This summer has been really hot, arguably one of the hottest ever. And, we expect the weather to get hotter and hotter. At this stage, I feel that air conditioning is no longer a luxury, but a must-have. Of course, there are many ways to cool down, but our preferred method is to use machines and special gases. However, repairing old-fashioned air conditioners can cost a lot of money, and they also consume a lot of electricity. Surprisingly, according to 2022 data, the United States uses a full 10% of its energy for cooling. That's a really big number!
We need to explore other ways to combat the heat. There is a unique way to cool the object, and it doesn't require any power or fuel. This is radiative cooling. By using special materials, we can make the object release more energy than it absorbs, allowing it to cool down by a few degrees. Sounds a little incredible. But it's real, thanks to some very interesting principles of physics. In fact, all objects emit light, which means they can transfer heat. It sounds a little weird, but imagine a light bulb. There are many ways to make objects glow, but the easiest way is to heat them. A traditional incandescent bulb works like this: the filament is heated by an electric current until it shines. It's a basic principle, but it's also because of this that incandescent light bulbs have been around for more than a century.
However, for things that aren't so hot, like potatoes, your favorite shoes, or doorknobs, do they glow too? Yes, they also emit a specific type of light. What we have to remember is that light is actually an electromagnetic wave, and they travel at the speed of light (3x10 8m s), just at different wavelengths. When the wavelength is between 400 and 700 nanometers, this is the visible light that our eyes can see. Whereas, the wavelength of electromagnetic waves emitted by potatoes at room temperature is mainly in 98 microns. This belongs to the infrared part of the electromagnetic spectrum. While we can't see infrared light directly, an infrared camera can capture it and display it in the image.
Use a dog as an example. His body temperature is slightly higher than that of the people around him, so his light waves in the infrared ** are slightly different. This makes him more eye-catching in ** rather than blending in with other objects. Now, let's take a closer look at the thermal interactions between objects. The main way to achieve this is through heat conduction. Imagine this: when two objects with different temperatures come into contact, heat is transferred from the hotter to the cooler object, as if you were holding an ice-cold soda can. The jar will get hot, and your hands will get cold.
So, what we're going to talk about is a type of heating called convection, which only happens in gases and liquids. Let's take air as an example. If you have a heat source, it's like a stove. When the air next to the stove comes into direct contact with the stove, it gets hotter. Now, these hot gases will be less dense than the cold air above it, so it will rise and occupy the position of the cold air. Next, this warm air interacts with objects above, such as the ceiling, to transfer heat indirectly from the stove to the ceiling, which is the process of convection.
The third type of heating is radiation, and when one hot object emits infrared rays, other objects can absorb them. Let's take an oven as an example. When you put food in a preheated oven, the heat element releases heat radiation. This radiation is absorbed by the food, causing it to heat up. When preheat the oven, then turn it off and put in the potatoes again. The heat radiation from the oven is gone, and most of it is absorbed by the potatoes. Then the potatoes will get hot, and the oven will get cold. This process shows how an object cools down when it releases thermal radiation.
If everything around us was emitting infrared rays, would everything get colder?Actually, not exactly, let's take an apple as an example to explain. When you place it on a table, it emits heat radiation, but it also absorbs light from all objects around it, such as tables, air, and walls. When the surrounding objects are all at the same temperature, they do not cool each other by radiation. To truly understand how radiative cooling works, there is one key thing to consider, and that is the difference between reflectance and emissivity. Like a perfect mirror, it reflects every bit of light that hits it. The mirror has a reflectivity of 1, which means that it reflects all the light it receives.
A piece of aluminum foil reflects some of the light, but not all of it. It may reflect back 88% of the light, which means that about 12% of the light will be absorbed, causing the foil to heat up. And those objects that don't reflect light at all. The light they emit is only because of their temperature, not the light coming back from their surface. These objects have an emissivity of 1 and are called"Perfect blackbody"。They are able to absorb all electromagnetic radiation. Therefore, emissivity and reflectivity are almost opposite.
Reflectance and emissivity vary with the wavelength of light. Some objects may not reflect light in the visible range, but may react differently in the infrared range. If you look at the puppy's infrared** again, you'll notice its reflection on the floor. This is because the floor reflects light in the infrared range, although it reflects very little in the visible range.
As an example, we can clearly contrast reflective and emitted surfaces. For example, there are two aluminum cans, both of which are at room temperature. Will they be any different. The aluminum can on the right has masking tape on the side, but not on the top. This tape prevents the canisters from reflecting infrared light, so both canisters are the same except for the emissivity.
The ordinary aluminum cans on the left glow brightly in the infrared region. They look hotter, but it's not the jar itself, it's the heat from my hands that is reflected in the infrared. And the jar on the right I sealed it with duct tape, which would increase its emissivity. The tape prevents the jar from reflecting infrared rays, so you can only see the temperature of the jar itself and not be affected by my hands. However, since there is no tape attached to the top, it still reflects light. That's why you see an orange spot on the top, and that's because the heat of my hand is reflected in that part.
Here's a real question: is it better to wear white or black on a hot day? White clothes are highly reflective and reflect more sunlight to keep them cool. Whereas, black clothes have a high emissivity, absorb a lot of light and generate heat. So, in general, choosing white clothes is a better choice, although some people may find black clothes cooler. Also on clear winter nights, the ground releases infrared energy to cool itself. However, not all infrared rays escape. Some specific infrared rays, located in the range of 8 to 13 microns, are able to pass through the atmosphere into space, which is known as the "infrared window".
This phenomenon only happens at night when it is clear. Clouds can obscure the infrared window, causing energy to return directly to the ground, keeping it comfortable. It's like the earth is wrapped in a blanket of fluffy clouds. However, this does not happen during the day. Of course, it is possible for thermal radiation to make some objects colder. But the point is that there is a huge source of heat for the sun. The radiation of the sun warms the object more than the effect of radiative cooling. So, in general, everything will be warmer.
When an object on the Earth's surface cools, it can defy the principles of physics. An object does not automatically get cold unless the other object gets hotter. It's like your air conditioner, which cools the indoor air by heating it out. Or like a soda can placed in an ice cooler, its temperature drops as the ice melts. So, when one object is cooled by radiation, the other is being heated. There is also radiation in space. This radiation can reach the Moon, warming it slightly, or continue to spread into the unknown.
When the sun is hot, is it possible to create a radiative cooling panel that can reduce the temperature of the object to a lower temperature than the surrounding temperature? Like a flat surface, it is extremely reflective in visible light and excels at emitting infrared light. It avoids absorbing heat by reflecting visible light and lowers its own temperature by emitting infrared rays. Both reflected light and infrared light escape into space, and they could warm another planet.
There are a few tricks you can use to get your radiant cooling panel working. One of the easy ways to do this is to use scotch tape to glue to reflective aluminum. The tape allows visible light to pass through and reflect on aluminum, but still allows the material to emit infrared light.
In the visible spectrum, the foil shimmers, while the tape section is less conspicuous. Now, let's switch to the infrared image, the normal foil looks duller, it reflects the infrared light from the sky, but the tape part is hotter. This is obviously because it is radiating heat from the foil.
3M has created a product called Radiant Cooling Tape, which works similarly to combining aluminum foil and tape. Another way is to use a special white paint. This paint reflects a lot of light in the visible range, but at the same time emits infrared light.
There is another method that involves the use of composite materials with nanoparticles or hydrogels. It is even possible to create clothing that reflects visible light and emits infrared light. There are two other ingenious applications of radiative cooling. One is to use the temperature difference between the cooler radiant panels and the warmer ground to generate electricity through a thermoelectric generator, a bit like solar panels running after sunset. Another application is to use the temperature difference of radiative cooling to condense water directly from the air, similar to Tatooine's moisture vaporizer.
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