‘Mantas’ that fly underwater could bring clean, predictable power to Alaska’s remote coastal communities that diesel has long held hostage
Beneath the surface of San Francisco Bay, a wing-shaped device traces silent figure-of-eight loops through the water, tethered to a small fishing boat. It isn’t swimming. It’s flying — pulled by tidal currents that barely seem to move.
For the remote coastal communities where energy arrives by diesel barge, or not at all, that quiet motion could matter. Researchers are now asking whether a device engineered to behave like a kite in the sky might do something those communities have never quite had: deliver clean, predictable power from the sea.
Why Tides Make a Surprisingly Powerful Energy Source
Tidal currents run on a schedule you can set a clock to. Solar panels go dark at night, wind turbines stall on calm days — but tidal flows are governed by the gravitational pull of the moon and sun, forces that don’t take days off. That predictability alone makes them attractive to engineers and the communities counting on them.
Instead of relying on high-ratio gearing to spin its generator, the Manta uses a twisted-string tether that drives the generator directly as the kite sweeps outward.
Water’s density adds another advantage. Because water is far heavier than air, a device moving through a tidal current harvests significantly more energy per swept area than a comparable wind turbine. As UC Berkeley professor Evan Variano puts it, “Underwater kites can be smaller because water is so much denser.”
The catch has always been speed. Most tidal channels around the world move water at less than 1.5 meters per second — too sluggish for fixed seafloor turbines to exploit economically. Slow currents simply don’t deliver enough force to spin those devices efficiently.
Underwater kites sidestep that problem entirely. By flying across the current rather than sitting stationary in it, they move much faster than the water around them. That relative speed drives the generator, transforming a slow, modest current into a usable energy source.
How an Underwater Kite Actually Works
The physics behind an underwater kite mirrors what keeps a kite aloft on a windy afternoon. A wing-shaped hull generates hydrodynamic lift as water flows over it, letting the device “fly” in a moving current the same way its airborne cousin rides the wind.
What makes modern underwater kites viable is the autopilot. Onboard sensors and robotics steer the kite along autonomous figure-of-eight paths, sweeping it back and forth across the current — each pass carrying the kite faster than the water itself is moving. That speed difference is where the energy comes from.
The tether connecting the kite to a generator is central to how power gets extracted. As the kite pulls outward, the tether unspools and spins a generator. Then the system reels the kite back in, using only a fraction of the power just harvested, before the cycle repeats. A continuous yo-yo rhythm, measured in sweeps rather than rotations.
Keeping that rhythm steady is harder than it sounds. University of Michigan professor Chris Vermillion explains the challenge plainly: “It must be continuously flying. That periodic motion requires substantial control.” The autopilot must manage six degrees of freedom on the kite — pitch, roll, yaw, and position — plus three more on the tether. Variano puts it bluntly: “Control algorithms are where rubber meets road.”
SRI International’s Manta: Testing in San Francisco Bay
SRI International’s Manta project introduces a mechanical twist — literally. Instead of relying on high-ratio gearing to spin its generator, the Manta uses a twisted-string tether that drives the generator directly as the kite sweeps outward. The result is a system that’s simpler to maintain, more compact, and less expensive to build.
In 2025, the team tested a 1-meter prototype in San Francisco Bay, anchored to a small fishing vessel so that only tidal currents moved the kite. At the bay’s typical peak flow of 1.5 m/s, the device drew over 100 watts. A 2-meter pilot system is now under evaluation, and simulations suggest it can average 1 kilowatt across a full tidal cycle — including the sluggish flows near slack tide.
The team watches each test closely. “We see the generator gathering power, pausing, expending a little energy to reload the string, then repeat,” says Variano. Across hundreds of sweeps, output has been tracked through different phases of the tidal cycle, with additional towing tests run at a steady 1 m/s to isolate variables.
Their U.S. Department of Energy contract sets a clear commercial benchmark: 1 kW at under $0.09 per kilowatt-hour. That figure shapes every design decision. “If we were far off,” Variano notes, “we wouldn’t still be trying.”
Minesto’s Megawatt Kite and the Spectrum of Ambition
SRI’s kilowatt-scale ambitions exist alongside a much larger proof of concept. Swedish firm Minesto operates a kite with a 12-meter wingspan connected to a 1.2-megawatt generator, already delivering grid electricity to the Faroe Islands. Where Manta keeps its generator at the tether’s base, Minesto mounts the generator directly on the wing and transmits power to shore via undersea cable.
CEO Martin Edlund argues that mean peak flows as low as 1.5 m/s carry genuine economic potential. “When you study tidal flows of the world’s oceans, you realize low to medium flows are in abundance,” he says. Being grid-connected and attracting capital backs that claim — though Edlund himself stops short of calling the company commercially successful just yet.
At the smaller end of the spectrum, BladeRunner Energy is testing a tethered corkscrew-style rotor in Alaskan rivers. Its 2-meter-diameter device generates 5 kW in flows between 1.8 and 2 m/s. If an upgraded 11-kW generator performs as expected, BladeRunner plans to deploy to the native village of Napaimute with one direct goal: replace 100 percent of the community’s diesel consumption.
Remote Communities as the Proving Ground
Alaska’s remote coastal and river communities are a telling test case. Many rely on diesel generators to fill the gaps that wind and solar leave — an arrangement that’s both expensive and carbon-heavy. Fuel must be barged in, sometimes seasonally, and the costs fall hardest on small populations with few alternatives.
Manta’s next planned trial targets the Metlakatla Indian Community in Alaska’s Alexander Archipelago. The community has shared traditional knowledge of the narrow tidal straits nearby, knowledge that will help the team site the device where flows are strongest. “We’re small, portable, and could provide clean power to people in these remote coastlines,” says Variano.
The match between kilowatt-scale tidal kites and off-grid household needs is practical, not incidental. A community that needs basic electricity — lighting, refrigeration, communication — doesn’t require megawatts. It requires reliability, which is precisely what tidal energy, by its nature, offers.
Full efficiency analysis of the Manta system across a complete tidal cycle is expected by the end of 2026. That milestone will determine whether the design is ready to move toward commercialization — and whether Alaska’s tidal straits might finally offer something the diesel barge never could: power that arrives on its own, every day, without being ordered.
Carlos is an engineer with strong expertise in technical and industrial topics. He previously worked at international companies such as Siemens and is multilingual.
