Piezoelectric energy harvesters are unproven at scale, so the California Energy Commission wants to see just how much power they could produce.
The roads of California literally hum with energy. Why not use that to make electricity?
Perhaps easier said than done — the cost of that energy could dictate its relevance, and there’s very little information available as to the details of what such a system would be able to deliver. Nonetheless, the state has started the process of testing the idea to see if it could work at scale.
The technology to make it happen already exists. It’s called piezoelectric energy harvesting. Crystalline structures can generate a charge when warped — as road vibrations and other movements could do — and then transmit electricity to power everything from roadside lights to household refrigerators.
As it stands, the energy from the road vibrations produced during millions and millions of daily car trips in California is simply wasted. But put enough piezoelectric energy harvesters just under the surface of the asphalt, and it could become an arguably zero-carbon energy source that could support the grid and potentially displace more polluting fuels.
After some initial research into the issue, the California Energy Commission concluded in 2014 that the amount of electricity such a system could generate depends heavily on frequency and type of traffic. So it’s unclear what the generation capacity of an installation would be, though one vendor claimed it could achieve a capacity of 13.6 megawatts per kilometer of road.
Using $2 million in research funding from the state’s Public Utilities Commission, the CEC has started soliciting bids to do between one and three lab tests to get at some of the questions underpinning whether piezoelectricity could be a viable energy source in the future.
There are a lot of questions. Research on the technology is scarce — aside from California, the only other places in the country where highway applications appear to have been studied are Virginia Tech and the Oregon Department of Transportation. Outside the U.S., a handful of countries — Italy, Israel, Japan — have started some small-scale testing.
“They were pretty hush-hush on their numbers and performance,” said Mike Gravely, deputy chief of the CEC Energy Research and Development Division.
Some of the things the CEC will be looking for are what kind of energy density they could expect and how variations on traffic would affect generation.
“[Applications] would be mainly driven by how much energy you get from the system," he said, "and that’s the type of research we want to do in this first stage."
Further down the line, there will be many other things to consider — how much maintenance would the harvesters need? How much might it cost to produce the electricity, and could that compete with the levelized cost of energy from other sources in a fluctuating marketplace?
That’s really the key — cost. It’s not enough that the technology would be harvesting energy that would otherwise go to waste, and it’s not enough that it wouldn’t add additional carbon to the atmosphere. There are plenty of choices for renewable energy now and, on the whole, they’re getting cheaper. Rather, piezoelectric energy harvesters would need to generate electricity at a rate at least competitive with other fuels so that it could pay for itself.
After some theoretical number-crunching, the CEC believes the levelized cost of piezoelectric energy could be between 8 and 18 cents per kilowatt-hour. The range is large enough that at its low end it might be able to compete with established fuels such as natural gas. But at its high end, it might be more expensive than most other sources.
There are other potential uses for government when it comes to vibration harvesting, however, albeit with somewhat intangible worth. Because the amount of energy coming out of a piezoelectric harvester field would be related to the number of vehicles traveling overhead, they might be able to give the state real-time information about traffic counts and congestion. That’s the kind of information the government is willing to pay private companies for.
“It would be useful for understanding the traffic patterns, high times, low times and maybe where roads need to be expanded, those types of things,” Gravely said.
Alternatively, it might provide data about vehicle weight and possibly even the health of roads. That could improve weigh stations for heavier vehicles and possibly save money on the cost of inspecting roads.
But it will likely be years before the state has more information available. The CEC anticipates that it will award contracts for lab testing in the spring of 2017, and would expect testing to take two years or so — and that’s all before doing a field test.
“I wouldn’t say it’s a sure thing,” Gravely said. “But I’d say it’s worth researching.”