We all know that cool pavements can help keep cities cool, right?
As with many questions that can be answered by the word "yes," the answer should really be "yes, but..."
New research done by the United States Department of Energy's Lawrence Berkeley National Laboratory in California, shows that many reflective pavements have some unexpected drawbacks considering the entire life cycle of the materials.
Looking at technologies over a span of 50 years, including manufacture, installation, use, and disposal or recycling, scientists found the extra energy and emissions embodied in cool pavement materials usually exceeds the expected energy and emissions savings from reduced cooling in buildings.
Reflective pavements have been considered as one of several strategies to mitigate what are known as urban heat islands, in which daytime temperatures rise due, in part, to dark, dry infrastructure such as roads and buildings. Other cooling strategies include urban trees that provide shade and reflective roofs and walls, which absorb less sunlight. The payoff for all three is the energy saved on air conditioning.
But in light of this latest research, Haley Gilbert, a researcher at Berkeley Lab, says he "cannot go to a city and say that cool pavements are good without letting them know that there could be negative environmental consequences in their use."
Pavements, including roads and parking lots, can cover one-third or more of a typical North American city and previous studies have shown that cool pavements can reduce the city's average outside air temperature by around half a degree Celsius.
They can also improve local air quality by reducing the formation of smog and the concentration of ozone.
Reducing the urban heat-island effect is becoming more important in a warming world. But more information is needed to identify the most effective cooling strategies, Gilbert says.
By far the most common urban pavement material is asphalt concrete, which is dark and has a low solar reflectance, or albedo. Cement concrete is lighter in colour and has a higher albedo. However, the high-temperature process for making cement is more energy- and carbon-intensive than the production of asphalt from petroleum.
In addition to asphalt concrete and cement concrete, the researchers also assessed a number of other alternatives, such as reflective coatings and the use of industrial waste products like slag and fly ash to replace some of the cement in cement concrete.
Running simulations of building energy consumption, researchers found that pavement albedo affects buildings directly by reflecting more or less sunlight to nearby buildings and indirectly by changing the outside air temperature. But they found in most cases the extra energy embodied in the cool material was far greater than the energy savings achieved from increasing the pavement's albedo.
Ronnen Levinson, another researcher, says that "over the life cycle of the pavement, the pavement material matters substantially more than the pavement reflectance.
"I was surprised to find that over 50 years, maintaining a reflective coating would require over six times as much energy as the slurry seal. The slurry seal is only rock and asphalt, which requires little energy to produce, while the reflective coatings contain energy-intensive polymer."
The research enabled scientists to produce a decision-support tool that can be used to estimate the energy and environmental consequences of various pavement types. They then used the tool to create case studies in Los Angeles and Fresno.
Their paper, Energy and Environmental Consequences of a Cool Pavement Campaign, has been published in the journal Energy and Buildings.
Levinson notes there is a major benefit to consider: global cooling.
Making roads more reflective and thereby sending more sunlight back into space helps cool the planet, offsetting some of the 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 says.
Korky Koroluk is an Ottawa-based freelance writer. Send comments to firstname.lastname@example.org.