Samples of an all-season smart-roof coating designed to keep homes warm during the winter and cool during the summer. Photo courtesy of Junqiao Wu/Lawrence Berkeley National Laboratory
Dec. 16 (UPI) -- An all-season "smart roof" coating keeps homes warm during the winter and cool during the summer, without using natural gas or electricity, a study published Thursday in the journal Science found.
The technology, called temperature-adaptive radiative coating, outperformed currently available commercial cool-roof systems in energy savings in cities representing 15 different climate zones across the continental United States, the researchers said.
In the study, it reflected about 75% of sunlight year-round, with a thermal emittance of approximately 90% in temperatures above 77 degrees Fahrenheit, releasing heat from the home into the sky, the data showed.
In cooler weather, the coating's thermal emittance automatically switches to about 20%, helping to retain heat from solar absorption and indoor heating.
With temperature-adaptive radiative coating installed, the average household in the United States could save up to 10% of electricity consumption annually, researchers said.
"Our all-season roof coating automatically switches from keeping you cool to warm, depending on outdoor air temperature," study co-author Junqiao Wu said in a press release.
"This is energy-free, emission-free air conditioning and heating, all in one device, said Wu, a professor of materials science and engineering at the University of California, Berkeley.
Currently available cool-roof systems, such as reflective coatings, membranes, shingles or tiles, have light-colored or darker surfaces that cool homes by reflecting sunlight.
These systems also emit some of the absorbed solar heat as thermal-infrared radiation as part of a natural process called radiative cooling, the researchers said.
However, many of these cool-roof systems continue to radiate heat in the winter, which drives up heating costs, they said.
Temperature-adaptive radiative coating is designed to create energy savings by automatically turning off the radiative cooling in the winter, overcoming the problem of overcooling, according to the researchers.
The coating is made from vanadium dioxide, a material that behaves like a metal in response to electricity, meaning it conducts it, but acts as an insulator to heat.
Below 153 degrees Fahrenheit, vanadium dioxide is also transparent and thus does not absorb thermal-infrared light.
However, above that temperature, it switches to a metal state, becoming an absorber of thermal-infrared light, according to the researchers.
This ability to switch from one phase to another is characteristic of what's known as a phase-change material. Wu and his colleagues were able to lower its phase-change threshold to 77 degrees Fahrenheit, a more common real-world temperature, by adding tungsten, they said.
By combining vanadium dioxide with the metal tungsten, a process called "doping," the researchers were able to engineer a top layer -- a coating -- for a roof system that also includes a reflective bottom layer made from silver and a transparent middle layer composed of barium fluoride.
That top layer, the temperature-adaptive radiative coating, "looks like Scotch tape, and can be affixed to a solid surface like a rooftop," Wu said.
As part of this study, the researchers set up a rooftop experiment at Wu's East Bay home last summer to demonstrate the technology's performance in a real-world environment.
A wireless measurement device set up on Wu's balcony continuously recorded responses to changes in direct sunlight and outdoor temperature with a temperature-adaptive radiative coating-based roof system and a commercially available product over multiple days.
The researchers then used the data from the outdoor experiment to simulate how temperature-adaptive radiative coating would perform year-round in 15 cities or climate zones across the country, they said.
In addition, using a set of more than 100,000 building energy simulations, the researchers predicted the annual energy savings generated by temperature-adaptive radiative coating, thanks to its ability to reduce the need for both cooling energy in summer and heating energy in winter.
The coating outperformed existing roof coatings for energy savings in 12 of the 15 climate zones, the data showed.
It was most effective in regions with wide temperature variations between day and night, such as the San Francisco Bay Area, or between winter and summer, such as New York City.
The researchers plan to develop temperature-adaptive radiative coating prototypes on a larger scale to further test its performance as a practical roof coating.
It may also have potential as a thermally protective coating to prolong battery life in smartphones and laptops, and shield satellites and cars from extremely high or low temperatures, according to the researchers.
It could also be used to make temperature-regulating fabric for tents, greenhouse coverings and even hats and jackets, the researchers said.
"Simple physics predicted temperature-adaptive radiative coating would work, but we were surprised it would work so well," Wu said.
"We originally thought the switch from warming to cooling wouldn't be so dramatic [but] our simulations, outdoor experiments, and lab experiments proved otherwise," he said.