High-Tech Mirror Could Reduce The Need For Air Conditioning

Chuck Bednar for redOrbit.com – Your Universe Online
High-tech mirror beams heat away from buildings into space
Engineers from Stanford University have developed a new ultrathin, multilayered material capable of cooling down buildings by radiating warmth from the inside out into space and reducing incoming heat by reflecting sunlight.
The revolutionary coating, which is described in a research paper published online Wednesday in the journal Nature, is designed to deal with both visible and invisible light in the form of infrared radiation. The authors explain that infrared radiation is one of the ways in which all living beings and all objects give off heat, but their new material diverts it away from buildings.
Stanford electrical engineering professor Shanhui Fan, research associate Aaswath Raman and their colleagues refer to the technique used by the coating as photonic radiative cooling, a combination of offloading infrared heat from inside a building while also reflecting the sunlight that would otherwise have caused it to grow warmer.
“This is very novel and an extraordinarily simple idea,” Eli Yablonovitch, director of the Center for Energy Efficient Electronics Science and an engineering professor at the University of California, Berkeley, said in a statement Wednesday. “As a result of professor Fan’s work, we can now [use radiative cooling], not only at night but counter-intuitively in the daytime as well.”
The material, which was designed to be cost-effective for large-scale deployment on building rooftops, could reduce the need for buildings to use air conditioning, the study authors said. While the technology is still in its formative stages, they believe it could reduce the demand for electricity, especially the 15 percent required to power air conditioning systems in the US.
Raman, who was the lead author of the Nature paper, said that there is a need to find cooling technologies that do not require power, and that “photonic radiative cooling makes off-grid cooling a possibility” in rural regions and across the developing world. It would also help meet “skyrocketing demand for air conditioning in urban areas,” he added.
Heat can be transferred in three different ways: through touch (conduction), through movement of air or fluids (convection) or as infrared light (radiation). The new coating developed by the Stanford engineers first radiates heat-bearing infrared light directly into space, sending it away from buildings at a frequency that allows it to pass through the atmosphere without causing harm to the air. It also serves like a mirror, reflecting sunlight away from the building.
“Together, the radiation and reflection make the photonic radiative cooler nearly 9 degrees Fahrenheit cooler than the surrounding air during the day,” the university explained. “The multilayered material is just 1.8 microns thick, thinner than the thinnest aluminum foil. It is 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.”
“This photonic approach gives us the ability to finely tune both solar reflection and infrared thermal radiation,” said Linxiao Zhu, a doctoral candidate in applied physics and co-author of the paper. Marin Soljacic, a physics professor at the Massachusetts Institute of Technology (MIT), added that he was “personally very excited about their results” and said that the research was “a great example of the power of nanophotonics.”
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