A few days ago, many short video platforms released content saying that relying on mineral water bottles and fans, you can make an energy-saving and emission-reducing "air conditioner". So is this true?
To do this, we did a little experiment, directly on the video:
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From the previous simple experiment video, it can basically explain the problem. However, for the sake of rigor, let's sort it out completely.
First of all, the "homemade air conditioner" device of "electric fan + half-cut mineral water bottle" that has been circulating on the Internet recently is not the latest invention.
As early as June 2016, there was a similar device on a foreign website, which was named "Zero Electricity" Air Conditioner or Eco-Cooler, claiming to be able to cool down by 5°C.
This foreign device is essentially the same as the "self-made air conditioner" of "electric fan + half section mineral water bottle" that has been circulated in China recently, the difference is that the ventilation device composed of plastic bottles and wooden boards is directly installed on the window of the room, using the natural wind outdoors, and even the electric fan is not used, so the gimmick is larger, if it can be realized, it will be a real "zero energy consumption".
Image and information source: https://wonderfulengineering.com/guy-invents-an-air-conditioner-that-works-without-electricity/
So does this "air conditioner" have any effect?
It doesn't work, and you can see it in the lab
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 discipline teaching (physics) at Shanghai Normal University and a professor of the Department of Physics of the School of Mathematics and Physics, the author supervised an undergraduate dissertation on physics "A Test of a "Non-power-consuming Air Conditioner", which also conducted in-depth theoretical analysis and experimental testing on this issue.
In the experiment, the students found that the temperature drop of the air after passing through the device is basically within 0.2°C, which is much lower than the 5°C reported on the Internet, and in most cases, the air temperature after passing through the device tends to rise.
As for the case of "fan + mineral water bottle", in the video at the beginning of this article, we can also find that the air temperature through the mineral water bottle does not change significantly.
However, there are indeed many friends who report that adding mineral water bottles does blow cooler, is this a delusion?
Perceived temperature ≠ temperature
Why does the addition of this device make people feel cooler? This starts with distinguishing between two concepts: the actual cooling effect and the somatosensory cooling effect.
The actual cooling effect refers to the cooling effect of the real gas, which can be detected by a thermometer. For actual objects, the temperature can be measured by placing a thermometer directly, or by means of an infrared thermometer or thermal imaging camera. As mentioned earlier, the addition of mineral water bottles does not have a noticeable cooling effect, and may even cause a slight increase in temperature.
The somatosensory cooling effect is caused by the evaporative cooling of the human body surface. Due to the addition of plastic bottles, the fan-shaped air outlet of the electric fan is smaller, and the air flow rate increases in a specific area, so that the surface of the human body evaporates faster, so the temperature is reduced. However, this comes at the cost of a reduction in the air outlet area of the electric 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 pellet, the decrease in the temperature of the pellet cannot be measured. Of course, if a layer of wet wipes/wet towels is wrapped around the ball, the temperature drop can also be detected due to the evaporative cooling effect. As we can see from the video at the beginning of this article, evaporation caused by the blowing of air can reduce the surface temperature of wet cardboard by several degrees Celsius.
In order to understand a phenomenon, we need both experimental data and theoretical support, so let's see how to explain it from a theoretical perspective.
Theoretical calculations
Is it theoretically possible for this window-mounted device to have a cooling effect? This is a complex problem that can only be explored by theoretically constructing an ideal physical model.
In 2017, there was a paper in the journal "Physical Bulletin" to discuss this problem, by assuming that the process of flowing gas entering the room through the shrunken bottle mouth is two adiabatic + two isobaric reversible refrigerators, the calculated refrigeration coefficient is:
where α is the ratio of the pressure of the gas after compression at the mouth of the bottle to the previous pressure, and γ is the specific heat capacity ratio, for air
γ=7/5=1.4
Where does the energy come from for chillers? For the sake of theoretical derivation, we assume that the kinetic energy of the moving air (breeze) provides the operating energy of the chiller. Make the following assumptions: 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, assuming that the wind speed is perpendicular to the window, then there is 5m3 of air (mass of 1.29kg/ m3×5 m3≈6.5kg) blowing towards the window at a speed of 5m/s within 1s per unit time, and its kinetic energy is:
Assuming that the room is 4 meters wide× 5 meters long × 3 meters high, the total surface area is 94 square meters, and the thermal conductivity κ of general building materials is found to be between 0.2~1 W/(m·K), and 0.5 W/(m·K) is taken (such as the thermal conductivity κ of dry bricks is about 0.27W/(m·K), assuming that the temperature difference between indoor and outdoor 5°C is maintained, and assuming that the wall thickness is 0.2m, then
dT/dz= 5℃/0.2m=25℃/m
Then the heat conducted from the outdoor to the indoor in the unit of 1s from Fourier's law is:
Q= 0.5W/(m·K) × 25(℃/m) ×94m2 =1175J≈1000J
This heat is to be transferred to the outside through the chiller, assuming that the kinetic energy of the breeze estimated earlier is all used to provide energy for the chiller to work (which is only theoretical, and practically impossible), by
It can be obtained: α≈ 1.4, that is, it is difficult to achieve the atmospheric pressure that requires the breeze air flow or electric fan to increase from 1 atmosphere to 1.4 atmospheres during the compression process from the bottom of the bottle to the mouth of the bottle. Therefore, from the theoretical analysis, there is a cooling effect, but it is far from a cooling effect of 5°C. The above discussion is to put this device on the window, through the outdoor breeze kinetic energy as energy, from the theoretical analysis, there is this possibility, but the cooling effect is very small.
Back to the "mineral water bottle + fan" device, that is, the device that installs the mineral water bottle at the outlet of the electric fan in the room, as can be seen above, this device has no cooling effect in theory. This is because the electric fan is placed indoors, and the electric fan needs to consume energy, and from the perspective of energy conservation, the indoor temperature will only get higher and higher, and it is impossible to cool the room like an air conditioner. This principle is equivalent to opening the door of the refrigerator indoors, which obviously does not make the room have a cooling effect, but will make the room hotter and hotter, and the temperature is getting higher and higher.
Editor's note: This experiment has its limitations, it uses civilian-grade measuring equipment, and it is not operated in a professional laboratory, there may be certain errors, everyone is welcome to actively think, question and try.