Smart roof coating helps regulate temperature

A team of engineers from Berkeley Labs developed roof coatings that can make buildings warmer or cooler depending on the weather.

When it's hot, the material reflects sunlight and heat. However, this reflective cooling automatically stops in winter. As a result, the new material reduces the amount of energy required for heating and cooling.

TARC coating specimen. (Image: Thor Swift/Berkeley Labs)

Reflective cooling works by drawing radiant heat out of the building and into the air. Since these wavelengths can travel through the atmosphere, the heat dissipates into space immediately. Other versions use reflective surfaces such as super white paint to disperse sunlight and heat, making the building cooler. While such systems work well at cooling homes in the summer, they also make the house cooler in the winter.

In a new study, published December 16 in the journal Science, a team of engineers at Berkeley Lab developed a coating that automatically switches to retaining heat when the temperature drops. They call the material temperature-customized reflective coating (TARC). The key to this technology is a strange compound called vanadium dioxide (VO2). In 2017, the team discovered an unusual feature of VO2. When it reaches 67 degrees Celsius, the material will conduct electricity but not heat, contrary to known laws of physics.

The team's idea is that as the weather warms, the material will absorb and reflect light. But in cold weather, the material allows heat to flow directly from the Sun to the building. The researchers experimented with TARC sheets as thin as two square centimeters, and then compared them with samples of dark and white roof materials. They use wireless equipment to measure changes in direct light and temperature.

TARC works better than expected. According to the measurement results, TARC reflects about 75% of sunlight regardless of the weather, but when the surrounding temperature is above 30 degrees Celsius, the material dissipates 90% of the heat into the air. If the weather is cold below 15 degrees Celsius, TARC only dissipates about 20% of the heat. Using the data they collected, the team simulated how TARC works year-round in 15 different climates across the US, and estimated that an average US household could save about 10% on electricity bills. by using TARC.

The team says that TARC could be adapted to be used as an air conditioning material in cars, electronics, satellites, even tents or clothing. Next, the researchers will conduct experiments using a larger TARC prototype to test the practicality of using the material as a roof covering.