We’ve covered a few underwater autonomous robots designed to make exploring the murky depths easier here on Gizmag, such as Snookie and the Talisman, but none that can generate its own power – until now. NASA, US Navy and university researchers have successfully demonstrated the first underwater vehicle to be powered entirely by natural, renewable, ocean thermal energy. Scalable for use on most robotic oceanographic vehicles, this technological breakthrough 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 Sounding Oceanographic Lagrangrian Observer Thermal RECharging (SOLO-TREC) autonomous underwater vehicle uses a novel thermal recharging engine powered by the natural temperature differences found at different ocean depths.

How it works

SOLO-TREC draws upon the ocean's thermal energy as it alternately encounters warm surface water and colder conditions at depth. Keys to its operation are the carefully selected waxy substances known as phase-change materials that are contained in ten external tubes, which house enough material to allow net power generation. As the vehicle 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.

Ocean Testing

The 84kg (183lbs) SOLO-TREC prototype was tested and deployed by researchers at NASA's Jet Propulsion Laboratory, Pasadena, California; and the Scripps Institution of Oceanography, University of California, San Diego on November 30, 2009, about 161km (100 miles) southwest of Honolulu. They completed the first three months of an ocean endurance test of the prototype vehicle off the coast of Hawaii in March.

Plot map of the path of the SOLO-TREC autonomous underwater vehicle since its deployment south of Hawaii on November 30, 2009 (Image: NASA/JPL/SIO/NOAA/US Navy/NGA/GEBCO/Google)

So far, SOLO-TREC has completed more than 300 dives from the ocean surface to a depth of 500m (1,640ft). Its thermal recharging engine produced about 1.7 watt-hours, or 6,100 joules, of energy per dive, enough electricity to operate the vehicle's science instruments, GPS receiver, communications device and buoyancy-control pump.

"People have long dreamed of a machine that produces more energy than it consumes and runs indefinitely," said Jack Jones, a JPL principal engineer and SOLO-TREC co-principal investigator. "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."

"Energy harvesting from the natural environment opens the door for a tremendous expansion in the use of autonomous systems for naval and civilian applications," said Thomas Swean, the Office of Naval Research program manager for SOLO-TREC. "This is particularly true for systems that spend most of their time submerged below the sea surface, where mechanisms for converting energy are not as readily available. The JPL/Scripps concept is unique in that its stored energy gets renewed naturally as the platform traverses ocean thermal gradients, so, in theory, the system has unlimited range and endurance. This is a very significant advance."

SOLO-TREC is now in an extended mission with the JPL/Scripps team planning to operate SOLO-TREC for many more months, if not years. The public can even keep tabs on SOLO-TREC's travels via an online map.

"The present thermal engine shows great promise in harvesting ocean thermal energy," said Russ Davis, a Scripps oceanographer. "With further engineering refinement, SOLO-TREC has the potential to augment ocean monitoring currently done by the 3,200 battery-powered Argo floats."

The international Argo array measures temperature, salinity and velocity across the world's ocean. NASA and the US Navy also plans to apply this thermal recharging technology to the next generation of submersible vehicles.