Scientists at the atomic energy commission (CEA) in Grenoble, France, have developed a technique that takes the mechanical force produced by falling raindrops and converts this force into electricity that can be used to power various electronic devices and sensors.
The system uses piezoelectric structures, which convert mechanical force to voltage, and can recover up to 12 milliwatts of power from one of the larger “downpour” raindrops.
“We thought of raindrops because they are one of the still-unexploited energy sources in nature,” said Jean-Jacques Chaillout, who led the research, in an interview with the magazine New Scientist.
Chaillout’s team began their research by examining data on different types of rainfall. Drizzle, they found, produces droplets of about 1 millimeter in diameter which have an impact energy of around 2 microjoules, while droplets from a downpour were typically 5 millimeters across and gave 1 millijoule of impact energy.
The researchers then used computer simulations to see how different-sized drops hit surfaces, concluding that a 25-micrometer-thick piezoelectric material would be the most efficient at harvesting energy from a range of raindrop sizes.
Finally, they mounted a 10-centimeter-long strip of a piezo plastic material called polyvinylidene fluoride on a rig and suspended a pipette above it that could be adjusted to create different size water droplets that fell at realistic rainfall velocities.
As the drops hit the piezo plastic, they found that it produced between 1 nanojoule and 25 microjoules of energy per raindrop, depending on the size of the raindrop. That equals about one microwatt of power for the smallest drops, enough to transmit a digital bit of information through 10 meters of air.
Although the output is tiny compared to that of solar panels, rain power has the advantage of working in the dark and could be used as a supplement to solar-powered devices.
The researchers said the first application could be inside the cooling towers of nuclear power stations, where a build-up of limescale reduces efficiency.
The team now plans to create a wireless limescale sensor that will be powered by the falling droplets that form when steam vented up the chimney condenses.
Other applications could include a self-powered rain detector for a car’s windshield wipers or wireless air-quality sensors that transmit pollution readings to a data center.
Environmental sensors that “scavenge” their own energy make good sense, Peter Tavner, head of engineering at Britain’s Durham University, told New Scientist. “We use far too much energy in simply exchanging information between devices. I think self-powering them is the future.”
However, Stephen Roberts of Perpetuum, a company that builds devices, which capture energy from vibrating machinery and bridges, noted that rain-powered piezoelectric sensors provide only intermittent power and may wear out quickly.
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