Twisted Tethers Make Tidal Energy Cheaper and Cleaner

Off-grid coastal communities that rely on weather-dependent solar and expensive diesel to meet their energy needs may soon have a new addition to their power production portfolio: Tidal energy. Researchers are testing a buoyant kite that flies underwater to determine whether it can squeeze enough power from intermittent, often slow-moving tidal currents to provide clean energy to small coastal communities. It’s

The rise and fall of Earth’s tides draws water through straits and channels each day with impeccable predictability, driving currents that can carry significant kinetic energy. And marine energy is power dense: It provides more power per swept area, compared to a similar wind field. “Underwater kites can be smaller because water is so much denser,” says Evan Variano, professor of civil and environmental engineering at UC Berkeley

Hydrodynamic lift causes a wing-shaped kite to “fly” in moving water in a similar manner to a kite in the air on a windy day. These underwater kites can autonomously fly figure-of-eight paths with the help of modern sensors and onboard robotics or electronics, allowing them to swoop across tidal currents much faster than the water itself is moving. Couple that movement to a generator and the system can harvest useful amounts of electricity, even close to slack tide—the tidal cycle’s slowest flows. That’s something other strategies for harvesting power, like a fixed seafloor turbine placed in a channel, can’t do.

Kites are beginning to find their wings. The furthest-along has installed a megawatt-scale demo system that delivers electricity from a sheltered bay to an electrical grid. Others, aiming for kW-scale systems, are starting to show what they can do in real-world settings. They’re measuring efficiency in different conditions and preparing pilot-trials of their gear in remote communities.

In one kite design, a generator sits at the base of the tether, either on a boat or an anchored mooring. The pull from the kite as the tide moves it extends the tether and spins the generator, producing power. Then, like a giant yo-yo, the generator reels it back in using a fraction of that power, thanks to the much lower force on the kite while it’s in its most streamlined position.

The Manta project from SRI International, a research institute in Menlo Park, Calif., adds a special twist. The Manta kite uses a twisted-string tether rather than a static tether to spin the generator without needing high-ratio gearing. That makes for a less-expensive system that’s simpler to maintain, more compact, and more efficient.

But any underwater kite faces the same fundamental challenge, says Chris Vermillion, a mechanical engineering professor at University of Michigan. “It must be continuously flying. That periodic motion requires substantial control.”

That means that kites require an autopilot that constantly adjusts flight path in response to generator load. It’s an intricate dance: There are six…