Mechanical energy from traffic can generate electricity

In September, California lawmakers passed a bill aimed at tapping a surprising source of green energy: traffic on the state’s busy freeways. Cars and trucks compress asphalt, and piezoelectric generators installed under road surfaces convert this compression into electricity. “The heavier the load, the better the electricity returns”, explains Randy Copperman of Channel Technologies (Santa Barbara, CA), which manufactures piezoelectrics. These subsurface generators can power lights along the road or plug into the existing electrical grids that power most neighborhoods in the state.

Piezoelectrics are not new – the underlying principles are the same as those in touchscreens on mobile devices – but using this technology to harvest energy from traffic is a novel application. To see how piezoelectric generators compare with other renewable energy sources, the bill (AB 306) requires pilot research to assess the output and cost of energy from such road installations.

The energy output is considerable: a 1-km stretch of a two-lane highway could generate enough power for 140 homes, according to a 2011 report that Copperman prepared for the city of San Francisco. “The payback period for highway installations is about 7–10 years, and the generators would work for 25 years or more”, Copperman estimates. Moreover, the cost of installing piezoelectrics under roads is small compared to that of building a new power plant.

Road tests of piezoelectric generators are already underway in other parts of the world. Israel, where the technology was developed, has several demonstration projects, and Italy plans to retrofit part of the Autostrada – the national roadway system – with piezoelectrics. The generators can also be embedded in rubber mats to produce electricity from foot traffic. A Tokyo train station uses this “pedestrian power” to illuminate its display boards, while a piezoelectric floor powers the lights of a San Francisco dance club.

A major benefit of this technology is that it eliminates the need for new transmission lines. Wind and solar power are often generated in remote places, complicating electricity transmission to urban areas. But because traffic is concentrated in the same places as people, piezoelectrics can create energy right where it is needed.
Frontiers in Ecology and the Environment, 2011