Putting a mineral water bottle on an electric fan can make it as effective as an air conditioner? The truth is...

Recently, many short video platforms have released content claiming that a mineral water bottle and a fan can be used to make an energy-saving and emission-reducing "air conditioner." Is this true?

First of all, the "homemade air-conditioning" device of "electric fan + half a mineral water bottle" that has been circulating on the Internet recently is not a new invention.

Mineral water bottles are used as "air conditioners". As early as June 2016, similar devices appeared on foreign websites. At that time, they were named "Zero Electricity" Air Conditioner or Eco-Cooler, claiming to be able to reduce the temperature by 5°C.

This foreign device is essentially the same as the "homemade air conditioner" of "electric fan + half a mineral water bottle" that has been circulating in China recently. The difference is that the foreign ventilation device made of plastic bottles and wooden boards is directly installed on the windows of the room, utilizing the natural wind outdoors and does not even require an electric fan. Therefore, it is a bigger gimmick. If it can be realized, it will be truly "zero energy consumption."

Image and information source: https://wonderfulengineering.com/guy-invents-an-air-conditioner-that-works-without-electricity/

So does this kind of "air conditioning" have any effect?

Whether it works or not, the truth will be revealed in the laboratory

Many people, including middle school physics teachers, have done similar experiments, but no significant cooling effect has been measured. In 2018, as the head of the graduate degree program of subject teaching (physics) at Shanghai Normal University and a professor in the Department of Physics, School of Mathematics and Physics, I supervised an undergraduate graduation thesis in physics, "Testing a "non-power-consuming air conditioner", and conducted a relatively in-depth theoretical analysis and experimental test on this issue.

During the experiment, students found that the temperature drop of air after passing through this device was basically within 0.2℃, which was far lower than the 5℃ circulated on the Internet, and in most cases the temperature of air tended to rise after passing through this device.

As for the case of "fan + mineral water bottle", we can also find that the air temperature passing through the mineral water bottle does not change significantly.

However, many friends have reported that adding a mineral water bottle does make the air cooler. Is this an illusion?

Feeling temperature ≠ temperature

Why do people often feel cooler after adding this device? First of all, we need to distinguish two concepts: actual cooling effect and perceived cooling effect.

The actual cooling effect refers to the cooling effect of real gas, which can be detected by temperature measuring instruments. For real objects, the temperature can be measured by placing a thermometer directly, or by using an infrared thermometer or infrared thermal imager. According to the above, adding a mineral water bottle does not have a significant cooling effect, and may even cause a slight increase in temperature.

The perceived cooling effect is caused by evaporative cooling on the human body surface. The addition of the plastic bottle makes the fan-shaped air outlet smaller, and the air flow rate in a specific area increases, making the human body surface evaporate faster, so the temperature feels lower. However, this is at the expense of a reduced air outlet area of ​​the fan.

In addition, the evaporative cooling effect and the cooling effect are two different concepts. If the human body is replaced with a dry metal ball, the temperature drop of the ball cannot be measured. Of course, if a layer of wet paper towel/wet towel is wrapped around the ball, the temperature drop can also be measured due to the evaporative cooling effect. From the video at the beginning of this article, we can also see that the evaporation caused by air flow can reduce the surface temperature of the wet cardboard by several degrees Celsius.

To understand a phenomenon, we need both experimental data and theoretical support. Next, let’s see how to explain it from a theoretical perspective.

Theoretical calculations

Is it possible for this device installed on the window to have a cooling effect in theory? This is a rather complicated question, which can only be explored by constructing an ideal physical model in theory.

In 2017, a paper in the journal "Physical Bulletin" discussed this issue. The process of the flowing gas entering the room through the shrinking bottle mouth was assumed to be two adiabatic + two isobaric reversible refrigerators. The calculated refrigeration coefficient was:

Among them, α is the ratio of the pressure of the gas after being compressed at the bottle mouth to the pressure before, and γ is the specific heat capacity ratio. For air,

γ=7/5=1.4

Where does the energy of the refrigerator come from? For theoretical derivation, we assume that the kinetic energy of the flowing air (breeze) provides the working energy of the refrigerator. Assume the following: the area of ​​the window where the mineral water bottle is installed is 1 square meter (m2), the electric fan or outdoor wind speed is generally 5m/s, and assuming that the wind speed is perpendicular to the window, then in a unit time of 1s, 5m3 of air (mass is 1.29kg/m3×5m3≈6.5kg) blows towards the window at a speed of 5m/s, and its kinetic energy is:

Assuming the room is 4 meters wide × 5 meters long × 3 meters high, the total surface area is 94 square meters. The thermal conductivity κ of general building materials is between 0.2 and 1 W/(m·K). Take 0.5 W/(m·K) (for example, the thermal conductivity κ of dry bricks is about 0.27W/(m·K). Assuming the temperature difference between indoor and outdoor is 5℃, and the wall thickness is 0.2m, then

dT/dz= 5℃/0.2m=25℃/m

According to Fourier's law, the heat transferred from the outside to the inside in 1 second is:

Q= 0.5W/(m·K) × 25(℃/m) ×94m2 =1175J≈1000J

This heat needs to be transferred to the outside through the refrigerator. Assuming that the kinetic energy of the breeze estimated above is all used to provide energy for the refrigerator (this is only theoretical and impossible to achieve in practice),

It can be obtained that: α≈1.4, that is, the atmospheric pressure of the breeze air flow or the breeze of the electric fan must increase from 1 atmosphere to 1.4 atmospheres during the compression process from the bottom of the bottle to the bottle mouth, which is difficult to achieve. Therefore, from a theoretical analysis, there is a cooling effect, but it is far from a cooling effect of 5℃. The above discussion is to place this device on the window and use the kinetic energy of the outdoor breeze as energy. From a theoretical analysis, it is possible, but the cooling effect is very small.

Back to this "mineral water bottle + fan" device, that is, a device that installs a mineral water bottle at the outlet of an indoor electric fan. As mentioned above, this device has no cooling effect in theory. This is because the electric fan is placed indoors, and it consumes energy. From the perspective of energy conservation, the indoor temperature will only get higher and higher, and it is impossible to achieve the effect of cooling the room like an air conditioner. This principle is equivalent to opening the door of a refrigerator indoors. Obviously, it will not have a cooling effect indoors, but will make the room hotter and hotter.

Editor's note: This experiment has its limitations. It used civilian-grade measuring equipment and was not conducted in a professional laboratory. There may be certain errors. Everyone is welcome to actively think, question and try.

Author: Fang Wei, Associate Professor, Department of Physics, School of Mathematics and Physics, Director of the Postgraduate Degree Program in Subject Teaching (Physics) at Shanghai Normal University

Review丨Zhang Zhongbin Member of the Science Popularization Committee of the Chinese Society of Refrigeration Professor of Nanjing Normal University