NASA, U.S. Navy and university researchers have successfully demonstrated the first robotic underwater vehicle to be powered entirely by the natural temperature differences found in varying depths of the ocean.

The new technology, found on an undersea robot called the Sounding Oceanographic Lagrangrian Observer Thermal RECharging (SOLO-TREC), is scalable for use on most other robotic oceanographic vehicles, NASA said.

The energy-reaping technology could usher in a new generation of autonomous underwater vehicles capable of virtually indefinite ocean monitoring for climate and marine animal studies, exploration and surveillance.

The performance of underwater robotic vehicles has traditionally been limited by power considerations. SOLO-TREC, with its novel thermal recharging engine, might offer a way around this problem.

"People have long dreamed of a machine that produces more energy than it consumes and runs indefinitely," Jack Jones, a NASA Jet Propulsion Laboratory (JPL) principal engineer in Pasadena, Calif. and SOLO-TREC co-principal investigator, said in a statement.

"While not a true perpetual motion machine, since we actually consume some environmental energy, the prototype system demonstrated by JPL and its partners can continuously monitor the ocean without a limit on its lifetime imposed by energy supply," Jones said.

The 183-pound (84-kilogram) SOLO-TREC prototype was tested and deployed by a joint JPL and Scripps Institution of Oceanography, University of California, San Diego team on Nov. 30, 2009, about 100 miles (161 kilometers) southwest of Honolulu, Hawaii. The three-month test wrapped up last month.

Energy from the briny warm and cold

SOLO-TREC draws upon the ocean's thermal energy as it alternately encounters warm surface water and colder conditions at depth.

Key to its operation is the carefully selected waxy substances known as phase-change materials that are contained in 10 external tubes.

As the robot surfaces and encounters warm temperatures, the material melts and expands; when it dives and enters cooler waters, the material solidifies and contracts. The expansion of the wax pressurizes oil stored inside the float.

This oil periodically drives a hydraulic motor that generates electricity and recharges the vehicle's batteries. Energy from the rechargeable batteries powers the float's hydraulic system, which changes the float's volume (and hence buoyancy), allowing it to move vertically.

The watery road ahead

So far, SOLO-TREC has completed more than 300 dives from the ocean surface to a depth of 1,640 feet (500 meters). Its thermal recharging engine produced about 1.7 watt-hours of energy per dive, enough electricity to operate the vehicle's science instruments, GPS receiver, communications device and buoyancy-control pump.

SOLO-TREC is now in an extended mission as the JPL-Scripps team plans to operate the autonomous underwater vehicle for many more months, if not years.

NASA and the U.S. Navy also plan to apply this thermal recharging technology to the next generation of submersible vehicles.

"Most of Earth is covered by ocean, yet we know less about the ocean than we do about the surface of some planets," said Yi Chao, a JPL principal scientist and SOLO-TREC principal investigator.

"This technology to harvest energy from the ocean will have huge implications for how we can measure and monitor the ocean and its influence on climate."