Underwater Kites

Most wind turbines need to be placed where there is a strong and reliable wind resource. But wind is not the only moving fluid that contains enormous amounts of energy. Ocean currents are constantly transporting water across the planet, and because water is hundreds of times denser than air, even relatively slow currents can exert substantial forces on underwater structures.

This has led engineers to develop underwater tidal kites. Instead of constructing a massive turbine on the seabed, a tidal kite consists of a wing-shaped structure attached to a tether. The wing flies through the water in large looping or figure-eight patterns. As it moves, water flows rapidly across its surfaces, generating lift. That lift pulls the kite through the water at speeds much greater than the surrounding current.

A turbine mounted on the kite converts some of this motion into electricity. The kite continuously changes its direction and follows a programmed trajectory, allowing it to extract energy from the current more efficiently than simply leaving a turbine stationary in the water. Once it reaches the end of its path, the system changes its configuration and repeats the cycle.

The idea is similar to how a sailboat can travel faster than the wind itself. The surrounding fluid does not need to move rapidly if the vehicle can create relative motion through its trajectory. Engineers can therefore extract useful energy from currents that might otherwise appear too slow to power a conventional turbine.

The challenge is making the system survive an environment that is extraordinarily difficult for machinery. Saltwater causes corrosion, marine organisms grow on exposed surfaces, underwater maintenance is expensive, and the tether must withstand enormous repeated forces. Every component must also survive millions of loading cycles without fatigue failure.

What makes tidal kites interesting is that they demonstrate how changing the motion of a machine can be just as important as changing its size or power source. Rather than building a gigantic turbine and waiting for the ocean to push water through it, engineers can make the machine move through the ocean itself. The energy source has not changed; only the way engineers interact with it has.