Researchers have created an all-season smart roof coating that keeps homes cool in the summer and warm in the winter. It does not require natural gas or electricity. The groundbreaking technology outperforms commercial cool roof systems in terms of energy savings, according to research findings published Science.
“Our all-season roof coating automatically switches from keeping you cool to warm, depending on outdoor air temperature. This is energy-free, emission-free air conditioning and heating, all in one device,” said Junqiao Wu, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a UC Berkeley professor of materials science and engineering who led the study.
Cool roofs today, such as reflective coatings membranes, tiles, and membranes, are light-colored or darker “cool colored” surfaces that reflect sunlight to cool homes. Some of the solar heat absorbed by these systems is also radiated off as thermal-infrared radiation. This natural process is known as radiative cooling.
Wu explained that the problem with many cool roof systems is their inability to emit heat in winter. This can lead to higher heating costs. He explained that the new material, called a temperature-adaptive radiation coating (TARC), can help energy savings by turning off radiative cooling in winter and overcoming the problem with overcooling. The first roof coating to automatically switch between cooling in hot and warming in cold temperatures is TARC. It regulates its rate of radiative cool.
A roof that can be used in all seasons
He said, ““A few years ago, I wondered, if only there were a material that could automatically switch between radiative cooling in hot weather to heat retention in cold weather. And then I thought, vanadium dioxide can do that.”
Metals are good conductors both of electricity and heat. Wu and his research group discovered in 2017 that vanadium dioxide electrons behave like a metal to electric current but as an insulator to heat. In other words, they conduct electricity efficiently without generating much heat. Wu explained that this behavior is different from other metals, where electrons conduct electricity and heat proportionally.
Below 67°C (153°F), vanadium dioxide is transparent to thermal-infrared radiation and therefore not absorbtive. Once vanadium dioxide has reached 67 degrees Celsius, however, it becomes a metal and absorbs thermal-infrared radiation. This is what is known as a phase change material.
Wu’s 2017 study found that replacing just 1.5% of vanadium in vanadium dioxide with tungsten (a technique known as “doping”) lowers the material’s phase-change threshold from 25 degrees Celsius to 77 degrees Fahrenheit, which is the ideal temperature for real-world application.
To see how vanadium dioxide would perform in a roof system, Wu and his team engineered a 2-centimeter-by-2-centimeter TARC thin-film device from three layers: a reflective bottom layer made from silver, a transparent middle layer composed of barium fluoride, and a top layer containing orderly blocks of vanadium dioxide “islands.”
Wu stated that TARC “looks like Scotch tape, and can be affixed to a solid surface like a rooftop”
This study shows vanadium dioxide’s remarkable all-weather versatility in TARC thin films. It also compares its performance to a commercial dark roofing coating and a white commercial roof coating.
Co-lead author Kechao Tan set up a roof experiment at Wu’s East Bay home last year to show the technology’s viability and practicality in a real-world setting.
An outdoor wireless measuring device was set up on Wu’s balcony and recorded over multiple days the responses to temperature changes and direct sunlight.
TARC’s energy savings
The data from the outdoor experiment was used by the researchers to simulate TARC’s performance year-round in cities that represent 15 climate zones throughout the continental U.S.
To improve their understanding of roof surface temperatures, Wu hired Ronnen Levy, a Berkeley Lab staff scientist and leader in the Heat Island Group’s Energy Technologies Area. Levinson, who has been studying the technology, benefits and policies of cool surfaces like reflective roofs, walls and pavements for almost three decades, created a method to estimate TARC Energy savings from a collection of more than 100,000 building simulations that the Heat Island Group had previously done to assess the benefits of cool rooftops and cool walls throughout the United States.
Wu and his team used this method for predicting the annual energy savings TARC offers by reducing both the use of cooling energy in summer, and heating energy during winter.
Finnegan Reichertz is a student at East Bay Innovation Academy in Oakland, who was a 12-year-old th year old. She worked remotely for Wu as a summer intern last year to simulate how TARC roof materials would perform during specific times or days of the year in each of the 15 climate zones that the researchers examined for their paper.
The researchers found that TARC is more efficient than existing roof coatings in energy savings in 12 of 15 climate zones. This includes regions where there are large temperature variations between day/night, such the San Francisco Bay Area, and between winter/summer, as well as New York City.
Tang stated that TARC could help average households in the U.S. save as much as 10% on electricity.
Cool roofs are standard and have high solar reflectance as well as high thermal emittance. This means that they can emit heat through thermal-infrared radiation even in cold weather.
The researchers found that TARC reflects approximately 75% of the sun year-round. However, its thermal emittance (about 90%) is higher when the temperature is above 25 degrees Celsius (77 degrees Fahrenheit), which promotes heat loss to the atmosphere. Levinson stated that TARC’s thermal emission automatically switches to low (about 20%) in cooler weather. This helps to retain heat from indoor heating and solar absorption.
The simulations were validated by infrared spectroscopy results from experiments at Berkeley Lab’s Molecular Foundry.
Wu stated that TARC was predicted to work by simple physics, but they were surprised that it worked so well. We initially thought that the transition from cooling to warming wouldn’t be as dramatic. It’s exciting, according to our simulations, lab experiments, and outdoor experiments.
TARC prototypes will be developed on a larger scale by the researchers to test its effectiveness as a roof coating. Wu stated that TARC could also be used as a thermally protected coating to extend the battery life of smartphones and laptops. It can also shield satellites from extreme high and low temperatures. It could also be used to make temperature-regulating fabric for tents, greenhouse coverings, and even hats and jackets.
Kaichen Dong, Jiachen Li and Jiachen L were the co-lead writers of this work.
Berkeley Lab’s Molecular Foundry hosts a number of nanoscience users.
This work was supported primarily by the DOE Office of Science, and a Bakar Fellowship.