Looks like a toy, but a larger-sized version of the same thing sometimes is known as a swamp cooler They work well in dry climates: It takes energy to make water evaporate. Water exposed to dry air will evaporate, and the vapor and the water will become cooler than the ambient air as a result. en.wikipedia.org/wiki/Enthalpy_of_vaporization

Having lived with swamp coolers as my primary cooling system for a number of years, I can attest that (A) thy work very well in dry places like southern New Mexico and (B) on the few humid days a year you either suffer or retreat to a place that has refrigerated air.

And how much degrees could a room with average humidity and 30°C be cooled down with this technique?

Starting from, say, 40 C and 10% RH (a nice summer day in the area where I lived with these thing) you'd get about 15 C cooling. Performance drops rapidly with increasing humidity and slowly with decreasing exterior temperature. But beware that a unit sized for a small building is an appreciable fraction of a cubic meter in size and the fan makes a lot of noise. Even in the places where they work well these things are understood to be a cheaper alternative to a real refrigeration unit: something you use if you can't afford the better option.

@dmckee - I have never regretted buying a house with refrigerated air (central NM). And my wife despised having to deal with her mother's swamp cooler every spring and fall...

@Jon I'm with you. When I told my wife I would probably be offered a job in New Mexico she though briefly before making a single demand: "No swamp-coolers." But they were what was available on the inexpensive rental market when I started grad school, and I was surprised and pleased by how well it worked: they'd be useless in the area where I grew up.

@dmckee - oh, that was a real demand by my wife (she had experience). Now it would be a real demand by me as well!

@SinOfficial - I've been using similar but larger units (nothing fancy from the web, just from your local Home Depot or equivalent) for about a decade now. Don't expect a temperature drop generally in the room, that's not the point. It simply gives you a breeze providing a much better working or living environment: that's the point and they do it well, better than the similar standalone or desk ventilators beause the louvers make it possible to achieve a much better directed airflow that blows very close near you, but still not directly onto you.

@SolomonSlow If it works best in dry climates, why is it called a "swamp cooler", considering that swamps are extremely humid?

@tparker I'm guessing (and this is pretty much a blind guess) that it has more to do with the verb swamp than the noun swamp. You "swamp" the air with humidity.

It says it's an evaporative cooler. Is there any reason to doubt that?

@OrangeDog There is if you don't really understand how evaporative cooling benefits you in this case. OP also mentioned that no heat was leaving, it's a pretty good question that isn't exactly straightforward.

@dmckee: I'm confused, doesn't higher humidity make the heat worse? Why would you want to increase the humidity in a hot house?

@BlueRaja-DannyPflughoeft Because cooler moister air can feel better than hotter dryer air. Sometimes. Depending on a lot of details. In the places where these things are common it is usually dry enough that the added humidity is less interesting than the reduced temperature. But as I said, on humid (for the region) days they were a lot less useful.

I live in the UK, which has a RH of 58% on our least humid month. At this humidity level, swamp coolers do not make an appreciable difference - when it gets hot enough that workers pressure the boss to get some AC equipment in, it's hot enough that a swamp cooler won't stop them sweating. If your area has humidity similar to that of the UK, I'd advise avoiding swamp coolers.

Just a note: you can achieve a similar effect by hanging up a few wet towels (or just your laundry) and pointing a fan at them. This works reasonably well if the air is dry and you don't have a better alternative.

@dmckee Ok, but once it has cooled the air down 15C, the air isn't going to just stay that cold - heat is going to slowly conduct into the structure and raise the temperature back up those 15C, only now the air is already more humid. How do you keep cooling the air when it's going to eventually reach the point where it has too much humidity?

@Michael When used at the scale of a room or building, these are forced air systems that are continuously drawing in a cooling fresh air which is then forced out again by the steady influx. It is common for building cooled by these things to deliberate leak around doors and windows for just that purpose. And yes, that is a pain come winter; just another drawback.

@Michael A ventilation system of some sort. Ideally any building will have some sort of ventilation with the outside, which would dump some portion of the humidity and exchange it with new dry air. Take hot dry air in, exhaust moist slightly cooler air out. The air which has increased humidity will now be lower temperature than the room; therefore heat transfers to that air before it is exhausted, and essentially is able to carry heat out of your house through the moisture.

One of the major questions I have reading some of these prior comments is how they're using it. I bought a similarly small one last summer for Chicago, and after some trial-and-error realized that most of the 1-star comments on Amazon indicated they weren't using it well. In a room that's ~85F (~30C), with about 60-65% humidity, having one of these blowing directly on you (and giving it a bit of time so your sweat can first evaporate) felt like ~70F (~21C). And they only use a few cups of water over several hours, so these small ones don't affect the room's humidity any noticeable amount.