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Underwater Kites
Tom Lombardo posted on November 17, 2013 |

As young engineering students, my friends and I used to joke about certain college athletes* who were apparently studying “underwater basket weaving” in order to maintain their athletic scholarships. I never would have imagined that thirty years later I’d be writing a serious engineering article about underwater kite flying! But with 25 GW of usable power available in just the Florida Gulf Stream, this idea is nothing to laugh about.

Researchers at Worcester Polytechnic Institute (WPI), led by mechanical engineering professor David Olinger, are investigating Tethered UnderSea Kites (TUSKs) as a source of renewable energy. Dr. Olinger has experience with airborne wind energy (AWE) systems, so taking the same principle and applying it under the ocean seemed like a natural extension of his prior work. Air and water both obey the same laws of fluid dynamics so the idea is essentially the same, except that the forces produced by water currents are much larger due to water’s higher density, so the underwater kites need rigid wings.

The above image shows the components of the TUSK system. In this model, the kite is tethered to a platform that’s anchored to the ocean floor through its mooring lines. The mooring lines double as conductors to transmit electricity to distribution lines. The kite moves in a cross-current figure-8 motion at velocities three to five times higher than the ocean current speeds. (Previous experiments with airborne wind energy systems produced these numbers.) Because these turbines are moving faster than the ocean current, they can generate more energy than stationary water turbines. (Since turbine power is a function of velocity cubed, four times the velocity will produce 64 times the power of a stationary turbine of the same size.)

The above configuration has the turbine-generator attached to the kite itself. Another proposed model would have the generator attached to the platform as shown here:

The kite moves away from the platform, pulling its tether and causing the generator to spin. When fully extended, the generator turns into a motor and retracts the kite. The retraction stage uses much less energy than is generated during the power phase, thanks to the control mechanism reducing the kite’s angle of attack. Again, this principle has been demonstrated using airborne wind energy kites.

Are you wondering how the rudder, control surfaces, ballast tanks, and angle of attack will be controlled? Several algorithms exist for airborne wind energy systems. One is described in this research article published earlier this year, in which the authors propose a new control algorithm while also giving references to earlier methods. If you’re interested in control systems, that should give you plenty of reading material! One goal of the TUSK team is to evaluate various control algorithms as they apply to underwater systems.

This research has multiple purposes. It examines the technical and economic feasibility of TUSKs as a viable renewable energy source. As a project-based learning activity, it gives engineering students an opportunity to perform experiments, collect data, develop computer models, and design control systems using a project that has a significant real-world application. The results will be used in a renewable energy course at WPI as well as a summer program for high-school students. If you’ve read my previous articles, you know I’m a big advocate of renewable energy and education, so any project that addresses both is a winner in my book!

Funded by a grant from the National Science Foundation, Dr Olinger’s team will begin conducting its study early in 2014. I’ve asked him to keep me up-to-date on findings. Hopefully I’ll have a follow-up article showing some of their results.

Images courtesy of Dr. David Olinger

*I realize that the majority of college athletes are serious students with real majors.