A new device could generate electricity from a previously untapped source of energy – dripping water.

An array of 100 small LED light bulbs was reportedly lit up by a drop falling onto the device, which was developed by a team of researchers led by the City University of Hong Kong (CityU).

A conventional droplet-based electricity generator (DEG) is based on the triboelectric effect, which can generate electricity induced by contact electrification and electrostatic induction when a droplet hits a surface. The amount of charge generated on the surface is limited by the interfacial effect however, normally leading to low energy conversion efficiency.

The new DEG uses a structure like a field-effect transistor (FET) for high energy-conversion efficiency. The structure consists of an aluminium electrode and an indium tin oxide (ITO) electrode with a film of PTFE deposited on it, which is responsible for charge generation, storage and induction. When a falling water droplet hits and spreads on the PTFE/ ITO surface, it naturally ‘bridges’ the aluminium electrode and the PTFE/ ITO electrode, creating a closed-loop electric circuit.

A high density of surface charges can accumulate as drops of water constantly fall on to the device. When spreading water connects the two electrodes, all the stored charges are fully released for the generation of electric current.

 “Our research shows that a drop of 0.1ml of water released from a height of 15cm can generate a voltage of over 140V. And the power generated can light up 100 small LED light bulbs,” said Professor Wang Zuankai from CityU.

The researchers claimed the instantaneous power density is thousands of times higher than other devices without the FET-like structure, reaching up to 50.1W/m². Professor Zuankai said that increase comes from the conversion of kinetic energy of the water. That energy is “free and renewable” and should be better utilised, he added.

Relative humidity reportedly does not affect the efficiency of power generation. Both rainwater and seawater can apparently be used to generate electricity.

Devices based on the generator could potentially be useful for some small-scale electricity generation, such as powering small lights or sensors.

Professor Zuankai said the design could be applied and installed on different surfaces to utilise low-frequency kinetic energy in water. This could include the hulls of boats, sea walls, or even the surfaces of umbrellas.

The research was published in Nature.

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