An Unexpected New Source Of Sustainable Energy: Humidity

Scientists at Columbia University have shown that changes in atmospheric humidity can be used to generate minor amounts of electricity – enough to power small gadgets like children’s toys or environmental sensors. The system, reported this week in Nature Communications, relies on bacterial spores that swell and shrink in response to changes in moisture. By adhering a large number of these spores to a polymer sheet, the researchers produced “artificial muscles” which could quadruple in length and then shrink back to their original size as the humidity level was fluctuated.

Bacillus subtilis: spores of this bacteria swell and shrink in response to changes in humidity. Image by Y tambe.Bacillus subtilis: spores of this bacteria swell and shrink in response to changes in humidity. Image by Y tambe.

The spores used are Bacillus subtilis, a common bacteria found in soil and the human gastrointestinal tract. Each spore can swell by 6% when moved from low to high humidity environments. To exploit this phenomenon, the dry spores were glued to one side of a curved sheet of plastic. When placed in wetter surroundings, the spores swelled causing the plastic to straighten out, and the opposite occurred when the sheet was moved back into dry air. This stretching and contracting motion, akin to the way our muscles operate, could then be used to power low-energy devices.

Spore-based humidity generator: coating one half of this Ferris Wheel-shaped engine in paper towel leads to a humidity imbalance that prompts motion. Image reproduced with permission from Nature Comm. 6, 7346, 2015.Spore-based humidity generator: coating one half of this Ferris Wheel-shaped engine in paper towel leads to a humidity imbalance that prompts motion. Image reproduced with permission from Nature Comm. 6, 7346, 2015.

To turn this basic principle into an actual proof-of-concept engine, the team constructed a sort of Ferris Wheel of artificial muscles with one side blanketed by paper towels to produce a humidity imbalance. This asymmetric environment caused the wheel to turn allowing the powering of devices. The video below shows a toy car the team constructed to demonstrate the idea. Though it seems rather rudimentary at first viewing, it is pretty remarkable when you consider that it is powered by a damp paper towel!

The team ran the new generator through a battery of tests to determine its full potential and see how it really stacks up against alternatives. They found that by using an extremely thin layer of spores on the order of a few microns – that is, thinner than your average red blood cell – the muscles could react extremely quickly (within 3 seconds) to environmental changes. This is important when considering the amount of electricity that can be generated in a given timeframe. They also analyzed the durability of the generator and found that over a million shrink-expand cycles could be completed with minimal degradation.

Ok, so it may be a while before changes in humidity become a major contributor to our energy needs. In fact, they may never be substantial players in the fuel economy. However, it is important to remember that any system which can generator power sustainably, no matter how small, can serve to alleviate some of the reliance on fossil fuels. In this era of increasing energy demand and decreasing energy supply, every small step could prove crucial and the creativity demonstrated in this research project is exactly what we need to meet the upcoming energy challenges.

Via Science News and Nature Communications.