Eco-Friendly Smart Coating Keep Buildings Cool Day and Night

Researchers have developed a smart coating to keep buildings cool without the use of air conditioners. Image courtesy of Hong Kong Polytechnic University.

A team of China- and U.S.-based researchers—many of whom are affiliated with Hong Kong Polytechnic University (Hung Hom, Kowloon, Hong Kong)—have developed a low-cost, eco-friendly smart coating designed to keep buildings cooler. According to their recent study published inAdvanced Materials, this new smart coating uses sub-ambient daytime radiative cooling (SDRC) in order to enhance daytime cooling and minimize nighttime heating loss, while also reducing the excessive electricity consumption and greenhouse gas causing emissions associated with air conditioners and other “active” cooling devices.

The coating consists of conventional building materials, including titanium dioxide nanoparticles, fluorescent microparticles, and glass microspheres, that were engineered to reflect most of the sunlight that hits a building, thereby minimizing its heat absorption while simultaneously re-emitting infrared radiation. While infrared radiation-based passive cooling has been explored by researchers for the past several years, cost and design challenges have limited its large scale and widespread application.

In addition, past “passive” cooling coatings, i.e., ones that use no electricity or generate greenhouse gas emissions, led to drastic day-night temperature differences in which more heat was lost than gained at night. However, the new smart coating contains titanium dioxide particles that reflect sunlight and fluorescent particles that convert absorbed sunlight into emissions that are dissipated away from the building. Also, the coating’s glass microspheres re-emit mid-infrared broadband radiation, which allows for not only heat dissipation, but also heat exchange between the building and the sky.

According to the university, the coating was field tested on the surface of a model concrete building, where it was found that daytime cooling was enhanced while nighttime cooling was suppressed. The building maintained an internal temperature of about 78.8 °F (26 °C), even when the outside temperature varied from 75.2-98.6 °F (24-37 °C) during the day. This artificially accelerated weathering test, equivalent to exposing the coating to the outdoors for two years, showed that the overall cooling performance degrades negligibly within this time frame.

Source: Hong Kong Polytechnic University,