In a development that is either a visionary breakthrough or a very expensive way to briefly light a small room, Seattle-based startup Endurance Energy has completed the world's first demonstration of a subsea geothermal power plant near an active underwater volcano almost a mile below the ocean surface. The three-week test deployment of the 3-ton prototype at the Axial Seamount in the northeast Pacific Ocean in September generated less than 10 seconds of net-positive electricity, but hey, it worked.
The company aims to eventually commercialize subsea geothermal power for Pacific island nations and other customers looking to avoid importing expensive fossil fuels. "The deep ocean gives us unlimited cold seawater for cooling, and its weight keeps the system and the well pressurized, so our plants can be compact," said Andrew Redd, founder and CEO of Endurance Energy, in an email to Ars.
But tapping geothermal power on the seafloor faces challenges distinct from land-based projects, which can also add up to higher installation and operational costs. Endurance Energy must demonstrate consistent underwater operations as well as prove the approach can be cost-effective. "It's probably technically feasible, but it would undoubtedly be more expensive than to do it on land," said Roland Horne, professor of energy science & engineering at Stanford University, in an emailed response to Ars. That's the academic equivalent of a shrug and a "good luck with that."
The deployment required a subsea lander drill deployed from the ship Ocean Guardian - operated by Stabbert Maritime - to drill two holes in the seafloor at a depth of about 5,000 feet (1,524 meters). That's more than 2.5 times the height of One World Trade Center, because apparently regular geothermal wasn't dramatic enough. Both holes started venting hot hydrothermal fluid measured at about 500° F (260° C). The team cased and cemented the second hole before installing a valve to create a well.
Next, the company deployed its 100-kilowatt power plant, named Adélie, designed to vaporize incoming hydrothermal fluid to spin a turbine and drive an electric generator. It used Canada's remotely operated submersible ROPOS to connect the power plant to the newly drilled well. Adélie also connected to Endurance Energy's Seattle command center via the Regional Cabled Array, a 559-mile (900-kilometer) network of undersea fiber-optic and electric cables funded by the US National Science Foundation and operated by the University of Washington.
That all set the stage for the Adélie power test. But a major hitch soon arose - the drilled well extending 52 feet (16 meters) down into the seafloor was not delivering enough hydrothermal fluid to run the power plant. "For our first well, the goal was to show we could drill, case, cement, and cap a well on the seafloor, so we kept it small and shallow," Redd explained. "That limited how much fluid it could deliver, and deeper wells should change that." The test well was also just 2 inches (5 centimeters) wide, compared to oil, gas, and onshore geothermal wells that typically have casings 20 to 36 inches (51 to 91 centimeters) across. In other words, they drilled a very expensive straw.
Endurance Energy plans to avoid this issue by drilling deeper wells to access more fluid. The company's current lander drill can reach about 260 feet (80 meters), five times deeper than the initial test well.
To try running Adélie anyway, the team redeployed it to a nearby natural vent where hydrothermal fluid was flowing out at about 538° F (281° C). On September 27, they gave it a try. Over nearly half a minute, the turbine reached about 26,000 rpm and the generator produced 60 watt-hours of electricity. The plant became net-positive - producing more electricity than it consumed - for about 8.7 seconds, with a median of about 2 kilowatts of net power and a peak of 16.5 kilowatts. "It's the proof point," Redd told Ars. "Net-positive is the threshold that matters: the plant made more power than it took to run itself on the seafloor, which no one had done before." It's also roughly enough to run a microwave for a few seconds, but progress is progress.
Then a software issue brought the run to a halt. Before the generator takes over, Adélie relies on a battery to run pumps, thrusters, and controls, and to spin the turbine up to speed. A battery module recorded a brief current spike during spin-up and then stopped reporting, prompting the software to automatically stop the plant as a safety precaution. When the team was restarting it, a power cable feeding the refrigerant pump failed when an imperfection in the cable casing allowed seawater to seep in and short two electrical conductors. That ended the trial, though Endurance Energy plans to use the experience to improve how it builds and tests subsea cables.
Placing the geothermal plant on top of a natural hydrothermal vent was a one-time test for research and demonstration purposes, Redd said. His company still plans to rely on drilled wells in future deployments.
Endurance Energy also described steps to reduce potential environmental impacts on the seafloor and marine life. The team chose the "least disruptive" site and continuously monitored the process by sampling water and recording the seafloor array of tubeworms, snails, crabs, and bacterial mats. It checked findings against scientific sensors run by independent researchers and sent water, rock, and mineral samples for independent lab analysis.
The company's next trial, planned for the winter of 2026 - 2027, will involve a refurbished and upgraded Adélie - renamed Gentoo in keeping with the penguin-inspired naming - aiming to sustain higher power generation. The company is also developing a larger 2-megawatt commercial generator for its first planned commercial deployment with the Kingdom of Tonga in 2027. The Polynesian island nation runs its power grid primarily on imported diesel fuel, which has significantly increased in price since the United States and Israel launched their war with Iran. "The 2 MW commercial generators will be about the size of a shipping container, and we are deploying up to six of these generators [to] offshore Tonga," Redd told Ars. "The physical footprint of this project will be relatively small."
Geothermal energy projects are already cost-effective on land but "on the edge of competitive pricewise," said Horne. So he cautioned that any factors making geothermal more expensive would "push it to the noncompetitive side." Still, other customers may consider Endurance Energy's approach if the company can demonstrate reliable operations over time with Tonga. The skyrocketing cost of imported fossil fuels is a huge factor that has already spurred multiple governments to invest in more renewable energy alternatives.
"Large markets with ample offshore geothermal resources include the United States - Pacific Northwest and Alaska - Taiwan, and Japan," Redd said. "As the technology matures, we expect to compete on cost and deliver continuous, weather-independent power to utilities and large energy users." So the next time someone tells you geothermal is just for Iceland, you can point them to the startup that briefly powered a turbine with volcano juice and called it a proof point.