It doesn't look like the most cutting-edge vehicle. A dinky toy car made out of what looks like Lego Technic creeps in slow jerks across a tabletop—and that's with the video sped up to twice speed. But this is one of the first examples of an engine powered by an unusual renewable resource: evaporation.
In a paper in Nature Communications, researchers at Columbia University and Loyola University Chicago demonstrate a way of harnessing the natural energy of water evaporating and turning it into power to run a rotary-like and piston-like engine and even produce enough electricity through a generator to power LEDs.
Co-author Ozgur Sahin told me in a phone call that the paper evolved from his vision to create a device "that would sit on a lake or a river, and as moisture gets from the water surface to the air, the device could capture energy from that process."
It's an attractive suggestion. Evaporation happens all the time, including at night, so an evaporation-powered energy source would have less of the the intermittency problem of renewable energy sources such as solar or wind, which need to store energy for down times.
Additionally, Sahin says that their evaporation method could be low-cost, as the key part of the energy-harnessing device is naturally-occurring bacterial spores.
"The complicated stuff is already made by the organism, so the technology is there."
The bacterial spores respond to humidity, so to make their devices, the researchers put a layer of Bacillus subtilis spores onto 8-micrometre-thick tape. The strips of tape then curve into arcs depending on how humid it is, so that they grow and shrink in overall length. Assembled together, the tapes are capable of lifting against gravity. The researchers call these tape mechanisms hygroscopy-driven artificial muscles, or HYDRAs.
"The complicated stuff is already made by the organism, so the technology is there," said Sahin.
In the videos, you can see that the various devices start up automatically when water is added and the spores start responding to the evaporating liquid and its effect on the air's humidity.
Sahin said that given the spores are dormant, they can work in quite varied conditions, similar to a dead material. "They can sit outside for hundreds of years and survive as a material, and this is what makes them technologically interesting and potentially useful," he said.
The devices the researchers built so far are obviously very small, but they show that these "muscles" can be put to use: there's a piston-type engine that can power a few tiny LED bulbs, and a "mill" device that can power the toy car. Given that many naturally occurring bodies of water have a much larger surface area than the small test samples, it's easy to see how the process could be scaled up.
"You can make it bigger in two dimensions, just wider basically, and cover the surface of water, and the power will go up proportionally," said Sahin. "Twice the area, twice the power, approximately."
There are other issues to overcome, but he envisions one application at existing hydroelectric power plants. They naturally have a lot of water to work with in the first place, which would make them a good place to position the systems, and the devices also offer secondary benefit: they slow evaporation down. Sahin explained that the devices basically mean more energy is required for evaporation, and compared it to putting a tollbooth on a highway. It's still the go-to route for the water to travel, but it's more difficult to get through. Plus, a hydroelectric power plant is already conveniently connected to the grid.
It's a long way to that sort of scale, and the next step for the researchers is to make improvements to their current devices. Sahin said that while the evaporation method didn't compete with solar power in terms of energy production, it could do it more cheaply.
As the researchers conclude in their paper, "Due to the ubiquity of evaporation in nature and the low cost of materials involved (plastic tapes, hygroscopic materials), the engines presented here may find applications as energy sources for a wide range of off-the-grid systems that function in the environment."