Harvesting power from raindrops

Downpours may be the next source of renewable energy

By Sandra Upson

When most of us see an overcast sky, we scramble for an umbrella. When French researchers looked to the cloudy heavens, they saw a potential source of energy. Their new technology could make so-called bad weather ideal for harvesting power.

Scientists from Europe’s Atomic Energy Commission, in Grenoble, France, have shown that vibrations from raindrops landing on a certain type of plastic can generate enough energy to operate some low-power wireless sensors, like battery-powered outdoor thermometers. But the results, published in the February issue of the journal Smart Materials and Structures, could do much more than save you the inconvenience of replacing a drained battery in your outdoor thermometer. The findings could help improve networks of wireless sensors that measure conditions like temperature, pressure, or the presence of pollutants. By continually monitoring the environment, these networks provide early warning systems for dangerous air quality, severe storms, or disease outbreaks. Networks that exist now use batteries that require annual replacement.  To be completely reliable—not to mention sustainable—sensor networks would have to power themselves. 

To address this shortfall, researchers have focused their efforts on capturing and storing energy from the environment. For example, solar-powered sensors are sometimes connected to a battery that stockpiles power collected by the cells in the daytime. But solar cells only work on clear, sunny days. That’s why it’s essential to find more ways to match sensors to the environments they monitor. “People think of light and wind when they think of free energy,” says Jean-Jacques Chaillout, one of the paper’s authors. “But there is much more out there. There is no one answer for powering sensors.”

To make electricity in rainy locales, Chaillout’s team is using a piezoelectric material—in this case, a plastic—that translates mechanical energy from the impact of the raindrop into electric energy that powers a sensor. During a rainstorm, the material dribbles electrical energy to a battery, storing it for later use. The scientists examined raindrops that range in diameter from a 1-millimeter drizzle to the 5-millimeter drops dumped in a downpour. Their experiments suggest they can collect up to 12.5 milliwatts of instantaneous power from one large droplet; you’d need nearly 5,000 of these drops to light up a 60-watt bulb, but the sensors require only a fraction of that power.

Piezoelectric sensors are already in use. Some cars use them to trigger airbags. Other devices capture vibrational energy from ocean waves and humans—including backpack-wearers, people pushing turnstiles, and pedestrians climbing stairs (see page 28 for more examples). Someday scientists hope the technology will power implantable biomedical devices, like pacemakers, that currently rely on batteries. But only recently have wireless sensors become sufficiently cheap and low-power to benefit from piezoelectric energy scavenging. “It’s an emerging area,” says Edward Sazonov, an assistant professor of electrical and computer engineering at Clarkson University. “It just has to gain acceptance in industry as a viable alternative to batteries alone.”

Vibrational energy harvesting may still have a way to go before it’s widely adopted. According to Daniel Inman, a mechanical engineer at Virginia Tech and director of the school’s Center for Intelligent Material Systems and Structures, the best materials harvest no more than a few milliwatts per strain. “There’s still quite a bit of materials-science research going on to find the best mechanics and configurations,” Inman says. Given how little power wireless sensors require, that isn’t an issue for Chaillout. From his point of view, the biggest challenge is the cost of the material: $460 for 1 kilogram. But that will change with time, he adds, hinting that one company has already expressed interest in his lab’s rain-power product.
Who knows, April showers may soon bring power.