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Research Paves Way for Environmental Benefits

Tuesday, May 23, 2017

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A comprehensive study by a national laboratory in California has dug deep into pavement, revealing that while the choice of materials can impact carbon emissions, the conventional wisdom about cool pavements may not always tell the whole story.

The U.S. Department of Energy’s Lawrence Berkeley National Laboratory agreed that laying certain types of pavement can help cities stay cool, but the study also found that many reflective pavements produce drawbacks in relation to conventional pavements, especially when considering the life cycle of materials used.

© / allgord

Research by Lawrence Berkeley National Laboratory found that laying certain types of pavement can help cities stay cool, but that many reflective pavements produce drawbacks in relation to conventional pavements, especially when considering the life cycle of materials used.

Scientists in Berkeley Lab’s Heat Island Group, in collaboration with the UC Pavement Research Center, the University of Southern California, and, conducted life-cycle assessments of conventional and cool pavements. By analyzing technologies over a 50-year span—manufacture, installation, use, and disposal/recycling were among the areas considered—researchers discovered extra energy and emissions exhibited by cool pavement materials exceeds the expected energy and emissions savings produced by reduced cooling and heating in buildings.

“This study provides an important perspective on the trade-offs of cool pavements and gives cities a tool to understand them for their particular setting,” said Berkeley Lab researcher Haley Gilbert. “I cannot go to a city and say, ‘Cool pavements are good,’ without letting them know that there could be negative environmental consequences from deployment.”

Degree of Interest

Paved areas, which include roads and parking lots, can sprawl over one-third or more of a typical U.S. city. Other studies have determined that cool pavements can reduce a city’s average outside air temperature by around 1 degree Fahrenheit, depending on the use of certain paving materials, city size, and city location. Reducing the air temperature also has a positive effect on local air quality; smog formation and ozone concentration are both lessened.

Reflective pavements emerged as a method to combat so-called “urban heat islands,” areas in which daytime temperatures rise due in part to dark, dry infrastructure such as roads and buildings. Shade-providing trees, as well as reflective roofs and walls, which absorb less sunlight, are other heat-reducing approaches. The payoff? All three save energy by reducing the need for air conditioning.

Gilbert said reducing the urban heat island effect has become increasingly vital, as cities scramble to counteract the inevitability of future warming. But Gilbert added that researchers and cities are still seeking the most effective cooling strategies.

In an effort to help cities estimate energy and environmental drawbacks by using particular types of pavement materials, the researchers created a decision-support tool for download. The study that generated the initial report was funded by the California Air Resources Board and the California Department of Transportation.

The researchers made use of the decision tool to conduct case studies in Los Angeles and Fresno, California. Their paper, "Energy and Environmental Consequences of a Cool Pavement Campaign," was published in the journal Energy and Buildings; Gilbert is the lead author, and co-authors include Pablo Rosado, Dev Millstein, and Ronnen Levinson of Berkeley Lab as well as collaborators from UCPRC, USC, and the New York City Mayor’s Office of Sustainability.

Comparing Shades

Most cities use asphalt concrete, which is dark and has a low solar reflectance (also known as albedo). Cement concrete has a lighter color, which gives it a higher albedo. But cement is made via a high-temperature process, which makes it more energy- and carbon-intensive than petroleum-produced asphalt.

Berkeley Lab

Researchers (from left) Pablo Rosado Ronnen Levinson, and Mel Pomerantz were part of a team that studied the effects of reflective pavements in comparison to traditional paving materials.

The researchers also experimented with other material alternatives—reflective coatings and the use of industrial waste products like slag and fly ash among them—as a replacement for some of the energy-intensive cement in cement concrete. UCPRC and thinkstep Inc. then calculated the energy and emissions associated with pavement materials and construction.

USC researchers applied this regional climate model to California, and Berkeley Lab ran simulations of building energy consumption. They found the pavement albedo affects buildings both directly (by reflecting the amount of sunlight to nearby buildings), and indirectly (by changing the outside air temperature).

In most cases, the extra energy expended in the creation and maintenance of the cool material was far greater than the energy savings from increasing the albedo.

“Over the life cycle of the pavement, the pavement material matters substantially more than the pavement reflectance,” Levinson said. “I was surprised to find that over 50 years, maintaining a reflective coating would require over six times as much energy as a slurry seal. The slurry seal is only rock and asphalt, which requires little energy to produce, while the reflective coating contains energy-intensive polymer.”

Cooling the Earth

However, Levinson pointed out another possible benefit to certain pavement materials: global cooling. More reflective roads would send sunlight back toward space, which he said helps cool the planet and offset atmospheric warming caused by greenhouse gas emissions.

“The one-time global cooling benefit of cool pavements is substantially larger than the 50-year life cycle carbon penalty or savings,” he said.


Tagged categories: Asphalt; Environmental Protection; Lawrence Berkeley National Laboratory; Research; Roads/Highways; U.S. Department of Energy

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