Using Rain to Generate Power

Here’s something residents of cloudy Northern Europe should appreciate: a way of using rain to generate power.

Jean-Jacques Chaillout and colleagues at the atomic energy commission (cea) in Grenoble, France, have shown that piezoelectric materials, which generate voltage in response to mechanical force, can be made to produce useful amounts of electrical power when hit by falling rain. “we thought of raindrops because they are one of the still- unexploited energy sources in nature,” Chaillout says.

His team started by looking up data on different types of rainfall. Drizzle, they found, produces droplets of about 1 millimetre in diameter which have an impact energy of around 2 microjoules, while droplets from a downpour were typically 5 millimetres across and gave 1 millijoule of impact energy.

The team then used computer simulations to see how different-sized drops hit surfaces and concluded that a 25-micrometre-thick piezoelectric material would be the most efficient at harvesting energy from a range of raindrop sizes. To put this to the test, they mounted a 10-centimetre-long strip of polyvinylidene fluoride, a piezo plastic, on a rig and suspended a pipette above it that could be adjusted to create different size water droplets that fell at realistic rainfall speeds. When the drops pounded the piezo plastic, they found that it produced between 1 nanojoule and 25 microjoules of energy per drop, depending on the size of the drop. That translates to about one microwatt of power for the smallest drops, which is enough to transmit a digital bit of information 10 metres of air.

Although the output is puny compared with that of solar panels, rain power has the advantage of working in the dark and could be used to supplement solar- powered devices.

The first application, the researchers say, could be inside the cooling towers of nuclear power stations, where a build-up of limescale reduces their efficiency. The team plans to create a wireless limescale sensor that will be powered by the falling droplets that form when the steam vented up the chimney condenses. Other applications might include a self-powered rain detector for a car’s windshield wipers or wireless air-quality sensors that beam back readings to a pollution data centre.

Environmental sensors that “scavenge” their own energy make good sense, says Peter Tavner, head of engineering at Durham university in the uk. “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 in Ssouthampton, UK, that builds devices which capture energy from vibrating machinery and bridges, points out that rain-powered piezoelectric sensors provide only intermittent power and fears that they may wear out quickly.

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