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This summer is one of the hottest on record, and it is likely to get hotter and hotter. The author believes that today's air conditioning is not only a good thing of life, but also a necessity. There are many ways to reduce the temperature of air conditioners, the most common one is to use compressors and refrigerants. However, this traditional air conditioning equipment is difficult to maintain (and expensive) and consumes a lot of power. In 2022, 10% of the energy in the United States will be used to cool air, which is no small number.

It is really necessary for us to consider other ways to cool down. There is a way to lower the temperature of an object without even energy or fuel-radiant cooling. By using the right material, you can make an object radiate more energy than it absorbs, thus lowering the temperature by several degrees. It sounds like boasting, but it's true, thanks to some really cool physics ideas.

Everything can glow, which means that everything can transfer heat. It may sound strange, but don't worry, let's start with the light bulb. There are several ways to make an object glow, and the easiest way is to make it super hot. This is how traditional incandescent bulbs glow: electricity passes through the filament inside the bulb, causing the filament to glow (the temperature is about 3600 degrees Fahrenheit, or 1982 degrees Celsius). This kind of light bulb has been around for more than 100 years because of its simple principle.

But what about things that are not very hot, such as potatoes, your favorite pair of shoes, or doorknobs? The answer is yes, they also produce a kind of light.

Don't forget that light is a kind of electromagnetic wave. All electromagnetic waves travel at the speed of light (3 x 108 mpg s), but the wavelengths are different. When the wavelength of the electromagnetic wave is between 400 nm and 700 nm, we call it visible light, which can be detected by the human eye. The maximum intensity of electromagnetic waves produced by potatoes (at room temperature) is 9.8 microns, and we call the electromagnetic waves in this electromagnetic spectrum infrared light. People can't detect it with their eyes, but they can use infrared cameras to generate images.

Take the author's dog as an example. Because its body temperature is slightly higher than its surroundings, the wavelength of light produced is also slightly different. This means that in the infrared image, it will stand out without blending into the background.

Image source: there are three ways of heat transfer between RHETT ALLAIN objects. The most common way is heat conduction. When two objects with different temperatures come into contact, heat is transferred from the hotter objects to the colder ones. For example, if you hold a can of cold soda in your hand, the jar will get hot and your hand will get cold.

The second way of heat transfer is convection, which applies only to fluids (that is, gases and liquids). Let's take air as an example. Suppose you have a heat source such as a stove, and through heat conduction, the air temperature near the stove will rise. Now, the hotter air density near the stove is lower than the colder air density away from the stove. The hot air will rise and the cold air will sink to its original position. The hot air then conducts heat with things above the space, such as the ceiling. The indirect heat transfer from the furnace to the ceiling is convection.

The third way of heat transfer is radiation, which is what we really want to talk about. When a hot object emits infrared radiation, the radiation is absorbed by other objects. That's how the oven works. If you put what you want to bake in the oven, the heating element in the oven will become very hot and produce thermal radiation (yes, infrared). When the food absorbs heat radiation, the temperature will rise.

Now imagine you preheat the oven, turn off the power, and put a potato in it. The hot oven emits thermal radiation, while potatoes absorb most of it. As a result, the potatoes get hot and the oven gets cold. This is not the normal way to bake potatoes, but the point here is that when objects (ovens) produce thermal radiation, they cool down.

But if everything around us is emitting infrared electromagnetic radiation, isn't everything around us supposed to be getting cold? Actually this is not so. If you put an apple on the table, it will emit heat radiation. But it also absorbs radiation from other objects: tables, air, walls. Therefore, when all objects near the same area have reached the same temperature, they will not be cooled by radiation.

Reflectivity vs. For emissivity to fully understand the principle of radiative cooling, another very important property needs to be considered: the difference between reflectivity and emissivity. Imagine you have a perfect mirror. All the light that hits it will be reflected. The reflectivity of this mirror is 1, which means that 100% of the light shining on it will be reflected.

Aluminum foil can also reflect a lot of light, but not all light. Its reflectivity may be around 0.88, that is, 88% of the light that hits the aluminum foil will be reflected. The other 12% of the light that falls on the aluminum foil is absorbed, raising the temperature of the aluminum foil.

Now imagine an object that does not reflect light at all. Of course, it still glows, but it is only caused by temperature, not by reflection. The emissivity of this object is 1, which we call "perfect blackbody", that is, it can absorb all electromagnetic radiation. Therefore, the emissivity is essentially opposite to the reflectivity.

Both reflectivity and emissivity depend on the wavelength of light. An object with low reflectivity in the visible spectrum (wavelength 400-700 nm) does not mean that it has the same low reflectivity in the infrared wavelength (about 10 microns). And look at the infrared image of the dog above. Did you notice its reflection on the floor? In the visible spectrum, the reflectivity of the floor is not high, but in the infrared, the reflectivity of the floor is very high.

Here is another way to observe the difference between the reflected surface and the emitted surface. The following is an infrared image of two aluminum cans at room temperature. The only difference is that the jar on the right has masking tape on the side, but not on the top. The tape prevents the jar on the right from reflecting infrared light, which means that the two objects are exactly the same except for the emissivity (you can see that the author's hand touched the top of the jar on the right).

Image source: the ordinary aluminum can on the left of RHETT ALLAIN has very high reflectivity in the infrared region. Although the orange part looks hotter, it is actually not the heat emitted by the jar itself, but the infrared reflection of the heat emitted by the author's hand when it touches another jar.

The author pasted the tape on the jar on the right in order to improve its emissivity. Because the tape does not reflect infrared, the color you see is based on the temperature of the jar, not from other hot objects, such as the author's hand (because there is no tape on the top of the jar on the right, so the reflectivity of that part is still very high. That's why you can also see an orange spot, which is reflected by the heat of the hand.

Here is another real-life example: on a hot sunny day, is it better to wear white or black? White shirts (with high reflectivity) reflect more sunlight and are less hot. On the other hand, black clothes (with high emission / emissivity) absorb most of the light and become very hot. Therefore, it is generally best to wear white clothes-although in some special cases, black clothes may actually be cooler.

Space is cold, a kind of radiative cooling that you may have experienced: in winter, you can tell whether it is cold tonight by looking at the sky. On cloudless nights, the ground radiates energy in the form of infrared, and the loss of this energy will significantly cool the ground. Not all energy can be lost: carbon dioxide in the atmosphere traps some wavelengths of infrared, and that's how Greenhouse Effect works. But a small number of wavelengths of infrared, that is, infrared rays with wavelengths between 8 and 13 microns, can pass through the atmosphere into space (this range is called "infrared window").

This method of judging whether a night is cold or not is only useful on a cloudless night. Because the clouds block the infrared window, the energy is reflected back to the ground. Therefore, the ground will stay warm. It's like the earth is covered with an infrared blanket made of fluffy clouds.

Besides, it doesn't work during the day. There is indeed thermal radiation during the day, which can lower the temperature of some objects. But there is another big heat source-the sun. The heating effect of sunlight on an object is much greater than the cooling effect of outward radiation. As a result, everything gets hot.

Here we consider a special assumption: there is an object on the earth that is cooling alone. This seems to violate the laws of physics, because this object will not get cold unless it makes something else hot. For example, an air conditioner cools indoor air by heating outdoor air; a can of soda in the freezer is cooled because the ice around it rises and melts.

Therefore, when one object cools through radiation, the temperature of other objects must rise. This warming object is space. Radiation sent into space may eventually reach the moon, raising the moon's temperature-or it may spread out forever.

Is it possible for the radiation cooling plate to make the object colder than the ambient temperature when the sun is shining? Yes, of course. You can make a radiation cooling plate. This is a plate with high reflectivity (to prevent sunlight from heating it) and high infrared emissivity (especially at wavelengths of 8 to 13 microns) in the visible spectrum. Visible light is reflected from the object so that it does not heat up, while infrared radiation lowers the temperature of the object. Both visible and infrared radiation will go into space (maybe at some point, they will radiate to another planet, causing it to heat up-but that's not something we need to worry about).

There are several ways to make the radiation cooling plate work. A very simple method is to use transparent tape on reflective aluminum. Visible light passes through transparent tape and is then reflected from aluminum foil (so aluminum foil has high reflectivity), but transparent tape also allows the material to emit infrared (that is, improve emissivity). This is very simple, the author tried it myself. Below is a visible and infrared image of aluminum foil with a wide transparent tape and a thin transparent tape.

Please note that the aluminum foil shines so brightly in visible light that it is not easy to see the parts with tape. In the infrared spectrum, ordinary foil looks dark because it only reflects infrared light from the sky (its emissivity is not high). However, the part with the tape looks hotter, indicating that it does make the aluminum foil radiate heat. This simple experiment does not actually make the aluminum foil colder than the air temperature, because the heating effect of the hot grass below may be stronger than radiation cooling-but the author thinks it is possible to succeed.

In fact, 3M has produced a radiation cooling tape that works similar to that of aluminum foil and tape (but may be better).

Another way is to use a special white paint. This paint has a strong reflectivity in the visible spectrum, but a high emissivity in the infrared. There are some great videos showing how it works and how it is made. Here is a video from Tech Ingredients. His method seems to be very effective, but it is difficult for you to make it without a laboratory. NightHawkInLight has a different version of radiation paint, which you might be able to make in a regular kitchen.

Another option is to use more complex materials such as nanoparticles or hydrogels. You can also make clothes that reflect visible light and radiate infrared light.

There are also two very cool applications for radiation cooling. You can use a thermoelectric generator to generate electricity (like a solar panel that works at night) by taking advantage of the temperature difference between the colder radiant cooling plate and the hotter ground. You can also use the temperature difference produced by radiant cooling to condense water directly from the air, just like the moisture evaporator on Tatooine (the desert planet in Star Wars).

Most importantly, all of these applications are zero power inputs. It's like cooling down from space for free. None of these methods is enough to replace air conditioners because they can only cool down by a few degrees. But it kind of works, doesn't it?

Author: RHETT ALLAIN

Translation: machine 7

Revision: floor-sweeping monk

Original link: How to Cool an Object Without Using Any Energy

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: RHETT ALLAIN

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