It’s a Dirty Job, But Somebody Has to Do It

While the potential of large-scale sensor networks is beginning to be realized in applications like environmental monitoring and smart cities, there are still some limitations holding back their widespread adoption. Chief among these limitations is supplying them with power — there are many logistical issues and environmental concerns that arise when trying to power hundreds or thousands of devices that are spread out over a large geographical area, especially where access to the electrical grid is unavailable.

These interconnected devices, capable of collecting and transmitting vast quantities of real-time data, could have a transformative impact that extends across diverse sectors if this problem could be resolved. Solar panels have been used to great effect for certain use cases, but that is not always a good option. Agriculture is arguably one of the areas that could benefit most from the use of sensor networks, but in a dirty environment like a farm, solar panels will quickly get covered with dirt, rendering them ineffective. And the maintenance required to keep hundreds or thousands of solar panels clean is unmanageable for most farming operations.

But that same dirt that fouls up solar harvesting operations may be an asset, thanks to the work of a group led by Northwestern University researchers. They have developed a novel type of fuel cell that can generate electricity with the help of microbes in the soil. This approach may not generate a large amount of electricity, but it has been demonstrated to be more than sufficient to sustain modern, energy-efficient sensing and wireless communications components.

Soil-based microbial fuel cells are hardly a new technology, having first been discovered over a century ago. These devices work much like a battery, but instead of utilizing a chemical electrolyte to produce electricity, they harvest electricity from electrons that are freely donated by bacteria in the soil. These bacteria are ubiquitous, and can be found in soil all over the world. However, in order for traditional microbial fuel cells to operate, they must remain in contact with both moisture and oxygen, which renders them inoperable under dry conditions.

Toward the goal of building a more robust fuel cell that can operate even under challenging environmental conditions, the team spent over two years designing and testing a wide range of options. By methodically testing these designs, they landed on a configuration in which the anode and cathode are perpendicular to one another, rather than in the traditional parallel configuration. Furthermore, a hole (protected by a cap) was built into the top of the device that runs to an air chamber around the cathode. This ensures a consistent supply of oxygen will be available.

The cathode was constructed to accommodate multiple situations. It is situated well below the surface of the ground such that it will remain hydrated even as the upper layers of soil dry out. Moreover, a portion of the cathode was treated with a waterproofing material, to make it resilient against flood conditions that otherwise starve the fuel cell of oxygen.

To confirm that the energy harvester would perform as hoped under real-world conditions, it was tested in a garden. Tasked with powering a sensor and wireless radio, it was discovered that the fuel cell could provide 68 times more power than what was required. It was also demonstrated that the device could operate under a wide range of conditions, ranging from completely underwater to modestly dry soil (41% water by volume).

It was noted that all of the components needed to build the fuel cell could be purchased at a typical hardware store. Given that simplicity, and the effectiveness of the device, it could make a real impact in precision agriculture in the future.