As wildfires once again rage across California, scientists say they have hit on a cheaper, easier way to detect and warn of such deadly blazes: a new type of forest fire sensor powered by nothing more than the swaying of trees. The device—the size of a soup can and costing just $20 to produce—would theoretically be much cheaper than satellite monitoring or manned patrols.
There have been more than 37,000 wildfires in the United States this year, charring nearly 11,000 square kilometers of land. Firefighters typically monitor such outbreaks with manned watchtowers, aircraft, ground patrols, and satellites. But all of these are costly and require significant manpower. And low-cost fire sensors, like the ones you might find in your home, require routine battery replacement—a real issue if, as in California alone, you’d need to regularly replace batteries in some 81,000 units. The batteries can also pollute the environment if the harmful metals they contain leak out.
The new prototype fire detector doesn’t need batteries; it is powered by a “triboelectric generator” that harnesses small motions to produce energy. The design consists of a top and bottom set of slightly different size concentric cylinders—one coated in copper film, the other in Teflon—that interlock and are connected by a rubber band. The bottom set is weighted such that when hung from a swaying branch, the cylinders move up and down against each other, converting the branch’s erratic motion into electrical power.
This design can “generate electricity from the slightest swinging of tree branches,” says study leader Changyong Cao, a mechanical engineer at Michigan State University. The device requires a breeze to provide power, but fires create air currents, meaning power will likely be readily available, he says. The technology is both fire- and waterproof, and because it has no batteries, there is no risk of leaking harmful metals, the team reports in Advanced Functional Materials.
The prototype is fitted with two different sensors: One measures temperature and the other the presence of carbon monoxide released from wildfires. The team envisages that the final design will also feature a wireless transmitter, so each sensor can relay information to a control center via local data hubs and then satellites in order to alert emergency services of fires. Additional sensors could also be added as needed, the researchers say, to improve the device’s fire detection capabilities. Extra sensors could also pick up toxic gases or monitor air pollution.
The device has a detection range on the order of tens of meters. But in practice, Cao says, a network of sensors would not need to offer full coverage. “We could arrange them in 100-meter intervals or more,” he says. “If we have a fire, we can determine the location and also have enough time and safe distance to take action.” The nanogenerator, he adds, should be able to charge quickly enough to provide one report every 3 minutes.
The device has several advantages over current fire surveillance approaches, Cao says. Not only would it be cheaper and easier to operate in the long run, he says, but it would provide more continuous monitoring than satellites, which often appear only periodically over specific parts of Earth. And, unlike satellites, the system would not be blinded by local weather conditions or the smoke and dust of wildfires.
The device has “great potential,” agrees Zhong Lin Wang, a materials scientist at the Georgia Institute of Technology who was not involved with the work. Triboelectric nanogenerators have revolutionized tech—from creating self-powered heart-rate monitors to battery-free intruder detectors—and the new device has the potential to do the same with forest fire monitoring, he says.
Stefan Doerr, a wildland fire scientist at Swansea University, praises the sensor as an “exciting development.” But he notes the technology has not yet been tested in the field. Furthermore, he adds, the devices “are really not meaningfully comparable” to optical techniques that can spot smoke plumes over vast areas from fire towers or aircraft, or satellite sensors that detect the infrared heat signature created by fires. One challenge, he says, is that you might need 100 of the new sensors to cover just 1 square kilometer of land, and that would be merely “a drop in the ocean in fire prone areas.”