The Slocum Thermal Glider has travelled the 4,000-metre-deep Virgin Islands Basin between St. Thomas and St. Croix more than 20 times, powered entirely by the ocean's heat energy. ((Webb Research))

A submersible robot that propels itself using heat energy from the ocean is the first underwater vehicle to travel great distances using only "green" energy, say a group of U.S. researchers.

Researchers at the Woods Hole Oceanographic Institution (WHOI) and Webb Research Corp. said Thursday their "thermal glider" has criss-crossed the 4,000-metre-deep Virgin Islands Basin between St. Thomas and St. Croix more than 20 times since it was launched in December, travelling a total distance of 1,400 kilometres so far.

Though called a glider, the autonomous robot doesn't skip across the ocean surface, instead diving and rising, tracing a saw-tooth pattern as it cuts through the ocean's layers. The glider reaches depths of about 1,250 metres before it begins its rise, said Doug Webb, a former WHOI researcher who came up with the thermal glider idea in the 1980s.

Traditionally, gliders use battery-powered motors and mechanical pumps to move ballast water or oil from inside the vehicle to a bladder on the vehicle's exterior. This changes its volume and buoyancy without changing its mass, causing it to sink or rise while at the same time pushing it forward.

The researchers say the same principle is at work, only their thermal glider gets its energy from the heat of the ocean. Warmer water closer to the surface warms wax-filled tubes inside the engine, which expand to push oil from the interior to the exterior of the glider. As the glider dives and reaches deeper, colder waters, the wax cools and contracts, bringing the oil back to the interior.

"We are tapping a virtually unlimited energy source for propulsion," said WHOI oceanographer Dave Fratantoni, a member of the research team, in a statement Thursday.

The glider moves relatively slowly, travelling horizontally at a speed of about a quarter of a metre per second, said Webb, or less than a kilometre per hour. But the researchers said it could be useful for conducting undersea research or reconnaissance, replacing battery-powered machines.

Although the glider propels itself with "green" energy," it still requires batteries to power external sensors used to monitor surroundings and chart a course using the global positioning system.

The researchers said they hope future models of the glider will be able to draw energy from the oceans to power the sensors as well.