Cities are caught in a hot, vicious cycle.

Surfaces in a given urban area absorb heat, and that heat causes city dwellers to use more air-conditioning, water, and other resources, which in turns calls for greater resources from power plants and water-treatment facilities.

There’s a need to break that energy-intensive cycle, says Sushobhan Sen, a postdoctoral associate from the University of Pittsburgh.

Sen, along with researchers at the University of Pittsburgh Swanson School of Engineering, used a Computational Fluid Dynamics model to find the optimal places to apply new materials that reflect, rather than absorb, heat.

How to Cool a Surface

There are several ways to cool a surface. The University of Pittsburgh study focused on just one way: by making a surface reflective.

Typically, urban surfaces fend off just 10 to 20% of sunlight falling on them; the rest is absorbed, raising the temperature of both the surface and the surrounding air. A reflective surface, on the other hand, turns away 50% or more sunlight, significantly lowering temperatures.

You can make a surface more reflective by tearing out a material and using something lighter — for example, an asphalt road could be replaced with concrete or covered with commercially available reflective coating.

While this kind of "grey infrastructure" reduces air temperature, changing up the roads is costly, time-consuming, and requires buy-in from property owners.

The team's paper, published in the journal Nature Communications , examined the possibility of applying cooler surfaces to just half of the surfaces in a city.

That’s where the study began, say Sen: understanding that cities have limited resources available for climate change adaptation and heat pollution mitigation.

"If the ideal solution of modifying every surface is too difficult, are there less-than-ideal ways, like modifying just a fraction of surfaces, to reduce air temperature and is there one among them that is the best?" Sen told Tech Briefs.

One ideal solution, it turns out, involves taking advantage of the wind.

Sen's study debunked a commonly held idea about urban cooling strategies: That surfaces near the edge of the city, along the direction of the wind, need reflective surfaces less than the inner parts of the city, which experience higher temperatures because the air blowing into them has already been heated by the outer surfaces.

"My study showed that this is actually a bad strategy, and the optimal strategy is to modify the surfaces at the edge of the city to pre-emptively cool the air before it can be heated up," Sen told Tech Briefs. "This way, relatively cooler air blows into the inner part of the city (and so doesn’t need to be cooled as much) and the overall cooling of the city is higher even as only a fraction of the surfaces are modified."

A "barrier" of cool surfaces preemptively cools the warm air, which then cools the rest of the city at a fraction of the cost.

Catching on to Cooling

Organizations like The Global Cool Cities Alliance  link cities around the world to share energy-efficiency ideas and cooling strategies. Cities like Los Angeles have adopted grey infrastructure initiatives, like funding a large reflective pavements program. New York City, similarly, introduced a program that applies reflective coatings to roofs.

But while these projects are a good start, they are not enough, Sen told Tech Briefs.

"The approach is still very haphazard, with little consideration for who suffers the most from urban heat and whether the solutions used do any good for them," said Sen.

In other words, equity is still largely missing from the picture, he said, and a lot of the projects may just turn into expensive demonstrations instead of creating widespread public good. Many in the developing world are building cities that are unplanned and based on a growth model from a different century, Sen told Tech Briefs.

"There’s a need to think creatively about solutions for those places too," said Sen.

Sen's new research gives urban planners and civil engineers an additional way to build resilient and sustainable infrastructure using limited resources.

The paper, titled “Limited application of reflective surfaces can mitigate urban heat pollution,” (DOI: 10.1038/s41467-021-23634-7 ) was coauthored by Sen and co-author Lev Khazanovich. The paper was recently featured  by Nature Communications in the Editors’ Highlight page on climate change impacts.

In an interview with Tech Briefs below, Sen explains more about the strategic approach, and how it's actually a bit like stopping a wildfire.

Tech Briefs: You mention the importance of selecting surfaces “strategically.” What should the criteria be for surfaces chosen to be modified/cooled? Which surfaces should be left alone?

Sushobhan Sen: A simple way to think of this solution is to compare it to fighting a wildfire. When you have finite resources, as is usually the case, you need to pre-emptively stop the fire from spreading instead of chasing it as it spreads. Similarly with urban heat: instead of looking only at parts of the city that are very hot, examine where that heat is coming from, and cut it off. This is how you strategically pick surfaces and use limited resources optimally.

Tech Briefs: How does your CFD model look for this criteria?

Sushobhan Sen: The key approach to use this criteria is to understand how wind is distributed within a city. Out in the countryside, where you don’t have tall buildings or rapidly changing surfaces, the wind is somewhat more predictable. But within a city, the combination of varying surface characteristics and turbulence can make things significantly more complicated. Indeed, turbulent fluid flow is one of the great challenges in physics and engineering, so this problem is in good company.

The CFD model, while far from perfect, is a good and practical approach towards this. Instead of fully calculating the wind speed and direction everywhere, down to micron-level accuracy, the approach uses an averaging technique called Reynolds Average Navier-Stokes or RANS modeling to evaluate the average temperature within a few meters of a point in the city. This is generally sufficient since people living in cities are also sensitive to a few meters of air around them, not just the air at the exact point they’re standing at.

So, the CFD model creates an approximate but solvable solution for the wind speed and direction in the city, and from there was can evaluate different ways to use the wind to carry cooler air and thus mitigate heat pollution. The study did just that: once I developed and validated the CFD model, I cooled the air in one part of the city, turned on the model, and let it tell me how that cooler air travels. I did this for a simple, prototypical urban neighborhood to demonstrate the point, but it can be scaled up to larger and more complicated cities using the exact same principles since the underlying CFD model remains the same.

Tech Briefs: Why are strategically placed reflective surfaces so important?

Sushobhan Sen: The key motivation for choosing reflective surfaces strategically is the realization, or maybe resignation, of the fact that there isn’t enough funding and resources for every part of every city to be modified for climate change adaptation. On top of that, there is inequity in how the scarce resources are spent; unsurprisingly, socio-economically disadvantaged neighborhoods tend to get the short end of the stick. This study shows that instead of haphazardly using reflective surfaces wherever convenient (or expedient), there are ways to use them strategically so as to maximize the benefits to the entire city. It’s as much an issue of scientific rigor as it is of equity.

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