The bottom layer of the Janus emitter absorbs the heat inside the vehicle and emits it through the top layer to the atmosphere (indicated in blue), causing a temperature drop. The right image shows an experimental model that simulates a vehicle. Credit: Young Min Song/GIST

Scientists engineer the first passive radiative device that absorbs heat from the inside of an enclosure and emits it on the outside.

If you have ever stepped into a car parked under the sun, you would be familiar with how hot it can get on the inside. This occurs because although sunlight can pass through the transparent windows, the thermal radiation re-emitted by the interior cannot, thereby creating a “greenhouse effect” and heating the inside of the car to temperatures as high as 82°C. Elderly people and children are at a particularly high risk of suffering heatstroke or hyperthermia at such temperatures.

This heat from a parked vehicle can be released by either spending energy in active cooling, which is not sustainable, or opening a window, which is not ideal on rainy days or when driving on a highway. Fortunately, scientists from Gwangju Institute of Science and Technology (GIST), Korea, have developed a new type of passive cooling technology to solve this issue.

In a new study, published in Science Advances, they present a device called “Janus emitter,” or JET. Named after the two-faced Greek god Janus, the JET comprises a stack of patterned quartz, silver, and polydimethylsiloxane thin layers. Each face of the JET has unique properties for passively cooling enclosed spaces. The bottom side absorbs a broad spectrum of thermal radiation from inside the enclosure and, through a quantum phenomenon called “spoof surface plasmon polaritons,” re-emits this energy to the atmosphere on the top side in a selective frequency range that maximizes emissivity. Prof Young Ming Song, who led this study, explains, “The ‘Janus’ thermal radiation characteristics of the JET allow it to function as a heat channel that efficiently draws heat from the enclosure and sends it outside.”

The JET requires no electricity at all and no conscious effort from the user, which makes it a sustainable way of keeping the temperature of stationary vehicles, building interiors, and solar cells low. Excited about the results, Dr. Song concludes, “Our work is the first to address passive radiative cooling for enclosed spaces, and we hope it creates a ripple effect that bolsters research in this field.”

Indeed, as this novel technology takes off, staying cool doesn’t have to be so hard anymore!

Reference: “A Janus emitter for passive heat release from enclosures” by Se-Yeon Heo, Gil Ju Lee, Do Hyeon Kim, Yeong Jae Kim, Satoshi Ishii, Min Seok Kim, Tae Joon Seok, Bong Jae Lee, Heon Lee and Young Min Song, 4 September 2020, Science Advances.DOI: 10.1126/sciadv.abb1906