Tag Archives: raindrops

Turning raindrops into usable electricity

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Harvesting energy from raindrops is of great interest to a lot of researchers on the hunt for new sources of energy. My last posting on the topic was a November 8, 2024 piece highlighting work from Korea. This latest work is from Singapore. From an April 16, 2025 news item on ScienceDaily,

When two materials come into contact, charged entities on their surfaces get a little nudge. This is how rubbing a balloon on the skin creates static electricity. Likewise, water flowing over some surfaces can gain or lose charge. Now, researchers reporting in ACS Central Science have harnessed the phenomenon to generate electricity from rain-like droplets moving through a tube. They demonstrate a new kind of flow that makes enough power to light 12 LEDs.

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An April 16, 2025 American Chemical Society (ACS) news release (also on EurekAlert), which originated the news item, delves further into the topic,

“Water that falls through a vertical tube generates a substantial amount of electricity by using a specific pattern of water flow: plug flow,” says Siowling Soh, the study’s corresponding author. “This plug flow pattern could allow rain energy to be harvested for generating clean and renewable electricity.”

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, an alternative is to harness charge separation, a phenomenon that produces electrical charges as water moves through a channel with an electrically conductive inner surface. But charge separation is extremely inefficient because it is restricted to the surface that the water moves over. Previously, scientists have tried to improve its efficiency by making more surface area available through micro- or nanoscale channels for a continuous stream of water. However, water doesn’t naturally pass through such tiny channels, and if pumped, it requires more energy than gets generated. So, Soh, Chi Kit Ao and colleagues wanted to produce electricity using larger channels that rainwater could pass through.

The team designed a simple setup whereby water flowed out the bottom of a tower through a metallic needle and spurted rain-sized droplets into the opening of a 12-inch-tall (32-centimeter-tall) and 2-millimeter-wide vertical polymer tube. The head-on collision of the droplets at the top of the tube caused a plug flow: short columns of water interspersed with pockets of air. As water flowed down the inside of the tube, electrical charges separated. The water was then collected in a cup below the tube. Wires placed at the top of the tube and in the cup harvested the electricity.

The plug flow system converted more than 10% of the energy of the water falling through the tubes into electricity. And compared to water flowing in a continuous stream, plug flow produced 5 orders of magnitude more electricity. Because the droplet speeds tested were much slower than rain, the researchers suggest the system could be used to harvest electricity from falling raindrops.

In another experiment, the researchers observed that moving water through two tubes, either simultaneously or sequentially, generated double the energy. Using this information, they channeled water through four tubes, and the setup powered 12 LEDs continuously for 20 seconds. The researchers say that plug flow energy could be simpler to set up and maintain than hydroelectric power plants, and it could be convenient for urban spaces like rooftops.

The authors acknowledge funding from the Ministry of Education, Singapore; the Agency for Science, Technology and Research [A*STAR]; and the Institute for Health Innovation & Technology at the National University of Singapore.

Here’s a link to and a citation for the paper,

Plug Flow: Generating Renewable Electricity with Water from Nature by Breaking the Limit of Debye Length by Chi Kit Ao, Yajuan Sun, Yan Jie Neriah Tan, Yan Jiang, Zhenxing Zhang, Chengyu Zhang, and Siowling Soh. ACS Central Science 2025, 11, 5, 719–733 DOI: https://doi.org/10.1021/acscentsci.4c02110 Published April 16, 2025 Copyright © 2025 The Authors. Published by American Chemical Society. Creative Commons Licence: CC-BY 4.0 .

This paper is open access.

Multifunctional smart windows that lower indoor temperatures without consuming power and can generate electricity from raindrops

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Caption: Figure 1: The main functions of the multifunctional smart windows for implementing Plus Energy (transparent radiative cooling, power generation, and fog and frost removal technology). Credit: © Seoul National University College of Engineering

I’m always a sucker for a ‘smart window’ story and this one from Korea with its reference to harvesting energy from raindrops seems particularly intriguing. From an August 13, 2024 Seoul National University (SNU) press release, also on EurekAlert but published August 22, 2024,

■ Research Necessity

o Recently, with the significant increase in cooling demand due to global warming, a vast amount of energy is being consumed for heat management inside buildings. Existing windows, which have a high solar absorption rate and low reflectance, lead to considerable energy loss. Therefore, energy-saving windows are emerging as a practical solution to global challenges such as responding to climate change and ensuring energy sustainability. These windows not only provide optimal thermal comfort to occupants but also contribute to economic development by reducing dependence on conventional cooling systems.

o For windows to effectively save energy in buildings, it is necessary to adopt energy-efficient cooling technology (Zero Energy) and further ensure energy harvesting methods (Plus Energy) that guarantee sustainable power supply. Additionally, windows must maintain high transparency, which is their fundamental function, even on cold or foggy days.

■ Research Achievements / Expected Effects

o The multifunctional smart windows developed in this research demonstrate their effectiveness as next-generation energy-saving devices by implementing three main functions.

o First, they provide radiative cooling that lowers indoor temperature on sunny days without energy input. Second, they generate electricity using raindrops on rainy days. Third, they implement a transparent heater function to quickly remove frost from the windows on cold days.

■ Research Details 

□ Research Content Overview

o The research team led by Professor Seung Hwan Ko from the Department of Mechanical Engineering at Seoul National University has developed “multifunctional smart window technology” that lowers indoor temperatures without electricity consumption and generates power using the frictional electricity from raindrops. This research is significant in that it pioneers new possibilities for Plus Energy technology, surpassing Zero Energy to contribute to improving energy self-sufficiency in response to global warming.

□ Background

o Recently, implementing Plus Energy Buildings (PEBs) that surpass Zero Energy has become a key task for achieving energy self-sufficiency in buildings. Next-generation PEBs are buildings that go beyond minimizing energy loads and can autonomously produce energy. Buildings inherently consume a massive amount of energy for heat management, and with the rise in cooling demand due to global warming, energy usage has surged dramatically. Furthermore, existing windows with high solar absorption and low reflectivity result in substantial energy losses during cooling. Therefore, to realize economically efficient next-generation Plus Energy Buildings, it is necessary to develop multifunctional smart windows equipped with transparent cooling technology (Zero Energy-based) and further energy-harvesting technology (Plus Energy-based) that ensures sustainable power supply.
o To address these issues, researchers worldwide are focusing on the development of smart windows that maximize energy savings. Smart windows are often thought to adjust internal temperatures by changing color to control sunlight. However, this method has limitations since the windows become opaque during the cooling process, thus failing to maintain high transparency, which is the window’s primary function.

□ Key Research Methods

o The research team is actively working on developing new technologies that improve energy efficiency while preserving the transparency of windows. As part of this effort, Professor Ko’s research team developed a Zero Energy-based “transparent radiative cooling technology” that maintains transparency while enabling cooling without using electricity. Additionally, they developed energy-harvesting technology that produces electricity through the friction generated when raindrops contact the window surface, introducing a Plus Energy-based smart window technology that surpasses Zero Energy. The team also developed a transparent heater technology that quickly clears frost from windows on cold or foggy days, thereby implementing three functions—radiative cooling, power generation, and frost removal—simultaneously in a single device for the first time in the world.
o The research team achieved these three functionalities in a single device by fabricating windows with a layered structure of silver and ITO (Indium Tin Oxide), materials with excellent electrical conductivity and unique optical properties. First, the “transparent radiative cooling technology” minimizes the absorption of sunlight entering indoors while emitting radiant heat outdoors to lower the temperature. Unlike conventional air conditioning systems that use refrigerants, this radiative cooling technology offers cooling performance without consuming electrical energy. The research team focused on allowing only the visible light spectrum from sunlight to pass through the window while selectively reflecting near-infrared sunlight to lower indoor temperatures and maximize cooling. Second, the “frictional electricity-based power generation technology” generates electricity when raindrops contact the window surface on rainy days. For this purpose, an electrode material covering the window surface is necessary, and thanks to the excellent electrical conductivity of the layered silver and ITO structure, the smart window can generate electricity through frictional electricity. Lastly, through “Joule heating,” the transparent electrodes also serve as a heater that quickly removes frost or ice from the window, ensuring clear visibility on cold days. The multifunctional smart windows developed by the research team can provide transparent radiative cooling on sunny days, generate power on rainy days, and remove frost or ice on cold days.

□ Results

o The research team led by Professor Seung Hwan Ko confirmed that the smart windows they developed maintained a temperature approximately 7 degrees lower than regular windows in hot environments under direct sunlight. In an experiment simulating rainy conditions, the smart windows generated 8.3 W m-2 of power with just a single raindrop, while also clearing frost from the window twice as fast as regular windows through Joule heating, demonstrating both high performance and multifunctionality.

□ Expected Effects

o Professor Seung Hwan Ko stated, “This achievement of presenting next-generation smart window technology optimized for responding to the depletion of fossil fuels and global warming offers valuable insights into the technological advancements for Plus Energy buildings and the eco-friendly electric vehicle industry. Smart windows are expected to be applied across various industries because they address environmental pollution, reduce cooling energy, and overcome the limitations of conventional battery technologies through self-power generation.”

□ Achievements

o This research was supported by the Basic Science Research Program through the National Research Foundation of Korea, and it has gained global attention, being published in the October 2024 issue of the prestigious journal Nano Energy (Impact factor: 16.8, Top 5.3%) under the title: “Energy-saving window for versatile multimode of radiative cooling, energy harvesting, and defrosting functionalities.”

o Meanwhile, Dr. Yeongju Jung, the lead author of this study, is currently conducting follow-up research at Professor Ko’s laboratory in the Department of Mechanical Engineering at Seoul National University and is preparing for a postdoctoral research fellowship abroad.

□ Introduction to the SNU College of Engineering

Seoul National University (SNU) founded in 1946 is the first national university in South Korea. The College of Engineering at SNU has worked tirelessly to achieve its goal of ‘fostering leaders for global industry and society.’ In 12 departments, 323 internationally recognized full-time professors lead the development of cutting-edge technology in South Korea and serving as a driving force for international development.

Here’s a link to and a citation for the paper,

Energy-saving window for versatile multimode of radiative cooling, energy harvesting, and defrosting functionalities by Yeongju Jung, Ji-Seok Kim, Junhyuk Bang, Seok Hwan Choi, Kangkyu Kwon, Min Jae Lee, Il-Kwon Oh, Jaeman Song, Jinwoo Lee, Seung Hwan Ko. Nano Energy DVolume 129, Part A, October 2024, 110004 DOI: https://doi.org/10.1016/j.nanoen.2024.110004 Available online 25 July 2024, Version of Record 25 July 2024

This paper is behind a paywall.