Harvesting Energy from Evaporation on a Leaf

Can electricity come from evaporation?

Dr. Michael Maharbiz and his colleagues¹ like to think so. Their recently published article shows that a small amount of energy can be scavenged from water evaporating from a "microfabricated leaf." The idea was inspired by water transport in plants, to wit, the researchers noticed that "trees pump water hundreds of feet into the air without active pumps by maintaining a large negative pressure gradient across their vasculature," but harnessing this action for human usage is a difficult task. There have been other researchers who have used the evaporation of water to drive flow in microchannels, but none have used this concept to generate electrical power.

In their paper, Dr. Maharbiz's team presents the idea for what they call "energy scavenging from evaporation–driven motion." But where does the energy come from? As a liquid flows across a surface, there is evaporation, creating a gas–lquid interface. This interface can be thought of as a moving dielectric surface, changing the electrical properties of the fluid. There is a 1 µF capacitor embedded in the surface and as more fluid evaporates, there is a potential increase of 2-5 µvolts induced across that storage capacitor.

These gradients, Dr. Maharbiz notes, occur in many places including the surface of the skin during perspiration, near the surface of bodies of water, and at the soil–air interface.

¹Charge–pumping in a synthetic leaf for harvesting energy from evaporation–driven flows, Ruba T. Borno, Joseph D. Steinmeyer, and Michel M. Maharbiz, Applied Physics Letters 95, 013705 (2009).