San Francisco Bay tidal kites generate electricity by moving faster than surrounding currents
In remote coastal communities, the rhythm of daily life often runs on diesel — generators humming through the night, solar panels idled by overcast skies. Energy is expensive, unreliable, and tethered to supply chains that stretch hundreds of miles.
Now, researchers are testing something unexpected: a kite that flies silently beneath the water’s surface, tracing figure-eight loops through tidal currents to generate clean electricity. Tides are among the most predictable forces in nature, yet their energy remains largely untapped. That may be starting to change.
Why tides make a compelling energy source
Tidal currents run on a schedule set by the Moon and Sun — gravitational forces that don’t vary with weather or season. That predictability alone sets tidal energy apart from wind and solar, which can go quiet for days. For communities that need reliable power, not just occasional power, that consistency matters enormously.
Rudders, elevators, and aileron-like blades control pitch and angle, while the generator’s load shifts constantly and the kite must maintain speed throughout.
Water also carries far more energy than air. Because it’s so much denser, a tidal current delivers more power per swept area than a comparable wind installation. As Evan Variano, a civil and environmental engineering professor at UC Berkeley, puts it: “Underwater kites can be smaller because water is so much denser.”
Many remote coastal communities — particularly in Alaska — currently run on diesel generators to fill the gaps when solar panels go dark or winds die down. That fuel arrives via supply chains stretching hundreds of miles, making electricity expensive and the whole system fragile. Fixed seafloor turbines offer one approach to tapping tidal flows, but they struggle near slack tide, when currents slow to a crawl.
Underwater kites sidestep that limitation by flying actively through the water rather than sitting still, extracting useful power even during the tidal cycle’s weakest moments.
How an underwater kite actually works
The principle behind an underwater kite is the same one that keeps a child’s kite aloft on a breezy afternoon. Hydrodynamic lift acts on a wing-shaped device moving through water just as aerodynamic lift acts on a kite moving through air. The water doesn’t need to be fast — the kite’s motion does the work, IEEE Spectrum told.
Instead of drifting passively with the current, the kite follows an autonomous figure-of-eight flight path. That looping trajectory lets it move much faster than the surrounding water, multiplying the effective flow it experiences. Couple that motion to a generator — via a tether that spins or extends — and the system produces usable electricity.
The engineering challenge lies in keeping the kite flying continuously. Its autopilot must manage nine degrees of freedom: six on the kite itself and three on the tether. Rudders, elevators, and aileron-like blades control pitch and angle, while the generator’s load shifts constantly and the kite must maintain speed throughout. “Control algorithms are where rubber meets road,” says Variano.
SRI International’s Manta: Testing in San Francisco Bay
SRI International’s Manta project introduces a mechanical twist — literally. Rather than using conventional gearing to connect tether motion to a generator, Manta uses a twisted-string tether that spins the generator directly, producing a simpler, more compact, and less expensive system.
In 2025, the team tested a 1-meter wingspan prototype attached to an anchored fishing vessel in San Francisco Bay. At the bay’s typical peak tidal flow of 1.5 meters per second, the kite drew over 100 watts — enough to confirm the concept works in real conditions, not just simulations.
Testing has since moved to a larger 2-meter wingspan pilot system with a 15-meter tether. Simulations project it should generate 1 kilowatt of average power across a full tidal cycle, including flows below 1 meter per second. Researchers watch the kite sweep back and forth like a wakeboarder, the tether untwisting with each pass. “We see the generator gathering power, pausing, expending a little energy to reload the string, then repeat,” Variano says.
The team’s Department of Energy contract sets a target of under $0.09 per kilowatt-hour. Variano is direct about where they stand: “If we were far off, we wouldn’t still be trying.”
Minesto’s Megawatt Milestone and What It Proves
While Manta works at the kilowatt scale, Swedish company Minesto has already deployed something far larger. Its 12-meter wingspan kite operates near the Faroe Islands in the Atlantic, driving a 1.2-megawatt generator connected directly to the local grid. Minesto places its generator on the wing itself, transmitting power to shore through an undersea cable — a different architecture from Manta’s tether-based approach.
CEO Martin Edlund argues that low-to-medium tidal flows — abundant across the world’s oceans — represent a viable economic target. He’s measured about what success means so far. “Calling us commercially ‘successful’ is a bit ahead of ourselves,” he says, but notes that being grid-connected and attracting continued investor interest signals real progress.
That grid connection matters. It demonstrates the technology can move beyond test tanks and research vessels into infrastructure that real communities depend on.
From labs to remote communities: Alaska as the proving ground
The Manta team’s next destination is Alaska’s Alexander Archipelago, where they plan to work alongside the Metlakatla Indian Community. Community members have shared traditional knowledge of the narrow straits that funnel tidal flows into strong, consistent currents — exactly the kind of site-selection insight that no simulation can replicate.
Elsewhere in Alaska, BladeRunner Energy is already testing a tethered rotor system in river currents. Its 2-meter-diameter corkscrew-style device generates 5 kilowatts in flows between 1.8 and 2 meters per second, tested at the University of Alaska’s Tanana River site. If trials with a new 11-kilowatt generator succeed, BladeRunner plans to ship the system to the native village of Napaimute. “We want to integrate with the Napaimute microgrid system and replace 100 percent of the diesel they consume,” says co-founder and CEO Moriel Arango.
University of Michigan professor Chris Vermillion pushes back against the idea that underwater kites are too immature or too risky to bet on. “If you compare underwater kites to other marine technologies, I don’t see them being behind the eight ball at all,” he says. Full efficiency analysis of Manta’s performance across a complete tidal cycle is expected by the end of 2026 — a milestone that could shape deployment decisions for years to come. The tides will keep moving. The question now is how much of that energy communities will be able to catch.
Daniel García is an Editor-in-Chief with strong expertise in structural work and engineering principles. He combines this technical foundation with deep knowledge of energy, spatial design, and emerging technologies, bringing a forward-thinking and analytical approach to editorial leadership.
