Smart roof coating reflects heat in summer and traps it in winter

Smart roof coating reflects heat in summer and traps it in winter
Samples of the new temperature-adaptive radiative coating (TARC)
Samples of the new temperature-adaptive radiative coating (TARC)
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Samples of the new temperature-adaptive radiative coating (TARC)
Samples of the new temperature-adaptive radiative coating (TARC)
TARC samples are tested in a rooftop experiment
TARC samples are tested in a rooftop experiment

Berkeley Lab engineers have developed a roof coating that can keep a building warmer or cooler, depending on the weather. When it’s warm out, the material will reflect sunlight and heat, but this radiative cooling automatically switches off in winter, reducing energy use for both heating and cooling.

Radiative cooling systems work by drawing thermal radiation (i.e. heat) out of a building, then emitting it to the sky. Because the atmosphere is transparent to these wavelengths, the heat escapes straight out into space. Other versions use reflective surfaces like super-white paints to bounce sunlight and heat away, keeping the building cooler.

But while these systems might work well to keep your house cool in summer, they also keep it cooler in winter. So for the new study, the team developed a coating that could automatically switch to trapping heat instead when the mercury drops. They call the material a temperature-adaptive radiative coating (TARC).

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 (153 °F), the material will conduct electricity but not heat, in apparent violation of known physics.

Now, the team has put this quirk to work. The idea is that when the weather warms up, the material absorbs and emits thermal-infrared light and so keeps it away from the building. But when the weather is cool, the material is transparent to heat, allowing it to pass right through from the Sun to the building.

The team tested the device using 2-cm2 (0.8-in2) thin-film patches of TARC, and compared them to samples of commercial dark and white roof materials. Wireless devices measured changes in direct sunlight and temperature.

TARC samples are tested in a rooftop experiment
TARC samples are tested in a rooftop experiment

And sure enough, TARC worked surprisingly well. According to measurements, TARC reflected about 75 percent of sunlight regardless of the weather, but when the ambient temperature was above 30 °C (86 °F), it emitted up to 90 percent of its heat to the sky. When the weather cooled to below 15 °C (59 °F), TARC emitted just 20 percent of its heat.

Using the data gathered, the team simulated how TARC would work year-round in 15 different climate zones across the continental US and estimated that the average US household could save up to 10 percent of their electricity bill using TARC.

The researchers say that TARC could also be adapted as a temperature-regulating material for cars, electronics, satellites, and even fabrics for tents or clothing. Interestingly, an independent team has just announced a similar coating for glass and windows, using vanadium dioxide nanoparticles as one of its active ingredients.

Next, the team plans to run experiments using larger TARC prototypes, to test how practical it might be as a roof coating.

The research was published in the journal Science.

Source: Berkeley Lab

If you're in an area where heating or cooling is important, you should have plenty of insulation between roof and ceiling, so this system shouldn't have much impact. A white--as opposed to any other colour--roof probably offers a much higher benefit per cost ratio.
Sure Gazer but who in their right mind wants white shingles? People do care about what their home looks like. As per usual, Atlas yet again answers every question except what most of us want to know. When & where is this expected to enter the market & at what cost? Looks like a whopping $500 or so annual utility bill reduction on the average home. Or just enough to re roof after 30 yrs. It's a start....
Trapping the heat in winter would just melt the snow off the roof. Since the snow itself is a fair insulator, the heat trapping would only work in the day time unless there was some form of active heat transfer besides radiant heat from the roof coating.
Talk is cheap. How expensive is it expected to be?

And as @TechGlazer notes, contemporary building technology loads up on insulation in the roof. So in order to "harvest" the heat absorbed, roof systems would need to be redesigned, perhaps with an accessible air chamber - at additional cost. Otherwise, seems this technology would be better suited to vehicles or perhaps RV's.
Lamar Havard
Dark shingles or other materials get sufficiently hot to dry properly. White, or light colors don't, and the roof tends to mold if it's in a wetter climate. So this material would have to be dark for that purpose alone.
Interesting properties but how to use it needs to be thought out. In climates where snow accumulates on the roof one would have to have a system to clear the snow to derive any benefit from the heat capture.
@techgazer, this coating keeps the roofing from conducting so much absorbed heat down into the attic. That lowers the heat island effect from each treated roof.
I don't see what kind of difference it would make on the cooling side. Heat trapped in the roof doesn't help inside the home, below that R38 insulation.