The motion of bacterial spores that expand and contract depending on humidity might offer a way to extract renewable energy from natural evaporation.

Evaporation is a process that takes place naturally at oceans, lakes and other sources of water, driven by the sun. Unlike boiling, it’s a passive process; a fraction of the molecules have heat energy above the liquid’s average, and escape from the surface as gas.

”We have the technology to harness energy from wind, water and the sun, but evaporation is just as powerful. We can now put a number on its potential. Evaporation is a source of great power in nature, but one that has mostly been overlooked because we have not known how to capture it”, says Ozgur Sahin, a biophysicist at Columbia University.

Evaporation engines

One possible implementation of an evaporation engine. Spores absorb moisture and elongate, then dry out and contract. The cyclical process generates circular motion.

In 2015, researchers were able to build tiny evaporation engines that generate power via evaporation from a body of water. The engines are based on spores of the bacteria Bacillus subtilis, which exhibit strong mechanical responses to changing relative humidity. The spores expand when they absorb water and contract when they dry out. In the engine, the expansions and contractions drive a rotary or piston motion that runs a generator. The Subtilis spores can lie dormant for hundreds of years, and they are so rigid that as the material’s shape changes it produces useful amounts of energy.

The machine is put on the evaporating water surface, with the spores glued to strips of a polymer tape that coordinate the force to generate circular motion. A shutter mechanism controls the passage of moisture, and opens and closes in response to the spores changing shape. When the shutter is open, moisture escapes to the air and the spore dries out; when it closes, humidity increases, the material expands, and the next cycle is set off.

Huge potential – at least in theory

The first prototypes produced way to little power per unit area to have any practical applications in the near term, but researchers were confident that the materials, technology and scalability had lots of room for improvement. By fall 2017, a study was released where researchers aimed to quantify the large-scale potential of the technology once it has matured. According to the paper, the devices would be able to provide more than 325 GW of power annually in the US alone. This is more than the current electricity-generating capacity of coal in the US – but the assumption was that the surface of every US lake and reservoir larger than 0.1 square kilometers, excluding the Great Lakes, would be covered with evaporation energy harvesters.

Of course, such a massive coverage is not realistic, and the study is more of a thought experiment to underscore the importance of advancing the technology beyond lab scale. Eventually, it could still make an actual contribution to clean energy production.

A non-intermittent addition to renewable energy

Muscle-like plastic strips equipped with spores, in a prototype developed by researchers at the University of Columbia. The strips can contract millions of times without failure. Photo: Xi Chen.

One possible use could be to create bricks of spores to act as batteries, activated to produce electricity by adding water. The first applications would likely include use cases in process industry, or on reservoirs already generating hydroelectric power where it would both contribute power and reduce water loss. A side effect of the energy-harvesting process is that the net evaporation is reduced, saving up to half of the water naturally lost and hence increasing the amount available for energy generation, irrigation, and other needs.

Outsider with benefits

The technology would have to compete directly with solar panels, which also can be placed on reservoirs. But evaporation engines have certain unique advantages: they save water, they could be made from cheap biological materials, and they avoid the intermittency limitations of wind and solar power since evaporation happens continously.

It is a technology far from commercialization, and the power per unit area is still too low to compete with established sources of energy such as wind and solar. Nevertheless, to reach an entirely renewable energy production, we will likely have to employ every tool in the toolbox. Evaporation power has tremendous potential if we can harness it, and the technology has certain unique features which may give it a competitive advantage. After all, the Earth is 70 percent water, constantly moving through a natural cycle of evaporation, condensation and precipitation.

The article was published in January 2018.