By now you are probably well familiar with the concept of the urban heat island effect, even if you can’t quite pinpoint the physics at play when your sneaker sole melts a little on a hot black street in July. Asphalt is an awesome material for storing the sun’s heat. On a steamy summer day, the surface of a road may be as hot as 140 degrees Fahrenheit. And it’ll stay that miserable long after the sun sets, pushing up the temperature of whole neighborhoods covered in this blacktop.
A lot of work has gone into figuring out how to combat the effect. We could plant more tree cover. We could paint black surfaces white. We could construct… artificial glaciers. But this idea might top them all: Why don’t we use that heat instead of fighting it?
"The bottom line is that roads get hot in summertime, even springtime," says Rajib Mallick, a professor of civil and environmental engineering at the Worcester Polytechnic Institute in Massachusetts. “They have a large surface area, which is collecting solar energy. Why not use that solar energy for something? It’s free energy, and if you use it, at the same time you can lower the temperature of the pavement.”
Mallick and other researchers have been developing a model that would harness the heat contained in asphalt and put it to productive uses. Asphalt, for instance, could heat water coursing through a series of pipes embedded in the road. And that process would both cool street surfaces and send their heat somewhere useful.
Everyone would win, Mallick says: Neighborhoods would cool off, the pavement itself would last longer, and that harnessed energy could power nearby buildings and cut down on the cost of utilities. Just think of buildings with vast parking lots, like theaters, malls or office complexes. All that hot water could do the laundry at a sizable hotel.
This heat could also be converted into different forms of energy. Other liquids that turn into vapor could be used to drive turbines generating electricity. Mallick and his colleagues have not only done the theoretical work to envision these possibilities, they’ve begun testing them as well with support from both the state of Massachusetts and the National Science Foundation. This, for instance, is a thermal image from a simulation of the kind of pavement that lines almost all of our neighborhoods (red, as you might have guessed, is hot):
By some estimates, this material covers 29-to-45 percent of urban areas. That equates to a huge urban heat island effect, but also a huge potential for untapped energy. Here is a thermal image of what such pavement with look like with a series of pipes removing all that heat:
“In parking lots, that’s a great idea, no one has any objection,” Mallick says. “But you don’t want to put a lot of pipes and water underneath highways.”
And so a lot of work still needs to be done to come up with other models that would take advantage of the full spectrum of asphalt surfaces in our cities. Perhaps, for example, we could embed a flexible cloth-like material that would conduct heat to pipes running down road medians.
Why not, though, just research an entirely new pavement material that doesn’t get so hot in the first place? Wouldn’t it be simpler to develop an alternative to asphalt, rather than looking for ways to capture the heat that is today asphalt’s biggest downside?
“Economics drives everything,” Mallick says. “And if you think about it, asphalt is very cheap. You can’t find a cheaper material to build pavement. Asphalt is a byproduct of petroleum, so as long as there will be petroleum, there will be asphalt.”
Asphalt is also an infinitely recyclable material, and so in that sense it’s a very valuable one. It’s highly unlikely in the short term, Mallick says, that we’ll be moving en masse to something else.
“Let’s say you do use another material that does not generate heat,” he adds. “You’re still losing a large part of real estate on a surface that is not doing anything except letting you travel over it.”
In that sense, even solar panels start to look a little wasteful. Sure they’re generating energy, but we can’t simultaneously use them for, say, transportation.