Sparks

Harnessing the energy of rain

 By Robin Wylie

In a study released this August, researchers in the UK have found a way to extract energy from H2O before it even touches the ground: by harnessing the kinetic energy of rain. To test the device, they dropped artificial raindrops onto the PVDF film (polyvinylidene fluoride) from a height to simulate a rainshower. “Now there are reasons to believe that rain power could someday be a viable option”, explains Robin Wylie…

Humans have been using water as a power source for thousands of years. The ancient Romans used water mills to grind flour. In the 18th and 19th centuries, water wheels helped drive the industrial revolution. And today, hydroelectric dams supply clean electricity to millions of people. But now scientists have found a way to extract energy from H2O before it even touches the ground: by harnessing the kinetic energy of rain.

The energy carried by the average raindrop is on the order of one milijoule (one thousandth of a joule), which is transferred to the Earth’s surface when the droplet lands. That’s not much; our bodies release 60 thousand times as much energy as heat every second! But in the course of a rainshower or storm, those little splashes add up to a considerable amount of energy. And scientists have just come one step closer to harnessing it.

In a study released this August, researchers in the UK developed an electronic device able to harvest energy from laboratory-generated raindrops. To convert the drops’ kinetic energy into electricity, they used a thin film made of PVDF (polyvinylidene fluoride) – a polymer with so-called “piezoelectric” properties, meaning that when it undergoes stress, an electric current is spontaneously produced within the material.

Forget damming a river or erecting turbines, soon generating clean power could be as simple as laying out a sheet

To test the device, the researchers dropped artificial raindrops onto the PVDF film from a height to simulate a rainshower. When the drops hit the film, the device generated around 2.5 microwatts of electrical power. This is admittedly a tiny amount, and wouldn’t be able to power any substantial electronic device (for comparison, the average smartphone needs more than 100,000 times as much power to run). But there are reasons to believe that rain power could someday be a viable option.

One reason for hope is the fact that the device is currently operating at extremely low efficiency — about 0.1% (of a theoretically possible 100%). No device can ever reach 100% efficiency, but it’s thought that for piezoelectric materials like PVDF, 40% efficiency might eventually be attainable. This would boost the output of raindrop harvester devices by 400 times.

Another way to get more power from rain could be to use a larger collecting surface. These would be exposed to more raindrops, and could thereby scavenge more kinetic energy from the falling liquid. The device in the August study had an area of just 3 cm(1.1 in). But the researchers calculated that by increasing this area to 10 m(32 ft2), the device could generate 1.5 miliwatts of power, 600 times more than the original device. If this was increased to 100m2, they estimate that the device could yield 20 miliwatts of power, an impressive 8000 fold improvement.

Raindrops will probably never be able to compete with a hydroelectric plant or windfarm. But they do have one major advantage — they’re free. Forget damming a river or erecting turbines, soon generating clean power could be as simple as laying out a sheet.

about the author
Robin Wylie
Freelance earth/space science journalist. Currently finishing off a PhD in volcanology at University College London.