Sometimes there’s a bit of unharnessed genius lurking under our noses. Think of those occasions when someone says, “It’s so hot you could fry an egg on the sidewalk,” or those summer days when you step barefoot onto the city streets and do your best to walk in the cooler shadows. Nestled in those moments of everyday life is a potential source of renewable energy now being investigated by scientists.

Instead of warming up cities and blistering your feet, hot asphalt could be used to generate electricity and heat water. The thermal energy absorbed by concrete has been used to warm eco-friendly swimming pools since the 1970s. The process is simple: Pool water is circulated through a network of pipes embedded in a driveway or patio; sunlight warms the pavement and in turn heats the water, which is then pumped into the pool.

Michael Hulen, founder of the tech company Novotech, thinks the concept could be implemented at the commercial level. “We view it as almost an entirely new source of energy,” says Hulen, who has patented an idea that he calls Roadway Energy Systems. The technology could have several lucrative applications. The most straightforward is simply to heat water without electricity—as with swimming pools—but on a larger scale. “Say you have a big hotel with a big parking lot, as well as a lot of laundry and people taking showers,” Hulen says. “You run a piping system under the pavement, heat the water up, and use it for your laundry or whatever you want.”

That captured heat can also create electrical power by producing high-pressure steam from water or other liquids that boil at lower temperatures. The pressure would spin a turbine and produce kilowatts—or possibly even megawatts—of power. Hulen hopes to begin commercial installations by 2010.

Another eco-friendly benefit to sucking the heat out of paved surfaces is that doing so could cool urban areas. During the summer, the heat trapped in concrete can warm cities and suburbs 2–to-10°F above the temperature in less-developed areas nearby. Water-filled pipes laid beneath streets could wick away up to 50 degrees from the surrounding asphalt, helping reduce cities’ swelter. “You’ve got all these man-made surfaces that can’t do anything with the sunlight that’s hitting  them,” says Jamie Paquette, of the nonprofit Solar One, describing what is known as the urban-heat island effect. “Right now all they can do is absorb it, hold onto it, and radiate it back.”

It sounds simple, but maximizing the temperature and heat transfer is critical. Optimal conditions include hot climates with long hours of sunshine, where embedded pipes can heat up water to nearly 210°F. Only 10-to-20 percent of the solar energy that hits the pavement can be transferred to the water. But that still equals up to 200 watts of power per square meter, according to Sankha Bhowmick, an associate professor of mechanical engineering at the University of Massachusetts and a member of Hulen's Roadway Energy Systems research team. Scale that up to the size of a major road or a large parking lot, and it could provide cheap, green electricity to help power commercial or industrial operations.

Bhowmick and his colleagues are trying to increase the heat transfer from the hot cement to water-filled copper pipes with a three-quarter-inch diameter. Much depends on the components of the cement and design of the pipes. For example, Worchester Polytechnic Institute graduate students found that replacing limestone with quartzite in the paving mix produced asphalt that had 1.5 times as much heat capacity and 2.5 times as much conductivity.

“This isn’t going to replace nuclear or coal in the United States, but the infrastructure is already there, except for the piping,” Bhowmick says. With more than 10,000 miles of new roads built in America each year, there is ample opportunity to install the technology. “Since asphalt systems exist, why not see if we can get anything useful out of them?”

Story by Jacoba Charles. This article originally appeared in "Plenty" in January 2009.

Copyright Environ Press 2009

Radiating roads
Capturing asphalt's heat for power.