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A self-charging buoy that uses nanogenerators to harness power from the movement of waves could be used to monitor water levels for early flood warning systems or long-term water quality.

Many sensing systems used in water have a problem: they rely on batteries that are often made from unsustainable materials聽and will need replacing at some point.

Looking for a cleaner, long-lasting alternative, at the Chinese Academy of Sciences, Beijing, and his colleagues have created a self-charging buoy. The device uses nanogenerators that harness the triboelectric effect, in which electricity is produced when materials become charged as they rub together.

The buoy consists of an acrylic ball about 10 centimetres across, containing four nanogenerators coupled to a module that harvests the produced charge. As the water moves back and forth through the nanogenerators 鈥� made from polyester film and copper twisted into a spiral 鈥� they generate around 24.5 milliwatts of power, enough to send a radio signal to a mobile phone 25 metres away.

The authors suggest that such self-charging buoys could be useful for checking water levels to warn of floods. However, there are already many effective and cheap alternatives for this task, says at Imperial College London.

Non-contact methods, such as lidar, are probably more suitable for sensing water levels, he says.

If used in rivers, there is also a high chance the buoys would be damaged by rising or turbulent water, especially if they are installed upstream of a settlement at risk of flooding as an early warning system. 鈥淚n these environments, there is often a high sediment load in the river during a flood and so any sensor actually in the river is likely to get destroyed,鈥� says at the University of Reading, UK.

The novel power generation method could prove useful for situations in which non-contact methods aren鈥檛 viable, though, says Buytaert, such as water quality sensing.

鈥淭here have been developments of using buoys very similar to the one described here to measure water quality variables. For applications like that, combined with the type of power harvesting they show here, it could be very, very promising,鈥� he says.