Previously, I tried to categorize all possible basic mechanisms for producing electricity. While I don’t think a recent invention by scientists working at Berkeley, MIT, and the University of Michigan uses any new principles, it is certainly a novel combination. Their artificial glass ‘leaves’ use ambient heat to move water, then exploit that to generate small amounts of electricity:
The leaf is transformed into a source of power by periodically interrupting the water flowing into the leaf with air bubbles. Thanks to the different electrical properties of air and water, every time a bubble passes between the plates the capacitance of the device changes and a small electric current is generated, which passes to an external circuit where it’s used to pump up the voltage on a storage capacitor.
While their prototypes produce minute amounts of energy (2 to 5 microvolts per bubble), the inventors hope that large trees made of these materials could generate electricity on the basis of changing humidity: something that could nicely counterbalance some of the variable output from wind or solar farms.
The research was published in Applied Physics Letters: Charge-pumping in a synthetic leaf for harvesting energy from evaporation-driven flows, Appl. Phys. Lett. 95, 013705 (2009); doi:10.1063/1.3157144, Published 7 July 2009.