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Tapping the hot asphalt jungle for energy

Massachusetts researchers argue in paper that blacktop roads would make better solar collectors than solar-electric panels by converting built-up heat into water-carrying pipes.

Pavement, it turns out, is a pretty good place to look for free energy.

A handful of Massachusetts researchers on Monday published a paper detailing a technique for using water-carrying pipes to convert the built-up heat in asphalt roads into usable energy.

Researchers measure ways to transfer heat from a source, such as this lamp shining over asphalt, to water. Worcester Polytechnic Institute

Released at the International Symposium on Asphalt Pavements and Environment in Zurich, Switzerland, the paper argues that asphalt roads have a number of advantages over solar-electric panels as a source of distributed energy.

"The significance of this concept lies in the fact that the massive installed base of parking lots and roadways creates a low-cost solar collector an order of magnitude more productive than traditional solar cells. The significantly high surface area can offset the expected lower efficiency (compared to traditional solar cells) by several orders of magnitude, and hence result in significantly lower cost per unit of power produced," according to the paper.

Blacktops can continue to generate energy after the sun goes down, and upgrades with heat exchangers could be fit into road constructions, which are done every 10 to 12 years, Rajib Mallick, associate professor of civil and environmental engineering at Worcester Polytechnic Institute, said in a statement.

Also, wicking heat away from roads could reduce the "heat island" effect in densely populated areas where temperatures rise when buildings and pavement release heat accumulated during the day.

The researchers used computer modeling and small-scale prototypes to test alternatives to pipes for transferring asphalt heat to water.

It found that the depth of the heat exchanger was critical and that a material with higher heat conductivity, such as quartzite, can be added to asphalt to improve heat transfer.

The hot water from the roads could be used in neighboring buildings, something that has already been done in the Netherlands. A more sophisticated approach would be to convert the heat into electricity using thermoelectric modules.

"Our preliminary results provide a promising proof of concept for what could be a very important future source of renewable, pollution-free energy for our nation. And it has been there all along, right under our feet," Mallick said.