Mirror keeps buildings cool by sending infrared radiation into space

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Researchers from Stanford University in the USA have developed a coating to help keep buildings cool in a more energy efficient way. By using a technique referred to as radiative active cooling, the ultrathin, multilayered material reflects sunlight and sends heat from inside the structure directly into space as infrared radiation, preventing the building from heating up and reducing the energy and costs needed to run air conditioning systems.

The research was published in a recent edition of the journal Nature.

As much as 15 per cent of the energy used in buildings in the United States is spent powering air conditioning systems. According to the researchers, the material was created to be cost-effective for large-scale deployment on building rooftops. Though still a young technology, they believe it could one day reduce demand for electricity. 

Infrared radiation is produced when sunlight is reflected off of objects and livings things and produces heat. Stanford's ultrathin coating was constructed to send this infrared light away from buildings at the same precise frequency that allows it to pass through the atmosphere without warming the air. The material can also act as an efficient mirror, preventing 97 per cent of sunlight from striking the building and heating it up.

Together, the radiation and reflection make the photonic radiative cooler nearly 9°F cooler than the surrounding air during the day.

The multilayered material is 1.8 microns thick and made of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver. These layers are not a uniform thickness, but are instead engineered to create a new material. Its internal structure is tuned to radiate infrared rays at a frequency that lets them pass into space without warming the air near the building.

‘This photonic approach gives us the ability to finely tune both solar reflection and infrared thermal radiation,’ said Linxiao Zhu, doctoral candidate in applied physics and a co-author of the paper.

In practice the researchers think the coating might be sprayed on a more solid material to make it suitable for withstanding the elements.

The material could prove to be particularly valuable in countries where resources are scarce, according to Aaswath Raman, lead author of the Nature paper: ‘Across the developing world, photonic radiative cooling makes off-grid cooling a possibility in rural regions, in addition to meeting skyrocketing demand for air conditioning in urban areas’. 

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Further Information

Stanford University