26 February 2008
Unlike motorized, 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 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.
“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 Fratantoni, an associate scientist in the WHOI Department of Physical Oceanography. “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. 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 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 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.
The engineering trial for the thermal glider is the first step in a broader plan by Fratantoni and colleagues to launch a fleet of gliders for studies of the waters in the subtropical gyre of the North Atlantic, a key region for assessing the ocean’s response to climate change.
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