New smart roof coating enables year-round energy savings

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Dec 16, 2021

(Nanowerk news) Scientists have developed a year-round smart roof coating that keeps houses warm in winter and cool in summer – without using natural gas or electricity. Research results published in the journal science (“Temperature-adaptive radiation coating for year-round heat regulation in the home”) indicate a breakthrough technology that outperforms commercial cooling roof systems in terms of energy savings.

“Our year-round roof coating automatically switches from cool to warm, depending on the outside air temperature. This is zero-energy, zero-emission air conditioning and heating in one device, ”said Junqiao Wu, faculty scientist in the materials science department at Berkeley Lab and professor of materials science and engineering at UC Berkeley, who led the study.

Modern, cool roof systems such as reflective coatings, membranes, shingles or roof tiles have light or darker “cool” surfaces that cool apartments by reflecting sunlight. These systems also give off part of the absorbed solar heat as thermal infrared radiation; In this natural process, known as radiation cooling, thermal infrared light is emitted from the surface.

The problem with many cooling roof systems is that they continue to radiate heat in winter, which drives up heating costs, Wu said. “Our new material – called temperature adaptive radiation coating, or TARC – can save energy by automatically turning off radiation cooling in winter and overcoming the problem of hypothermia,” he said. TARC is the first roof coating to automatically switch between cooling in hot weather and heating in cold weather by regulating the rate of radiant cooling.

Pattern of the temperature adaptive radiation coating. The material looks like tape and can be attached to a roof. (Image: Thor Swift / Berkeley Lab)

A roof for all seasons

“A few years ago I asked myself if there was only one material that could automatically switch between radiant cooling in hot weather and heat storage in cold weather,” he said. “And then I thought vanadium dioxide can do it.”

Metals are usually good conductors of electricity and heat. In 2017, Wu and his research team discovered that electrons in vanadium dioxide act like a metal to electricity, but like an insulator to heat – in other words, they conduct electricity well without conducting much heat. “This behavior is in contrast to most other metals, in which electrons conduct heat and electricity proportionally,” explained Wu.

Vanadium dioxide below about 67 degrees Celsius (153 degrees Fahrenheit) is also transparent (and therefore non-absorbent) to infrared thermal light. However, as soon as vanadium dioxide reaches 67 degrees Celsius, it changes to a metallic state and absorbs the thermal infrared light. This ability to change from one phase to another – in this case from an insulator to a metal – is characteristic of so-called phase change materials.

Wu’s 2017 study found that replacing just 1.5% of the vanadium in vanadium dioxide with tungsten, a technique called “doping,” lowers the material’s phase change threshold to 25 degrees Celsius, or 77 degrees Fahrenheit – an ideal temperature for real Applications.

To see how vanadium dioxide behaves in a roof system, Wu and his team constructed a 2 centimeter by 2 centimeter TARC thin-film device from three layers: a reflective lower layer made of silver, a transparent middle layer made of barium fluoride, and an upper layer that is orderly Contains blocks of vanadium dioxide “islands”.

TARC “looks like tape and can be attached to a solid surface like a roof,” said Wu.

The current study shows the amazing all-weather versatility of vanadium dioxide in a thin TARC film and compares its performance to a commercial dark roof coating and a commercial white roof coating.

In a key experiment, co-lead author Kechao Tang set up a rooftop experiment on Wu’s house in East Bay last summer to demonstrate the viability of the technology in a real-world setting.

A wireless meter placed on Wu’s balcony continuously recorded responses to changes in direct sunlight and outside temperature from a TARC sample, a commercial dark roof sample, and a commercial white roof sample over several days.

How TARC outperforms in energy savings

The researchers then used the data from the outdoor experiment to simulate how TARC would fare year-round in cities representing 15 different climates across the continental United States

Wu hired Ronn Levinson, a co-author of the study who is a research fellow and head of the Heat Island Group in the Energy Technologies Area of ​​the Berkeley Lab, to refine her model of roof surface temperature. Levinson, who has studied the technology, benefits, and policies of cool surfaces such as reflective roofs, walls, and walkways for nearly three decades, developed a method of estimating TARC energy savings from a series of more than 100,000 building energy simulations made by the Heat Island Group has previously conducted to evaluate the benefits of cool roofs and cool walls in the United States (Energy and Buildings, “Potential Benefits of Cooling Wall Panels in Residential and Commercial Buildings Across California and the United States: Save Energy, Save Money, and Reduce Greenhouse Gas and Air Pollution Emissions”).

Wu’s team used this method to predict the annual energy savings that TARC will offer by reducing both the need for cooling energy in summer and heating energy in winter.

Finnegan Reichertz, a 12th grade student at the East Bay Innovation Academy in Oakland who worked remotely as a summer intern for Wu last year, helped simulate how TARC and the other roofing materials would perform at certain times and days Year for each of the 15 cities or climates that the researchers examined for the paper.

According to the simulation experiments, TARC outperforms existing roof coatings in terms of energy savings in 12 of the 15 climate zones, especially in regions with strong temperature fluctuations between day and night, such as the San Francisco Bay Area, or between winter and summer, such as New York City, the researchers report.

“With TARC installed, the average household in the US could save up to 10% on electricity,” said Tang.

Standard cooling roofs have a high level of solar reflection and high heat emission (the ability to give off heat by emitting heat infrared radiation) even in cool weather.

According to the researchers’ measurements, TARC reflects about 75% of sunlight year-round, but its thermal emittance is high (about 90%) when the ambient temperature is warm (above 25 degrees Celsius 77 degrees Fahrenheit), causing heat loss to the sky . In cooler weather, TARC’s thermal emittance automatically switches to low (around 20%), which helps retain the heat from solar absorption and indoor heating, Levinson said.

Findings from infrared spectroscopy experiments with advanced tools at Berkeley Lab’s Molecular Foundry validated the simulations.

“Simple physics predicted TARC would work, but we were surprised it would work that well,” said Wu. “We originally thought the switch from heating to cooling wouldn’t be that dramatic. Our simulations, outdoor experiments and laboratory experiments have proven the opposite – it’s really exciting. ”

The researchers plan to develop TARC prototypes on a larger scale to further test its performance as a practical roof coating. Wu said that as a thermal barrier coating, TARC could also have the potential to extend battery life in smartphones and laptops, and to protect satellites and cars from extremely high or low temperatures. It could also be used to make temperature regulating fabrics for tents, greenhouse covers, and even hats and jackets.


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