Scientists from Harvard University and Columbia University have developed a new type of electrical generator which is capable of harvesting energy from change in humidity levels via the use of soil bacteria.

The prototype generator, developed by a team of scientists working at Harvard University’s Wyss Institute of Biologically Inspired Engineering, involves the use of rubber sheets plastered on a single side with bacterial spores.

The bacterial spores cause the rubber material to deform in response to changing levels of humidity, much like the leaves or petals of many moisture-sensitive plants.

When the air is dry, the sheet straightens out and when exposed to evaporation, the contraction of the spores causes the sheet to curl up. The resulting back and forth movement is vigorous enough for the spore-laden materials to serve as actuators for the generation of electricity.

Ozgur Sahin Associate Professor

Ozgur Sahin Associate Professor

The initial prototype, developed by Ozgur Sahin, associate professor of biological sciences and physics at Columbia University and leader of the Wyss Institute research team, is extremely modest and simple, consisting of Lego pieces, a magnet, a spore-coated cantilever and a miniature fan. When exposed to moisture the movement of the cantilever drives the rotating magnet, which generates the electricity to power the fan.

While the prototype may appear to be little more than novelty toy, Sahin said the new technology has a “very promising endgame” given the ubiquity of the evaporation process in the natural world, and the vast amount of potential power it harbours.

According to Wyss Institute founding director Don Ingber, humidity has the potential to become a third major source of climate-based renewable energy, helping to compensate for the fickle nature of solar and wind power.

Sahin’s efforts to develop a humidity-driven generator first made strides when he discovered that Wyss Institute faculty member L Mahadevan and Harvard University professor Lola England de Valpine had been conducting research into how moisture warps the form of different materials, including organic matter such as pine cones, leaves and flower petals, as well as artificial materials such as tissue paper.

Joint research by Sahin and Mahadevan led to the discovery of a bacterium which was the ideal candidate for use in a humidity-driven generator. The soil bacterium Bacillus subtilisis shrinks into a hardy, dormant spore when dessicated, yet expands into its original form almost instantaneously following exposure to moisture.

Sahin realized that the ability for the bacterium to contract and expand repeatedly made it a potent energy storage vessel for an electrical generator running on changes in moisture levels.

Initial experiments produced remarkable results. When a tiny silicon plank was coated with a solution of the spores, the humidity changes induced by Sahin’s breathing were enough to cause the material to curve and straighten.

Subsequent research further uncovered the remarkable energy potential of the spores. According to Sahin, a pound of dry spores would be capable of generating sufficient force to lift an automobile a metre above the ground following exposure to moisture. The shift in humidity levels represented by the transition from hot, dry weather to humid, foggy conditions can enable a spore-covered plank to produce 1,000 times the force of a human muscle.

The generator still possesses tremendous potential for improvement, as the prototype developed by Sahin only harvests a tiny amount of the energy embodied by the evaporation process. Genetic engineering or selective breeding of the spores promises to make them more efficient by heightening their elasticity, with early experiments already throwing up a strain of the bacteria capable of storing twice the energy of its peers.