Researchers have developed a technology that generates electricity from raindrops falling on rooftops, enabling the operation of drainage systems or alarm systems during heavy rainfall.

A research team led by Professor Park Young-bin at the Department of Mechanical Engineering at Ulsan National Institute of Science and Technology (UNIST) announced Wednesday that they have developed a carbon fiber composite-based droplet electricity generator (S-FRP-DEG) that produces electricity from raindrops.

The reason lotus leaves repel water lies in the microscopic protrusions on their surface. Under a microscope, the leaf surface reveals densely packed protrusions measuring 3 to 10 micrometers in size. These protrusions minimize the contact area with water, causing droplets to bead up rather than spread.

The research team applied this "lotus leaf effect" to their generator. They processed microscopic irregularities on the carbon fiber composite surface and applied a special coating. The design maximizes the contact area when raindrops land while allowing them to quickly transform into bead-like shapes and bounce off.

Electricity is generated at precisely this moment. Raindrops carry a positive charge while the composite surface carries a negative charge. During the instant when a raindrop contacts and separates from the surface, charged particles travel through the carbon fibers, creating an electrical current—a principle similar to static electricity.

In experiments, a single 92-microliter raindrop generated up to 60 volts and several microamperes of current. When four generators were connected in series, 144 LED bulbs lit up instantaneously.

Conventional metal-based droplet generators corroded easily from pollutants in rainwater. The research team solved this problem by using corrosion-resistant carbon fiber composites. Carbon fiber composites are lightweight yet strong, making them suitable as exterior roofing materials for buildings.

The lotus leaf effect-inspired coating offers another advantage. Urban soot and pollutants do not adhere to the surface, allowing performance to be maintained over extended outdoor use.

The research team also successfully demonstrated real-time rainfall detection by attaching the generators to roof edges and drainage ducts. The principle is that more falling raindrops generate more frequent electrical signals.

"Without external power sources, buildings and urban infrastructure such as bridges can be managed and flood damage prevented using only rainwater," Professor Park said. "We plan to expand this technology to self-powered systems for mobility applications where carbon fiber composites are used, such as aircraft and automobiles."

Dr. Lee Sung-hwan and researcher Kim Jae-jin participated as co-first authors in this study. The research was conducted with support from the National Research Foundation of Korea (NRF) Mid-career Researcher Program funded by the Ministry of Science and ICT. The results were published online on November 20 in Advanced Functional Materials, an international academic journal in the materials science field.