Bath Engineers Bet on Dirt for Micropower

A thimbleful of soil can contain a universe of microorganisms, up to 10 billion by some estimates. Now a group of researchers in Bath, United Kingdom, are building prototype technologies that harvest electrons exhaled by some micro-species.

The idea is to power up low-yield sensors and switches, and perhaps help farmers digitally optimize crop yields to meet increasing demand and more and more stressful growing conditions. There could be other tasks, too, that might make use of a plant-and-forget, low-yield power source—such as monitoring canals for illegal waste dumping.

The research started small, based out of the University of Bath, with field-testing in a Brazilian primary school classroom and a green pond near it—just before the onset of the pandemic.

“We had no idea what the surroundings would be. We just packed the equipment we needed and went,” says Jakub Dziegielowski, a University of Bath, U.K. chemical engineering Ph.D. student. “And the pond was right by the school—it was definitely polluted, very green, with living creatures in it, and definitely not something I’d feel comfortable drinking from. So it got the job done.”

The experiments they did along with kids from the school and Brazilian researchers that summer of 2019 were aimed at running water purifiers. It did so. However, it also wasn’t very efficient, compared to, say, a solar panel.

So work has moved on in the Bath labs: in the next weeks, Dziegielowski will both turn 29 and graduate with his doctorate. And he, along with two other University of Bath advisors and colleagues recently launched a spinoff company—it’s called Bactery—to perfect a prototype for a network of soil microbial fuel cells for use in agriculture.

A microbial fuel cell is a kind of power plant that converts chemical energy stored in organic molecules into electrical energy, using microbes as a catalyst. It’s more often used to refer to liquid-based systems, Dziegielowski says. Organics from wastewater serve as the energy source, and the liquid stream mixes past the electrodes.

A soil microbial fuel cell, however, has one of its electrodes—the anode, which absorbs electrons—in the dirt. The other electrode, the cathode, is exposed to air. Batteries work because ions move through an electrolyte between electrodes to complete a circuit. In this case, the soil itself acts as the electrolyte—as well as source of the catalytic microbes, and as the source of the fuel.

The Bath, U.K.-based startup Bactery has developed a set up fuel cells powered by microbes in the soil—with, in the prototype pictured here, graphite mats as electrodes. University of Bath

Fields full of Watts

In a primary school in the fishing village of Icapuí on Brazil’s semi-arid northeastern coast, the group made use of basic components: graphite felt mats acting as electrodes, and nylon pegs to maintain spacing and alignment between them. (Bactery is now developing new kinds of casing.)

By setting up the cells in a parallel matrix, the Icapuí setup could generate 38 milliwatts per square meter. In work since, the Bath group’s been able to reach 200 milliwatts per square meter.

Electroactive bacteria—also called exoelectrogens or electricigens—take in soluble iron or acids or sugar and exhale electrons. There are dozens of species of microbes that can do this, including bacteria belonging to genera such as Geobacter and Shewanella. There are many others.

But 200 milliwatts per square meter is not a lot of juice: enough to charge a mobile phone, maybe, or keep an LED nightlight going—or, perhaps, serve as a power source for sensors or irrigation switches. “As in so many things, it comes down to the economics,” says Bruce Logan, an environmental engineer at Penn State who wrote a 2007 book, Microbial Fuel Cells.

A decade ago Palo Alto engineers launched the MudWatt, a self-contained kit that could light a small LED. It’s mostly marketed as a school science project. But even now, some 760 million people do not have reliable access to electricity. “In remote areas, soil microbial fuel cells with higher conversion and power management efficiencies would fare better than batteries,” says Sheela Berchmans, a retired chief scientist of the Central Electrochemical Research Institute in Tamil Nadu, India.

Korneel Rabaey, professor in the department of biotechnology at the University of Ghent, in Belgium, says electrochemical micro-power sources—a category that now includes the Bactery battery—is gaining buzz in resource recovery, for uses such as extracting pollutants from wastewater, with electricity as a byproduct. “You can think of many applications that don’t require a lot of power,” he says, “But where sensors are important.”