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Wednesday, July 24, 2024

How to Cool an Object Without Using Any Energy

This summer has been hot—one of the hottest on record—and it's probably just going to keep getting hotter. I feel like we are getting to the point where air-conditioning isn't just something nice to have, it's a necessity. There are several ways to cool things off, but the most common method is using a compressor and refrigerant. However, these traditional AC units are difficult (and expensive) to repair, and they consume quite a bit of electricity. For the United States in 2022, 10 percent of the nation’s energy use was devoted to cooling air. That's a lot.

We really need to think about alternative ways of reducing heat. There is another way to decrease the temperature of an object—and it doesn't even require energy or fuel. It's called radiative cooling. By using the right materials, you can get an object to radiate more energy than it absorbs, dropping its temperature by a few degrees. It seems too good to be true, but it works, thanks to some very cool physics ideas.

All Things Emit Light

Everything emits light, and that means everything can transfer thermal energy. That might sound weird, but let’s start by thinking about light bulbs. There are several ways you can make an object emit light, but the simplest method is to just get it super hot. This is what happens with traditional incandescent bulbs: An electric current runs through a filament inside the bulb and makes the filament so hot that it glows. (That temperature is around 3,600 degrees Fahrenheit.) It's simple, which is why that kind of bulb has been around for more than 100 years.

But what about things that are not super hot, like a potato or your favorite pair of shoes or a doorknob? Yes, they also produce a type of light.

Remember that light is an electromagnetic wave, and all of these waves travel at the speed of light (3 x 108 m/s) but with different wavelengths. If the wavelength of this electromagnetic wave is between 400 and 700 nanometers, then we call this visible light, and it is detectable with human eyes. That potato (at room temperature) produces electromagnetic waves with the maximum intensity at 9.8 micrometers. We call this region of the electromagnetic spectrum infrared light. You can't detect it with your eyes, but we can produce an image with an infrared camera.

Here's an example of my dog. Since he's a little bit warmer than his surroundings, he produces slightly different wavelengths of light. This means that in an infrared image, he doesn't just blend into the background.

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There are three ways that objects can have a thermal interaction with other objects. The most common method is through heat conduction. This happens when two objects of different temperatures are in contact, and thermal energy is transferred from the warmer object to the colder object—like when you hold a can of cold soda in your hand. The can warms up and your hand cools down.

The next heat transfer method is convection, and it only works with gases and fluids. Let's use air as an example. Suppose you have a heat source like a stovetop. The air near the stove burner will increase in temperature through a heat conduction interaction. This hotter air now will have a lower density than the colder air above it. It will rise and colder air will take its place. Then the hot air can have another heat conduction interaction with the stuff above it, like maybe the ceiling. The indirect transfer of heat from the stove to the ceiling is convection.

The third type of thermal interaction is radiation—and this is the one we really want. When a hot object emits infrared radiation, that radiation can be absorbed by other objects. This is exactly how your oven works. You put stuff that you want to cook inside, and the heating elements get very hot, producing thermal radiation. (Yes, that's the same as infrared.) The food absorbs this and increases in temperature.

Now imagine that you preheat your oven, then turn it off and stick a potato inside. The hot oven emits thermal radiation and the potato absorbs most of it. The result: The potato gets hotter and the oven gets cooler. This isn’t really a normal way to bake a potato, but the point is that when objects produce thermal radiation, they cool off.

But if everything around us is emitting electromagnetic radiation in the infrared, then shouldn't everything be getting cooler? Not really. If you take an apple and place it on a table, it emits thermal radiation. But it also absorbs radiation from everything else: the table, the air, the walls. So when all the objects in the same vicinity are already the same temperature, they aren't going to cool off by radiation.

Reflectivity vs. Emissivity

There's another very important property to consider to fully understand how radiative cooling works: the difference between reflectivity and emissivity. Imagine you have a perfect mirror. All the light that hits it reflects off of it. That mirror would have a reflectivity of 1, which means that 100 percent of the light that hits it bounces off.

A sheet of aluminum foil also reflects quite a bit of light—but not all the light. It might have a reflectivity of around 0.88, meaning that 88 percent reflects. The other 12 percent of light that falls on the foil is absorbed, increasing the temperature of the foil.

Now imagine an object that doesn't reflect light at all. Of course it still emits light, but only because of its temperature and not because light is reflecting from it. This object would have an emissivity of 1 and we would call it a “perfect black body,” meaning that it absorbs all electromagnetic radiation. So emissivity is essentially the opposite of reflectivity.

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Both reflectivity and emissivity depend on the wavelength of light. Just because something is not very reflective in the visible spectrum (400-700 nm wavelength) doesn't mean it acts the same way for infrared wavelengths (around 10 micrometers). Look again at the infrared image of the dog above. Did you notice that you can see his reflection on the floor? That floor is not very reflective in the visible spectrum; however, it is reflective for infrared.

Here's another way to see the difference between a reflective and emissive surface. Below is an infrared image of two aluminum cans, both at room temperature. The only difference is that the one on the right side has masking tape covering the side of the can, but not the very top. The tape prevents the can on the right from reflecting infrared light, which means these two objects are identical, except for their emissivity. (You can see my hand touching the top of the can on the right.)

The plain aluminum can on the left is very reflective in the infrared region. Although the parts of it shown in orange appear to be hotter, that’s not actually heat from the can—it’s actually an infrared reflection of the heat from my hand, which is touching the other can.

I put the tape on the can on the right just so I could increase its emissivity. Since the tape doesn’t reflect infrared light, the color you see is based on the can’s temperature and not due to hot things like my hand. (Because the very top of this righthand can isn’t covered in tape, that part is still very reflective. That’s why you can still see an orange spot, which is reflecting the heat from my hand.)

Here’s another real world example: On a hot sunny day, is it better to wear white or black clothes? A white shirt (with high reflectivity) reflects more of the sunlight and doesn't get as hot. A black shirt (with high emissivity), on the other hand, absorbs much of the light and gets hot. So that means it’s generally best to wear white—although there are some strange cases where the black clothing might actually be cooler.

Space Is Cold

You have probably already experienced a form of radiative cooling: During the winter, you can tell if it's going to be a cold night by looking at the sky. On a cloudless night, the ground radiates infrared energy, and the loss of this energy makes the ground significantly colder. Not all of that energy escapes: Carbon dioxide in the atmosphere can trap some infrared wavelengths. That’s what causes the greenhouse effect. But a small range of infrared wavelengths, those between 8 and 13 micrometers, can pass through the atmosphere and out into space. (This range is called the “infrared window.”)

This only works if the night is cloudless. Clouds block that infrared window, so the energy just gets reflected back to the ground. As a result, the ground stays warm. It's like the planet is wearing a nice infrared blanket made of fluffy clouds.

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And it doesn’t work during the day. During the day, there is indeed thermal radiation that could decrease the temperature of some objects. However, there is also this other big heat source—the sun. The light from the sun increases the temperature of objects more than the cooling effect from radiation. Overall, everything gets hot.

There’s one odd thing to consider—if you have an object on the surface of the Earth that’s cooling, it seems like that would violate the laws of physics. Stuff doesn't just get colder unless you make something else hotter. For example, your air conditioner cools the inside of your house by warming up the outside air. A can of soda in a cooler of ice decreases in temperature because the ice increases in temperature and melts.

So when an object cools through radiation, some other object must increase in temperature. That object is space. This radiation emitted into space might eventually hit the moon and increase the moon’s temperature—or maybe it will just travel outwards forever.

Radiative Cooling Panels

Is it possible to make things get colder than ambient temperature while the sun is shining? Yup. You can make or build a radiative cooling panel. This would be a flat surface with a high reflectivity in the visible spectrum (to prevent the sunshine from making it warmer) and high infrared emissivity (especially in the 8 to 13 micrometer wavelength). The visible light will reflect off the object so that it doesn’t cause thermal heating, and the infrared radiation will cause it to decrease in temperature. Both the reflected visible light and the infrared radiation go out into space. (Maybe at some point they hit another planet and cause it to heat up—but that’s not really our problem to worry about).

There are a couple of ways to make a radiative cooling panel work. One very simple method uses clear tape on top of reflective aluminum. The visible light passes through the tape and then reflects off the aluminum (so it has high reflectivity), but the tape also allows the material to be emissive in the infrared. This is simple enough that I could try it myself. Below is both a visible and infrared image of a sheet of aluminum foil with a piece of clear packing tape and also a strip of normal clear tape.

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Notice that in the visible spectrum, the foil is very shiny (that's good) and you can't easily see the part with the tape on it. In the infrared, the plain foil looks dark since it's reflecting the infrared from the sky (so it's not very emissive). However, the part with the tape looks much hotter and shows that it is indeed radiating the temperature of the foil. This simple experiment didn't actually get colder than the air temperature since the hot grass below it was probably heating it up more than the radiative cooling effect—but I think it's possible to get this to work.

3M actually makes a radiative cooling tape. This seems to do something similar (but probably better) than the aluminum-foil-plus-tape method.

Another method uses a special white paint. This paint is very reflective in the visible spectrum but emissive in the infrared. There are a couple of great videos that show how this works and how to make it. Here's one from Tech Ingredients. His method seems to work well, but it's not something you could easily make without a laboratory. NightHawkInLight has a different version of the radiative paint that you might be able to do in a normal kitchen.

Another option would include much more complicated materials using nanoparticles or hydrogels. It’s also possible to make clothes that reflect visible light and radiate infrared.

There are two other very cool applications of radiative cooling. You could use the temperature difference between a colder radiative cooling panel and the hotter ground to generate power with a thermoelectric generator. (It would be like a solar panel that also works at night.) Or you could also use the temperature difference created by radiative cooling to condense water right out of the air, just like the moisture vaporators on Tatooine.

The best part is that all of these applications have zero electric input. It's like free cooling—from the sky. None of these methods alone would be enough to replace an air conditioner, because they only shave off a few degrees of heat. But every little bit helps, right?

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