Samples of an all-weather smart roof covering designed to keep homes warm in the winter and cool in the summer. Photo courtesy of Junqiao Wu / Lawrence Berkeley National Laboratory
December 16 (UPI) – An all-season “smart roof” coating keeps homes warm in the winter and cool in the summer, without the use of natural gas or electricity, according to a study published in the journal Thursday. Science find.
The technology, called temperature-adaptive radiative coating, has outperformed currently available commercial cold roof systems in terms of energy savings in cities representing 15 different climate zones across the continental United States, the U.S. said. researchers.
In the study, it reflected about 75% of sunlight all year round, with a thermal emittance of about 90% at temperatures above 77 degrees Fahrenheit, releasing heat from the house into the sky, according to the researchers. data.
In cooler weather, the thermal emittance of the coating automatically drops to around 20%, helping to retain heat from solar absorption and interior heating.
With the installation of a temperature-adaptive radiative coating, the average household in the United States could save up to 10% of their electricity use per year, researchers say.
“Our all-weather roof covering automatically switches from keep cool to keep warm, depending on the outside air temperature,” study co-author Junqiao Wu said in a press release.
“This is energy-free, emissions-free air conditioning and heating all in one device,” said Wu, professor of materials science and engineering at the University of California, Berkeley.
Currently available cold roof systems, such as reflective coatings, membranes, shingles or tiles, have light or darker colored surfaces that cool homes by reflecting sunlight.
These systems also emit some of the absorbed solar heat as thermal infrared radiation in a natural process called radiative cooling, the researchers said.
However, many of these cold roof systems continue to radiate heat in the winter, pushing up heating costs, they said.
The temperature-adaptive radiative coating is designed to create energy savings by automatically disabling radiative cooling in winter, thereby overcoming the problem of overcooling, the researchers said.
The coating is made up of vanadium dioxide, a material that behaves like a metal in response to electricity, which means it conducts it, but acts as a thermal insulator.
Below 153 degrees Fahrenheit, vanadium dioxide is also transparent and therefore does not absorb thermal infrared light.
However, above that temperature, it turns metallic, becoming an absorber of thermal infrared light, the researchers say.
This ability to switch from one phase to another is characteristic of what is called a phase change material. Wu and his colleagues were able to lower its phase change threshold to 77 degrees Fahrenheit, a temperature more common in the real world, by adding tungsten, they said.
By combining vanadium dioxide with metallic tungsten, a process called “doping,” the researchers were able to design an upper layer – a coating – for a roof system that also includes a reflective silver lower layer and a compound transparent middle layer. of barium fluoride.
This top layer, the temperature adaptive radiative coating, “looks like tape and can be attached to a solid surface like a roof,” Wu said.
As part of the study, researchers set up an experiment on the roof of Wu’s house in East Bay last summer to demonstrate the performance of the technology in a real environment.
A wireless measuring device installed on Wu’s balcony continuously recorded responses to changes in direct sunlight and outside temperature with a temperature-adaptive radiative coating-based roofing system and a product available in trading over several days.
The researchers then used the data from the outdoor experiment to simulate the performance of the year-round temperature-adaptive radiative coating in 15 cities or climate zones across the country, they said.
Additionally, using a set of more than 100,000 building energy simulations, the researchers predicted the annual energy savings generated by the temperature-adaptive radiative coating, thanks to its ability to reduce cooling energy requirements by summer and heating energy in winter.
The coating outperformed existing roof coverings in terms of energy savings in 12 of 15 climate zones, according to the data.
It was most effective in areas with large 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 prototypes of the temperature-adaptive radiative coating on a larger scale to further test its performance as a practical roof coating.
According to the researchers, it could also have potential as a thermal protective coating to extend the battery life of smartphones and laptops, and protect satellites and cars from extremely high or low temperatures.
It could also be used to make temperature-regulating fabrics for tents, greenhouse coverings and even hats and jackets, the researchers said.
“Simple physics predicted that the temperature adaptive radiative coating would work, but we were surprised that it worked so well,” Wu said.
“Initially, we thought that the switch from warming to cooling would not be so dramatic [but] our simulations, outdoor experiments and lab experiments have proven otherwise, ”he said.