Converting differences in air humidity into movement

New 'evaporation engines' based on the reaction of bacteria to air humidity could become mini power sources for creating momentum or as mini electricity generators. These engines start and work autonomously; all that is needed are spores and water vapour.

Contrary to nature, mechanical systems make only sporadic use of vapour as a source of energy. Researchers at Columbia University in New York City now want to convert differences in air humidity into movement in order to generate energy from the environment, according to Mechatronica & Machinebouw and based on an article published in Nature.

Bacteria in action

The scientists based their system on a Bacillus Subtilis bacteria spore formation, which is a 'sleeping' state that enables it to survive unfavourable environments. In high humidity conditions, the spores start to swell and with low humidity, they contract again. In order to harness this phenomenon for generating movement, the researchers put a layer of spores on to one side of a plastic membrane. This resulted in the membrane curling up in low humidity conditions, and stretching out in conditions of high humidity. By repeatedly placing a layer on one side and then the other over an elongated membrane, the researchers were able to create an 'artificial muscle' that could contract into an S-shape just a quarter of its original length.

Rotational and linear movements

The researchers then developed a method for extracting energy from it. Both rotational and linear movements were achieved (similar to a piston) from the reactions of these so-called muscles to the changing levels of humidity.

In the initial design the artificial muscles were placed inside a small enclosed water tank and connected to valves in the lid. When the valves were closed, the air humidity increased and they started to open. This in turn caused the air humidity in the tank to decrease and so the valves closed again.

Strips were attached to a wheel in yet another design. By attaching a piece of damp tissue paper onto one half of the wheel, the strips stretched out and the torque increased. On the other side of the wheel the strips curled up and the torque reduced. This in turn caused the wheel to rotate.

In a demonstration, the researchers powered a little Lego car (weighing 0.1 kg) by evaporating water placed inside it, and at the same time they generated enough current to light up an LED from a generator placed on water.

These 'evaporation engines' could be used to drive sensors and small actuators placed in narural environments.

View the possibilities in the following film clip:

Within the TETRA-IPEH project, Sirris in collaboration with VUB is also investigating the possibilities of generating biological power. Find out more about this project here!

Contact Vrije Universiteit Brussel: An Braeken, professor 'Vakgroep Industriële Ingenieurswetenschappen (INDI)', tel. +32 2 559 02 63

(Photo: Chen et al.)