Berkeley Lab Develops Smart-Roof Coating

THURSDAY, FEBRUARY 24, 2022


Scientists from the Lawrence Berkeley National Laboratory recently announced their development of an all-season smart-roof coating, designed to keep homes warm during the winter months and cool throughout the summer without consuming natural gas or electricity.

“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.

“Our new material – called a temperature-adaptive radiative coating or TARC – can enable energy savings by automatically turning off the radiative cooling in the winter, overcoming the problem of overcooling.”

The coatings research has since been published in the journal Science and was primarily supported by the Department of Energy Office of Science and a Bakar Fellowship.

About TARC

According to Berkeley Lab, the problem with current cool roof systems, most notably reflective coatings, membranes, shingles or tiles that have light-colored or darker “cool-colored” surfaces, is that they still naturally emit some of the absorbed solar heat as thermal-infrared radiation. Because of this, heat is also radiated in the winter months and can lead to increased heating costs.

To find a more effective alternative, researchers looked to metals, as the material is typically a good conductor of electricity and heat.

In 2017, Wu and his team discovered that electrons in vanadium dioxide behave like a metal to electricity but an insulator to heat, meaning that the electrons could better conduct electricity without conducting much heat. In further research, the team found that vanadium dioxide below roughly 67 C (153 F) was transparent to (and hence not absorptive of) thermal-infrared light.

However, when vanadium dioxide reached 67 C, it switched to a metal state, thus absorptive of thermal-infrared light.

“This behavior contrasts with most other metals where electrons conduct heat and electricity proportionally,” Wu explained. The ability to switch an insulator to a metal, is characteristic of what’s known as a “phase-change material.”

To see how the material might be able to mitigate flaws in current cool roof systems, Wu and his team engineered a 2 cm by 2 cm TARC thin-film device. According to Wu, the material is similar to that of Scotch tape and can easily be fixed to solid surfaces.

For the experiment, co-lead author Kechao Tang placed the material at Wu’s East Bay home in the summer of 2020 to demonstrate the technology’s viability in a real-world environment. A wireless measurement device was also placed with the material so that changes in direct sunlight and outdoor temperature could be recorded for the TARC sample and dark roof and light roof commercial samples as a comparison.

The experiment was recorded over a span of several days.

From the rooftop data collected, the teams then simulated how TARC would perform year-round in cities representing 15 different climate zones across the continental United States. For this portion of the research, Wu was reported to bring on Ronnen Levinson, a co-author on the study and staff scientist and leader of the Heat Island Group in Berkeley Lab’s Energy Technologies Area, to help them refine their model of roof surface temperature.

To achieve this, Levinson developed a method to estimate TARC energy savings from a set of more than 100,000 building energy simulations that the Heat Island Group previously used to evaluate the benefits of cool roofs and cool walls in a seperate study.

In taking things even farther, East Bay Innovation Academy student and intern for Wu, Finnegan Reichertz, worked to simulate how TARC and the other roof materials would perform at specific times and on specific days throughout the year for the observed cities.

Results from the study revealed that the average household with TARC installed on the roof could save up to 10% in electricity usage and reflected around 75% of sunlight year-round. The thermal emittance of the material, however, was high (about 90%) when the ambient temperature was warm (above 25 C or 77 F), promoting heat loss to the sky. In cooler temperatures, the thermal emittance switched to low, and further promoted the retention of heat from solar absorption and indoor heating.

“Simple physics predicted TARC would work, but we were surprised it would work so well,” said Wu. “We originally thought the switch from warming to cooling wouldn’t be so dramatic. Our simulations, outdoor experiments, and lab experiments proved otherwise – it’s really exciting.”

Moving forward, the team plans to develop TARC prototypes on a larger scale to further test its performance as a practical roof coating. Wu said that TARC 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. The material could also be used to make temperature-regulating fabric for tents, greenhouse coverings and even hats or jackets.

Berkeley Lab Cool Wall Pilot

In May 2021, the Berkeley Lab confirmed that the U.S. Green Building Council was piloting a new credit for the installation of cool exterior walls in new homes, schools and commercial buildings for the purpose of mitigating urban heat islands.

Berkeley noted that with the pilot credit, this was a test integration for cool walls into the USGBC’s LEED ratings for buildings. The credit was written by Berkeley Lab scientist Ronnen Levinson based on peer-reviewed research by the University of South California and Berkeley Lab. That study found that reflective walls could lower urban air temperature much like reflective “cool roofs.”

The newly issued pilot credit means buildings that satisfy specific cool wall requirements will earn an innovation point in LEED. After a trial period, USGBC will decide whether to add the pilot credit to its permanent library.

Berkeley noted that idea is to surface exterior walls with a solar-reflective paint or cladding. Reflecting sunlight out of the city reduces urban heat. To qualify for the LEED pilot credit, buildings must cover 75% of a building’s exterior with a cool wall material that reflects at least 60% of sunlight, among other specifications.

   

Tagged categories: Asia Pacific; Coating Materials; Coating Materials - Commercial; Coatings Technology; Coatings Technology; Cool roof coatings; EMEA (Europe, Middle East and Africa); Latin America; Lawrence Berkeley National Laboratory; North America; Research; Research and development; Roof coatings; Roof coatings; Z-Continents

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