The kinetic energy of falling water has been turned into electricity since the 1870s. Current hydropower systems depend, however, on rain falling at high altitudes and collecting together into rivers to reach masses large enough to make turbine harvesting efficient. A new technology seeks instead to use the energy of the falling drops directly, making it useable wherever rain falls.

"The kinetic energy entailed in falling water is due to gravity and can be regarded as free and renewable,” said Professor Wang Zuankai of the City University of Hong Kong in a statement. "It should be better utilized.”

There have been many efforts to make something similar in the past, along with attempts to harvest the chemical energy of materials dissolved within the raindrops. All have been so inefficient they were suitable only as toys or curiosities, not sources of useful energy. Wang’s version may not be ready to power a city, but with 100 times the energy density of its predecessors, it’s in the right ballpark for practical use.

Wang and co-authors announced their success in Nature, demonstrating their generator’s potential by dripping water onto a polytetrafluoroethylene film over an indium tin oxide substrate. As each drop of water spreads across the film it connects aluminum electrodes, allowing charges built up in the film to flow as electricity.

As each drop hits the generator it provides enough energy to light up 100 commercial LEDs. Novel forms of renewable energy often produce tiny voltages. Wang, however, claimed: "Our research shows that a drop of 100 microlitres (1 microlitre = one-millionth liter) of water released from a height of 15 cm (6 inches) can generate a voltage of over 140V.”

Using tap water Wang achieved peak power densities of 50 watts per square meter (4.7 W/foot²), although rainwater was somewhat lower. That’s less than a third of a typical photovoltaic panel on a sunny day, but hundreds or even thousands of times greater than other raindrop harvesting devices. Probably not a competitor for, or even a complement to, solar panels in Los Angeles, but London might be a different matter.

Moreover, the best may be yet to come. The authors calculate they are converting 2.2 percent of each drop’s kinetic energy into electricity, so there is plenty of room for improvement. Wang envisages umbrellas coated with suitable films so if you’re caught in the rain you at least get to charge your phone.

Obstacles remain, though. A system that works with drops of the same size falling from a consistent height may not do as well under varying conditions. More seriously still, the authors acknowledge degradation of surface charge may reduce the generator’s efficiency with time.