In recent years, farmers worldwide increasingly have adopted precision agriculture as a strategy to improve crop yields. The tech-driven approach relies on measuring precise levels of moisture, nutrients and contaminants in soil to make decisions that enhance crop health. This requires a widespread, dispersed network of electronic devices to continuously collect environmental data.
This technological approach is constrained by putting a battery in it or harvesting solar energy. Batteries often run out of power and solar panels do not work well in dirty environments. There is an alternative solution that has emerged.
New technology, only the size of standard paperback book, harvests energy from microbes in soil to power sensors and different forms of communications. This is based on a fuel cell that harnesses naturally occurring microbes to generate electricity.
Soil-based microbial fuel cells (MFCs) operate like a battery — with an anode, cathode and electrolyte. But instead of using chemicals to generate electricity, MFCs harvest electricity from bacteria that naturally donate electrons to nearby conductors. When these electrons flow from the anode to the cathode, it creates an electric circuit.
Design
Made of carbon felt (an inexpensive, abundant conductor to capture the microbes’ electrons), the anode is horizontal to the ground’s surface. Made of an inert, conductive metal, the cathode sits vertically atop the anode. A 3D-printed cap rests on top of the device to prevent debris from falling inside. Plus, there is a hole on top and an empty air chamber running alongside the cathode enable consistent airflow. The lower end of the cathode remains nestled deep beneath the surface, ensuring that it stays hydrated from the moist, surrounding soil — even when the surface soil dries out in the sunlight.
Application
As examples of this Internet of Things technology, soil-powered sensors successfully monitor soil moisture. In trials, the system was robust enough to withstand drier soil conditions and flooding. Going forwards, it is hoped the fuel cell could replace batteries in sensors used for precision agriculture.
The technology potentially offers a sustainable, renewable alternative to batteries, which hold toxic, flammable chemicals that leach into the ground, are fraught with conflict-filled supply chains and contribute to the ever-growing problem of electronic waste.
To test the new fuel cell, the researchers used it to power sensors measuring soil moisture and detecting touch, a capability that could be valuable for tracking passing animals. To enable wireless communications, the researchers also equipped the soil-powered sensor with a tiny antenna to transmit data to a neighbouring base station by reflecting existing radio frequency signals.
On average, the resulting fuel cell generated 68 times more power than needed to operate its sensors. It also was robust enough to withstand large changes in soil moisture — from somewhat dry (41% water by volume) to completely underwater.
This demonstrated that the fuel cell will work in both wet and dry conditions. In addition, its power outlasted similar technologies by 120%. In terms of a further advantage, provided there is organic carbon in the soil for the microbes to break down, the fuel cell can potentially last forever.
The study appears in Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable and Ubiquitous Technologies. The research is titled “Soil-Powered Computing: The Engineer’s Guide to Practical Soil Microbial Fuel Cell Design.”
