Sea glider takes flight

Researchers from the Woods Hole Oceanographic Institution (WHOI) and Webb Research Corporation of Falmouth, Massachusetts have successfully flown the first thermally powered robotic vehicle through the sea.

Gliders propel themselves through the sea by changing their buoyancy to dive and surface. Wings generate lift, while a vertical tail fin and rudder allow the vehicles to be steered horizontally.

Most gliders rely on battery-powered motors and mechanical pumps to move ballast water or oil from inside the vehicle’s pressure hull to outside, increasing or decreasing the displacement of the glider without changing its mass.

The new thermal glider draws its energy for propulsion from the differences in temperature (thermal stratification) between warm surface waters and colder, deeper layers of the ocean. The heat content of the ocean warms wax-filled tubes inside the engine. The expansion of the warming wax converts heat to mechanical energy, which is stored and used to push oil from a bladder inside the vehicle’s hull to one outside, changing its buoyancy. Cooling of the wax at depth completes the cycle.

‘We are tapping a virtually unlimited energy source for propulsion,’ said Dave Fratantoni an associate scientist in the WHOI Department of Physical Oceanography. The computers, radio transmitters, and other electronics on the glider are powered by alkaline batteries, which are, for now, the principal limit on the length of operation. Webb Research is working to reduce the electrical needs of the instruments, while also developing the capability to convert some of the thermal energy to power for the electronics.

In December 2007, a research team led by Fratantoni and Roy Watlington of the University of the Virgin Islands launched a prototype ‘thermal glider’ off the coast of St Thomas. The vehicle is said to have been traveling uninterrupted ever since, crisscrossing the 4,000m deep Virgin Islands Basin between St Thomas and St Croix more than 20 times.

(Photo: John Lund)

Engineers and researchers, including research associate John Lund and postdoctoral investigator Ben Hodges from WHOI, and engineers Clayton Jones and Tod Patterson of Webb Research, project that the thermal glider could continue its current mission for up to six months.