New Photonic Structure Enables Cooling Without Using Electricity

New Photonic Structure Enables Cooling Without Using Electricity

11 February 2014

A research team at Stanford University has developed a photonic structure that could make cooling during sweltering days possible without using electricity, reports.

The structure was created with the help of a technique called "daytime radiative cooling," which uses a range of materials and layers together with a photonic crystal, enabling high-performance passive radiative cooling in daylight hours. The structure releases its heat in the form of thermal electromagnetic waves to the cold of outer space while reflecting sunlight at the same time.

The project, led by professor Shanhui Fan, makes use of an atmospheric transparency window that exists between 8 and 13 µm, which the researchers used as a heat sink that allows thermal waves to dissipate into outer space. That way, the device can rest at an equilibrium temperature significantly below the ambient, creating a passively maintained cold surface.

The structure design employs two thermally-emitting photonic-crystal layers of silicon carbide and quartz, with squares engraved in the layers with 6-µm periodicity that are located on top of a broadband solar reflector. Three sets of five bi-layers of magnesium fluoride and titanium dioxide form each of the squares, which also have varying periods on a silver substrate to shape the cooler.

The emissivity of the atmospheric transparency window is further maximized thanks to phonon-polariton modes used in the 2-D, two-layer photonic crystal part of the structure, while the absorption of solar radiation is minimized by the chirped 1-D photonic crystal reflector.