In planning smart cities, a number of strategies are used. They may include urban forestry, solar PV, cool roofs, and the use of high albedo (high solar reflectance) “cool” pavements” – all programs intended to help us reduce energy demand, lower carbon emissions and reduce the heat-island effect.
This is not exactly true, according to a recent study led by the Heat Island Group at Lawrence Berkeley National Laboratory and the University of California Pavement Research Center. Researchers concluded that “cool pavements” promise more than they can deliver.
While cool pavements can reduce the heat-island effect in urban settings, lower greenhouse gas emissions, reduce buildings’ energy consumption and improve air quality, their benefits may be outweighed by their environmental costs.
Using a holistic approach to research on cool pavements
To determine if cool pavements stand up to their promised results, the researchers took a holistic approach, not only considering both the positive environmental impacts of increased roadway reflectivity, but an analysis of pavement materials over a 50-year timeframe, including the extraction of the raw materials, the manufacturing of the pavement, energy use in buildings, and the removal and recycling of the pavement.
Researchers reached out to Thinkstep, Inc, a global company that provides industry-leading sustainability software, data and consulting services. to develop a pavement life-cycle assessment tool for California cities.
With the tool, they were able to assess how a choice of pavement materials affects “lifecycle. energy consumption; life-cycle emission of greenhouse gases, particulate matter, and smog precursors; urban air temperature; and urban ozone concentration.”
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Findings on use of cool pavements
One big takeaway from the study is that geographical location has a lot to do with cool city strategies. Ronnen Levinson, a staff scientist with the Heat Island Group at the Berkeley Lab and the principal investigator for the study said, “If water is plentiful and the air is dry, even something like abundant lawns can help mitigate city heat.”
“Vegetation increases the evaporation of water, and water vapor cools the air. It’s the same principle as a swamp cooler. Trees typically don’t evaporate as much water as grass—the water has to travel up the trunk to the leaves, and that’s a much longer trip than going through a blade of grass—but they generally require less water than lawns and they can shade buildings, which reduces the heat from sunlight absorbed through walls and windows.”
Materials used for cool pavements usually require more energy and carbon to manufacture than conventional pavement materials. And while cement is a promising material for pavements because of its light color, cement requires more energy to produce than the bitumen used in asphalt.
The energy and carbon saved in buildings was found to be much less than the extra energy and carbon needed to make the cooler pavements, while the savings in air conditioning energy due to lowered air temperature came to less than 1 kWh (saving less than US$0.60) a year per square mile of pavement modified.
The study further discusses the need for further research into the use of different types of materials for use in cool pavements. The researchers also mention the “cool roof” sector, writing the technology and engineering for cool roofs is fairly mature and reflective roofs have been proven to reduce sunlight absorption and heat convection.
