Water droplets falling through a tube have generated enough electricity to power 12 LED lights. Such an approach could one day be used in roof-based systems to harvest lots of clean power from rain.

“Rain falls on Earth every day. All the energy is wasted due to the lack of a system to harvest rain energy,” says Siowling Soh at the National University of Singapore.

Normally, when we generate electricity from water, we use the movement of lots of it to drive a turbine in a river, the sea or even in drinking water pipes. But water flowing over an electrically conductive surface can generate its own electrical charge through a process called charge separation. This is driven by positively charged protons of the water molecules staying in the liquid and negatively charged electrons being donated to the surface, much as you can generate static electricity by rubbing a balloon on your hair.

The phenomenon is usually an inefficient way of generating electricity because the electric charge is created only on the surface the water touches, and if you use micro or nanoscale channels to increase the surface area, you end up requiring more energy to pump the water into them than you get back out.

Now, Soh and his colleagues have created a simple set-up that relies on gravity to move water down a vertical tube that is 32 centimetres tall with an inner diameter of 2 millimetres

Water flows out of the bottom of a container via a horizontal, stainless steel needle, then falls towards the tube below. As the rain-like water droplets collide at the top of the tube, they capture pockets of air, creating what is called a plug flow as they fall. This disjointed flow seems to help the electrical charges of the water molecules separate as they travel down the tube. Wires at the top and bottom of the tube then harvest the generated electricity.

In an experiment, one tube produced 440 microwatts. When the researchers used four tubes at once, they could power 12 LEDs for 20 seconds.

“We can, for the first time, harvest the energy of rain, or other natural sources such as rivers or waterfalls, via charge separation at the solid-liquid interface,” says Soh.

The amount of electricity generated might not seem particularly impressive, but Soh says the set-up converted more than 10 per cent of the energy of the water falling through the tubes into electricity, which is five orders of magnitude more electricity than obtained from water flowing through the tubes in a continuous stream.

“Rain falls from a few kilometres up in the sky to earth, so there is a lot of room in three-dimensional space to harvest rain energy,” he says. This suggests that the system could be used to generate electricity from rain, perhaps on rooftops.

“If it could be developed in a way that could be useful on a house-by-house basis, that could be a really useful thing,” says Shannon Ames at the Low Impact Hydropower Institute in Boston.