Electric fan + mineral water bottle instantly turns into a "cooling artifact", is there any scientific basis for this?

Recently, a short video of "loading mineral water bottles on electric fans" has sparked heated discussions on the Internet. From the video, we can see that the operator bundled several mineral water bottles together, cut off the bottom of the bottles, and pointed the cut surface at the air outlet of the electric fan, with the bottle mouth facing outward, indicating that this will blow out cold air, "without electricity, and the cooling effect is comparable to air conditioning." Is this method really effective? In this regard, we invited Yan Dadong, professor and doctoral supervisor of the School of Physics and Astronomy of Beijing Normal University, to interpret it for the public from the perspective of physics.

Q: Is there any scientific basis for the claim that homemade gadgets can serve as a drop-in replacement for air conditioners?

Answer : First of all, it should be made clear that the claim circulating online that adding mineral water bottles to electric fans to achieve cooling effects instead of air conditioners has no scientific basis.

Generally speaking, electric fans input electrical energy. According to the law of conservation of energy, this will only increase the total energy in the room and will not bring heat to the outside. Therefore, it cannot be called air conditioning.

One misunderstanding is that some people think that the air will be compressed in the bottle and then diffuse out at the mouth of the bottle, which can be regarded as "adiabatic expansion", resulting in a decrease in temperature. The reason for this misunderstanding is that the air has already heated up during the compression process, and the cooling down of the air when it expands outward at the mouth of the bottle is only relative to the already increased temperature of the mouth of the bottle, not to mention that such a temperature change is very small.

Q: How does the air flow in a plastic bottle?

Image source: Video screenshot

Answer : The wind blows into the bottle from the bottom of the mineral water bottle. Since the bottle mouth is small, the wind will be accelerated at the bottle mouth, so the wind flow rate is fast. Some people may say that according to Bernoulli's principle, the pressure of the air blown out of the bottle mouth decreases. At the same time, the volume of compressed air will also decrease, so according to the ideal gas state equation, the temperature will decrease. This view confuses the applicability of the ideal gas equation, which is applicable to equilibrium, and the pressure reduction caused by Bernoulli's principle is not an equilibrium state.

To sum up, because the wind flows at a fast speed, it is equivalent to turning up the speed of the fan. The human body will feel cooler, but the temperature will not drop significantly.

Question: Some people also say that pointing the fan out the window is more effective than pointing it toward the room. Is there any scientific basis for this?

A : If the surrounding environment can provide a lower temperature than the air (for example, if you live in a bungalow, the temperature of the ground and walls is lower than the air temperature), then if the wind is blown outward, the supplementary air will flow through the ground and walls, thereby lowering the temperature. The premise is to make a good seal between the fan and the window, otherwise the hot air will flow back in quickly. This approach is equivalent to installing an exhaust fan on the wall, so it has little practical value.

Question: From a physics perspective, could you please recommend some tips for cooling down at home?

A: When I was studying at Zhejiang University in the 1980s, I lived in a dormitory on the first floor. The indoor temperature was as high as 32 degrees Celsius in summer. I once placed a basin of water on the desk and used a small ceiling fan to blow air at the basin of water (while closing the curtains), which could reduce the room temperature by 2 degrees.

Physics is a basic subject and the foundation of modern science and technology. The objective world around us, from the quarks that make up protons and neutrons to the entire universe, is made up of matter. I hope that through popular science, we can help everyone better use physics knowledge to rationally view the scientific principles and actual effects of the "air conditioners" circulated on the Internet, and stimulate everyone's curiosity and spirit of exploration of the unknown world.

Expert: Yan Dadong, professor and doctoral supervisor at the School of Physics and Astronomy, Beijing Normal University

Produced by: Science Popularization China x Xinhuanet

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.