By Jonathan Fildes
Science and technology reporter, BBC News
A sea-going robotic glider that harvests heat energy from the ocean has been tested by US scientists.
The yellow, torpedo-shaped machine has been combing the depths of seas around the Caribbean since December 2007.
The team which developed the autonomous vehicle say it has covered "thousands of kilometres" during the tests.
The team believe the glider - which needs no batteries - could undertake oceanographic surveys for up to six months at a time.
"We are tapping a virtually unlimited energy source for propulsion," said Dave Fratantoni of the Wood's Hole Oceanographic Institute (WHOi).
But Steve McPhail, an expert in autonomous underwater vehicles at the National Oceanography Centre (NOC), Southampton, said the machine would not totally do away with batteries.
"You still need to provide power for the sensors, for the data-logging system and for the satellite communications system to get the data back," he said.
As a result, the vehicle would have to intermittently return to a ship or shore to recharge its batteries.
"It's always a trade-off between the power used for the propulsion system and the power used for the sensors," said Mr McPhail.
Oceanographers are increasingly looking at ways to study the oceans over long periods and in real time.
Researchers wired the Atlantic in 2004 for the Rapid project
This is important for understanding natural variations in circulation, for example, and for looking for any changes.
Already scientists have deployed large networks of sensors across the oceans.
For example, in 2004, NOC researchers strung sets of instruments across the Atlantic to measure circulation patterns.
The Rapid project, as it was known, painted the first detailed picture of Atlantic Ocean currents and showed how they vary throughout the year.
Its successor - Rapid Watch - has just received £16m from the Natural Environment Research Council and will monitor the Gulf Stream until 2014.
Scientists are also in the process of wiring the Pacific.
One project, the Argo network, will consist of an array of 3,000 floats strung out every 300km across the vast ocean.
Sensors on the floats will provide 100,000 temperature and salinity profiles every year.
Another network, the Monterey Accelerated Research System (Mars), will connect a research station in California with a sensor array deployed on the edge of Monterey Canyon, the deepest submarine canyon off the continental West Coast.
The new vehicles could add to that knowledge by filling in the gaps between the sensors.
The glider has been tested in the waters of the Virgin Islands
For example, it is proposed that Rapid Watch will use an armada of gliders alongside stationary sensors.
The machines are already used in oceanography and propel themselves through the ocean by changing their buoyancy to dive and surface. Wings generate lift and a vertical tail fin and rudder is used to steer.
The latest glider has been developed by Webb Research Corporation and WHOi.
It generates its energy for propulsion from the differences in temperature between warm surface waters and colder, deeper layers of the ocean.
Wax-filled tubes inside the craft expand when it is gliding through warmer water. This heat is used to push oil from a bladder inside the hull to one outside, changing its buoyancy.
Cooling of the wax at depth reverses the cycle.
Since December 2007, the prototype machine has been crisscrossing a 4,000m-deep basin in the Virgin Islands of the Caribbean.
The machine traces a saw-tooth profile through the water column as it lazily glides through the ocean, surfacing periodically to fix its positions via GPS and to relay data back to base.
According to WHOi researchers the vehicle crossed the basin between St Thomas and St Croix more than 20 times studying local currents.
The eventual aim of the project is to deploy a fleet of vehicles to study much larger flows in the North Atlantic.
"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 Dr Fratantoni. "They can work around the clock in all weather conditions."