PITT Swanson Engineering Feature: Click to enlarge
A top paper from 2021 was one of the highlighted research stories in
University of Pittsburgh Swanson School of Civil & Environmental Engineering Newsletter:
CREATING COOLER CITIES—Pitt Civil Engineers Examine
Urban Cooling Strategies Using Reflective Surfaces:
2021 PAPER: Sushobhan Sen, Lead Author, Postdoctoral Associate-University of Pittsburgh;
and Lev Khazanovich, Co-Author, Anthony Gill Chair Professor of Civil and Environmental Engineering:
“Limited application of reflective surfaces can mitigate urban heat pollution”
(DOI: 10.1038/s41467-021-23634-7)
ABSTRACT:
Elevated air temperatures in urban neighborhoods due to the Urban Heat Island effect is a form of heat pollution that causes thermal discomfort, higher energy consumption, and deteriorating public health. Mitigation measures can be expensive, with the need to maximize benefits from limited resources. Here we show that significant mitigation can be achieved through a limited application of reflective surfaces. We use a Computational Fluid Dynamics model to resolve the air temperature within a prototypical neighborhood for different wind directions, building configurations, and partial application of reflective surfaces. While reflective surfaces mitigate heat pollution, their effectiveness relative to cost varies with spatial distribution. Although downstream parts experience the highest heat pollution, applying reflective surfaces to the upstream part has a disproportionately higher benefit relative to cost than applying them downstream.
SUMMARY
If you’ve been in a city’s central core in the middle of summer, you know the heat can be brutal—and much hotter than in the surrounding region! Researchers at the Pitt Swanson School of Engineering used a Computational Fluid Dynamics model to find ways to decrease cost and increase usage of cooler surfaces. The paper, also published in the journal Nature Communications, examined the possibility of applying cooler surfaces to just half the surfaces in a city. This research gives urban planners and civil engineers an additional way to build resilient and sustainable infrastructure using limited resources.
Temperatures in cities tend to be several degrees warmer than in its rural areas, a phenomenon called the Urban Heat Island (UHI) effect. Many cities have been observed to be 2-4ºC (3.6°-7.2°F) warmer than the countryside in virtually every inhabited continent. This phenomenon occurs because urban infrastructure, especially pavements, absorbs a lot of heat as compared to natural vegetated surfaces. This heat pollution causes higher air conditioning and water costs, while also posing a public health hazard.
One mitigation strategy called gray infrastructure involves the modification of impermeable surfaces (walls, roofs, and pavements) to counter their conventional heating effect. Typical urban surfaces have a solar reflectance (albedo) of 0.20, which means they reflect just 20% of sunlight and absorb as much as 80%. By contrast, reflective concrete and coatings can be designed to reflect 30-50% or more. Cities like Los Angeles have already used reflective coatings on major streets to combat heat pollution, although the solution can be expensive to implement city-wide: According to a Business Insider video, “Dark-colored or black asphalt absorbs between 80% and 95% of the sun’s rays, heating up LA’s streets up to 150° F. The light-coated streets … reflect much more of the sun’s rays and are an average of 10 to 15° F cooler than regular asphalt streets.”
Lead author Sen said, “This could be an effective solution if the surfaces selected were upstream of the dominant wind direction. A ‘barrier’ of cool surfaces preemptively cools the warm air, which then cools the rest of the city at a fraction of the cost. On the other hand, if the surfaces are not strategically selected, their effectiveness can decline substantially.”
Coauthor Khazanovich said ,“It’s important for the health of the planet and its people that we find a way to mitigate the heat produced by urban infrastructure. Strategically placed reflective surfaces could maximize the mitigation of heat pollution, while using minimal resources.”
Please click below for important links:
PAPER: www.nature.com/articles/s41467-021-23634-7 or in
Nature Communications PORTFOLIO Editors’ Highlight page on climate change impacts:
www.nature.com/collections/hcfhgcahdc
Business Insider VIDEO titled “Los Angeles is spending $40,000 per mile to paint streets white — and it could have a surprising ripple effect on the city”: www.businessinsider.com/los-angeles-is-spending-40000-per-mile-to-paint-streets-white-2018-3