Cool Roofs in China Can Save Energy and Reduce Emissions

September 1, 2017

Berkeley Lab-led study confirms efficacy of cool roofs in parts of China.

Working with Chinese researchers, the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has conducted the first comprehensive study of cool roofs in China and concluded that they would be effective in substantially reducing energy use and greenhouse gas emissions in climate zones with hot summers.

The team simulated both residential and office buildings in seven Chinese cities in five climate zones, evaluating energy use throughout the year. They also conducted short-term experiments on an office building in Chongqing and a factory in Foshan (near Guangzhou). They found that for the region of China roughly from Shanghai on south, light-colored “cool” roofs that reduce the need for air conditioning can lower annual energy cost, as well as annual emissions of CO2, NOx, and SO2. The results were recently published in a paper in Energy Policy titled, “Cool roofs in China: Policy review, building simulations, and proof-of-concept experiments.”

“Cool roofs have been well demonstrated in the United States, Europe, and elsewhere,” said lead author and Berkeley Lab scientist Ronnen Levinson. “While the concept is the same everywhere, we wanted to show that cool roofs would also be effective for Chinese construction, in Chinese climates, and with Chinese building operation practices.”

By reflecting more sunlight, cool roofs reduce heat flow into the building, which lowers energy consumption and power-plant emissions if the building is air-conditioned. For buildings without air conditioning, the sunlight absorbed by a dark roof heats the space below, making it less comfortable. Hot, dark roofs on any building also warm the city air, aggravating what is known as the urban heat island effect. Additionally, while nearly 80 percent of the sunlight reflected from a roof can escape to outer space, the “thermal infrared” energy radiated by a hot, dark roof is trapped by greenhouse gases, such as CO2 and water vapor, warming the atmosphere.

Co-authors of the paper included Berkeley Lab researchers Tengfang (Tim) Xu and Jing (Jane) Ge, and researchers from Chongqing University and the Guangdong Provincial Academy of Building Research. The team worked under the auspices of the U.S.-China Clean Energy Research Center Building Energy Efficiency (CERC-BEE) Consortium, which is led in the U.S. by Berkeley Lab. Additional funding was provided by the DOE’s Office of Energy Efficiency and Renewable Energy.

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