This smart roof covering reflects heat in summer and captures it in winter

Berkeley Lab engineers have developed a roof cladding that can keep a building warmer or cooler, depending on the weather. When it's hot, the material will reflect sunlight and heat, but this radiative cooling turns off automatically in the winter, reducing energy consumption for both heating and cooling.

Radiative cooling systems work attracting thermal radiation (i.e. heat) from a building, then emitting it towards the sky. Since the atmosphere is transparent to these wavelengths, the heat escapes directly into space. Other versions use reflective surfaces such as super-white paints to bounce sunlight and heat, keeping the building cooler.

The key to the technology is a strange compound called vanadium dioxide (VO2). In 2017, the team discovered an unusual property of VO2: when it reaches 67 ° C, the material conducts electricity but not heat, in apparent violation of known physics. The team put this oddity to work. The idea is that when it is warmer, the material absorbs heat and emits thermo-infrared light, thus keeping the heat away from the building. But when the weather cools, the material is transparent to heat, allowing it to pass directly from the sun to the building.



The team tested the device using TARC's thin-film patches and compared with samples of commercial dark and white roofing materials. The wireless devices measured changes in direct sunlight and temperature. According to measurements, TARC reflected about 75% of the sunlight regardless of the weather, but when the ambient temperature rises above 30 ° C, it emits up to 90 percent of its heat into the sky. When the weather cooled below 15 ° C, TARC only gave off 20% of its heat.

Using the collected data, the team simulated how TARC would work year-round in 15 different climate zones in the continental United States and estimated that the average U.S. household could save up to 10% of their utility bill electrical using TARC.

Researchers say TARC could also be adapted as a temperature regulating material for cars, electronics, satellites, and even curtain or clothing fabrics. Interestingly, an independent team just announced a similar coating for glass and windows, using vanadium dioxide nanoparticles as one of its active ingredients. The team plans to run experiments using larger TARC prototypes, to test how practical it can be as a roofing cladding.