"Asphalt has a lot of advantages as a solar collector," said Professor Rajib Mallick of Worcester Polytechnic Institute in Massachusetts.

"For one, blacktop stays hot and could continue to generate energy after the sun goes down, unlike traditional solar-electric cells."

"In addition, there is already a massive acreage of installed roads and parking lots that could be retrofitted for energy generation, so there is no need to find additional land for solar farms."

Professor Mallick is convinced that adapting roads for solar heat collection could be part of the natural cycle of resurfacing roads and car parks.

"Roads and lots are typically resurfaced every 10 to 12 years and the retrofit could be built into that cycle," he said.

He also believes that by drawing heat from the roads, the microclimate in cities could benefit as a result.

"Extracting heat from asphalt could cool it, reducing the urban ‘heat island’ effect," Professor Mallick said.

But one of the greatest benefit may be the lack of visual impact -- an aspect that is often used as an objection to renewable energy.

"Unlike roof-top solar arrays, which some find unattractive, the solar collectors in roads and parking lots would be invisible."

Researchers created a testbed that used copper pipes as a heat exchanger to create hot water that could be used to supply homes.

To make the technology viable, the team are now working on more efficient heat exchangers.

"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," said Professor Mallick.

"And it has been there all along, right under our feet."