Don’t Be Afraid Of The Damp

Charging your cell phone might one day be as easy as exposing it to humidity.

In 2013, Nenad Miljkovic, a postdoctoral researcher at the Massachusetts Institute of Technology, and Evelyn Wang, an MIT associate professor, along with two other researchers learned that when drops of water bounce off superhydrophobic, or highly water-repellent, surfaces, they develop an electric charge.

More recently, they learned that this process can generate extremely small amounts of electricity that, if enhanced, could be used to charge cell phones and other electronic devices simply by using ambient humidity in the air.

According to their research, published in the journal Applied Physics Letters, such a device would be simple: layers of thin, flat metal sheets such as copper, though any conductive metal would work, including less-costly aluminum. The layers of metal alternate between those that attract water and those that repel it, causing the droplets to bounce back and forth, rapidly constantly generating electricity.

Yet early tests show the amount of electricity produced was a mere 15 picowatts, or 15 trillionths of a watt, per square centimeter of metal. But Miljkovic says this doesn't make his team's discovery a mere theoretical exercise. He says the system can be adjusted to generate at least 1 microwatt, or millionth of a watt, per square centimeter.

That's comparable to what's found in other devices that have been proposed for gathering waste heat and other sources of ambient energy. And, he says, it's enough to provide useful power for small electronic gadgets in certain remote areas.

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That's good news if you're camping by a river in the mountains. Miljkovic explains that at 1 microwatt per square centimeter, a 20-inch cube-shaped array of the metal layers would be enough to charge a cell phone in about 12 hours - slow, but acceptable given the locale.

But this scenario highlights another limitation of the MIT discovery: The air needed to cause the reaction must be more humid and cooler than the overall ambient air, such as a cave or a riverside.

Still, Miljkovic says for some small electronic devices, even a minuscule amount of energy should be capable of producing several hours of power, especially in areas with morning. "Water will condense out from the atmosphere, it happens naturally," he says.

The key, Miljkovic explains, is temperature contrast, which causes humid air to condense. As an example, he cites a glass containing a cold beverage developing beads of condensation on a warm summer day. In other words, if you have a temperature differential, you have electricity.

By Andy Tully of Oilprice.com