In the latest of fantastic endless renewable energy schemes: energy harvested from falling raindrops on a surface (think roof tile and gutter) as devised by Yajuan Sun, Xu Huang and Siowling Soh (DOI) from the National University of Singapore (annual precipitation: 2,340 mm). The basics: water droplets sliding down a surface become positively charged and hence become a potential energy source. Based on the age-old triboelectric series the ideal surface is teflon. The scientific explanation according to no other than George M. Whitesides is that hydroxide ions in the water migrate out of the water onto the surface of the solid. When the droplet eventually becomes detached from the surface, it is permanently positively charged. The charge accumulation can be measured in another age-old concept: the Faraday cup. The scientists cleverly call their invention SLIDE as in Solid-Liquid Interface for Developing Electrification. Downside: salts dissolved in the water droplet diminish the effect. More data: more power with a smaller diameter teflon tube, no increase in power with a tube in excess of 30 cm. Power decreases when flow rate in excess of 100 mL/min.
Premium power at 170 microwatts and 3 to 4% efficiency. Interesting question: at one point the surface is saturated with negative charge. By experiment a water droplet is found to accumulate 20 nC of positive charge while all the teflon surface has to show for is a measly 2 nC. So where does all the negative charge go to? According to the researchers it all goes up to air, as ionised atmospheric molecules. It would nicely explain the power decrease at higher flow rates because surface exposure to the atmosphere also decreases.