The Little Cooling Device That Could: Stanford Scientists Invent AC Unlike Any Other

With much of the world's population centered around the equator, the ability to cool a structure during the heat of the day without the use of electricity-gobbling air conditioners is a puzzle scientists have long tried to solve, until now: Stanford University researchers announced recently that they believe they have finally cracked the code through a device designed to reflect sunlight back into space.

"People usually see space as a source of heat from the sun, but away from the sun outer space is really a cold, cold, place," said Shanhui Fan, a professor of electrical engineering and the paper's senior author. "We've developed a new type of structure that reflects the vast majority of sunlight, while at the same time it sends heat into the coldness, which cools manmade structures even in the daytime."

The device solves two main obstacles: reflecting as much light as possible, and not absorbing too much sunlight. If either one fails, the cooling effect is lost.

To answer the first challenge the group used a broadband mirror for solar light, capable of not only reflecting the majority of the rays it receives, but of emitting thermal radiation within the wavelength range needed to escape Earth's atmosphere.

Second, the scientists employed nanostructured photonic materials that can be engineered to enhance or suppress light reflection of certain wavelengths. Using this the scientists were able to limit the heat-inducing sunlight absorbed by the panel while still allowing the heat to escape in the right frequencies.

By combining the two - radiation and dispersion - in this way, the group says they have developed a passive model that is scalable and affordable. A standard one-story, single-family house, for example, would need to cover just 10 percent of its roof with the radiative cooling panels to offset 35 percent of the structure's entire air conditioning needs, even during the hottest days of the summer.

Such a device, the team believes, has the capability of a broad social impact, especially among the hottest and poorest areas of the world.

Fan said he and his group "foresee applications for radiative cooling in off-the-grid areas of the developing world where air-conditioning is not even possible at this time."