an air-conditioning unit being developed for developing countries which didn’t require electricity, dubbed the “Eco-Cooler.” It’s not clear to me exactly who or what is behind the idea: the “official” website gives little confidence that it is a serious project, and the videos, while well-produced, seem to be posted through third party accounts and get taken down after a while (this is the original link that was shared with me). In any case, you should be able to search for “eco cooler” on Google or YouTube to find the information I’m talking about, even if these links become defunct.
The science used to explain how the Eco-Cooler works in the video is wrong, and there are others who have already explained this elsewhere (1,2), although unfortunately it seems there’s a lot of noise – incorrect explanations are given along with the correct ones, and skeptics still aren’t sure what to believe. While I’ll take some time to explain why the explanation is wrong, I’m more interested (impressed really) in the experiment the video suggests you try in order to explain the working principle of the Eco-Cooler.
So how does the Eco-Cooler work, according to the video? Air is forced through a nozzle into the house, which pressurizes the air, therefore cooling it. This is bogus. First, pressurizing air heats it, while lowering pressure will lead to a lower temperature. For a real life example of this, you can look at what happens when you use a can of compressed air (or an air horn, or spray paint): as you spray, the can gets cold. When you release air from the can, you’re reducing the pressure inside, and that expansion of gas (not compression/pressurizing) is associated with lowering temperature. Second, the increase in pressure from air flowing through a water bottle nozzle would lead to a negligible change in temperature. If you want to calculate the magnitude of the change yourself, you can use the Joule-Thomson effect, the Bernoulli equation, and conservation of mass – with a back of the envelope calculation I get that air squeezed through the bottle nozzle should heat up by about 0.0001 °C. Finally, even if the air changes temperature as it is squeezed through the nozzle, it will expand as it flows into the room, so the temperature will return to its original value.
So if the scientific explanation they give doesn’t make any sense, why did the video go viral? I think the video’s success is due to the extremely convincing (albeit misleading) “try this yourself” experiment. In the video, the viewer is invited to breathe onto their open palm, first slowly with an open mouth, then quickly with pursed lips. Blowing with pursed lips feels cooler, and they (falsely) claim that this is the same principle which the Eco-Cooler runs on. So what’s actually going on? The air in our bodies is typically warmer than the ambient environment, so if you breathe that air onto your skin, you’ll feel warm (this is step 1 of the video’s experiment). When you purse your lips and blow, the air comes out of your mouth at higher velocity. This leads to more entrained air, that is, the air from your mouth drags along air from the environment with it (this is also how Dyson fans work). As the ambient air is entrained, it mixes with the air from your mouth, lowering its temperature. Overall, the air hitting your hand will be at a higher temperature than the environment (since it’s a mix of high temperature air from your body and ambient air), but it still feels cool since moving air can pull heat out of your body more effectively than still air (this is why sitting in front of a fan feels cool even though the fan doesn’t cool the air at all). To experience this first hand, you can try holding your palm at different distances as you blow with pursed lips. Holding your palm further away should feel cooler, since the hot air from your mouth will have longer to mix with colder ambient air.
So how does the Eco-Cooler actually work? First, I’m not convinced that it does. The video claims the Eco-Cooler can lower the temperature inside a house by 5 °C, but the other content in the video is full of falsehoods, so there’s no reason they couldn’t have just lied about that point as well. That being said, it’s possible that Eco-Cooler could lead to a lower temperature. Air flow through a house will keep the temperature closer to the outside temperature (the house can be hotter than outside because of absorbed sunlight – the same way a car sitting in the sun can get much hotter than its surroundings), and the Eco-Cooler might be more effective than a window because the white panel will reflect sunlight away. However “possible” does not mean “true,” and without much stronger evidence, I am not convinced that the Eco-Cooler is an idea worth pursuing.