Microbial Methane – New Fuel for Ocean Robots?

Imagine if the same marine microbes we study with ocean robots and autonomous underwater vehicles(AUVs) could help power those same vehicles?

Researchers at WHOI and Harvard University are working on it. They’re collaborating with Maritime Applied Physics Corporation (MAPC) -- which is leading the effort with support from the Defense Advanced Research Projects Agency (DARPA) -- on an energy harvesting platform that extracts methane produced by microbes and converts it to electricity. The system could be an answer to power-hungry robots that are being asked to explore increasingly larger swaths of the ocean.

“Deep sea microbes make tons of methane each year” says WHOI adjunct scientist and Harvard professor Peter Girguis. “So, we’re developing these harvesting systems that can be deployed above methane seeps to see if we can generate electricity from this methane.”

When it comes to powering AUVs—or other underwater ocean technologies for that matter—methane is an ideal choice given its abundance. It's also free, and tends to hang around.

“It’s a crazy stable molecule,” says Girguis. “You can put it in a glass vial, and thousands of years later it will still be methane.”

WHOI adjunct scientist and Harvard professor Peter Girguis (Photo courtesy of Harvard University)

It is, however, a potent greenhouse gas—the U.S. Environmental Protection Agency suggests that methane has a heat-trapping power 25 times greater than CO₂. But fortunately, very little of it ever leaves the ocean, thanks to the expansive communities of marine microbes that eat it.

Using methane to give ocean robots a power boost may sound like sci-fi, but it may be closer than you think. A prototype of what the researchers refer to as a ‘seafloor generator’ is being built for testing later this year. It’s roughly the size of a large dorm room fridge, and when deployed, sits above methane seeps bubbling up from the seafloor. As the gas bubbles enter the system, a device recovers the methane through a membrane. The new device is being developed by MAPC, in conjunction with Girguis and WHOI scientist Anna Michel, who has been collaborating with Girguis since 2013.

“We utilize similar approaches for in situ chemical sensing of methane and carbon dioxide,” says Michel. “We extract gases from seawater and then measure them using infrared spectroscopy or mass spectrometry. These instruments require much less gas than we aim to use here. In my own lab, we’re especially interested in finding ways to power sensors underwater. So, working with WHOI Engineer Jason Kapit, we are investigating ways to scale up our extraction processes.”

Once the methane is in gas form, the system combusts the gas to drive an engine and generator. This is a common approach to converting chemical energy from the gas to electrical energy, but this would be the first time it’s been done on the seafloor for re-charging vehicles and powering sensors.

“The exhaust gases produced are cooled and recirculated back to the inlet of the generator,” explains Tom Bein, a principal engineer with MAPC. This novel approach, he says, minimizes the power required by the system which maximizes the energy available to recharge AUVs or to power sensor networks.

The seafloor generator, depicted here, is designed to continuously generate one kilowatt of power from methane seeps—enough power to recharge AUVs on long-endurance missions without having to resurface. (Illustration by MAPC)

From Girguis’ perspective, the new system will help address a key question that’s been lingering over the ocean science community for decades: How do we sustain our presence in the deep sea? The need for AUVs, for example, to travel over longer distances—and longer time periods—without having to surface to charge up, is very real. Particularly in endurance-sapping applications like geologic surveys, search and rescue missions, and oil spill monitoring.

Girguis sees value in the “cabled observatories we all clamored for” but says their capabilities are limited to the regions of the seafloor that they can reach. There have been advances in battery technologies, and in low-power instrument design, that have spurred the launch of new high-endurance vehicles.  WHOI’s Long Range Autonomous Underwater Vehicles (LRAUVs), for example, areultramarathoners: they can operate continuously for more than two weeks over a distance of 620 miles (1,000 kilometers).

But Girguis says that for autonomous vehicles to reach their potential, they will ultimately need underwater charging capabilities. He refers to the concept as a “Supercharger Network”—a network of underwater charging ports that provides rapid charging for an AUV on a mission—ideally in remote and deep locations throughout the global ocean. These networks could also power underwater sensors and other instruments.

“Today, we have vehicle charging stations that make it possible for us to drive cross-country with an electric car,” says Girguis. “If I had my druthers, we’d have a supercharger highway beneath the surface that helps keep AUVs going as far as they need to.”

This work is sponsored by the Defense Advanced Research Projects Agency (DARPA) under contract number W912CG‐20‐C‐0015. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied of DARPA. (Approved for Public Release, Distribution Unlimited 3/8/21)