rain powered smartphone
New technology could harness the power of rain and water droplets to charge smartphones and other electronics

That age-old problem of mobile phones dying from lack of power could be about to end. Researchers at the Massachusetts Institute of Technology (MIT) have demonstrated a process in which smartphones can be charged using electricity generated using just the humidity in the air.

The technology was demonstrated after a discovery that when water droplets are repelled from superhydrophobic (water-repellent) surfaces they gain an electrical charge.

The findings by Nenad Miljkovic and Evelyn Wang from the NanoEngineering Group at MIT were published in the journal Applied Physics Letters.

Harnessing the Power of the Atmosphere

The charging system can be used with water droplets from rain, or simply from condensation in a humid environment.

"Water will condense out from the atmosphere, it happens naturally," Miljkovic said. "The atmosphere is a huge source of power, and all you need is a temperature difference between the air and the device."

This approach of generating electricity utilises a series of flat metal plates. By layering hydrophobic surface plates beside a hydrophilic (water-attracting) surface, water droplets jump from one to the other carrying a charge. Connecting the two plates to an external circuit then allows the electricity to be harnessed.

rain charge smartphone
False-color time-lapse images captured via high-speed imaging show a droplet jumping (green) from a superhydrophobic surface to a hydrophilic surface (orange)  MIT

Charging Devices in Remote Locations

Miljkovic believes that this method could be used to charge smartphones, wearable devices and other electronics in locations where power sources are scarce.

It is estimated that one square centimetre of metal plate could generate one microwatt - or one trillionth of a watt - of energy. Miljkovic calculates that a metal cube about the size of a camping cooler could provide enough power to fully charge a smartphone in 12 hours.

"This work provides a new approach for energy-harvesting, which can be used to power small electronic devices." said Chuanhua Duan, an assistant professor of mechanical engineering at Boston University who was not involved in the research.

"Getting power from a condensation process is definitely a novel idea, as condensation is mainly used for thermal management. Recent studies of condensation on superhydrophobic surfaces (have) extended its applications in self-cleaning and anti-icing, but no one has correlated condensation with energy-harvesting before."