The water trickled, and the lights blinked on.
In a new study, researchers in Singapore describe a way to turn falling water into electricity using nothing more than droplets, a narrow plastic tube, and a surprising flow pattern called “plug flow.”
The setup, they say, can turn something like rainfall into a source of clean, renewable energy — enough to light a dozen small bulbs.
“We’re not talking about waterfalls or dams,” said Siowling Soh, a materials scientist at the National University of Singapore and the study’s senior author. “Water that falls through a vertical tube generates a substantial amount of electricity by using a specific pattern of water flow: plug flow. This plug flow pattern could allow rain energy to be harvested for generating clean and renewable electricity.
A different take on hydroelectricity
The method is so simple it seems crazy no one thought of it before: allow water to fall through a narrow, millimeter-scale tube in short, discrete plugs — columns of water separated by air. The result is a power generation method that sidesteps one of electrochemistry’s most stubborn barriers, and potentially unlocks a new way to produce clean, continuous electricity from rain.
The science behind this power generation is rooted in the same physics that lets a balloon stick to your hair. When two materials touch, their surfaces exchange electrical charges — a phenomenon known as contact electrification. It’s also why rubbing a balloon on your arm makes it crackle. When water slides across certain materials, a similar charge exchange happens.
Soh and his colleagues built a simple 32-centimeter-tall plastic tube just 2 millimeters wide. At the top, they fired tiny droplets of water — the size and speed of rain — directly into the opening through a metal needle.
Instead of forming a continuous stream, the droplets lined up like pearls: short columns of water separated by air pockets. This rhythmic movement, called plug flow, created ideal conditions for charge separation inside the tube.
The inside surface of the tube helped accumulate opposite charges as each water “plug” passed. Wires at the top and bottom collected the resulting electricity. The researchers called it a kind of “falling rain battery.”
In a scaled-up version of the experiment, the team fed water through four tubes in parallel. The system lit up 12 LED lights for 20 seconds.
That might not sound particularly impressive, but the practicality is there. The setup didn’t require massive infrastructure or a roaring river. It needed only gravity and a steady trickle — something easily available from rooftops or in rainy climates.
The researchers envision a future where plug flow energy systems could complement urban power supplies, especially in regions where conventional hydroelectric power isn’t practical. “It could be convenient for urban spaces like rooftops,” they write.
A Century-Old Limit, Broken
It’s no secret to physicists that electricity can arise where water touches a solid surface. The interface between a liquid and a solid spontaneously separates charges — negative ions cling to the wall, while positives flow nearby. This process creates what’s called an electric double layer.
But there’s a catch. The region where this happens — the so-called Debye length — is vanishingly small, on the order of a few nanometers to microns. This severely limits how much charge can be harvested, especially in wider channels like pipes or natural flows. As a result, so-called “streaming current” devices that rely on this interface typically produce negligible power. The efficiency essentially becomes completely negligible beyond 10 microns for practical uses.
That’s why the team’s results are so unexpected. They showed that when water flows not continuously but in discrete, air-separated slugs it can bypass the Debye limitation entirely. Their setup, using tubes just 2 millimeters in diameter and 32 centimeters long, achieved energy conversion efficiencies over 10% and power densities around 100 watts per square meter.
That’s not just better than previous streaming current approaches. It’s five orders of magnitude better.
Not Quite Hydroelectric, Not Quite Solar
Traditional hydroelectric dams rely on large volumes of water and expensive engineering. While efficient, they’re limited by geography — they work only where water can be gathered and stored in bulk.
This new system sidesteps that entirely. Instead of rotating turbines, it harvests electricity from the movement of water itself — not its force. It’s part of a growing field known as triboelectric nanogeneration, where scientists study how materials can generate power from friction, bending, or (in this case) droplets.
Until now, most attempts at such methods struggled with scale and efficiency. Soh’s team seems to have crossed that barrier with a simple, elegant workaround: let water fall naturally, in just the right rhythm.
The team tested their system under a range of conditions. Tap water, saline water, hot water, cold water — it all worked. They scaled it up by using multiple tubes and found that power output scaled linearly.
Perhaps most enticingly, the researchers found that their system can be powered by natural rainfall. Since raindrops have a higher terminal velocity than the flow rate used in the lab, real rain might generate even more power.
It also helps explain longstanding mysteries in atmospheric electricity, such as the Lenard effect, where air near waterfalls or crashing waves becomes negatively charged. The team suggests that natural plug flows — splashes, droplets, spray — might play a key role.
Of course, much remains to be tested. Real-world durability, integration into existing systems, and long-term output under variable weather will all need to be explored.
“Rain is abundant and free,” Soh said. “We just need to find better ways to use it.”
The findings appeared in the journal ACS Central Science.
