The black asphalt roads of urban centres are notorious for soaking up the sun, often helping make cities uncomfortably hot during the summer. Special piping technology from the Worcester Polytechnic Institute in Massachusetts, however, is offering a way to trap this heat and use it elsewhere, potentially transforming urban streets into giant solar collectors.
The idea is simple: the sun-warmed asphalt can be used to heat up water, which is pumped through tubes embedded a few centimetres below the road surface. This has the dual effect of cooling the asphalt, prolonging the lifespan of the road, and heating water which can be used either as is, or to produce electricity.
A prototype using copper piping pointed to an average asphalt temperature drop of around 10°C. The next step for the researchers will be to experiment with different materials for the piping, while different conductive aggregates will also be added to the asphalt in a bid to improve the overall rate of heat absorption.
“Our preliminary results provide a promising proof of concept for what could be a very important future source of renewable energy”, says Rajib Mallick, the associate professor leading the research at Worcester’s Civil and Environmental Engineering Department.
The concept is not without its drawbacks, however. For starters, installing and maintaining piping within roads would be costly, and the potential returns are yet to be quantified.
“It would seem to make sense: take heat from the road using water then store it for another purpose – such as distributing it to buildings”, says solar energy expert Tim Anderson, a senior lecturer of mechanical engineering at the Auckland University of Technology. However, Anderson doubts the temperature of the water will be high enough to give a good return on investment in most cases. He suggests that the system might be better employed for niche applications where the temperature does not need to be high, such as the de-icing of airport runways or roads. “There may be an economic case for this,” he says, “as it could lead to increased productivity.”