This hydropower harnesses energy one water drop at a time | Science News for Students

This hydropower harnesses energy one water drop at a time

The friction generated by a single droplet can light 15 LEDs
Feb 9, 2018 — 7:20 am EST
David Ma
David Ma, an engineer at the University of Hawaii, led a team that developed a method to harvest electricity from the friction of a water droplet rolling across a surface.
UH Staff

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

A rainy day can be a chance to recharge — literally. While you relax on the couch with a movie, the raindrops falling on your windows might one day provide the power for your TV. This is the idea behind an invention that harvests energy from water, one drop at a time.

The technology is based on the triboelectric (TRY-boh-ee-LEK-trik) effect, explains David Ma. The triboelectric effect is the generation of an electric charge due to friction. An engineer at the University of Hawaii in Manoa, Ma knew that it’s possible to generate electricity by rubbing two things together. So, he thought, “Why don’t we use water?”

A drop of water sliding across a surface coated with two different materials would generate enough friction to create an electrical charge. By placing metal wires that the droplet touched as it moved, it should be possible to harvest electricity, he reasoned.

LED lights
One drop of water provided enough power to light up all of these LEDs.
Jian Yu and Enze Ma

“It’s a very simple idea,” Ma says. “We basically used a water drop as an electrode.”

And it worked. In fact, the researchers lit up 15 LED bulbs with a single moving water droplet.

Ma was an author on a December paper describing the technique. It appeared in Scientific Reports.

This is not the first time scientists have captured electricity from water-generated friction. Earlier experiments, though, harvested the charge induced in a surface by a sliding drop of water. There, the surface had acted as an electrode. This is different. The energy of friction is being harvested from the water itself.

And that proved to be a clever innovation.

“It turns out,” Ma says, “the charge in the water drop is way more than the charge induced in the other electrode.” In fact, his team’s prototype generated almost 100 times more power than previous experiments from a single droplet of water.

The technology could someday power phones, sensors or other small electronics, says Christopher Oshman. He works as a mechanical engineer at the Colorado School of Mines in Golden. “This work is a step toward harvesting the energy of moving objects all around us, including ourselves, to power the electronic devices we use every day,” he says.

electrifying droplet
In this prototype, low-level vibration helped move a water drop back and forth across a surface coated with two different materials. This created a small but useful amount of electricity.
Jian Yu and Enze Ma

Ma has shown that the technology can work in a lab, Oshman says. Next, the Colorado researcher would like to see it tried on a larger scale, such as on an umbrella.

Another step will be to create prototypes, or models, that test the technology under real-world conditions. For instance, maybe it could be turned into a coating that would extract power as rain drips down a building or hits car windows. Triboelectric-energy harvesting also could charge electrical devices far from conventional power sources. Think of sensors placed along a highway or stream.

“We can use salt water,” Ma adds. “Maybe even blood.” That suggests that one day this technology might even help power biomedical devices, such as heart pacemakers. Currently, these are battery-powered. Think about it: Patients now need surgery every time those batteries run out of power. In the future, the natural flow of fluids in the body might be tapped to power such devices.

Power Words

(for more about Power Words, click here)

application     A particular use or function of something.

battery     A device that can convert chemical energy into electrical energy.

biomedical     Having to do with medicine and how it interacts with cells or tissues.

electric charge     The physical property responsible for electric force; it can be negative or positive.

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

electrode     A device that conducts electricity and is used to make contact with non-metal part of an electrical circuit, or that contacts something through which an electrical signal moves. (in electronics) Part of a semiconductor device (such as a transistor) that either releases or collects electrons or holes, or that can control their movement.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

friction     The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage a surface of some material as it rubs against another.

innovation     (v. to innovate; adj. innovative) An adaptation or improvement to an existing idea, process or product that is new, clever, more effective or more practical.

LED     (short for light emitting diode) Electronic components that, as their name suggests, emit light when electricity flows through them. LEDs are very energy-efficient and often can be very bright. They have lately been replacing conventional lights for home and commercial lamps.

literally     A term that means precisely what it says. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.

mechanical     Having to do with the devices that move, including tools, engines and other machines (even, potentially, living machines); or something caused by the physical movement of another thing.

pacemaker     A small medical device implanted in the body to help control abnormal heart rhythms. This device sends an electrical signal. It stimulates the heart to beat at a regular and healthy rate.

prototype     A first or early model of some device, system or product that still needs to be perfected.

sensor     A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

triboelectric     A term for an electric charge that develops when two things rub against each other, causing friction.


Journal: J. Yu et al. Harvesting energy from low-frequency excitations through alternate contacts between water and two dialectic materials. Scientific Reports. Vol. 7, December 7, 2017. doi: 10.1038/s41598-017-17522-8.