Reflecting on the suitability of cool pavements

Lawrence Berkeley National Laboratory (LBNL) and the U.S. Department of Energy (DOE) are putting some heat on the concept of ‘cool’ pavements, exploring whether the energy savings they offer throughout their life cycles outbalance their emissions. The organizations’ study examines not just short-term benefits of such pavements, but also their long-term effects.

According to previous studies, cool pavements reduce average outside air temperature by approximately 0.5 C (0.9 F), with the exact cooling effect varying by city conditions and pavement extent. They are also said to contribute to reducing smog and ozone concentration. Typically, cement concrete is thought of as ‘reflective,’ given its lighter color and higher albedo (i.e. solar reflectance). Its more-common counterpart, asphalt concrete, is darker and reflects less sunlight.

In this study, the manufacturing, installation, use, and disposal of both cool and conventional pavements were observed over a span of 50 years. The research was funded by the California Air Resources Board (CARB) and California Department of Transportation (Caltrans), and conducted by:

• LBNL’s Heat Island Group;
• University of California Pavement Research Center (UCPRC);
• University of Southern California (USC); and
• thinkstep.

UCPRC and thinkstep calculated each material’s energy use and emissions, while LBNL ran simulations of building energy consumption and USC created a regional climate model. It was discovered that while high albedo (i.e. solar reflectance) has the short-term effects of directing less sunlight to buildings and mitigating outdoor air temperature, the embodied energy of high-albedo materials tends to exceed the savings they provide in the long term.

“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 Haley Gilbert, a researcher with Berkeley Lab. “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.”

From the research, a report was created and published, and a tool was developed to facilitate estimation of various pavements’ energy and environmental consequences. The researchers used the tool themselves to conduct case studies in Los Angeles and Fresno, and will publish the results in Energy and Buildings as a white paper entitled “Energy and Environmental Consequences of a Cool Pavement Campaign.” The paper is bylined by Gilbert, with Pablo Rosado, Dev Millstein, and Ronnen Levinson of Berkeley Lab as co-authors. (Other contributors represent UCPRC, USC, and New York City Mayor’s Office of Sustainability.)

“Over the life cycle of the pavement, the pavement material matters substantially more than the pavement reflectance,” said Ronnen Levinson of Berkeley Lab. “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.”

This study corroborates the findings of another Berkeley Lab Heat Island Group researcher, Melvin Pomerantz. When calculating energy savings and carbon dioxide (CO₂) reductions using data on electricity supplied from seven cities’ power companies, Pomerantz discovered cool pavements annually generate:

• energy savings of less than 1 kWh/m²;
• cost savings of less than 60 cents; and
• avoided CO₂ of less than one cent.

His results, in the form of an Urban Climates white paper titled “Are Cooler Surfaces a Cost-effective Mitigation of Urban Heat Islands?”, are available online.

“This particular analysis has not been done before,” said Pomerantz. “It’s important to know what does not work, as well as what does work. Not every city should have white pavements.”

It is true cool pavements are only one strategy used to reduce heat island effect—other examples include planting trees for shade or employing reflective roofs and walls to prevent sunlight absorption.

All this being said, however, there may still be some benefit to employing cool pavements—short-term or not, their high albedo does reduce greenhouse gas (GHG) emissions. This may become more significant when different considerations are made, such as:

• future increases in global warming;
• economic and health benefits; and
• vehicle/road interactions.

“The development of lower-energy, lower-carbon cool pavement solutions is still necessary,” said Levinson.

FOLLOW
Concrete