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Radiative Cooling Without Power
Radiative Cooling Without Power
Imagine a device that can sit outside under sunlight on a clear day, and without using any electric power cool things down.
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Imagine a device that can sit outside under blazing sunlight on a clear day, and without using any electric power cool things down by more than 23 degrees Fahrenheit. It almost sounds like magic, but a new system designed by a team of Chilean and MIT researchers can do exactly that.

The device, which has no moving parts, works by a process called radiative cooling. It blocks incoming sunlight to keep from heating it, and at the same time, efficiently radiates infrared light—which is essentially heat—that passes straight out into the sky and into space, cooling the device significantly below the ambient air temperature.

The key to the functioning of this simple, inexpensive system is a special kind of insulation, made of a polyethylene foam called an aerogel. This lightweight material, which looks and feels a bit like marshmallow, blocks and reflects the visible rays of sunlight so that they don’t penetrate through it. But it’s highly transparent to the infrared rays that carry heat, allowing them to pass freely outward.

The new system described recently in the journal Science Advances could be used as a way to keep vegetables and fruit from spoiling, potentially doubling the time the product could remain fresh, in remote places where reliable power for refrigeration is not available. Radiative cooling is simply the primary process that most hot objects use to cool down. They emit midrange infrared radiation, which carries the heat energy from the object straight off into space because air is highly transparent to infrared light.

The new device is based on a concept the researchers demonstrated a year ago, which also used radiative cooling but employed a narrow strip of metal, to shade the device from direct sunlight to prevent it from heating up. That device worked, but it provided less than half the amount of cooling power that the new system achieves because of its highly efficient insulating aerogel layer.

Aerogels are lightweight materials that consist mostly of air and provide very good thermal insulation, with a structure made up of microscopic foam-like formations of some material. The team’s new insight was to make an aerogel out of polyethylene, the material used in many plastic bags. The result is a soft, squishy, white material that’s so lightweight that a given volume weighs just 1/50 as much as water.

The key to its success is that while it blocks more than 90 percent of incoming sunlight, thus protecting the surface below from heating, it is very transparent to infrared light, allowing about 80 percent of the heat rays to pass freely outward. The result is that it can dramatically cool down a plate, made of a material such as metal or ceramic, placed below the insulating layer, which is referred to as an emitter. That plate could then cool a container connected to it, or cool liquid passing through coils in contact with it.

To confirm its effectiveness, the team set up a proof-of-concept device in Chile’s Atacama desert. It achieved a cooling of 23 degrees Fahrenheit under full sunlight at solar noon. That’s enough cooling to make a significant difference in preserving produce in remote locations, the researchers say. Also, it could be used to provide an initial cooling stage for electric refrigeration, thus minimizing the load on those systems to allow them to operate more efficiently with less power.

Theoretically, such a device could achieve a temperature reduction of as much as 90 degrees Fahrenheit. So, the researchers are continuing to work on ways of further optimizing the system so that it could be expanded to other cooling applications such as building air conditioning without the need for any source of power.

With increasing affluence in the tropics and global temperatures forecast to rise in the coming decades, this low-cost cooling solution could be a game-changer.

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