‘Glider’ uses ocean power to fly

An undersea robot “glider” that harvests heat energy from the sea to propel itself across oceans? You have to be kidding – surely?

But this is exactly what scientists have achieved at America’s world-famous Woods Hole Oceanographic Institute. Working with US company Webb Research Corporation, they have successfully flown the first environmentally powered robotic vehicle through the ocean.

Together with specialists at the University of the Virgin Islands, they launched a prototype “thermal glider” off the coast of St Thoma in the Caribbean Sea.

The vehicle has apparently been travelling ever since, crisscrossing the 4,000m deep Virgin Islands Basin time and again between the islands of St Thomas and St Croix.

A trip from St Thomas to Bermuda is planned over the next few weeks.

Unlike motorised, propeller-driven vehicles, gliders propel themselves through the ocean 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.

Gliding underwater vehicles trace a saw-tooth (up and down zigzag) profile through the ocean’s layers, surfacing periodically to fix their positions via the global positioning satellite system now commonly used in the family car and to communicate via satellite to a shore lab.

“Gliders can be put to work on tasks that humans wouldn’t want to do or cannot do because of time and cost concerns,” says Dave Fratantoni, oen of the Woods Hole scientists.

“They can work around the clock in all weather conditions. The vehicles can carry a variety of sensors to collect measurements such as temperature, salinity and biological productivity.

“Gliders also operate quietly, which makes them ideal for acoustic studies.”

Though the thermal glider is not the first autonomous underwater vehicle to traverse great distances or stay at sea for long periods, it is the first to do so with green energy.

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 glider’s 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,” says Fratantoni.

The thermal glider concept was originally hatched back in the 1980s by Doug Webb when he was at Woods Hole.

He worked closely with oceanographer Henry Stommel, who championed the idea to the US Navy and the oceanographic community.

Stommel even wrote a science fiction story about a fleet of these gliders quietly cruising the oceans. Webb and Stommel named the vehicles “Slocum” gliders for Joshua Slocum, the first man to sail single-handed.