Many sensing systems used in water have a problem: They rely on batteries that are often made from unsustainable materials, so such batteries will need replacing at some point.
To look for a cleaner, more long-lasting alternative, Zhonglin Wang at the Chinese Academy of Sciences，Beijing，and his colleagues have created a self-charging buoy (浮标). It uses nanogenerators (纳米发电机) to gain power from the movement of waves.
The buoy consists of an acrylic (丙烯酸的) ball about 10 centimeters across and four nanogenerators connected to a part that collects the produced charge. As the water moves back and forth through the nanogenerators, they generate around 24.5 milliwatts of power.
It is suggested that the buoy could be used to check and monitor water levels for early flood warning systems. However, there are already many effective and cheap alternatives for this task, says Wouter Buytaert at Imperial College London. Non-contact methods, such as lidar (激光雷达), are probably more suitable for sensing water levels， he explains.
"The new power generation method could prove useful for situations in which non-contact methods aren't workable, though, such as water quality sensing," Buytaert says. "There have been devices similar to the buoys in measuring and checking long-term water quality. But if such application is combined with the function of power collecting shown here, it could be more promising, "Buytaert says.
However, if used in rivers, the buoy would possibly be damaged by rising or violently moving water, especially if they are fixed at the rapids of the flood as an early warning system. "In these environments, there is often a high sand load in the river during a flood, so any sensor actually in the river is likely to get destroyed, "says Liz Stephens at the University of Reading, UK.
