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A hot new field study aims to uncover just how cool pavement surfaces can be when coated with reflective coatings.
Scientists at Lawrence Berkeley National Laboratory’s Heat Island Group have converted part of a temporary parking lot into a “cool pavement” exhibit that will allow them to evaluate solar-reflective coatings over time under real-life conditions.
Hot Spots, Cool Solutions
Paved surfaces, including exposed parking lots, account for about 35% to 50% of surface area in a typical city, making them a leading contributor to higher temperatures in urban areas—a phenomenon known as the “heat island effect,” reported Berkeley Lab.
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Photos: Roy Kaltschmidt / Berkeley Lab |
| Jordan Woods of the Berkeley Lab Heat Island Group takes measurements of a new cool pavement coating using an albedometer. |
Dark pavements in particular also contribute to global warming, formation of smog, and increasing energy loads, the researchers noted.
“It’s amazing how hot these pavements get and how we’ve let them cover most of our urban surfaces,” said Haley Gilbert, a researcher in the Heat Island Group.
“Because dark pavements absorb almost all of the sun’s energy, the pavement surface heats up, which, in turn, also warms the local air and aggravates urban heat islands.”
That’s why the scientists are studying cool-pavement technologies.
The Technologies
Like lighter-colored “cool” roofs that help cool the air inside and outside a building by reflecting more of the sun’s energy, cool pavements reflect as much as 50% of the sun’s energy, compared to only 5% for new asphalt (and 10% to 20% for aged asphalt), Berkeley Lab said.
Cool pavements can be made from traditional pavement materials, such as cement concrete, that are lighter in color and therefore have a higher solar reflectance (SR of 30-50%), according to Berkeley Lab.
There are also novel “cool-colored” coatings or surface treatments for asphalt surfaces that reflect about 50% of sunlight.
The Heat Island Group works with both the asphalt and cement industries in analyzing these technologies. “An ideal design goal would be a pavement with solar reflectance of at least 35 percent,” Gilbert said. “How you get there will vary by project.”
One Cool Exhibit
In this field study, the Berkeley Lab is testing cool-pavement coatings that were applied directly to existing paved surfaces in the lab’s lot.
Six different coatings donated by two manufacturers are included. The manufacturers are Emerald Cities Cool Pavement and StreetBond. A request for more information regarding the coatings tested was not immediately returned.
However, the Berkeley Lab reported that it is also coordinating with additional manufacturers to apply their technologies to the lot in the coming months.
During the evaluation period, researchers say they will reach an equilibrium at which the solar reflectance won’t degrade much anymore.
“We’re also very interested to see what happens when it rains, which may help the coatings self-clean and restore higher reflectance,” said Benjamin Mandel, another researcher in the group.
Replacing Traditional Sealants?
Another area of focus is the development of alternatives to conventional sealants.
Sealants are commonly used to maintain parking lots and schoolyards, since the asphalt pavement structure degrades over time.
Traditional sealants provide a protective layer that helps keep water out and slow the oxidation of the asphalt. However, they also contribute to the Heat Island Effect by restoring the aged-asphalt surface to its jet-black color, Berkeley Lab said.
“The cool pavement coatings we’re exhibiting can be applied to existing asphalt or cement,” Gilbert said. “They can be used in lieu of a seal coat, and are a good strategy for cities looking to introduce cool-pavement technologies.”
Beyond White
Cool coatings for pavements aren’t just white. Instead, they come in a variety of hues, including green, blue and yellow. The scientists noted that a coating’s solar reflectance value depends on both color and material. The team coated the lot in different hues for testing.
“There are some colors that look dark, but are actually more reflective in the near-infrared spectrum,” said Mandel.
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| Sharon Chen and Ana Paula Werle of the Berkeley Lab Heat Island Group use a reflectometer to take measurements of cool pavement solar-reflective coatings. |
“These products have higher solar reflectance values than one might initially believe, because they are specially designed to reflect invisible infrared light.”
Benefits for Building Owners
More reflective parking lots could allow building owners and cities to save on energy needed to illuminate streets and parking lots, Berkeley Lab said.
“Chicago has reported energy savings from using solar-reflective pavements in its alleys,” said Gilbert. “Quantifying that would be something a business, such as Walmart, could literally take to the bank.”
While scientific models affirm many of these benefits, field studies like Berkeley Lab’s are needed to verify and quantify the results.
“Better data may also help sell cool-pavement coatings, since they tend to be more expensive than traditional sealants,” the lab said.
Cool pavements also tend to benefit the public more than the building owner, making them a tougher sell to owners, researchers said. But Mandel said cool pavements could eventually pay for themselves.
“The benefits are less immediately tangible than for cool roofs,” he said. “But the initial cost premium can potentially be offset over the lifespan of the product with increased durability and less need for ongoing maintenance, which are factors we are working with manufacturers to investigate further.”
More Research, Outreach
The group is also leading a study with the UC Pavement Research Center that will monitor the solar-reflectance values and temperatures of pavement on a residential street on the University of California Davis campus. Those 20-by-24-square-foot pavement sections are made of six different materials.
The scientists hope to better understand how changes in solar reflectance over time affect heat transfer throughout the pavement structure. The goal is to inform decisions by owners and policy makers on cool pavement requirements for building codes and project specifications.
Because cool pavements benefit the community, the Heat Island Group is focusing much of its technical assistance and outreach efforts on local governments.
To that end, the group held a seminar in June to teach local officials about the benefits of cool pavements.
Founded in 1931, the Lawrence Berkeley National Laboratory is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. The lab “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.”
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