Taking the heat off Valley parking lots

A two-degree decrease in temperature might not sound like much until you do the math. According to David Sailor, it could reduce energy costs in Phoenix by millions of dollars.

Sailor is an Arizona State University professor and director of the Southwest Urban Corridor Integrated Field Laboratory (SW-IFL), a U.S. Department of Energy-funded center that works to mitigate extreme heat and poor air quality across desert Southwest cities. 

This summer, amid record-high temperatures in the Phoenix metro area, Sailor and a team of students embarked upon a yearlong study of a cool pavement product put down in the parking lot of the Desert Ridge Marketplace in north Phoenix. 

The idea behind the study is that lighter-colored seals can take the heat off some paved roads, creating cooler subsurface, surface and above surface air temperatures.

Sensors were strategically placed under the paving, on light posts, attached to cars and even on helicopters to gather information. Initial data already indicates that lighter asphalt coatings can create surfaces up to 20 degrees cooler than the darker surfaces. This can translate into temperature reductions above the surface of up to two degrees Fahrenheit, which makes for a meaningful neighborhood-scale effect in many respects, including avoided energy costs for air conditioning. 

“These measurements were made in mid-September,” said Sailor, who is also the director of ASU's School of Geographical Sciences and Urban Planning. "The benefits are likely to be larger during the peak of the summer, with higher solar radiation and air temperatures.”

The project was initiated by Vestar, owners of the Phoenix mall. The retail management company replaced a basic black sealcoat covering the 60,000-square-foot parking lot with CoolSeal, a light-gray colored product. 

The project builds on previous work that Sailor and ASU conducted in partnership with Phoenix residential street-paving projects, but it’s the first time an area this large has been studied.  

ASU News reached out to Sailor to learn more about the cooling pavement treatment and its potential to reduce energy costs in the future.  

Editor's note: Answers have been lightly edited for length and clarity.

Question: How did ASU get involved in this project?

Answer: Vestar initially approached me for guidance as they were considering a cool paving project and knew that I had done prior work on the topic.

In that conversation, I saw a unique opportunity for a technology testbed at a scale that would allow us to really isolate and measure the effects of the cool paving treatment. Since this aligns well with the objectives of SW-IFL, I volunteered the resources necessary to monitor and analyze the performance of the project.

Our measurements include background weather conditions at the site as well as subsurface, surface and near-surface air temperatures over conventional and cool paving parts of the Desert Ridge Marketplace parking lots.

Q: The cool paving project could create an oasis from the Valley’s sweltering summer heat. How would that work?

A: The sun is, by far, the largest input of energy into the urban system. So we are doing what we can to reject that heat before it can be absorbed by materials in the built environment. The cool paving reflects about 30% of the sun’s energy, while the conventional dark seal coat in the adjacent parking lot reflects less than 4%. As a result, the cool paving absorbs less heat throughout the day. As air flows over the cooler paving, it picks up less heat, so the surface and air temperatures are reduced. 

Q:  How does this project differ from your previous work?

A: The challenge that we faced in residential neighborhoods was really one of isolating the effects of cool paving, given that every neighborhood is so different and that the application area was relatively small. The cool paving project at the mall is great because it covers more than 60,000 square feet of surface, directly adjacent to an even larger conventional dark parking lot. As such, we are much better able to isolate and quantify the effects of the cool paving, not only on surface temperatures, but on air temperatures.

Q: A lighter-colored surface doesn’t absorb the heat that a darker surface does. But the lighter surface reflects heat back into the air. In the end, is it a wash?

A: The atmosphere is relatively transparent to solar radiation, so the majority of reflected radiation will simply be transmitted upward through the atmosphere and out into space. The catch is that some of that radiation will be intercepted by objects in the parking lot, mainly cars and people briefly walking across the parking lot to the mall. 

This is complicated because the air temperature benefits will persist for hours throughout the day and into the nighttime hours, while any adverse effect on pedestrians is limited to the 30 seconds or so that they spend walking from their car to the covered walkway or the mall.

It is important to note a few things here — your thermal comfort is dictated more by the color of the clothing you are wearing than the color of the parking lot you are walking on, and any adverse effect related to reflected radiation only occurs durng the peak daylight hours.

Q: What are your findings so far? 

A: We actually conducted an intensive measurement campaign a few days before and after the installation of the cool seal. We took surface temperature measurements from both instrumented vehicles and from an infrared camera in a helicopter flying overhead. The surface temperature of the conventional paving in the middle of a sunny day in early September was about 14 degrees (Fahrenheit) hotter than the cool paving. We also measured near-surface air temperatures that were roughly two degrees (Fahrenheit) cooler over the newly paved areas.

Q:  Can the Valleywide application of cooler seals impact heat islands and significantly reduce summer heat?

A: These sorts of “cool surfaces” at the ground level can be hugely beneficial by reducing air temperatures. Even a small reduction of air temperature across Phoenix can save millions of dollars each year in avoided air conditioning energy costs and reduced water use.

At the same time, we need to be cautious with the choice of location for such products.

For example, they won’t have much impact in neighborhoods with lots of trees or tall buildings – because the paved surfaces may be shaded much of the day. We also shouldn’t ignore the fact that the reflected radiation can be intercepted by people who may spend much time on the surface. This is particularly a concern, say, for paved playgrounds at schools and parks where people, and children in particular, may spend long periods on the surface during the heat of the day.

I do think the application in large parking lots is particularly attractive. Consider, for example, the very large parking lots at major sports venues such as State Farm Stadium that sit empty much of the time, simply soaking up heat from the sun and releasing it into the air right in the heart of the city.

These kinds of spaces would benefit from the use of a product like this.

Top photo: Infrared image taken from above the Desert Ridge Marketplace in north Phoenix. Photo by ASU graduate student Edwin Ramirez