Researchers have efficiently harvested the kinetic energy of falling water droplets for the first time. The team, led by Zuankai Wang at the City University of Hong Kong, demonstrated the conversion through a device that both generates a current and charges a polymer surface as it is hit by falling droplets. Their technology could become an important source of renewable energy, capable of generating electricity in a wide variety of situations (Nature 10.1038/s41586-020-1985-6).
The motion of water, particularly in rivers, has long been an important source of renewable energy. Currently, this hydroelectricity is mostly produced through electromagnetic generators, but these are incredibly bulky, and become highly inefficient when water supplies are low. Alternatively, recent studies have attempted to harvest the kinetic energy of water using electrets – materials that remain charged for indefinite periods of time and can become charged through electrostatic interactions with water. So far, however, this technique has proven to be highly inefficient.
Wang’s team proposed that this performance could be improved through the use of the electret polymer material PTFE, which is known to be a highly stable reservoir for storing densely packed charges. In the researchers’ droplet-based electricity generator (DEG), PTFE is deposited onto a layer of indium tin oxide (ITO), itself deposited onto a glass substrate. Furthermore, the ITO coating is wired to a tiny aluminium electrode, separated from the PTFE by a small gap.
As falling droplets hit the DEG, they spread out across its PTFE surface, imparting an electrical charge. In addition, the interaction temporarily bridges the gap between the aluminium electrode and the PTFE/ITO electrode, creating a closed-loop circuit. Since the PTFE’s charge generates an equal and opposite charge in the ITO layer, this allows charges to migrate to the aluminium electrode, generating a current. Then, as the droplet slides off the surface, its area shrinks. This reverses the direction of this current, fully restoring charge to the ITO layer, so the cycle can repeat. After around 16,000 droplets, the PTFE’s surface charge saturates, and the effect stabilises.
By fine-tuning factors including the film thickness, Wang’s team achieved a peak power density of over 50 W/m2 before saturation; as well as an average energy conversion efficiency of 2.2%. Both of these values are thousands of times higher than those reachable in previous electret-based generators – significantly enhancing the DEG’s output voltage and current. Through their experiments, the researchers showed that when just four 100 μl droplets were dropped onto their device from heights of 15 cm, it could power 400 commercial LEDs to light up instantaneously.
Since the DEG harvests electrical energy purely from the kinetic energy of water, the team’s approach could greatly expand the range of possible applications of hydroelectricity. For the first time, the device opens up routes to the generation of renewable energy from sources where water impinges on surfaces periodically – including raindrops and ocean waves. Through longer-term improvements, Wang and colleagues hope that their technology could be applied to surfaces as far-ranging as the hulls of boats, the surfaces of umbrellas and the insides of water bottles.