Aaswath Raman holds a thin, silvery disc. It looks like a very clean mirror, but it’s hardly ordinary: it gets colder under direct sunlight and stays about 5 °C cooler than the surrounding air.

Raman is a practical person with a gentle personality; his button-down shirt and flip-flops blend in on the campus of Stanford, where he is a postdoctoral researcher. This mirror, he calmly explains, has a coating that sends heat into the vastness of outer space—which could make it ideal for air-conditioning and refrigeration systems that would require very little or no electricity.

The cooling material takes advantage of a fascinating phenomenon. Objects are always cooling down by radiating heat—this is why dew forms on blades of grass at night. Some of the radiation occurs at frequencies that send the energy right through Earth’s atmosphere and into space, allowing the object’s temperature to drop below that of the surrounding air.

During the day, the sun’s heat usually overwhelms the cooling effect. But while reading through old papers on the subject from the 1960s, Raman thought of a way around that. He applied his knowledge of nanoscale manufacturing techniques that didn’t exist decades ago to make something with optimum levels of thermal radiation and solar reflection. It is a multilayered film of hafnium dioxide, silica, and other materials deposited at carefully controlled thicknesses. It can be made over large areas using the same manufacturing techniques that are used to coat windows.

Coating the roof of a small structure with some of his material would wick heat away and keep the inside cool without electricity, as long as the roof wasn’t insulated. Since most buildings in developed areas have insulated roofs, Raman is working on integrating the material into existing air-conditioning infrastructure. He has a prototype on the roof of Stanford’s Packard Electrical Engineering Building. It is made up of a sheet of the passive cooling material about a square meter in area, mounted in a custom-machined plexiglass box patterned with water channels. In a finished system, the water would circulate through the building air-conditioning system, then go into the cooler box to chill and back into the building system. However, he still needs to demonstrate that his prototype can chill a substantial volume of water.

He has already partnered with a manufacturer that can produce large sheets of the cooling material for further development. He jokes that many researchers in his branch of physics tend to stay in their labs all day and “don’t like to go outside.” But he adds: “If you just go outside, there’s opportunity.”

—Katherine Bourzac

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