New droplet-based electricity generator: A drop of water generates 140V power, lighting up 100 LED bulbs

A research team led by scientists from the City University of Hong Kong (CityU) has recently developed a droplet-based electricity generator (DEG) with a field-effect transistor (FET)-like structure that allows for high energy conversion efficiency and instantaneous power density thousands of times that of its counterparts without FET technology. With the new droplet-based electricity generator, a drop of water released from a height of 15 cm can generate a voltage of over 140V, lighting up 100 small LED bulbs.The density of surface charges on the PTFE film fluctuates throughout the continuous droplet impinging. The increase in instantaneous power density does not result from additional energy, but from the conversion of kinetic energy of water itself.

Low-frequency kinetic energy contained in waves, tides, and even raindrops are not efficiently converted into electrical energy due to limitations in current technology. For example, a conventional droplet energy generator based on the triboelectric effect can generate electricity induced by contact electrification and electrostatic induction when a droplet hits a surface. However, the amount of charges generated on the surface is limited by the interfacial effect, and as a result, the energy conversion efficiency is quite low. In order to improve the conversion efficiency, the research team has spent two years developing the DEG. Its instantaneous power density can reach up to 50.1 W/m2, thousands times higher than other similar devices without the use of FET-like design. And the energy conversion efficiency is markedly higher.

There are two crucial factors for the invention:

• First, the team found that the continuous droplets impinging on PTFE, an electret material with a quasi-permanent electric charge, provides a new route for the accumulation and storage of high-density surface charges. They found that when water droplets continuously hit the surface of PTFE, the surface charges generated will accumulate and gradually reach a saturation. This new discovery helped to overcome the bottleneck of low charge density encountered in previous work.
• Another key feature of their design is a unique set of structures similar to a FET, the basic building block of modern electronic devices. The device consists of an aluminum 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. When a falling water droplet hits and spreads on the PTFE/ITO surface, it naturally "bridges" the aluminum electrode and the PTFE/ITO electrode, translating the original system into a closed-loop electric circuit. With this special design, a high density of surface charges can be accumulated on the PTFE through continuous droplet impinging. Meanwhile, when the spreading water connects the two electrodes, all the stored charges on the PTFE can be fully released for the generation of electric current. As a result, both the instantaneous power density and energy conversion efficiency are much higher.

Their research also shows that the reduction in relative humidity does not affect the efficiency of power generation. Also, both rainwater and seawater can be used to generate electricity.

The research was led together by Professor Wang Zuankai from CityU's Department of Mechanical Engineering, Professor Zeng Xiao Cheng from University of Nebraska-Lincoln, and Professor Wang Zhong Lin, founding director and chief scientist from Beijing Institute of Nanoenergy and Nanosystems of Chinese Academy of Sciences. Their findings were published in Nature in a study titled "A droplet-based electricity generator with high instantaneous power density."

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