Adapted from a BBC online article: Friday 8 February 2008

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 that developed the autonomous vehicle say it has covered "thousands of kilometres" during the tests.

The team believe the glider - which needs no batteries for propulsion - 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 the machine cannot totally do away with batteries. They are still needed for powering the sensors, for the data-logging system and for the satellite communications system to get the data back. As a result, the vehicle would have to return to a ship or shore intermittently to recharge its batteries.

Oceanographers are increasingly looking at ways to study the oceans over long periods of time and in real time.

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.

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."