Cool roofs in China offer enhanced benefits during heat waves

December 22, 2015

It is well established that white roofs can help mitigate the urban heat island effect, reflecting the sun's energy back into space and reducing a city's temperature under normal weather conditions. In a new study of Guangzhou, China, Lawrence Berkeley National Laboratory (Berkeley Lab) researchers working with Chinese scientists found that during a heat wave, the effect is significantly more pronounced.

Using a regional climate model combined with an urban model that allowed researchers to adjust roof reflectance, they found that the average urban midday temperature was lowered by 1.2 degrees Celsius (2.2 degrees Fahrenheit) during heat waves, or 50 percent more than the 0.8 degrees Celsius reduction for typical summer conditions.

The study, "Cool Roofs in Guangzhou, China: Outdoor Air Temperature Reductions during Heat Waves and Typical Summer Conditions," was published recently in the journal Environmental Science & Technology. The authors were Berkeley Lab researchers Dev Millstein, Ronnen Levinson, and Pablo Rosado; and Meichun Cao and Zhaohui Lin of the Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing.

"The hotter it is, the more cooling you get with cool roofs--and it is a significant difference, compared to the margin of error," said Millstein. "We found that the stagnant conditions of a heat wave, where the air is just sitting over the city, was one of the main factors."

Reflective roofs, also called cool roofs, save energy by keeping buildings cooler, thus reducing the need for air conditioning. Hot surfaces such as dark roofs that warm the outside air contribute to the urban heat island effect. Previous Berkeley Lab research in China found that cool roofs could substantially reducing energy use and greenhouse gas emissions in climate zones with hot summers.

The reasons for studying heat waves have to do with both health and energy. "That's when reducing the hottest temperatures can have the most health benefit," Millstein said. "It's also when the electric grid is the most stressed. Air conditioners are running at full speed and with no break, so a small change on the margin can have a bigger impact."

In addition to reducing city temperatures more during a heat wave, the researchers also found that cool roofs can decrease the intensity of the urban heat island effect more during extreme conditions. "Looking at the average difference in temperature between every grid cell in the city and the adjacent rural area, cool roofs had a more dramatic effect during heat waves," Millstein said.

Guangzhou is a sprawling megacity in southern China, near Hong Kong, with a population of more than 8.5 million. Researchers simulated conditions from six of the strongest historical heat waves over the last decade, and compared them to 25 typical summer weeks between 2004 and 2008.

For the purposes of the study, the researchers made all the roofs in the city as reflective as an aged white roof. While it is unlikely that will ever occur, it was necessary to have a statistically significant signal. A government policy, Millstein said, would likely be necessary to encourage use of cool roofs.

"It wouldn't have to be all at once, just as they're replaced," he said. "That's one of the reasons we think so much about cool roofs--because it's free or inexpensive to put a cool roof on when you're putting a new roof on anyway."
-end-
The research was funded by DOE's Building Technologies Office, through the U.S.-China Clean Energy Research Center Building Energy Efficiency (CERC-BEE), the Chinese Academy of Sciences, and the National Natural Science Foundation of China. The researchers used the computing facilities of the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility.

Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit http://www.lbl.gov.

DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

DOE/Lawrence Berkeley National Laboratory

Related Heat Waves Articles from Brightsurf:

Building walls that will make summer heat waves more bearable
A research team in Korea has developed a new material for buildings walls that can help reduce the penetration of heat from the outside.

Wound-healing waves
How do cells in our bodies ask for directions? Without any maps to guide them, they still know where to go to heal wounds and renew our bodies.

Remembrance of waves past: memory imprints motion on scattered waves
Now, it appears that between relativity and the classical (stationary) wave regime, there exists another regime of wave phenomena, where memory influences the scattering process.

Data analytics can predict global warming trends, heat waves
New data analytics process evaluates how global energy consumption, as well as urban green infrastructure, can affect climate change.

Urban heat waves imperil LA's most vulnerable communities
Some of LA's most disadvantaged communities are vulnerable to extreme heat waves, which are increasingly common due to global warming.

Deep learning accurately forecasts heat waves, cold spells
Using an advanced form of deep learning, Rice University researchers created a computer system that learned how to accurately predict extreme weather events, like heat waves, up to five days in advance using minimal information about current weather conditions.

Researchers generalize Fourier's heat equation, explaining hydrodynamic heat propagatio
Researchers have developed a novel set of equations for heat propagation that explain why and how heat propagation can become fluid-like, rather than diffusive.

Heat waves could increase substantially in size by mid-century, says new study
Scientists found that by mid-century, in a middle greenhouse emissions scenario, the average size of heat waves could increase by 50%.

Shocking heat waves stabilize single atoms
Single atom catalysts are highly desirable, but difficult to stabilize.

Deep snow cover in the Arctic region intensifies heat waves in Eurasia
Variations in the depth of snow cover in the Arctic region from late winter to spring determines the summer temperature pattern in Eurasia, according to Hokkaido University researchers.

Read More: Heat Waves News and Heat Waves Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.