The unbelievable power of a droplet – innovations in sustainable energy
By : Pinky Choi
Renewable energy is essential for tackling the global energy crisis. However, there is still a long way to go. To play their part, CityU scientists are exploring innovative ways of using water and rain to generate more accessible and efficient environmental energy.
A research team led by scientists at CityU has developed a new form of a droplet-based electricity generator (DEG), advancing scientific research and technology development for harvesting future energy from water. Incredibly, a single droplet can light up 100 small LED light bulbs.
This international collaboration is led by Professor Wang Zuankai from CityU’s Department of Mechanical Engineering. Members of the team comprise academics from the University of Nebraska-Lincoln in the US and from the Beijing Institute of Nanoenergy and Nanosystems at the Chinese Academy of Sciences in mainland China.
Their findings were published in the prestigious journal Nature under the title “A droplet-based electricity generator with high instantaneous power density”.
“Our research shows that a drop of 100 microlitres of water released from a height of 15cm can generate over 140 volts, and the power generated can light up 100 small LED lights,” says Professor Wang. One microlitre equals one-millionth of a litre.
Instantaneous power density
Professor Wang Zuankai (middle) and his research team. A conventional DEG can generate electricity based on the triboelectric effect, a classical phenomenon occurring when two objects become electrically charged once they are in contact with each other and then separate. In this case, the charge is from a droplet hitting a surface. However, the amount of charge generated on a typical surface is limited by the interfacial effect, i.e. the contact between two surfaces. As a result, the efficiency of the energy conversion is low, making it more difficult to convert the energy of a falling droplet into electricity compared to, for example, energy created by waterfalls or tides.
In order to improve efficiency, the research team spent two years developing the DEG. Their major challenge was to significantly increase instantaneous power density in a small and convenient device.
Two key factors
Professor Wang pointed out the two crucial factors for the team’s invention. First, they found that the continuous droplets impinging on PTFE, an electret material with a quasi-permanent electrical charge, provides a new route for the accumulation and storage of high-density surface charges. They found that when water droplets continuously hit a PTFE surface, the surface charge generated will accumulate and gradually reach saturation point. This new discovery has helped to overcome the bottleneck of the low-charge density encountered in previous work.
Another key feature is a unique set of configurations similar to the field-effect transistor (FET), an electronic device that won the Nobel Prize in Physics in 1956 for its inventors. FET has three terminals, i.e. source, drain and gate, and is the basic building block for modern electronic devices. The DEG consists of an aluminium electrode and an indium tin oxide (ITO) electrode with a film of PTFE deposited on it. The PTFE/ITO electrode is responsible for the charge generation, storage, and induction, similar to the source in FET. When a falling water droplet hits and spreads on the PTFE/ITO surface, it naturally “gates” the aluminium electrode (drain) and the PTFE/ITO electrode, translating the original system into a closed-loop electric circuit.
A droplet can generate electricity with high instantaneous power density. With this special design, a high density of surface charge can be accumulated on the PTFE through continuous droplets impinging until reaching capacity. Meanwhile, when the spreading water connects the two electrodes, all the stored charges on the PTFE can be fully released to generate electricity with instantaneous power density much more efficiently, increasing the capability of a single droplet to light up 100 small LED lights. This instantaneous power density can reach up to 50.1 watts per square metre, which is thousands of times higher than that of similar devices without the use of the FET-like design.
The findings show how a small amount of water falling from a height of 15cm can generate electricity efficiently.
Wherever the water
Professor Wang hopes that the outcome of this research will help to harvest water energy to respond to the global renewable energy shortage. He believes that in the long run, the new design can be applied anywhere water comes into contact with a solid, for example, the surface of the hull of a ship, or even umbrellas.