A team of researchers from the National University of Singapore unveiled a novel water generation system that can convert raindrops into electricity. The system utilizes a phenomenon called plug flow to improve on current options by an order of magnitude. Here’s how plug flow systems could open the door for a greener and more sustainable future.
Harnessing Water Power: From Ancient Mills to Plug Flow
Mankind has always looked towards water as a way to generate power. In ancient times, cultures like the Romans and Egyptians would utilize water to power tasks like grinding grain. As time progressed, these systems became more complex.
Today, hydroelectricity is a main power source that has found use globally. It relies on gravity and the force of running water to move turbines, which generate power. These systems can generate a lot of energy for a long time, making them ideal for large-scale applications.
Why Traditional Hydroelectric Power Isn’t Always Practical
Hydroelectricity has been a blessing to millions of people in the world. Massive dams can power major population centers without producing as much environmental waste as fossil fuel alternatives. However, they still produce waste and have several limitations that have hindered their expansion.
For one, they must be located near large bodies of water. These systems require dams, flowing water, or even ocean waves to make energy. As such, they are only practical for large-scale applications, and building these massive facilities is both expensive and resource-intensive.
The Struggle to Scale Down Hydropower for Small Systems
Scientists have thought up some creative alternatives to these large plants as a way to generate electricity on a smaller scale. The process of Charge Separation is one such science that has seen considerable interest.
Charge separation occurs whenever you have water moving over an electrically charged surface. You can think of this process a lot like how static electricity will generate power as you rub your feet over the carpet. The friction of the two surfaces produces electrical charges as they interact.
Understanding Charge Separation for Energy Generation
There have been several attempts to make charge separation generators that could produce enough energy to be useful. However, none have achieved this goal. In the past, engineers have studied different approaches to try and advance the generation capabilities of this approach.
Notably, scientists have examined different approaches, including changing the flow in the channel, splashing, angled drops, and varying droplet flows. Of these methods, the most effective to date has been the constant flow method. This method utilizes a small channel where water is run through continuously.
Why Previous Charge Separation Systems Fell Short
The problem with this approach is that the electricity generated via charge separation is limited by the amount of surface space the water covers. These restrictions have led engineers to utilize nanotechnology to provide more channels, equaling more surface space.
This approach has proven to be futile as water droplets will not naturally progress through nanoscale channels. Additionally, adding a pump to the process raises energy requirements without providing additional benefits. One of the main restrictions in this equation is a concept called the Debye Length.
What Is the Debye Length and Why Does It Matter?
The Debye length refers to the characteristic distance of this layer of free ions from the charged surface. There is a length at which the water and surface create an electric double layer that can attract oppositely charged free ions. Notably, every liquid has a Debye length.
For example, the Debye length for pure water at pH 7 is roughly 1 μm. If you raise the CO2 content of the water and change the atmospheric pressure, the Debye length is shortened to nanoscales.
Breakthrough Study: Generating Power Using Plug Flow
Notably, a team of engineers has shown that it’s possible to overcome the Debye length obstacle by using specific flow patterns—most notably plug flow—in millimeter-scale channels. This breakthrough eliminates the need for nanoscale confinement, which is impractical for natural water sources. The five flow patterns that were tested include full flow, plug-dripping flow, dripping flow, Rivulet, and plug flow.
The “Plug Flow: Generating Renewable Electricity with Water from Nature by Breaking the Limit of Debye Length” study1 was published in ACS Central Science this month. The paper highlights a new approach to charge separation for electricity generation. It introduces a novel method that leverages larger channels through which rainwater can flow naturally to generate electricity.
Source – ACS Cent. Sci. 2025
How Plug Flow Creates Electricity Using Water and Air
Plug flow refers to a type of spacing within drops in a tube that takes into account the unique shape of each drop. The drops naturally form as water exits a horizontally oriented metallic needle and impact the side of a vertically positioned polymer tube. This orientation facilitates the creation of plug flow without requiring engineered or custom-shaped droplets.
Unlike streaming current systems, which lose efficiency in larger channels, plug flow maintains high output in millimeter-scale tubes, overcoming a fundamental limitation that has stymied progress for decades.
This strategy initiates a powerful interfacial chemistry with a large chemical potential of charge separation. Keenly, the engineers determined that this strategy allowed aqueous H+ and OH– ions to be separated without creating the electric double layer that limited previous attempts.
The paper describes how engineers were able to determine that millimeter-sized channels were needed to allow water from natural sources to leverage gravity effectively to make their way through the predetermined channels.
Unlike previous systems that relied on electrostatic induction from pre-existing surface charges, the plug flow system generates electricity through direct charge separation at the solid–liquid interface. This occurs specifically at the trailing edge of water droplets, where hydrogen (H⁺) and hydroxide (OH⁻) ions separate naturally, enabling power generation without the need for a pre-charged surface. This capability allowed the system to convert 10% of the energy of the water falling through the tubes into electricity.
Plug Flow Device: Simple, Scalable, and Sustainable
The plug flow system consisted of a plastic container to hold the water, a purpose-built needle tip that was fitted to the container, a tube, and a cup at the bottom of the device. This layout leveraged deionized water and an optimal flow pattern to achieve consistent plug flow.
Creating Plug Flow with a Custom Metallic Needle
The metallic needle tip that fits onto the water storage was built to produce rain-sized drops. These drops were spurted out. As such, they fall at a much slower rate than actual raindrops, allowing engineers to determine that rain could easily enter and exit the system via gravity.
Using a 2mm Polymer Tube for Optimal Plug Flow
The engineers set the metallic needle to drop the deionized water into the opening of a 12in x 2mm vertical polymer tube. This tube was sized exactly to allow sustainable plug flow. At the bottom of the tube, a cup with an electrical node was set up. This cup collected the fallen water. Whereas, electricity was harvested through wires connected to both the collection cup and the top of the tube.
Testing the Plug Flow Generator: Proof of Concept
The engineers conducted several tests to accurately measure the capabilities of their system. They began by creating a miniature generator and testing its capabilities. The first test involved setting up 12 LED lights. The generator was hooked up and successfully powered these lights for 20 seconds.
An electrometer was hooked up to the Faraday cup wires. This setup allowed the team to accurately measure the electricity in the cup after the water fell through the tube. The team determined that the added electrical charge was due to high-performance charge separation at the solid–liquid interface.
Scaling the Plug Flow System for Real-World Use
The team then decided to see how their generators would work in series. They set up two plug flow system tubes and observed their reaction over time. In one experiment, the team operated the generators 5x a day, for 1 min each, for a week. Their test produced some impressive results.
Plug Flow Test Results: Efficiency and Power Output
The plug flow system test results were eye-opening. For one, the system provided 5x orders of magnitude more energy production than all previous methods. It was unanimously proven that it was more effective than the stream methods and highly efficient.
Specifically, the plug flow system converted >10% of the water droplets’ energy into electricity. Additionally, the team registered a power density of ∼100 W/m2. The researchers found that kinetic energy played a negligible role, with over 98% of power generation derived from the gravitational potential energy of the falling water. This emphasizes the plug flow system’s ability to harness natural water flow without external energy inputs. As such, the device demonstrated a new level of performance.
How Plug Flow Delivers Uninterrupted Power Supply
The team ensured that the plug system generator was capable of providing consistent power after repeated and long-term use. It can operate without pumps and interruptions, producing constant energy, even though the water flow is not continuous. The study showed that this was possible thanks to an accumulated charge within the system. This charge naturally flows out during the moment that air is flowing instead of water.
Plug Flow Energy: Compact, Clean, and Cost-Effective
There is a long list of benefits that the plug flow system study brings to light. For one, it offers the world a glimpse into future power generation systems. This approach provides sustainability and can be easily installed in tight spaces. As such, it’s ideal for urban power generation.
No Pumps, No Problem: A DIY-Friendly Energy Solution
This setup is one of the easiest and most affordable options to hit the green energy sector. There are far fewer costs associated with the plug flow approach versus traditional hydroelectric options. Additionally, this option is simple enough to be set up and operated by anyone.
Low-Cost Energy Generation for Global Deployment
The affordability of this approach is another benefit that can’t be understated. There are no alternatives to small-scale hydroelectric energy creation. This option doesn’t require expensive pumps, generators, or even a power supply. As such, it can be affordably deployed anywhere in the world with minimum costs.
Real-World Plug Flow Applications and What Comes Next
There are endless applications for this technology. The world is in dire need of green energy solutions. As governments around the globe seek to hit net-zero carbon goals this century, the need for green energy options is on the rise.
This solution would help both companies and everyday people reduce their reliance on fossil fuels and other environmentally damaging energy options. Here are some applications for this tech.
Generating Electricity from Rain and Gravity Alone
The plug flow system could become a valuable part of your home or even vehicles. These systems are capable of generating electricity from naturally falling water sources such as rain or gravity-fed clean water. While the plug flow system performs best with deionized water, it has demonstrated strong performance with tap water and low-salt solutions, broadening its potential applications. This approach will allow electricity to become common in places where the infrastructure has never existed. Additionally, it will help to reduce the demand on the current system, which is in desperate need of repairs and maintenance.
When Will Plug Flow Systems Hit the Market?
The ability to generate electricity from water flow, without the need for additional motors, pumps, or power supplies, is a huge advantage over today’s alternatives. You can expect to see these systems make their way into the global economy in the next 5-10 years as the engineers optimize their power generation and layout.
Meet the Researchers Behind the Plug Flow Breakthrough
This study was led by a team of engineers from the National University of Singapore. Specifically, Siow Ling Soh and Chi Kit Ao are listed as the main authors. They worked alongside a team of students and researchers.
Additionally, support was secured 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.
The Future of Plug Flow: What’s Next for This Tech?
Now, engineers want to take their study to the next level by improving electrical production and performance. The next steps will include researching additional materials, building processes, and liquids. The hope is to make the system as efficient and affordable as possible.
Investing in the Renewables Market
The green energy sector has become more competitive over the last decade. More companies see the value in becoming the first to create energy creation methods that fall in line with the global sustainability movement. Here’s one example that has led the charge towards green energy and is positioned to leverage the tech in this study.
Clearway Energy: A Renewable Powerhouse to Watch
Clearway Energy (CWEN +3.16%) group entered the market after several acquisitions. Primarily, the acquisition of NRG energy. NRG Energy started service in 1989 as an energy provider.
After it was acquired alongside several other firms, it made Clearway Energy one of the largest energy producers in the US. Today, it retains that role, providing wholesale generation services to a global community.
In 2018, Clearway Energy began trading under its new ticker, CWEN. This marked an uptick in business. Today, the company has an electrical production capacity of approximately 11.8 GW and operates in 26 states.
Clearway Energy, Inc. (CWEN +3.16%)
Clearway is ideally positioned to take advantage of any green energy initiatives and scientific breakthroughs due to its positioning and network. The company has departments that offer wind, solar, and battery storage systems. Those seeking a well-known energy stock that has shown stability, CWEN, is worth further investigation.
Latest on Clearway Energy
Plug Flow Systems: Turning Rain into Renewable Energy
The plug flow system could be a game changer across industries. This technology has the potential to transform a rainy day into the best way to power your home and electronics. For these reasons and many more, you have to commend these engineers on their hard work and determination.
Learn about more cool energy projects here.
Studies Referenced:
1. Ao, C. K., Sun, Y., Tan, Y. J. N., Jiang, Y., Zhang, Z., Zhang, C., & Soh, S. (2025). Plug flow: Generating renewable electricity with water from nature by breaking the limit of Debye length. ACS Central Science. https://doi.org/10.1021/acscentsci.4c02110