When two materials come into contact, charged entities on their surfaces shift slightly. This is how rubbing a balloon on the skin creates static electricity. In a similar manner, water flowing over certain surfaces can either acquire or release charge.
When running water moves a turbine, it generates electricity. However, hydroelectricity is constrained to locations with large volumes of water, like rivers. For smaller and slower volumes of water, another method involves harnessing charge separation. This phenomenon produces electrical charges as water moves through a channel with an electrically conductive inner surface. However, charge separation is inefficient because it is confined to the surface over which the water moves.
Previous attempts to improve efficiency focused on creating more surface area through micro- or nanoscale channels, but water does not naturally flow through such tiny channels. Pumping the water through these channels requires more energy than it generates. To address this, Soh, Chi Kit Ao, and colleagues aimed to generate electricity using larger channels suitable for rainwater flow.
The team designed a simple system in which water exited from the bottom of a tower through a metallic needle, creating rain-sized droplets that fell into the opening of a 12-inch (32-centimeter) tall, 2-millimeter-wide vertical polymer tube. The droplets collided head-on at the top of the tube, creating a "plug flow" - short columns of water separated by air pockets. As the water flowed down the inside of the tube, electrical charges separated. The water was then collected in a cup below the tube, and wires placed at the top and inside the cup harvested the resulting electricity.
The plug flow system converted more than 10 % of the energy from the falling water into electricity. Compared to continuous water flow, the plug flow system produced five orders of magnitude more electricity. Since the droplet speeds tested were slower than actual rain, the researchers suggest that this system could be used to harvest electricity from falling raindrops.
In a separate experiment, the researchers discovered that moving water through two tubes, either simultaneously or sequentially, generated twice the energy. Building on this, they directed water through four tubes, powering 12 LEDs continuously for 20 seconds. The researchers suggest that plug flow energy could be easier to implement and maintain compared to traditional hydroelectric power plants, making it suitable for urban environments, such as rooftops.
The authors acknowledge funding from the Ministry of Education, Singapore; the Agency for Science, Technology and Research; and the Institute for Health Innovation & Technology at the National University of Singapore.
Journal Reference:
Ao, C. K., et al. (2025). Plug Flow: Generating Renewable Electricity with Water from Nature by Breaking the Limit of Debye Length. ACS Central Science. doi.org/10.1021/acscentsci.4c02110.