Glider Moves by Harvesting Ocean’s Heat Energy

Researchers from the Woods Hole Oceanographic Institution (WHOI) and Webb Research Corp. have successfully flown the first hybrid robotic vehicle through the ocean. The new robotic glider harvests heat energy from the ocean to propel itself across thousands of kilometers of water.

Since December 2007, the vehicle has been traveling uninterrupted, crisscrossing the 4,000-meter-deep Virgin Islands Basin between St. Thomas and St Croix.

“Gliders can be put to work on tasks that humans wouldn’t want to do or cannot do because of time and cost concerns,” said Dave Fratantoni, an associate scientist in the WHOI Department of Physical Oceanography. The vehicles can carry a variety of sensors to collect measurements.

Gliding underwater vehicles trace a saw-tooth profile through the ocean’s layers, surfacing periodically to fix their positions via the Global Positioning System and to communicate via Iridium satellite to a shore lab.

The thermal glider is not the first autonomous underwater vehicle to traverse great distances or stay at sea for long periods. However, it differs from other gliders in how it draws its energy for propulsion. 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. The idea is to increase or decrease the displacement (volume) of the glider without changing its mass.

The thermal glider relies on 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 Fratantoni. 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, and to find a way to convert some of the thermal energy to power for the electronics.

The current mission is a test-drive, preparing Fratantoni and colleagues for a launch of a fleet of gliders in the subtropical gyre of the North Atlantic, a key region for assessing the ocean’s response to climate change.

Woods Hole Oceanographic Institution (WHOI)
www.whoi.edu

Webb Research Corp.
www.webbresearch.com